Chytridiomycosis: Difference between revisions

From Wikipedia, the free encyclopedia
Browse history interactively
← Previous editNext edit →
Content deleted Content added
VisualWikitext
Chaleur (talk | contribs)
190 edits
No edit summary
Restored content by User:FNR498, along with edits and revisions, + integrating sections from Batrachochytrium. Still needs cleanup and more can be added but this is a good start.
Line 1: Line 1:
[[Image:Chytridiomycosis.jpg|thumb|150px|A chytrid-infected frog.]]
[[Image:Chytridiomycosis.jpg|thumb|150px|A chytrid-infected frog.]]
[[Image:Chytridiomycosis2.jpg|thumb|175px|Chytridiomycosis in ''[[Atelopus varius]]'' - two [[Sporangium|sporangia]] containing numerous [[zoospore]]s are visible]]
[[Image:Chytridiomycosis2.jpg|thumb|175px|Chytridiomycosis in ''[[Atelopus varius]]'' - two [[Sporangium|sporangia]] containing numerous [[zoospore]]s are visible]]
'''Chytridiomycosis''' is an [[infectious disease]] of [[amphibians]], caused by the [[chytrid]] ''[[Batrachochytrium dendrobatidis]]'', a non-hyphal zoosporic [[fungus]]. Chytridiomycosis has been linked to dramatic population [[Amphibian declines|declines]] or even extinctions of amphibian species in western [[North America]], [[Central America]], [[South America]], eastern [[Australia]], and [[Dominica]] and [[Montserrat]] in the [[Caribbean]]. The fungus is capable of causing sporadic deaths in some amphibian populations and 100% mortality in others. There is no effective measure for control of the disease in wild populations. The disease has been proposed as a contributing factor to a global [[decline in amphibian populations]] that apparently has affected 30% of the amphibian species of the world.<ref>Stuart, S. N., J. S. Chanson, ''et al.'' (2004). "Status and trends of amphibian declines and extinctions worldwide." ''Science'' 306: 1783-1786.</ref>
'''Chytridiomycosis''' is an [[infectious disease]] of [[amphibians]], caused by the [[chytrid]] ''[[Batrachochytrium dendrobatidis]]'', a non-hyphal zoosporic [[fungus]]. Chytridiomycosis has been linked to dramatic population [[Amphibian declines|declines]] or even extinctions of amphibian species in western [[North America]], [[Central America]], [[South America]], eastern [[Australia]], and [[Dominica]] and [[Montserrat]] in the [[Caribbean]]. Much of the new world is also at risk of seeing the disease arrive within the coming years.<ref name="Olson">{{cite journal|last=Olson|first=Deanna H.|coauthors=Aanensen, David M.; Ronnenberg, Kathryn L.; Powell, Christopher I.; Walker, Susan F.; Bielby, Jon; Garner, Trenton W. J.; Weaver, George; Fisher, Matthew C.; Stajich, Jason E.|title=Mapping the Global Emergence of ''Batrachochytrium dendrobatidis'', the Amphibian Chytrid Fungus|journal=PLoS ONE|year=2013|volume=8|issue=2|pages=e56802|doi=10.1371/journal.pone.0056802|url=http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0056802|editor1-last=Stajich|editor1-first=Jason E|pmid=23463502|pmc=3584086}}</ref>
The fungus is capable of causing sporadic deaths in some amphibian populations and 100% mortality in others. There is no effective measure for control of the disease in wild populations. Various clinical signs are seen by individuals affected by the disease. There are a number of options for controlling this disease-causing fungus, though none have proved to be feasible on a large scale. The disease has been proposed as a contributing factor to a global [[decline in amphibian populations]] that apparently has affected approximately 30% of the amphibian species of the world.<ref>Stuart, S. N., J. S. Chanson, ''et al.'' (2004). "Status and trends of amphibian declines and extinctions worldwide." ''Science'' 306: 1783-1786.</ref>


==History==
==History==
The disease in its [[epizootic]] form was first discovered in 1993 in dead and dying frogs in [[Queensland]], [[Australia]]. Research since then has shown that it had been present in the country since at least 1978 and is widespread across Australia. It is also found in [[Africa]], the [[Americas]], [[Europe]], [[New Zealand]] and [[Oceania]]. In Australia, [[Panama]], and New Zealand, the fungus seemed to have suddenly ‘appeared’ and expanded its range at the same time frog numbers declined. However, it may simply be that the fungus occurs naturally and was only identified recently because it has become more virulent or more prevalent in the environment, or because host populations have become less resistant to the disease. The fungus has been detected in four areas of Australia — the east coast, [[Adelaide]], south-west [[Western Australia]] and the [[Kimberley]] — and is probably present elsewhere.<ref name=AustraliaGov>{{cite web|title=Chytridiomycosis (Amphibian Chytrid Fungus Disease)|url=http://www.environment.gov.au/biodiversity/invasive/publications/c-disease/pubs/c-disease.pdf|publisher=Australian Government Department of Sustainability, Environment, Water, Population and Communities|accessdate=14 October 2013}}</ref>
The disease in its [[epizootic]] form was first discovered in 1993 in dead and dying frogs in [[Queensland]], [[Australia]]. Research since then has shown that it had been present in the country since at least 1978 and is widespread across Australia. It is also found in [[Africa]], the [[Americas]], [[Europe]], [[New Zealand]] and [[Oceania]]. In Australia, [[Panama]], and New Zealand, the fungus seemed to have suddenly ‘appeared’ and expanded its range at the same time frog numbers declined. However, it may simply be that the fungus occurs naturally and was only identified recently because it has become more virulent or more prevalent in the environment, or because host populations have become less resistant to the disease. The fungus has been detected in four areas of Australia — the east coast, [[Adelaide]], south-west Western Australia and the Kimberley — and is probably present elsewhere.<ref name=AustraliaGov>{{cite web|title=Chytridiomycosis (Amphibian Chytrid Fungus Disease)|url=http://www.environment.gov.au/biodiversity/invasive/publications/c-disease/pubs/c-disease.pdf|publisher=Australian Government Department of Sustainability, Environment, Water, Population and Communities|accessdate=14 October 2013}}</ref>


The oldest documented case of ''Batrachochytrium'' is from a [[Biological specimen|specimen]] of an [[African clawed frog]] (''Xenopus laevis'') collected in 1938, and this species also appears to be essentially unaffected by the disease, making it a suitable [[Vector (epidemiology)|vector]].<ref name=Xenopuslaevis>Weldon; du Preez; Hyatt; Muller; and Speare (2004). ''[http://wwwnc.cdc.gov/eid/article/10/12/03-0804_article.htm Origin of the Amphibian Chytrid Fungus.]'' Emerging Infectious Disease 10(12).</ref> The first well-documented method of human [[pregnancy testing]] involved this species, and as a result large-scale international trade in living African clawed frogs began more than 60 years ago.<ref name=Xenopuslaevis/> If ''Batrachochytrium'' originated in Africa, it has been theorized that the African clawed frog was the vector of the initial spread out of the continent.<ref name=Xenopuslaevis/> The earliest documented case of the disease chytridiomycosis was an [[American bullfrog]] (''Rana catesbeiana'') collected in 1978.<ref name=Xenopuslaevis/> It is still not clear if it is a new emergent pathogen or if it is an old pathogen with recently increased [[virulence]].
The oldest documented occurrence of ''Batrachochytrium'' is from a [[Biological specimen|specimen]] of an [[African clawed frog]] (''Xenopus laevis'') collected in 1938, and this species also appears to be essentially unaffected by the disease, making it a suitable [[Vector (epidemiology)|vector]].<ref name=Xenopuslaevis>Weldon; du Preez; Hyatt; Muller; and Speare (2004). ''[http://wwwnc.cdc.gov/eid/article/10/12/03-0804_article.htm Origin of the Amphibian Chytrid Fungus.]'' Emerging Infectious Disease 10(12).</ref> The first well-documented method of human [[pregnancy testing]] involved this species, and as a result large-scale international trade in living African clawed frogs began more than 60 years ago.<ref name=Xenopuslaevis/> If ''Batrachochytrium'' originated in Africa, it has been theorized that the African clawed frog was the vector of the initial spread out of the continent.<ref name=Xenopuslaevis/> The earliest documented case of the disease chytridiomycosis was an [[American bullfrog]] (''Rana catesbeiana'') collected in 1978.<ref name=Xenopuslaevis/> It is still not clear if it is a new emergent pathogen or if it is an old pathogen with recently increased [[virulence]].


