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'''''Neorickettsia risticii''''' (formerly ''Ehrlichia risticii'')<ref>{{cite journal |vauthors=Dumler JS, Barbet AF, Bekker CP, etal |title=Reorganization of genera in the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales: unification of some species of Ehrlichia with Anaplasma, Cowdria with Ehrlichia and Ehrlichia with Neorickettsia, descriptions of six new species combinations and designation of Ehrlichia equi and 'HGE agent' as subjective synonyms of Ehrlichia phagocytophila |journal=Int. J. Syst. Evol. Microbiol. |volume=51 |issue=Pt 6 |pages=2145–65 |year=2001 |pmid=11760958 |doi=10.1099/00207713-51-6-2145|doi-access=free }}</ref> is a Gram-negative bacterium that causes [[Potomac horse fever]].<ref>{{Cite journal|last=Xiong|first=Qingming|last2=Bekebrede|first2=Hannah|last3=Sharma|first3=Pratibha|last4=Arroyo|first4=Luis G.|last5=Baird|first5=John D.|last6=Rikihisa|first6=Yasuko|date=2016-09-16|title=An Ecotype of Neorickettsia risticii Causing Potomac Horse Fever in Canada|url=https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5038023/|journal=Applied and Environmental Microbiology|volume=82|issue=19|pages=6030–6036|doi=10.1128/AEM.01366-16|issn=0099-2240|pmc=5038023|pmid=27474720}}</ref>
'''''Neorickettsia risticii''''' (formerly ''Ehrlichia risticii'')<ref>{{cite journal |vauthors=Dumler JS, Barbet AF, Bekker CP, etal |title=Reorganization of genera in the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales: unification of some species of Ehrlichia with Anaplasma, Cowdria with Ehrlichia and Ehrlichia with Neorickettsia, descriptions of six new species combinations and designation of Ehrlichia equi and 'HGE agent' as subjective synonyms of Ehrlichia phagocytophila |journal=Int. J. Syst. Evol. Microbiol. |volume=51 |issue=Pt 6 |pages=2145–65 |year=2001 |pmid=11760958 |doi=10.1099/00207713-51-6-2145|doi-access=free }}</ref> is an obligate intracellular gram negative bacteria that typically lives as an endosymbiont to parasitic flatworms, specifically flukes.<ref name=":2">{{Cite journal|last=Teymournejad|first=Omid|last2=Lin|first2=Mingqun|last3=Bekebrede|first3=Hannah|last4=Kamr|first4=Ahmed|last5=Toribio|first5=Ramiro E.|last6=Arroyo|first6=Luis G.|last7=Baird|first7=John D.|last8=Rikihisa|first8=Yasuko|date=2020-02-25|title=Isolation and Molecular Analysis of a Novel Neorickettsia Species That Causes Potomac Horse Fever|url=https://mbio.asm.org/content/11/1/e03429-19|journal=mBio|language=en|volume=11|issue=1|doi=10.1128/mBio.03429-19|issn=2150-7511|pmid=32098825}}</ref> ''N. risticii'' is the known causative agent of equine neorickettsiosis (also known as [[Potomac horse fever]] (PHF)),<ref>{{Cite journal|last=Gibson|first=Kathryn E.|last2=Rikihisa|first2=Yasuko|last3=Zhang|first3=Chunbin|last4=Martin|first4=Carole|date=2005|title=Neorickettsia risticii is vertically transmitted in the trematode Acanthatrium oregonense and horizontally transmitted to bats|url=https://sfamjournals.onlinelibrary.wiley.com/doi/abs/10.1111/j.1462-2920.2004.00683.x|journal=Environmental Microbiology|language=en|volume=7|issue=2|pages=203–212|doi=10.1111/j.1462-2920.2004.00683.x|issn=1462-2920}}</ref> which gets its name from its discovery near the [[Potomac River]] in Maryland and Virginia<ref name=":13">{{Cite journal|last=Chaichanasiriwithaya|first=W.|last2=Rikihisa|first2=Y.|last3=Yamamoto|first3=S.|last4=Reed|first4=S.|last5=Crawford|first5=T. B.|last6=Perryman|first6=L. E.|last7=Palmer|first7=G. H.|date=1994-12-01|title=Antigenic, morphologic, and molecular characterization of new Ehrlichia risticii isolates.