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. 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.
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.
The oldest documented case 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. 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. 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. The earliest documented case of the disease chytridiomycosis was an American bullfrog (Rana catesbeiana) collected in 1978. It is still not clear if it is a new emergent pathogen or if it is an old pathogen with recently increased virulence.
Chytridiomycosis is believed to adhere to the following course: zoospores first encounter amphibian skin and quickly give rise to sporangia, which produce new zoospores. 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).
Laboratory studies suggest that the amphibian chytrid fungus grows best between 17-25°C, and that exposure of infected frogs to high temperatures can cure the frogs. 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. This may explain why chytridiomycosis-induced amphibian declines have occurred primarily at higher elevations and during cooler months. 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.
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. 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. 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. In Guatemala, several thousands of tadpoles perished from an unidentified pathogen distinct from B. dendrobatidis. Such researchers stress the need for a broader understanding of the host-parasite ecology that is contributing to the modern day amphibian declines.
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