==Geographic Range==
⚫
==Disease progression==
The geographic range of chytridiomycosis is difficult to ascertain. If it occurs, the disease is always present where the fungus ''B. dendrobatidis'' is present. However, the disease is not always present where the fungus is. Reasons for amphibian declines are often termed ‘enigmatic’ because the cause is unknown. It is not fully understood why some areas are affected by the fungus while others are not. Oscillating factors such as climate, habitat suitability, and population density may be factors which cause the fungus to result in chytridiomycosis in amphibians of a given area. Therefore, when considering the geographic range of chytridiomycosis it is important to consider the range of ''B. dendrobatidis'' occurrence.4.<ref name=amphibiaweb>Whittaker, Kellie, and Vance Vredenburg. "An Overview of Chytridiomycosis." AMPHIBIAWEB. University of California, 17 May 2011. Web. 23 Oct 2013. <http://www.amphibiaweb.org/chytrid/chytridiomycosis.html.</ref> ).


The geographic range of ''B. dendrobatidis'' has recently been mapped, and spans much of the world. This could be seen as a portentous, though it is important to remember that chytridiomycosis does not always occur where the the fungus does. ''B. dendrobatidis'' has been detected in 56 of 82 countries, and in 516 of 1240 (42%) species using a data set of more than 36,000 individuals. ''B. dendrobatidis'' is widely distributed in the Americas, and detected sporadically in Africa, Asia, and Europe.<ref name="Olson" /> Asia, for example, has only 2.35% B. dendrobatidis prevalence<ref name="Swei et al 2011">{{cite journal|last=Swei|first=A.|coauthors=Rowley, J.J. L.; Rödder, D.; Diesmos, M. L. L.; Diesmos, A. C.; Briggs, C. J.; Brown, R.; ''et al.''|title=Is Chytridiomycosis an Emerging Infectious Disease in Asia?|journal=PLoS ONE|year=2011|volume=6|issue=8|pages=e23179|doi=10.1371/journal.pone.0023179|url=http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0023179|editor1-last=Arlettaz|editor1-first=Raphaël|displayauthors=30|pmid=21887238|pmc=3156717}}</ref>
⚫
Chytridiomycosis is believed to adhere to the following course: [[zoospore]]s first encounter amphibian skin and quickly give rise to [[sporangia]], which produce new zoospores.<ref>Berger, L., Hyatt, A.D., Speare, R. & Longcore, J.E. Dis. Aquat. Org. 68, 51-63 (2005).</ref> The disease then progresses as these new zoospores reinfect the host. Morphological changes in amphibians infected with the fungus include a reddening of the ventral skin, convulsions with extension of hind limbs, accumulations of sloughed skin over the body, sloughing of the superficial [[Epidermis (zoology)|epidermis]] of the feet and other areas, slight roughening of the surface with minute skin tags, and occasional small [[Ulcer (dermatology)|ulcer]]s or [[hemorrhage]]. Behavioral changes can include lethargy, a failure to seek shelter, a failure to flee, a loss of [[righting reflex]], and abnormal posture (e.g. sitting with the hind legs away from the body).<ref>{{cite web|last=Padgett-Flohr|first=G.E.|title=Amphibian Chytridiomycosis: An Informational Brochure|url=http://ccadc.us/docs/AmphibianDiseaseBrochure.pdf|publisher=California Center for Amphibian Disease Control|accessdate=14 October 2013|year=2007}}</ref>

The range suitable for ''B. dendrobatidis'' in the new world is vast. Regions with the highest suitability for ''B. dendrobatidis'' include habitats that contain the world’s most diverse amphibian faunas. Areas at risk are the Sierra Madre Pine Oak Occidental Forest, the Sonoran and Sinaloan dry forest, the Veracruz moist forest, Central America east from the Isthmus of Tehuantepec, the Caribbean Islands, the temperate forest in Chile and western Argentina south of latitude 300 S, the Andes above 1000 m of altitude in Venezuela, Colombia, and Ecuador, eastern slopes of the Andes in Peru and Bolivia, the Brazilian Atlantic forest, Uruguay, Paraguay, and Northeastern Argentina, as well as the southwestern and Madeira-Tapaj Amazonian ranforests.<ref name=Ron2005>{{cite journal|last=Ron|first=Santiago R.|title=Predicting the Distribution of the Amphibian Pathogen ''Batrachochytrium dendrobatidis'' in the New World|journal=Biotropica|year=2005|month=June|volume=37|issue=2|pages=209–221|doi=10.1111/j.1744-7429.2005.00028.x}}</ref>

The range of chytridiomycosis is dependent upon the range of ''B. dendrobatidis''. Even if there are not visible effects of the disease in a given area, the disease may simply be in a latent state, capable of affecting local amphibians as a result of unknown environmental or behavioral factors. Currently, the effects of chytridiomycosis are being seen most readily in Central America, eastern Australia, South America, and Western America.<ref name="Olson" />

==Causative agents==
{{Main|Batrachochytrium}}
Chytridiomycosis is caused by the fungus ''B. dendrobatidis''. ''B. dendrobatidis'' affects the layers of the skin that harbor keratin.<ref name=amphibiaweb/> When most species reach a ''B. dendrobatidis'' threshold of 10,000 zoospores they are not able to breathe, hydrate, osmoregualte, or thermoregulate correctly. This is proven by blood samples that show a lack of certain electrolytes, such as sodium, magnesium, and potassium. ''B. dendrobatidis'' is currently known to have two life stages. The first is the asexual zoosporangial stage.<ref name="Parker">Parker, J. M., I. Mikaelian, N. Hahn, and H. E. Diggs. 2002. Clinical diagnosis and treatment of epidermal chytridiomycosis in African clawed frogs (Xenopus tropicalis). American Association for Laboratory Animal Science 52:265-268.</ref> When a host first contracts the disease, spores penetrate the skin and attach themselves using microtubule roots.<ref name="Longcore">Longcore, J. E., A. P. Pessier, and D. K. Nichols. 1999. Batrachochytrium dendrobatidis gen. et sp. nov., a chytrid pathogenic to amphibians. Mycologia 91:219-227.</ref> The second stage takes place when the initial asexual zoosporangials produce motile zoospores.<ref name="Parker" /> To disperse and infect epidermal cells an aqueous surface is needed.<ref name="Parker" />
A second species of ''Batrachochytrium'', ''B. salamandrivorans'' was discovered in 2013 and is known to cause chytridiomycosis in salamanders.<ref name="Martel et al 2013">{{cite journal|last=Martel|first=A.|coauthors=Spitzen-van der Sluijs, A.; Blooi, M.; Bert, W.; Ducatelle, R.; Fisher, M. C.; Woeltjes, A.; Bosman, W.; Chiers, K.; Bossuyt, F.; Pasmans, F.|title=Batrachochytrium salamandrivorans sp. nov. causes lethal chytridiomycosis in amphibians|journal=Proceedings of the National Academy of Sciences of the United States of America |year=2013|volume=110|issue=38|pages=15325–15329|doi=10.1073/pnas.1307356110|pmid=24003137|pmc=3780879}}</ref>

⚫
==Disease transmission and progression==
''B. dendrobatidis'' is a waterborne pathogen that disperses zoospores into the environment.<ref>Morgan, J. A. T., V. T. Vredenburg, L. J. Rachowicz, R. A. Knapp, M. J. Stice, T. Tunstall, R. E. Bingham, J. M. Parker, J. E. Longcore, C. Moritz, C. J. Briggs, and J. W. Taylor. 2007. Population genetics of the frog-killing fungus ''Batrachochytrium dendrobatidis''. Proceedings of the National Academy of Sciences of the United States of America 104:13845-13850.</ref> The zoospores use flagella for locomotion through water systems until it reaches a new host and enter cutaneously.<ref name="Longcore" /> ''B. dendrobatidis''’ life cycle continues until new zoospores are produced from the zoosporangium and exit to the environment or reinfect the same host.<ref name="Longcore" /> Once the host is infected with ''B. dendrobatidis'' it can potentially develop chytridiomycosis, but not all infected hosts will develop chytridiomycosis.<ref name="Longcore" /> Other forms of transmission are currently unknown; however, it is postulated that chytridiomycosis can be transmitted by ''B. dendrobatidis'' through direct contact of hosts or through an intermediate host. <ref name="Longcore" />