|url=https://jcm.asm.org/content/32/12/3026|journal=Journal of Clinical Microbiology|language=en|volume=32|issue=12|pages=3026–3033|issn=0095-1137|pmid=7533780}}</ref>. ''N. risticii'' was first recovered from horses in this region in 1984 but was not recognized as the causative agent of PHF until 1979.<ref name=":13" /> Potomac horse fever is currently endemic in the United States but has also been reported with lower frequency in other regions, including Canada, Brazil, Uruguay, and Europe.<ref name=":02">{{Cite journal|last=Xiong|first=Qingming|last2=Bekebrede|first2=Hannah|last3=Sharma|first3=Pratibha|last4=Arroyo|first4=Luis. G.|last5=Baird|first5=John. D.|last6=Rikihisa|first6=Yasuko|date=|editor-last=Dozois|editor-first=C. M.|title=An Ecotype of Neorickettsia risticii Causing Potomac Horse Fever in Canada|url=https://aem.asm.org/content/aem/82/19/6030.full.pdf|journal=Applied and Environmental Microbiology|volume=82(19)|pages=6030 - 6036|doi=10.1128/AEM.01366-16|issn=0099-2240|pmc=PMC5038023|pmid=27474720|via=American Society for Microbiology}}</ref> ''N. risticii'' is typically acquired in the middle to late summer near freshwater streams or rivers, as well as on irrigated pastures.<ref name=":11">{{Citation|last=Pusterla|first=Nicola|title=Neorickettsia risticii|date=2014|url=https://linkinghub.elsevier.com/retrieve/pii/B9781455708918000403|work=Equine Infectious Diseases|pages=347–351.e2|publisher=Elsevier|language=en|doi=10.1016/b978-1-4557-0891-8.00040-3|isbn=978-1-4557-0891-8|access-date=2020-11-15|last2=Madigan|first2=John E.}}</ref> Although ''N. risticii'' is a well known causative agent for PHF in horses, it may act as a potential pathogen in cats and dogs as well.<ref>{{Cite journal|last=Ayllón|first=Tania|last2=Diniz|first2=Pedro Paulo V.P.|last3=Breitschwerdt|first3=Edward Bealmear|last4=Villaescusa|first4=Alejandra|last5=Rodríguez-Franco|first5=Fernando|last6=Sainz|first6=Angel|date=2012-02|title=Vector-Borne Diseases in Client-Owned and Stray Cats from Madrid, Spain|url=https://www.liebertpub.com/doi/full/10.1089/vbz.2011.0729?casa_token=rFJvg_ILPL4AAAAA:g6cadVZNx_ICY-ghGGsQFtGkI3bklaqO-gmz4fn4WhIdW1ugxKoO_MWGTlZV-VpUBXjBl3dmKQs|journal=Vector-Borne and Zoonotic Diseases|volume=12|issue=2|pages=143–150|doi=10.1089/vbz.2011.0729|issn=1530-3667}}</ref> Not only has ''Neorickettsia risticii'' been successfully cultured from monocytes of dogs and cats,<ref name=":11" /> but cats have become clinically ill after experimental infection with the bacteria. <ref>{{Cite journal|last=Lappin|first=Michael R.|last2=Griffin|first2=Brenda|last3=Brunt|first3=Jane|last4=Riley|first4=Allen|last5=Burney|first5=Derek|last6=Hawley|first6=Jennifer|last7=Brewer|first7=Melissa M.|last8=Jensen|first8=Wayne A.|date=2006-04-01|title=Prevalence of Bartonella species, haemoplasma species, Ehrlichia species, Anaplasma phagocytophilum, and Neorickettsia risticii DNA in the blood of cats and their fleas in the United States|url=https://doi.org/10.1016/j.jfms.2005.08.003|journal=Journal of Feline Medicine and Surgery|language=en|volume=8|issue=2|pages=85–90|doi=10.1016/j.jfms.2005.08.003|issn=1098-612X}}</ref> In addition, ''N. risticii'' has been isolated and cultured from human histiocytic lymphoma cells. <ref name=":11" />

== Taxonomy, Morphology, and Identification ==