Much of how ''B. dendrobatidis'' is successfully transmitted from one host to the next is largely unknown.<ref name="Nichols" />Research has found that once released into the aquatic environment, zoospores traveled less than 2 cm within 24 hours before they encysted.<ref name="Piotrowski">Piotrowski, J. S., S. L. Annis, and J. E. Longcore. 2004. Physiology of ''Batrachochytrium dendrobatidis'', a chytrid pathogen of amphibians. Mycologia 96:9-15.</ref> The limited range of ''B. dendrobatidis'' zoospores suggest that there is some unknown mechanism by which they transmit from one host to the next.<ref name="Piotrowski" /> Abiotic factors such as temperature, pH level, and nutrient levels affect the success of ''B. dendrobatidis'' zoospores.<ref name="Piotrowski" />B. dendrobatidis zoospores can survive within a temperature range of 4-25 °C and within a pH range of 6-7.<ref name="Piotrowski" />

⚫
Chytridiomycosis is believed to adhere to the following course: [[zoospore]]s first encounter amphibian skin and quickly give rise to [[sporangia]], which produce new zoospores.<ref>Berger, L., Hyatt, A.D., Speare, R. & Longcore, J.E. Dis. Aquat. Org. 68, 51-63 (2005).</ref> The disease then progresses as these new zoospores reinfect the host. Morphological changes in amphibians infected with the fungus include a reddening of the ventral skin, convulsions with extension of hind limbs, accumulations of sloughed skin over the body, sloughing of the superficial [[Epidermis (zoology)|epidermis]] of the feet and other areas, slight roughening of the surface with minute skin tags, and occasional small [[Ulcer (dermatology)|ulcer]]s or [[hemorrhage]]. Behavioral changes can include lethargy, a failure to seek shelter, a failure to flee, a loss of [[righting reflex]], and abnormal posture (e.g. sitting with the hind legs away from the body).<ref>{{cite web|last=Padgett-Flohr|first=G.E.|title=Amphibian Chytridiomycosis: An Informational Brochure|url=http://ccadc.us/docs/AmphibianDiseaseBrochure.pdf|publisher=California Center for Amphibian Disease Control|accessdate=14 October 2013|year=2007}}</ref>

==Clinical signs==
Amphibians infected with ''B. dendrobatidis'' have been known to show many different types clinical signs. Perhaps the earliest sign of infection is anorexia, occurring as little as 8 days after being exposed .<ref name="Nichols">Nichols, D. K., E. W. Lamirande, A. P. Pessier, and J. E. Longcore. 2001. Experimental transmission of cutaneous chytridiomycosis in dendrobatid frogs. Journal of Wildlife Diseases 37:1-11.</ref>. Individuals infected are also commonly found in a lethargic state, characterized by slow movements and refuse to move when stimulated. Excessive shedding of skin is seen in most frog species that are affected by''B. dendrobatidis''.<ref name=amphibiaweb/>. These pieces of shed skin are described as opaque, gray-white and tan.<ref name=amphibiaweb/>Some of these patches of skin are also found adhered to the skin of the amphibians.<ref name=amphibiaweb/>. These signs of infection are often seen 12-15 days following exposure.<ref name="Nichols" /> The most typical symptom of chytridomycosis is thickening of skin, which promptly leads to the death of the infected individuals because those individuals cannot take in the proper nutrients, release toxins, or, in some cases, breathe.<ref name=amphibiaweb/> Other common signs of B. dendrobatidis are reddening of the skin, convulsions, and a loss of righting reflex .<ref name="Nichols" />In tadpoles ''B. dendrobatidis'' affects the mouthparts, where keratin is present, leading to abnormal feeding behaviors or discoloration of the mouth.<ref name=amphibiaweb />


==Research==
==Research==
Laboratory studies suggest that the amphibian chytrid fungus grows best between 17-25°C,<ref>Piotrowski, J. S., Annis, S. L. & Longcore, J. E. (2004) Physiology of Batrachochytrium dendrobatidis, a chytrid pathogen of amphibians. Mycologia 96, 9–15</ref> and that exposure of infected frogs to high temperatures can cure the frogs.<ref>Woodhams, D. C., R. A. Alford, ''et al.'' (2003). "Emerging disease of amphibians cured by elevated body temperature." Diseases of aquatic organisms 55: 65-67.</ref> In nature, the more time individual frogs were found at temperatures above 25°C, the less likely they were to be infected by the amphibian chytrid.<ref>Rowley, J.J.L. & Alford, R.A. (2013) Hot bodies protect amphibians against chytrid infection in nature. Scientific Reports 3, 1515. {{doi|10.1038/srep01515}}</ref> This may explain why chytridiomycosis-induced [[amphibian declines]] have occurred primarily at higher elevations and during cooler months.<ref>Woodhams, D. C. & Alford, R. A. (2005) The ecology of chytridiomycosis in rainforest stream frog assemblages of tropical Queensland. Conserv. Biol. 19, 1449–1459 </ref> It has been shown that naturally produced cutaneous [[peptides]] can inhibit the growth of '' B. dendrobatidis'' when the infected amphibians are around temperatures near {{convert|10|C|F}}, allowing species like the [[northern leopard frog]] (''Rana pipiens'') to clear the infection in about 15% of cases.<ref>Voordouw, M. J., D. Adama, B. Houston, and P. Govindarajulu. "Prevalence of the Pathogenic Chytrid Fungus, Batrachochytrium Dendrobatidis, in an Endangered Population of Northern Leopard Frogs, Rana Pipiens." BMC Ecol. 6th ser. 10.1 (2010). Print.</ref>
Laboratory studies suggest that the amphibian chytrid fungus grows best between 17-25°C,<ref name="Piotrowski"/> and that exposure of infected frogs to high temperatures can cure the frogs.<ref>Woodhams, D. C., R. A. Alford, ''et al.'' (2003). "Emerging disease of amphibians cured by elevated body temperature." Diseases of aquatic organisms 55: 65-67.</ref> In nature, the more time individual frogs were found at temperatures above 25°C, the less likely they were to be infected by the amphibian chytrid.<ref>Rowley, J.J.L. & Alford, R.A. (2013) Hot bodies protect amphibians against chytrid infection in nature. Scientific Reports 3, 1515. {{doi|10.1038/srep01515}}</ref> This may explain why chytridiomycosis-induced [[amphibian declines]] have occurred primarily at higher elevations and during cooler months.<ref>Woodhams, D. C. & Alford, R. A. (2005) The ecology of chytridiomycosis in rainforest stream frog assemblages of tropical Queensland. Conserv. Biol. 19, 1449–1459 </ref> It has been shown that naturally produced cutaneous [[peptides]] can inhibit the growth of ''B. dendrobatidis'' when the infected amphibians are around temperatures near {{convert|10|C|F}}, allowing species like the [[northern leopard frog]] (''Rana pipiens'') to clear the infection in about 15% of cases.<ref>Voordouw, M. J., D. Adama, B. Houston, and P. Govindarajulu. "Prevalence of the Pathogenic Chytrid Fungus, Batrachochytrium Dendrobatidis, in an Endangered Population of Northern Leopard Frogs, Rana Pipiens." BMC Ecol. 6th ser. 10.1 (2010). Print.</ref>


Although many declines have been credited to the fungus ''B. dendrobatidis'', there are species that resist the infection and some reports have found that some populations can survive with a low level of persistence of the disease.<ref>Retallick, R. W. R., H. McCallum, ''et al.'' (2004). "Endemic Infection of the Amphibian Chytrid Fungus in a Frog Community Post-Decline." PLoS Biology 2(11): e351.</ref> In addition, some species that seem to resist the infection may actually harbor a non-pathogenic form of ''Batrachochytrium dendrobatidis''.
Although many declines have been credited to the fungus ''B. dendrobatidis'', there are species that resist the infection and some reports have found that some populations can survive with a low level of persistence of the disease.<ref>Retallick, R. W. R., H. McCallum, ''et al.'' (2004). "Endemic Infection of the Amphibian Chytrid Fungus in a Frog Community Post-Decline." PLoS Biology 2(11): e351.</ref> In addition, some species that seem to resist the infection may actually harbor a non-pathogenic form of ''Batrachochytrium dendrobatidis''.


Some researchers contend that the focus on chytridiomycosis has made amphibian conservation efforts dangerously [[myopia|myopic]]. A review of the data in the [[IUCN Red List]] found that the threat of the disease was assumed in most cases, but that there was no evidence that it is, in fact, a threat.<ref>Heard M, Smith KF, Ripp K, 2011 Examining the Evidence for Chytridiomycosis in Threatened Amphibian Species. PLoS ONE 6(8): e23150. {{doi|10.1371/journal.pone.0023150}}</ref> Conservation efforts in New Zealand continue to be focused on curing the critically endangered native [[Archey's frog]], ''Leiopelma archeyi'', of chytridiomycosis even though research has shown clearly that they are immune from infection by ''B. dendrobatidis'' and are dying in the wild of other still-to-be identified diseases.<ref>[http://frogs.orcon.net.nz/archeys-encounters.pdf Waldman B (2011) Brief encounters with Archey's Frog.] FrogLog 99:39-41.</ref> In Guatemala, several thousands of tadpoles perished from an unidentified pathogen distinct from ''B. dendrobatidis''.<ref>Di Rosa, Ines et al. "The Proximate Cause of Frog Declines?" ''Nature'' 447.31 (2007) E4-E5.</ref> Such researchers stress the need for a broader understanding of the host-parasite ecology that is contributing to the modern day amphibian declines.
Some researchers contend that the focus on chytridiomycosis has made amphibian conservation efforts dangerously [[myopia|myopic]]. A review of the data in the [[IUCN Red List]] found that the threat of the disease was assumed in most cases, but that there was no evidence that it is, in fact, a threat.<ref>Heard M, Smith KF, Ripp K, 2011 Examining the Evidence for Chytridiomycosis in Threatened Amphibian Species. PLoS ONE 6(8): e23150. {{doi|10.1371/journal.pone.0023150}}</ref> Conservation efforts in New Zealand continue to be focused on curing the critically endangered native [[Archey's frog]], ''Leiopelma archeyi'', of chytridiomycosis even though research has shown clearly that they are immune from infection by ''B. dendrobatidis'' and are dying in the wild of other still-to-be identified diseases.<ref>[http://frogs.orcon.net.nz/archeys-encounters.pdf Waldman B (2011) Brief encounters with Archey's Frog.] FrogLog 99:39-41.</ref> In Guatemala, several thousands of tadpoles perished from an unidentified pathogen distinct from ''B. dendrobatidis''.<ref name="Di Rosa">{{cite journal|last=Di Rosa|first=Ines|coauthors=Simoncelli, Francesca; Fagotti, Anna; Pascolini, Rita|title=Ecology: The proximate cause of frog declines?|journal=Nature|year=2007|volume=447|issue=7144|pages=E4–E5|doi=10.1038/nature05941}}</ref> Such researchers stress the need for a broader understanding of the host-parasite ecology that is contributing to the modern day amphibian declines.


==Treatment options==
==Treatment options==


Treatment options for chytridiomycosis include antifungal drugs and heat induced therapy. <ref name="Woodhams">Woodhams, D. C., C. C. Geiger, L. K. Reinert, L. A. Rollins-Smith, B. Lam, R. N. Harris, C. J. Briggs, V. T. Vredenburg, and J. Voyles. 2012. Treatment of amphibians infected with chytrid fungus: learning from failed trials with itraconazole, antimicrobial peptides, bacteria, and heat therapy. Diseases of Aquatic Organisms 98:11-25.</ref> The antifungal drug, itraconazole, is the most popular form of treatment.<ref name="Woodhams" /><ref name="Parker" /><ref name="Une">Une, Y., K. Matsui, K. Tamukai, and K. Goka. 2012. Eradication of the chytrid fungus Batrachochytrium dendrobatidis in the Japanese giant salamander Andrias japonicus. Diseases of Aquatic Organisms 98:243-247.</ref><ref name="Jones">Jones, M. E. B., D. Paddock, L. Bender, J. L. Allen, M. D. Schrenzel, and A. P. Pessier. 2012. Treatment of chytridiomycosis with reduced-dose itraconazole. Diseases of Aquatic Organisms 99:243-249.</ref> Individuals infected with chytridiomycosis from ''B. dendrobatidis'' are bathed in intraconazole solutions, and within a few weeks previously infected individuals test negative for ''B. dendrobatidis'' using PCR assays.<ref name="Parker" /><ref name="Une" /><ref name="Jones" /> Heat therapy is also used to neutralize ''B. dendrobatidis'' in infected individuals<ref name="Woodhams" /><ref name="Chatfield">Chatfield, M. W. H. and C. L. Richards-Zawacki. 2011. Elevated temperature as a treatment for Barachochytrium dendrobatidis infection in captive frogs. Diseases of Aquatic Organisms 94:235-238.</ref> Temperature controlled laboratory experiments are used to increase the temperature of an inidividual past the optimal temperature range of ''B. dendrobatidis'' . <ref name="Chatfield" />Experiments, where the temperature is increased beyond the upper bound of the ''B. dendrobatidis'' optimal range of 25 °C to 30 °C, show that ''B. dendrobatidis'' presence will dissipate within a few weeks and individuals infected with chytridiomycosis will return to normal. <ref name="Chatfield" /> Formalin/malachite green have also been used to successfully treat individuals infected with chytridiomycosis. <ref name="Parker" /> [[Archey's frog]], ''Leiopelma archeyi'', was successfully cured of chytridiomycosis by applying [[chloramphenicol]] topically.<ref name="Bishop et al 2009">{{cite journal|last=Bishop|first=PJ|coauthors=Speare, R; Poulter, R; Butler, M; Speare, BJ; Hyatt, A; Olsen, V; Haigh, A|title=Elimination of the amphibian chytrid fungus ''Batrachochytrium dendrobatidis'' by Archey's frog ''Leiopelma archeyi''|journal=Diseases of Aquatic Organisms|date=9 March 2009|year=2009|volume=84|pages=9–15|doi=10.3354/dao02028|url=http://www.int-res.com/articles/dao_oa/d084p009.pdf|pmid=19419002|issue=1}}</ref> However, the potential risks of using antifungal drugs on individuals are high, and additional research is being conducted to reduce these risks.<ref name="Woodhams" />
Reid Harris of James Madison University has found that coating frogs with ''[[Janthinobacterium lividum]]'' appears to protect them from chytridiomycosis.<ref>[http://www.newscientist.com/article/mg20227115.300-probiotic-bug-is-a-frog-lifesaver.html Probiotic bug is a frog lifesaver], Linda Geddes, [[New Scientist]], Issue 2711, June 8, 2009</ref>

==Immunity hypothesis==<!--from [[Batrachochytrium]]-->
Due to the fungus' immense impact on amphibian populations, considerable research has been undertaken to devise methods to combat its proliferation in the wild. Among the most promising is the revelation that amphibians in colonies that survive the passage of the chytrid epidemic tend to carry higher levels of the bacterium ''[[Janthinobacterium lividum]]''.<ref>[http://news.bbc.co.uk/2/hi/science/nature/7438205.stm (BBC News) Richard Black, " Bacteria could stop frog killer"] Accessed 7 June 2008.</ref> This bacterium produces [[Fungicide|antifungal]] compounds, such as indole-3-carboxaldehyde and violacein, that inhibit the growth of ''B. dendrobatidis'' even at low concentrations.<ref name="pmid18949519">{{cite journal |author=Brucker RM, Harris RN, Schwantes CR, Gallaher TN, Flaherty DC, Lam BA, Minbiole KP |title=Amphibian chemical defense: antifungal metabolites of the microsymbiont ''Janthinobacterium lividum'' on the salamander ''Plethodon cinereus'' |journal=J. Chem. Ecol. |volume=34 |issue=11 |pages=1422–9 |date=November 2008 |pmid=18949519 |doi=10.1007/s10886-008-9555-7}}</ref> Similarly, the bacterium ''[[Lysobacter gummosus]]'' found on the red-backed salamander (''[[Plethodon cinereus]]''), produces the compound 2,4-diacetylphloroglucinol that is inhibitory to the growth of ''B. dendrobatidis''.<ref name="pmid18058176">{{cite journal |author=Brucker RM, Baylor CM, Walters RL, Lauer A, Harris RN, Minbiole KP |title=The identification of 2,4-diacetylphloroglucinol as an antifungal metabolite produced by cutaneous bacteria of the salamander ''Plethodon cinereu''s |journal=J. Chem. Ecol. |volume=34 |issue=1 |pages=39–43 |date=January 2008 |pmid=18058176 |doi=10.1007/s10886-007-9352-8 |url=}}</ref>