''Neorickettsia risticii'' is a member of the Order Rickettsiae and Family Anaplasmataceae<ref name=":10">{{Citation|last=Yu|first=Xue-Jie|title=The Order Rickettsiales|date=2006|url=http://link.springer.com/10.1007/0-387-30745-1_20|work=The Prokaryotes|pages=493–528|editor-last=Dworkin|editor-first=Martin|place=New York, NY|publisher=Springer New York|language=en|doi=10.1007/0-387-30745-1_20|isbn=978-0-387-25495-1|access-date=2020-11-15|last2=Walker|first2=David H.|editor2-last=Falkow|editor2-first=Stanley|editor3-last=Rosenberg|editor3-first=Eugene|editor4-last=Schleifer|editor4-first=Karl-Heinz}}</ref>. Members of this order are [[Proteobacteria|ą-Proteobacteria]]<ref name=":10" />. ''N. risticii'' are obligate intracellular organisms<ref name=":10" /> of equine monocytes/macrophages and glandular intestinal epithelium<ref name=":112">{{Citation|last=Pusterla|first=Nicola|title=Neorickettsia risticii|date=2014|url=https://linkinghub.elsevier.com/retrieve/pii/B9781455708918000403|work=Equine Infectious Diseases|pages=347–351.e2|publisher=Elsevier|language=en|doi=10.1016/b978-1-4557-0891-8.00040-3|isbn=978-1-4557-0891-8|access-date=2020-11-15|last2=Madigan|first2=John E.}}</ref>. Individual organisms are small, pleomorphic (coccoid to ellipsoidal) gram negative aerobes that lack LPS and peptidoglycan in their cell walls<ref>{{Cite journal|last=Lin|first=Mingqun|last2=Zhang|first2=Chunbin|last3=Gibson|first3=Kathryn|last4=Rikihisa|first4=Yasuko|date=2009-10-01|title=Analysis of complete genome sequence of Neorickettsia risticii: causative agent of Potomac horse fever|url=https://academic.oup.com/nar/article-lookup/doi/10.1093/nar/gkp642|journal=Nucleic Acids Research|language=en|volume=37|issue=18|pages=6076–6091|doi=10.1093/nar/gkp642|issn=1362-4962|pmc=PMC2764437|pmid=19661282|via=}}</ref><ref name=":12">{{Cite journal|last=Rikihisa|first=Y.|date=2006-10-01|title=New Findings on Members of the Family Anaplasmataceae of Veterinary Importance|url=http://doi.wiley.com/10.1196/annals.1374.083|journal=Annals of the New York Academy of Sciences|language=en|volume=1078|issue=1|pages=438–445|doi=10.1196/annals.1374.083|issn=0077-8923}}</ref>. They have two cell forms: a reticular body type form that is large and light, and a smaller elementary body type form that is electron-dense<ref name=":112" /><ref name=":12" /><ref name=":10" />. They can exist as individual cells or as morulae (colonies similar in appearance to [[Morus (plant)|mulberries]]) and replicate in parasitophorous vacuoles via binary fission within the host cell<ref name=":12" /><ref name=":10" />. Identification can be done based on cell culture isolation, serology, or PCR assay<ref name=":22">{{Cite journal|last=Teymournejad|first=Omid|last2=Lin|first2=Mingqun|last3=Bekebrede|first3=Hannah|last4=Kamr|first4=Ahmed|last5=Toribio|first5=Ramiro E.|last6=Arroyo|first6=Luis G.|last7=Baird|first7=John D.|last8=Rikihisa|first8=Yasuko|date=2020-02-25|title=Isolation and Molecular Analysis of a Novel Neorickettsia Species That Causes Potomac Horse Fever|url=https://mbio.asm.org/content/11/1/e03429-19|journal=mBio|language=en|volume=11|issue=1|doi=10.1128/mBio.03429-19|issn=2150-7511|pmid=32098825}}</ref><ref name=":112" /><ref>{{Cite journal|last=Lappin|first=Michael R.|last2=Griffin|first2=Brenda|last3=Brunt|first3=Jane|last4=Riley|first4=Allen|last5=Burney|first5=Derek|last6=Hawley|first6=Jennifer|last7=Brewer|first7=Melissa M.|last8=Jensen|first8=Wayne A.|date=2006-04-01|title=Prevalence of Bartonella species, haemoplasma species, Ehrlichia species, Anaplasma phagocytophilum , and Neorickettsia risticii DNA in the blood of cats and their fleas in the United States|url=http://journals.sagepub.com/doi/10.1016/j.jfms.2005.08.003|journal=Journal of Feline Medicine and Surgery|language=en|volume=8|issue=2|pages=85–90|doi=10.1016/j.jfms.2005.08.003|issn=1098-612X|via=}}</ref>. However, ''N. risticii'' is rarely identified in monocytes from peripheral blood smears<ref name=":112" />. PCR assays can be performed on blood and feces (antemortem identification) or on fresh or formalin fixed tissues (postmortem identification)<ref name=":112" />. ''N. risticii'' can be viewed using light microscopy with a variety of stains: [[Giemsa stain|Giesma]] and [[Romanowsky stain|Romanowsky's]] will stain dark blue to purple<ref name=":12" />, Machiavello will stain red, and [[H&E stain|hematoxylin eosin]] (H&E) will stain pale blue<ref name=":112" />.


== Transmission ==
== Transmission ==
[[File:Proposed life cycle and mode transmission of Neorickettsia risticii.png|Proposed life cycle and mode transmission of Neorickettsia risticii.|left|frame]]
[[File:Proposed life cycle and mode transmission of Neorickettsia risticii.png|Proposed life cycle and mode transmission of Neorickettsia risticii.|left|frame]]
The exact mode of transmission of ''N. risticii'' has been undetermined for many years due to its complex life cycle. ''N. risticii'' is transmitted vertically inside a trematode (fluke), ''Acanthatrium oregonense'', and the transmission of ''N. risticii'' is closely related to the life cycle of the trematode.<ref name=":0">{{Citation|last=Sellon|first=Debra C.|title=Acknowledgments|date=2014|url=http://dx.doi.org/10.1016/b978-1-4557-0891-8.00078-6|work=Equine Infectious Diseases|pages=xi|publisher=Elsevier|isbn=978-1-4557-0891-8|access-date=2020-11-15|last2=Long|first2=Maureen T.}}</ref><ref name=":1">{{Cite journal|last=Madigan|first=J. E.|last2=Pusterla|first2=N.|last3=Johnson|first3=E.|last4=Chae|first4=J.-S.|last5=Pusterla|first5=J. Berger|last6=Derock|first6=E.|last7=Lawler|first7=S. P.|date=July 2000|title=Transmission of Ehrlichia risticii, the agent of Potomac horse fever, using naturally infected aquatic insects and helminth vectors: preliminary report|url=http://dx.doi.org/10.2746/042516400777032219|journal=Equine Veterinary Journal|volume=32|issue=4|pages=275–279|doi=10.2746/042516400777032219|issn=0425-1644}}</ref> As the trematode develops and gets into the next host, N. risticii is passed on to the next host along with it.<ref name=":1" /> The proposed mode of transmission involves two intermediate hosts (an aquatic snail and an aquatic insect such as caddisflies and mayflies.<ref name=":0" /> The egg of the fluke is ingested by an aquatic snail and cercaria form of the fluke develops and emerges from the snail into the environment.<ref name=":0" /> The cercaria then invades the second intermediate host, the larval or nymph stage of an aquatic insect. Inside the insect, the fluke develops into a metacercaria.<ref name=":0" /> The adult insect is then ingested by an insectivore such as a bat or a swallow.<ref name=":0" /><ref name=":1" /> It is thought that bats and swallows serve as the definitive hosts for the fluke and natural reservoirs of ''N. risticii''.<ref name=":0" />
The exact mode of transmission of ''N. risticii'' has been undetermined for many years due to its complex life cycle. ''N. risticii'' is transmitted vertically inside a [[Trematoda|trematode]] (fluke), ''Acanthatrium oregonense'', and the transmission of ''N. risticii'' is closely related to the life cycle of the trematode.<ref name=":0">{{Citation|last=Sellon|first=Debra C.|title=Acknowledgments|date=2014|url=http://dx.doi.org/10.1016/b978-1-4557-0891-8.00078-6|work=Equine Infectious Diseases|pages=xi|publisher=Elsevier|isbn=978-1-4557-0891-8|access-date=2020-11-15|last2=Long|first2=Maureen T.}}</ref><ref name=":1">{{Cite journal|last=Madigan|first=J. E.|last2=Pusterla|first2=N.|last3=Johnson|first3=E.|last4=Chae|first4=J.-S.|last5=Pusterla|first5=J. Berger|last6=Derock|first6=E.|last7=Lawler|first7=S. P.|date=July 2000|title=Transmission of Ehrlichia risticii, the agent of Potomac horse fever, using naturally infected aquatic insects and helminth vectors: preliminary report|url=http://dx.doi.org/10.2746/042516400777032219|journal=Equine Veterinary Journal|volume=32|issue=4|pages=275–279|doi=10.2746/042516400777032219|issn=0425-1644}}</ref> As the trematode develops and gets into the next host, N. risticii is passed on to the next host along with it.