Understanding the interactions of microbial communities present on amphibians’ skin with fungal species in the environment can reveal why certain amphibians, such as the frog ''[[Rana muscosa]]'', are susceptible to the fatal effects of ''B. dendrobatidis'' and why others, such as the salamander ''[[Hemidactylium scutatum]]'', are able to coexist with the fungus. As mentioned before, the antifungal bacterial species ''[[Janthinobacterium lividum]]'', found on several amphibian species, has been shown to prevent the effects of the pathogen even when added to another amphibian that lacks the bacteria (''B. dendrobatidis''-susceptible amphibian species).<ref name="Harris, R. 2009">{{cite journal | doi = 10.1038/ismej.2009.27 | author = Harris R., Brucker R., Minbiole K., Walke J., Becker M., Schwantes C. ''et al.'' | year = 2009 | title = Skin microbes on frogs prevent morbidity and mortality caused by a lethal skin fungus | url = | journal = ISME Journal | volume = 3 | issue = 7| pages = 818–824 | pmid = 19322245 }}</ref> Interactions between cutaneous microbiota and ''B. dendrobatidis'' can be altered to favor the resistance of the disease, as seen in past lab studies concerning the addition of the violacein-producing bacteria ''J. lividum'' to amphibians that lacked sufficient violacein, allowing them to inhibit infection.<ref name="Becker, M. 2009">{{cite journal | author = Becker M., Brucker R., Schwantes C., Harris R., Minbiole K. | year = 2009 | title = The bacterially-produced metabolite violacein is associated with survival in amphibians infected with a lethal disease | url = | journal = AEM | volume = 21 | issue = | pages = 1294–1302 }}</ref><ref>{{cite journal | author = Brucker R., Harris R., Schwantes C., Gallaher T., Flaherty D., Lam B. ''et al.'' | year = 2008 | title = Amphibian Chemical Defense: Antifungal Metabolites of the Microsymbiont ''Janthinobacterium lividum'' on the Salamander ''Plethodon cinereus'' | url = | journal = Springer | volume = 34 | issue = | pages = 1422–1429 }}</ref> Although the exact concentration of violacein ([[Fungicide|antifungal]] metabolite produced by ''J. lividum'') needed to inhibit the effects of ''B. dendrobatidis'' is not fully confirmed, violacein concentration can determine whether or not an amphibian will experience [[morbidity]] (or mortality) caused by the chytrid fungus ''B. dendrobatidis''. The frog ''[[Rana muscosa]]'', for example, has been found to have very low concentrations of violacein on its skin, yet the concentration is so small that it is unable to facilitate increased survivability of the frog; furthermore, ''J. lividum'' has not been found to be present on the skin of ''[[Rana muscosa]]''.<ref name="Harris, R. 2009"/><ref name="Lam, B. 2009">{{cite journal | doi = 10.1016/j.biocon.2009.11.015 | author = Lam B., Walke J., Vredenburg V., Harris R. | year = 2009 | title = Proportion of individuals with anti-''Batrachochytrium dendrobatidis'' skin bacteria is associated with population persistence in the frog ''Rana muscosa'' | url = | journal = Biological Conservation | volume = 143 | issue = 2| pages = 529–531 }}</ref> This implies that the antifungal bacteria ''J. lividum'' (native to other amphibians' skin, such as ''Hemidactylium scutatum'') is able to produce a sufficient amount of violacein to prevent infection by ''B. dendrobatidis'' and allow coexistence with the potentially deadly fungus.

A recent study has postulated that the water flea ''[[Daphnia magna]]'' eats the spores of the fungus.<ref>{{cite journal |url=http://www.springerlink.com/content/87544h87052r77p2/ |title=Predation by zooplankton on ''Batrachochytrium dendrobatidis'': biological control of the deadly amphibian chytrid fungus? |journal=Biodiversity and Conservation |doi=10.1007/s10531-011-0147-4 |volume=20 |issue=14 |pages=3549–3553 |year=2011 |last1=Buck |first1=Julia |last2=Truong |first2=Lisa |last3=Blaustein |first3=Andrew}}</ref>


==Interactions with pesticides==
Archey's frog, ''Leiopelma archeyi'', was successfully cured of chytridiomycosis by applying [[chloramphenicol]] topically.<ref>[http://www.int-res.com/articles/dao_oa/d084p009.pdf Elimination of the amphibian chytrid fungus Batrachochytrium dendrobatidis by Archey’s frog Leiopelma archeyi], Phillip J. Bishop, [[Diseases of Aquatic Organisms]], Vol. 84: 9–15, 2009 {{doi|10.3354/dao02028}}</ref>
The hypothesis that [[pesticide]] use has contributed to declining amphibian populations has been suggested several times in the literature.<ref name="isbn0-691-10251-1">{{cite book |author=Cohen, Nathan W.; Stebbins, Robert A. |title=A Natural History of Amphibians |publisher=Princeton University Press |location=Princeton, N.J |year=1995 |pages= |isbn=0-691-10251-1 |oclc= |doi= }}</ref><ref>{{cite journal | doi = 10.1890/1051-0761(2001)011[0464:DOTCRL]2.0.CO;2 | author = Daividson C, Shaffer HB, Jennings MR | year = 2001 | title = Declines of the California red-legged frog: climate, UV-B, habitat, and pesticides hypotheses | url = | journal = Ecological Applications | volume = 11 | issue = 2| pages = 464–479 }}</ref><ref name="pmid16818245">{{cite journal |author=Hayes TB, Case P, Chui S, Chung D, Haeffele C, Haston K, Lee M, Mai VP, Marjuoa Y, Parker J, Tsui M |title=Pesticide mixtures, endocrine disruption, and amphibian declines: are we underestimating the impact? |journal=Environ. Health Perspect. |volume=114 |issue=Suppl 1 |pages=40–50 |date=April 2006 |pmid=16818245 |pmc=1874187 |doi= 10.1289/ehp.8051|url=http://www.ehponline.org/docs/2006/8051/abstract.html}}</ref> Interactions between pesticides and chytridiomycosis were examined in 2007, and it was shown that sublethal exposure to the pesticide [[carbaryl]] (a [[cholinesterase]] inhibitor) increases susceptibility of foothill yellow-legged frogs (''[[Rana boylii]]'') to chytridiomycosis. In particular, the skin [[peptide]] defenses were significantly reduced after exposure to carbaryl, suggesting that pesticides may inhibit this innate immune defense, and increase susceptibility to disease.<ref name="pmid17396672">{{cite journal |author=Davidson C, Benard MF, Shaffer HB, Parker JM, O'Leary C, Conlon JM, Rollins-Smith LA |title=Effects of chytrid and carbaryl exposure on survival, growth and skin peptide defenses in foothill yellow-legged frogs |journal=Environ. Sci. Technol. |volume=41 |issue=5 |pages=1771–6 |date=March 2007 |pmid=17396672 |doi= 10.1021/es0611947|url=}}</ref>


==See also==
==See also==
Line 43: Line 72:
* [http://www.msnbc.msn.com/id/17166620 'Amphibian Ark' aims to save frogs from fungus]
* [http://www.msnbc.msn.com/id/17166620 'Amphibian Ark' aims to save frogs from fungus]


[[Category:Amphibian diseases]]
[[:Category:Amphibian diseases]]
[[Category:Fungal diseases]]
[[:Category:Fungal diseases]]


[[he:צ'יטרידיומיקוסיס]]
[[he:צ'יטרידיומיקוסיס]]

Revision as of 00:38, 24 December 2013

A chytrid-infected frog.
Chytridiomycosis in Atelopus varius - two sporangia containing numerous zoospores are visible

Chytridiomycosis is an infectious disease of amphibians, caused by the chytrid Batrachochytrium dendrobatidis, a non-hyphal zoosporic fungus. Chytridiomycosis has been linked to dramatic population declines or even extinctions of amphibian species in western North America, Central America, South America, eastern Australia, and Dominica and Montserrat in the Caribbean. Much of the new world is also at risk of seeing the disease arrive within the coming years.[1] The fungus is capable of causing sporadic deaths in some amphibian populations and 100% mortality in others. There is no effective measure for control of the disease in wild populations. Various clinical signs are seen by individuals affected by the disease. There are a number of options for controlling this disease-causing fungus, though none have proved to be feasible on a large scale. The disease has been proposed as a contributing factor to a global decline in amphibian populations that apparently has affected approximately 30% of the amphibian species of the world.[2]

History

The disease in its epizootic form was first discovered in 1993 in dead and dying frogs in Queensland, Australia. Research since then has shown that it had been present in the country since at least 1978 and is widespread across Australia. It is also found in Africa, the Americas, Europe, New Zealand and Oceania. In Australia, Panama, and New Zealand, the fungus seemed to have suddenly ‘appeared’ and expanded its range at the same time frog numbers declined. However, it may simply be that the fungus occurs naturally and was only identified recently because it has become more virulent or more prevalent in the environment, or because host populations have become less resistant to the disease. The fungus has been detected in four areas of Australia — the east coast, Adelaide, south-west Western Australia and the Kimberley — and is probably present elsewhere.[3]

The oldest documented occurrence of Batrachochytrium is from a specimen of an African clawed frog (Xenopus laevis) collected in 1938, and this species also appears to be essentially unaffected by the disease, making it a suitable vector.[4] The first well-documented method of human pregnancy testing involved this species, and as a result large-scale international trade in living African clawed frogs began more than 60 years ago.[4] If Batrachochytrium originated in Africa, it has been theorized that the African clawed frog was the vector of the initial spread out of the continent.[4] The earliest documented case of the disease chytridiomycosis was an American bullfrog (Rana catesbeiana) collected in 1978.[4] It is still not clear if it is a new emergent pathogen or if it is an old pathogen with recently increased virulence.

Geographic Range

The geographic range of chytridiomycosis is difficult to ascertain. If it occurs, the disease is always present where the fungus B. dendrobatidis is present. However, the disease is not always present where the fungus is. Reasons for amphibian declines are often termed ‘enigmatic’ because the cause is unknown. It is not fully understood why some areas are affected by the fungus while others are not. Oscillating factors such as climate, habitat suitability, and population density may be factors which cause the fungus to result in chytridiomycosis in amphibians of a given area. Therefore, when considering the geographic range of chytridiomycosis it is important to consider the range of B. dendrobatidis occurrence.4.[5] ).

The geographic range of B. dendrobatidis has recently been mapped, and spans much of the world. This could be seen as a portentous, though it is important to remember that chytridiomycosis does not always occur where the the fungus does. B. dendrobatidis has been detected in 56 of 82 countries, and in 516 of 1240 (42%) species using a data set of more than 36,000 individuals. B. dendrobatidis is widely distributed in the Americas, and detected sporadically in Africa, Asia, and Europe.[1] Asia, for example, has only 2.35% B. dendrobatidis prevalence[6]

The range suitable for B. dendrobatidis in the new world is vast. Regions with the highest suitability for B. dendrobatidis include habitats that contain the world’s most diverse amphibian faunas. Areas at risk are the Sierra Madre Pine Oak Occidental Forest, the Sonoran and Sinaloan dry forest, the Veracruz moist forest, Central America east from the Isthmus of Tehuantepec, the Caribbean Islands, the temperate forest in Chile and western Argentina south of latitude 300 S, the Andes above 1000 m of altitude in Venezuela, Colombia, and Ecuador, eastern slopes of the Andes in Peru and Bolivia, the Brazilian Atlantic forest, Uruguay, Paraguay, and Northeastern Argentina, as well as the southwestern and Madeira-Tapaj Amazonian ranforests.[7]

The range of chytridiomycosis is dependent upon the range of B. dendrobatidis. Even if there are not visible effects of the disease in a given area, the disease may simply be in a latent state, capable of affecting local amphibians as a result of unknown environmental or behavioral factors. Currently, the effects of chytridiomycosis are being seen most readily in Central America, eastern Australia, South America, and Western America.[1]

Causative agents

Main article: Batrachochytrium

Chytridiomycosis is caused by the fungus B. dendrobatidis. B. dendrobatidis affects the layers of the skin that harbor keratin.[5] When most species reach a B. dendrobatidis threshold of 10,000 zoospores they are not able to breathe, hydrate, osmoregualte, or thermoregulate correctly. This is proven by blood samples that show a lack of certain electrolytes, such as sodium, magnesium, and potassium. B. dendrobatidis is currently known to have two life stages. The first is the asexual zoosporangial stage.[8] When a host first contracts the disease, spores penetrate the skin and attach themselves using microtubule roots.[9] The second stage takes place when the initial asexual zoosporangials produce motile zoospores.[8] To disperse and infect epidermal cells an aqueous surface is needed.[8] A second species of Batrachochytrium, B. salamandrivorans was discovered in 2013 and is known to cause chytridiomycosis in salamanders.[10]

Disease transmission and progression

B. dendrobatidis is a waterborne pathogen that disperses zoospores into the environment.[11] The zoospores use flagella for locomotion through water systems until it reaches a new host and enter cutaneously.[9] B. dendrobatidis’ life cycle continues until new zoospores are produced from the zoosporangium and exit to the environment or reinfect the same host.[9] Once the host is infected with B. dendrobatidis it can potentially develop chytridiomycosis, but not all infected hosts will develop chytridiomycosis.[9] Other forms of transmission are currently unknown; however, it is postulated that chytridiomycosis can be transmitted by B. dendrobatidis through direct contact of hosts or through an intermediate host. [9]

Much of how B. dendrobatidis is successfully transmitted from one host to the next is largely unknown.[12]Research has found that once released into the aquatic environment, zoospores traveled less than 2 cm within 24 hours before they encysted.[13] The limited range of B. dendrobatidis zoospores suggest that there is some unknown mechanism by which they transmit from one host to the next.[13] Abiotic factors such as temperature, pH level, and nutrient levels affect the success of B. dendrobatidis zoospores.[13]B. dendrobatidis zoospores can survive within a temperature range of 4-25 °C and within a pH range of 6-7.[13]

Chytridiomycosis is believed to adhere to the following course: zoospores first encounter amphibian skin and quickly give rise to sporangia, which produce new zoospores.[14] The disease then progresses as these new zoospores reinfect the host. Morphological changes in amphibians infected with the fungus include a reddening of the ventral skin, convulsions with extension of hind limbs, accumulations of sloughed skin over the body, sloughing of the superficial epidermis of the feet and other areas, slight roughening of the surface with minute skin tags, and occasional small ulcers or hemorrhage. Behavioral changes can include lethargy, a failure to seek shelter, a failure to flee, a loss of righting reflex, and abnormal posture (e.g. sitting with the hind legs away from the body).[15]

Clinical signs

Amphibians infected with B. dendrobatidis have been known to show many different types clinical signs. Perhaps the earliest sign of infection is anorexia, occurring as little as 8 days after being exposed .[12]. Individuals infected are also commonly found in a lethargic state, characterized by slow movements and refuse to move when stimulated. Excessive shedding of skin is seen in most frog species that are affected byB. dendrobatidis.[5]. These pieces of shed skin are described as opaque, gray-white and tan.[5]Some of these patches of skin are also found adhered to the skin of the amphibians.[5]. These signs of infection are often seen 12-15 days following exposure.[12] The most typical symptom of chytridomycosis is thickening of skin, which promptly leads to the death of the infected individuals because those individuals cannot take in the proper nutrients, release toxins, or, in some cases, breathe.[5] Other common signs of B. dendrobatidis are reddening of the skin, convulsions, and a loss of righting reflex .[12]In tadpoles B. dendrobatidis affects the mouthparts, where keratin is present, leading to abnormal feeding behaviors or discoloration of the mouth.[5]

Research

Laboratory studies suggest that the amphibian chytrid fungus grows best between 17-25°C,[13] and that exposure of infected frogs to high temperatures can cure the frogs.[16] In nature, the more time individual frogs were found at temperatures above 25°C, the less likely they were to be infected by the amphibian chytrid.[17] This may explain why chytridiomycosis-induced amphibian declines have occurred primarily at higher elevations and during cooler months.[18] It has been shown that naturally produced cutaneous peptides can inhibit the growth of B. dendrobatidis when the infected amphibians are around temperatures near 10 °C (50 °F), allowing species like the northern leopard frog (Rana pipiens) to clear the infection in about 15% of cases.[19]

Although many declines have been credited to the fungus B. dendrobatidis, there are species that resist the infection and some reports have found that some populations can survive with a low level of persistence of the disease.[20] In addition, some species that seem to resist the infection may actually harbor a non-pathogenic form of Batrachochytrium dendrobatidis.