<ref name=":1" /> The proposed mode of transmission involves two intermediate hosts (an aquatic snail and an aquatic insect such as caddisflies and mayflies.<ref name=":0" /> The egg of the fluke is ingested by an aquatic snail and cercaria form of the fluke develops and emerges from the snail into the environment.<ref name=":0" /> The cercaria then invades the second intermediate host, the larval or nymph stage of an aquatic insect. Inside the insect, the fluke develops into a metacercaria.<ref name=":0" /> The adult insect is then ingested by an insectivore such as a bat or a swallow.<ref name=":0" /><ref name=":1" /> It is thought that bats and swallows serve as the definitive hosts for the fluke and natural reservoirs of ''N. risticii''.<ref name=":0" />


It is proposed that horses can be exposed to N. risticii through two routes. Horses can be exposed through skin penetration by infected cercaria form of the fluke. This is commonly seen when horses are grazing near contaminated water sources.<ref>{{Cite web|title=Potomac Horse Fever: A Water-borne Equestrian Worry|url=https://horse-canada.com/magazine/health/potomac-horse-fever-a-water-borne-worry/|access-date=2020-11-15|website=Horse Canada|language=en-CA}}</ref> Another route of exposure is through directly ingesting the second intermediate host, aquatic insects, containing the infected metacercariae.<ref name=":0" />
It is proposed that horses can be exposed to N. risticii through two routes. Horses can be exposed through skin penetration by infected cercaria form of the fluke. This is commonly seen when horses are grazing near contaminated water sources.<ref>{{Cite web|title=Potomac Horse Fever: A Water-borne Equestrian Worry|url=https://horse-canada.com/magazine/health/potomac-horse-fever-a-water-borne-worry/|access-date=2020-11-15|website=Horse Canada|language=en-CA}}</ref> Another route of exposure is through directly ingesting the second intermediate host, aquatic insects, containing the infected metacercariae.<ref name=":0" />


== Pathogenesis ==
== Pathogenesis ==
It is proposed that ''N. risticii'' is an obligate intracullular bacterium which prefer replicating in host monocytes.<ref>{{Cite web|title=Potomac Horse Fever - Digestive System|url=https://www.merckvetmanual.com/digestive-system/intestinal-diseases-in-horses-and-foals/potomac-horse-fever?query=neorickettsia%20risticii|access-date=2020-11-15|website=Merck Veterinary Manual|language=en-US}}</ref> Once phagocytized by the monocyte, the pathogen prevents lysosomal fusion with the phagosome thus escaping the host defense mechanism.<ref name=":0" /> Once N. risticii establishes infection inside the host, the primary targets are the epithelial cells, mast cells, and macrophages of the host's large intestine.<ref name=":0" /> The infected intestinal epithelial cells suffer from impaired electrolyte transport and loss of microvilli, which contribute to decreased electrolyte absorption and increased water loss in the large intestine.<ref>{{Cite journal|last=Rikihisa|first=Y.|last2=Johnson|first2=G.C.|last3=Wang|first3=Yu-Zhong|last4=Reed|first4=S.M.|last5=Fertel|first5=R.|last6=Cooke|first6=H.J.|date=May 1992|title=Loss of absorptive capacity for sodium and chloride in the colon causes diarrhoea in Potomac horse fever|url=http://dx.doi.org/10.1016/0034-5288(92)90037-3|journal=Research in Veterinary Science|volume=52|issue=3|pages=353–362|doi=10.1016/0034-5288(92)90037-3|issn=0034-5288}}</ref> Therefore, diarrhea is one of the common clinical signs of equine neorickettsiosis.