Some researchers contend that the focus on chytridiomycosis has made amphibian conservation efforts dangerously myopic. A review of the data in the IUCN Red List found that the threat of the disease was assumed in most cases, but that there was no evidence that it is, in fact, a threat.[21] Conservation efforts in New Zealand continue to be focused on curing the critically endangered native Archey's frog, Leiopelma archeyi, of chytridiomycosis even though research has shown clearly that they are immune from infection by B. dendrobatidis and are dying in the wild of other still-to-be identified diseases.[22] In Guatemala, several thousands of tadpoles perished from an unidentified pathogen distinct from B. dendrobatidis.[23] Such researchers stress the need for a broader understanding of the host-parasite ecology that is contributing to the modern day amphibian declines.

Treatment options

Treatment options for chytridiomycosis include antifungal drugs and heat induced therapy. [24] The antifungal drug, itraconazole, is the most popular form of treatment.[24][8][25][26] Individuals infected with chytridiomycosis from B. dendrobatidis are bathed in intraconazole solutions, and within a few weeks previously infected individuals test negative for B. dendrobatidis using PCR assays.[8][25][26] Heat therapy is also used to neutralize B. dendrobatidis in infected individuals[24][27] Temperature controlled laboratory experiments are used to increase the temperature of an inidividual past the optimal temperature range of B. dendrobatidis . [27]Experiments, where the temperature is increased beyond the upper bound of the B. dendrobatidis optimal range of 25 °C to 30 °C, show that B. dendrobatidis presence will dissipate within a few weeks and individuals infected with chytridiomycosis will return to normal. [27] Formalin/malachite green have also been used to successfully treat individuals infected with chytridiomycosis. [8] Archey's frog, Leiopelma archeyi, was successfully cured of chytridiomycosis by applying chloramphenicol topically.[28] However, the potential risks of using antifungal drugs on individuals are high, and additional research is being conducted to reduce these risks.[24]

Immunity hypothesis

Due to the fungus' immense impact on amphibian populations, considerable research has been undertaken to devise methods to combat its proliferation in the wild. Among the most promising is the revelation that amphibians in colonies that survive the passage of the chytrid epidemic tend to carry higher levels of the bacterium Janthinobacterium lividum.[29] This bacterium produces antifungal compounds, such as indole-3-carboxaldehyde and violacein, that inhibit the growth of B. dendrobatidis even at low concentrations.[30] Similarly, the bacterium Lysobacter gummosus found on the red-backed salamander (Plethodon cinereus), produces the compound 2,4-diacetylphloroglucinol that is inhibitory to the growth of B. dendrobatidis.[31]

Understanding the interactions of microbial communities present on amphibians’ skin with fungal species in the environment can reveal why certain amphibians, such as the frog Rana muscosa, are susceptible to the fatal effects of B. dendrobatidis and why others, such as the salamander Hemidactylium scutatum, are able to coexist with the fungus. As mentioned before, the antifungal bacterial species Janthinobacterium lividum, found on several amphibian species, has been shown to prevent the effects of the pathogen even when added to another amphibian that lacks the bacteria (B. dendrobatidis-susceptible amphibian species).[32] Interactions between cutaneous microbiota and B. dendrobatidis can be altered to favor the resistance of the disease, as seen in past lab studies concerning the addition of the violacein-producing bacteria J. lividum to amphibians that lacked sufficient violacein, allowing them to inhibit infection.[33][34] Although the exact concentration of violacein (antifungal metabolite produced by J. lividum) needed to inhibit the effects of B. dendrobatidis is not fully confirmed, violacein concentration can determine whether or not an amphibian will experience morbidity (or mortality) caused by the chytrid fungus B. dendrobatidis. The frog Rana muscosa, for example, has been found to have very low concentrations of violacein on its skin, yet the concentration is so small that it is unable to facilitate increased survivability of the frog; furthermore, J. lividum has not been found to be present on the skin of Rana muscosa.[32][35] This implies that the antifungal bacteria J. lividum (native to other amphibians' skin, such as Hemidactylium scutatum) is able to produce a sufficient amount of violacein to prevent infection by B. dendrobatidis and allow coexistence with the potentially deadly fungus.

A recent study has postulated that the water flea Daphnia magna eats the spores of the fungus.[36]

Interactions with pesticides

The hypothesis that pesticide use has contributed to declining amphibian populations has been suggested several times in the literature.[37][38][39] Interactions between pesticides and chytridiomycosis were examined in 2007, and it was shown that sublethal exposure to the pesticide carbaryl (a cholinesterase inhibitor) increases susceptibility of foothill yellow-legged frogs (Rana boylii) to chytridiomycosis. In particular, the skin peptide defenses were significantly reduced after exposure to carbaryl, suggesting that pesticides may inhibit this innate immune defense, and increase susceptibility to disease.[40]

See also

References

  1. ^ a b c Olson, Deanna H. (2013). Stajich, Jason E (ed.). "Mapping the Global Emergence of Batrachochytrium dendrobatidis, the Amphibian Chytrid Fungus". PLoS ONE. 8 (2): e56802. doi:10.1371/journal.pone.0056802. PMC 3584086. PMID 23463502. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: unflagged free DOI (link)
  2. ^ Stuart, S. N., J. S. Chanson, et al. (2004). "Status and trends of amphibian declines and extinctions worldwide." Science 306: 1783-1786.
  3. ^ "Chytridiomycosis (Amphibian Chytrid Fungus Disease)" (PDF). Australian Government Department of Sustainability, Environment, Water, Population and Communities. Retrieved 14 October 2013.
  4. ^ a b c d Weldon; du Preez; Hyatt; Muller; and Speare (2004). Origin of the Amphibian Chytrid Fungus. Emerging Infectious Disease 10(12).
  5. ^ a b c d e f g Whittaker, Kellie, and Vance Vredenburg. "An Overview of Chytridiomycosis." AMPHIBIAWEB. University of California, 17 May 2011. Web. 23 Oct 2013. <http://www.amphibiaweb.org/chytrid/chytridiomycosis.html.
  6. ^ Swei, A. (2011). Arlettaz, Raphaël (ed.). "Is Chytridiomycosis an Emerging Infectious Disease in Asia?". PLoS ONE. 6 (8): e23179. doi:10.1371/journal.pone.0023179. PMC 3156717. PMID 21887238. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help); Unknown parameter |displayauthors= ignored (|display-authors= suggested) (help)CS1 maint: unflagged free DOI (link)
  7. ^ Ron, Santiago R. (2005). "Predicting the Distribution of the Amphibian Pathogen Batrachochytrium dendrobatidis in the New World". Biotropica. 37 (2): 209–221. doi:10.1111/j.1744-7429.2005.00028.x. {{cite journal}}: Unknown parameter |month= ignored (help)
  8. ^ a b c d e f Parker, J. M., I. Mikaelian, N. Hahn, and H. E. Diggs. 2002. Clinical diagnosis and treatment of epidermal chytridiomycosis in African clawed frogs (Xenopus tropicalis). American Association for Laboratory Animal Science 52:265-268.
  9. ^ a b c d e Longcore, J. E., A. P. Pessier, and D. K. Nichols. 1999. Batrachochytrium dendrobatidis gen. et sp. nov., a chytrid pathogenic to amphibians. Mycologia 91:219-227.
  10. ^ Martel, A. (2013). "Batrachochytrium salamandrivorans sp. nov. causes lethal chytridiomycosis in amphibians". Proceedings of the National Academy of Sciences of the United States of America. 110 (38): 15325–15329. doi:10.1073/pnas.1307356110. PMC 3780879. PMID 24003137. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  11. ^ Morgan, J. A. T., V. T. Vredenburg, L. J. Rachowicz, R. A. Knapp, M. J. Stice, T. Tunstall, R. E. Bingham, J. M. Parker, J. E. Longcore, C. Moritz, C. J. Briggs, and J. W. Taylor. 2007. Population genetics of the frog-killing fungus Batrachochytrium dendrobatidis. Proceedings of the National Academy of Sciences of the United States of America 104:13845-13850.
  12. ^ a b c d Nichols, D. K., E. W. Lamirande, A. P. Pessier, and J. E. Longcore. 2001. Experimental transmission of cutaneous chytridiomycosis in dendrobatid frogs. Journal of Wildlife Diseases 37:1-11.
  13. ^ a b c d e Piotrowski, J. S., S. L. Annis, and J. E. Longcore. 2004. Physiology of Batrachochytrium dendrobatidis, a chytrid pathogen of amphibians. Mycologia 96:9-15.
  14. ^ Berger, L., Hyatt, A.D., Speare, R. & Longcore, J.E. Dis. Aquat. Org. 68, 51-63 (2005).
  15. ^ Padgett-Flohr, G.E. (2007). "Amphibian Chytridiomycosis: An Informational Brochure" (PDF). California Center for Amphibian Disease Control. Retrieved 14 October 2013.
  16. ^ Woodhams, D. C., R. A. Alford, et al. (2003). "Emerging disease of amphibians cured by elevated body temperature." Diseases of aquatic organisms 55: 65-67.
  17. ^ Rowley, J.J.L. & Alford, R.A. (2013) Hot bodies protect amphibians against chytrid infection in nature. Scientific Reports 3, 1515. doi:10.1038/srep01515
  18. ^ Woodhams, D. C. & Alford, R. A. (2005) The ecology of chytridiomycosis in rainforest stream frog assemblages of tropical Queensland. Conserv. Biol. 19, 1449–1459
  19. ^ Voordouw, M. J., D. Adama, B. Houston, and P. Govindarajulu. "Prevalence of the Pathogenic Chytrid Fungus, Batrachochytrium Dendrobatidis, in an Endangered Population of Northern Leopard Frogs, Rana Pipiens." BMC Ecol. 6th ser. 10.1 (2010). Print.
  20. ^ Retallick, R. W. R., H. McCallum, et al. (2004). "Endemic Infection of the Amphibian Chytrid Fungus in a Frog Community Post-Decline." PLoS Biology 2(11): e351.
  21. ^ Heard M, Smith KF, Ripp K, 2011 Examining the Evidence for Chytridiomycosis in Threatened Amphibian Species. PLoS ONE 6(8): e23150. doi:10.1371/journal.pone.0023150
  22. ^ Waldman B (2011) Brief encounters with Archey's Frog. FrogLog 99:39-41.
  23. ^ Di Rosa, Ines (2007). "Ecology: The proximate cause of frog declines?". Nature. 447 (7144): E4–E5. doi:10.1038/nature05941. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  24. ^ a b c d Woodhams, D. C., C. C. Geiger, L. K. Reinert, L. A. Rollins-Smith, B. Lam, R. N. Harris, C. J. Briggs, V. T. Vredenburg, and J. Voyles. 2012. Treatment of amphibians infected with chytrid fungus: learning from failed trials with itraconazole, antimicrobial peptides, bacteria, and heat therapy. Diseases of Aquatic Organisms 98:11-25.
  25. ^ a b Une, Y., K. Matsui, K. Tamukai, and K. Goka. 2012. Eradication of the chytrid fungus Batrachochytrium dendrobatidis in the Japanese giant salamander Andrias japonicus. Diseases of Aquatic Organisms 98:243-247.
  26. ^ a b Jones, M. E. B., D. Paddock, L. Bender, J. L. Allen, M. D. Schrenzel, and A. P. Pessier. 2012. Treatment of chytridiomycosis with reduced-dose itraconazole. Diseases of Aquatic Organisms 99:243-249.
  27. ^ a b c Chatfield, M. W. H. and C. L. Richards-Zawacki. 2011. Elevated temperature as a treatment for Barachochytrium dendrobatidis infection in captive frogs. Diseases of Aquatic Organisms 94:235-238.
  28. ^ Bishop, PJ (9 March 2009). "Elimination of the amphibian chytrid fungus Batrachochytrium dendrobatidis by Archey's frog Leiopelma archeyi" (PDF). Diseases of Aquatic Organisms. 84 (1): 9–15. doi:10.3354/dao02028. PMID 19419002. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: date and year (link)
  29. ^ (BBC News) Richard Black, " Bacteria could stop frog killer" Accessed 7 June 2008.
  30. ^ Brucker RM, Harris RN, Schwantes CR, Gallaher TN, Flaherty DC, Lam BA, Minbiole KP (November 2008). "Amphibian chemical defense: antifungal metabolites of the microsymbiont Janthinobacterium lividum on the salamander Plethodon cinereus". J. Chem. Ecol. 34 (11): 1422–9. doi:10.1007/s10886-008-9555-7. PMID 18949519.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  31. ^ Brucker RM, Baylor CM, Walters RL, Lauer A, Harris RN, Minbiole KP (January 2008). "The identification of 2,4-diacetylphloroglucinol as an antifungal metabolite produced by cutaneous bacteria of the salamander Plethodon cinereus". J. Chem. Ecol. 34 (1): 39–43. doi:10.1007/s10886-007-9352-8. PMID 18058176.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  32. ^ a b Harris R., Brucker R., Minbiole K., Walke J., Becker M., Schwantes C.; et al. (2009). "Skin microbes on frogs prevent morbidity and mortality caused by a lethal skin fungus". ISME Journal. 3 (7): 818–824. doi:10.1038/ismej.2009.27. PMID 19322245. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  33. ^ Becker M., Brucker R., Schwantes C., Harris R., Minbiole K. (2009). "The bacterially-produced metabolite violacein is associated with survival in amphibians infected with a lethal disease". AEM. 21: 1294–1302.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  34. ^ Brucker R., Harris R., Schwantes C., Gallaher T., Flaherty D., Lam B.; et al. (2008). "Amphibian Chemical Defense: Antifungal Metabolites of the Microsymbiont Janthinobacterium lividum on the Salamander Plethodon cinereus". Springer. 34: 1422–1429. {{cite journal}}: Explicit use of et al. in: |author= (help)CS1 maint: multiple names: authors list (link)
  35. ^ Lam B., Walke J., Vredenburg V., Harris R. (2009). "Proportion of individuals with anti-Batrachochytrium dendrobatidis skin bacteria is associated with population persistence in the frog Rana muscosa". Biological Conservation. 143 (2): 529–531. doi:10.1016/j.biocon.2009.11.015.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  36. ^ Buck, Julia; Truong, Lisa; Blaustein, Andrew (2011). "Predation by zooplankton on Batrachochytrium dendrobatidis: biological control of the deadly amphibian chytrid fungus?". Biodiversity and Conservation. 20 (14): 3549–3553. doi:10.1007/s10531-011-0147-4.
  37. ^ Cohen, Nathan W.; Stebbins, Robert A. (1995). A Natural History of Amphibians. Princeton, N.J: Princeton University Press. ISBN 0-691-10251-1.{{cite book}}: CS1 maint: multiple names: authors list (link)
  38. ^ Daividson C, Shaffer HB, Jennings MR (2001). "Declines of the California red-legged frog: climate, UV-B, habitat, and pesticides hypotheses". Ecological Applications. 11 (2): 464–479. doi:10.1890/1051-0761(2001)011[0464:DOTCRL]2.0.CO;2.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  39. ^ Hayes TB, Case P, Chui S, Chung D, Haeffele C, Haston K, Lee M, Mai VP, Marjuoa Y, Parker J, Tsui M (April 2006). "Pesticide mixtures, endocrine disruption, and amphibian declines: are we underestimating the impact?". Environ. Health Perspect. 114 (Suppl 1): 40–50. doi:10.1289/ehp.8051. PMC 1874187. PMID 16818245.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  40. ^ Davidson C, Benard MF, Shaffer HB, Parker JM, O'Leary C, Conlon JM, Rollins-Smith LA (March 2007). "Effects of chytrid and carbaryl exposure on survival, growth and skin peptide defenses in foothill yellow-legged frogs". Environ. Sci. Technol. 41 (5): 1771–6. doi:10.1021/es0611947. PMID 17396672.{{cite journal}}: CS1 maint: multiple names: authors list (link)

External links

Wikinews has related news:

Category:Amphibian diseases Category:Fungal diseases

Retrieved from "https://en.wikipedia.org/w/index.php?title=Chytridiomycosis&oldid=587448598"