It is proposed that ''N. risticii'' is an obligate intracullular bacterium which prefer replicating in host [[Monocyte|monocytes]].<ref>{{Cite web|title=Potomac Horse Fever - Digestive System|url=https://www.merckvetmanual.com/digestive-system/intestinal-diseases-in-horses-and-foals/potomac-horse-fever?query=neorickettsia%20risticii|access-date=2020-11-15|website=Merck Veterinary Manual|language=en-US}}</ref> Once phagocytized by the monocyte, the pathogen prevents lysosomal fusion with the [[phagosome]] thus escaping the host defense mechanism.<ref name=":0" /> Once N. risticii establishes infection inside the host, the primary targets are the epithelial cells, mast cells, and macrophages of the host's large intestine.<ref name=":0" /> The infected intestinal epithelial cells suffer from impaired electrolyte transport and loss of microvilli, which contribute to decreased electrolyte absorption and increased water loss in the large intestine.<ref>{{Cite journal|last=Rikihisa|first=Y.|last2=Johnson|first2=G.C.|last3=Wang|first3=Yu-Zhong|last4=Reed|first4=S.M.|last5=Fertel|first5=R.|last6=Cooke|first6=H.J.|date=May 1992|title=Loss of absorptive capacity for sodium and chloride in the colon causes diarrhoea in Potomac horse fever|url=http://dx.doi.org/10.1016/0034-5288(92)90037-3|journal=Research in Veterinary Science|volume=52|issue=3|pages=353–362|doi=10.1016/0034-5288(92)90037-3|issn=0034-5288}}</ref> Therefore, diarrhea is one of the common clinical signs of equine neorickettsiosis.

== Clinical Significance ==
''Neorickettsia risticii'' is the infectious cause of Equine neorickettsiosis, or the colloquially termed [[Potomac horse fever|Potomac Horse Fever]] (PHF) <ref name=":4">{{Cite journal|last=Roier|first=Erica C. R.|last2=Costa|first2=Renata L.|last3=Pires|first3=Marcus S.|last4=Vilela|first4=Joice A. R.|last5=dos Santos|first5=Tiago M.|last6=Santos|first6=Huarrisson A.|last7=Baldani|first7=Cristiane D.|last8=Massard|first8=Carlos L.|date=|title=Epidemiological survey of Neorickettsia risticii in equids from the State of Rio de Janeiro, Brazil|url=|journal=Pesq. Vet. Bras.|volume=36(10)|pages=939-946|doi=10.1590/S0100-736X2016001000004|via=SciELO}}</ref>. This disease is acquired when horses ingest a trematode host that is infected with the bacteria <ref name=":4" />. The trematodes are inside intermediate hosts, such as aquatic insects and molluscs, that can be located on pasture and are accidentally grazed by the horses <ref name=":4" />. Because of the presence of the arthropod vector, it seems that this disease is of concern in the summer, when warmer weather permits these vectors to be present on pasture <ref>{{Cite journal|last=McLaughlin|first=Brent|last2=Gough|first2=Jiggs|date=|year=1996|title=Potomac horse fever in southwestern Ontario|url=|journal=Can Vet J|volume=37|pages=367-368|via=}}</ref>. ''N. risticii'' is able to stay inside these trematodes through their development stages, and can also be transmitted to future generations through a [[transovarial transmission]] route <ref name=":5">{{Cite journal|last=Lin|first=Mingqun|last2=Zhang|first2=Chunbin|last3=Gibson|first3=Kathryn|last4=Rikihisa|first4=Yasuko|date=|year=2009|title=Analysis of complete genome sequence of Neorickettsia risticii: causative agent of Potomac horse fever|url=|journal=Nucleic Acids Research|volume=37(18)|pages=6076-6091|doi=10.1093/nar/gkp642|via=}}</ref>. Once infected, the horse is not contagious to other horses, as the infection must be spread by the intermediate host ingestion <ref name=":4" />.

This disease can cause horses to become feverish, experience liquid diarrhea, they can show a quiet demeanor, go off their food, which can lead to colic and [[laminitis]] <ref name=":4" />. Intestinal lesions previously seen with PHF include pronounced enterocolitis with ulcerative erosions and evident reduction in villus projections<ref name=":6">{{Cite journal|last=Heller|first=Meera C.|last2=McClure|first2=J.|last3=Pusterla|first3=Nicola|last4=Berger Pusterla|first4=Jeannine|last5=Stahel|first5=Simone|date=|year=2004|title=Two cases of Neorickettsia (Ehrlichia) risticii infection in horses from Nova Scotia|url=|journal=Can Vet J|volume=45|pages=421-423|via=}}</ref>. The lesions will result in the horse having severe gestrointestinal disease, and reduced ability to absorb nutrients. Lesions that were linked to the presence of the bacteria in the blood include hepatitis, kidney tubule inflammation, loss of renal arterial blood supply, and inflammation of the adrenal glands<ref name=":6" />. There are also changes to the blood in response to infection, and early PHF can result in reduced white blood cells, including low neutrophils and low lymphocytes, while progression of PHF can result in an inflammatory response and increased white blood cells <ref name=":6" />. Replication of ''N. risticii'' happens within the [[Epithelium|epithelial lining]] of the intestines, and within myeloid cells such as macrophages, monocytes, and mast cells <ref name=":5" />. The condition can be fatal in 3 out of 10 horses that are infected, but up to 1/3 of horses infected have been shown to be asymptomatic, so there is variability in the disease presentation <ref name=":4" />. Due to the severe symptoms associated with the disease, and fatality rate, this condition is of concern for horse owners.

If a pregnant mare is infected with ''N. risticii'' and has symptoms of PHF, there is a chance that it may cause abortion and lesions in the fetus. Studies have been done on mares that were infected either naturally or artificially with ''N. risticii'' and experienced abortion. The fetuses had lesions of liver inflammation, heart inflammation, enterocolitis, and inflammation of the mesenteric lymph nodes <ref name=":14">{{Cite journal|last=Coffman|first=Elizabeth A.|last2=Abd-Eldaim|first2=Mohamed|last3=Craig|first3=Linden E.|date=|year=2008|title=Abortion in a horse following Neorickettsia risticii infection|url=|journal=J Vet Diagn Invest|volume=20|pages=827 - 830|doi=https://doi.org/10.1177/104063870802000622|via=}}</ref>. Abortion seems to be a notable complication of PHF, as was showcased in a study that infected pregnant mares with ''N. risticii'' resulting in abortion in just over half of the subjects <ref name=":14" />. This infection does not cause inflammation of the placenta, and lesions are generally absent from the placenta or very minor, yet it does cause lesions in the fetus itself, which is quite unique <ref name=":14" />. PHF is not a well documented cause for equine abortion, and is often not chosen as a differential diagnosis <ref name=":14" />. However, it could be argued that ''N. risticii'' requires more research in this area, and could be a more common cause of equine abortion than was previously thought

== Diagnostics ==
Diagnosis of Potomac Horse Fever (PHF) is most commonly based on serological techniques and therefore requires collecting a blood sample from an infected horse. ''Neorickettsia risticii'' is detected in the blood using an indirect [[Immunofluorescence|immunofluorescent]] antibody (IFA) test or by [[Polymerase chain reaction|PCR]] identification.<ref>{{Cite journal|last=Gibson|first=Kathryn E.|last2=Pastenkos|first2=Gabrielle|last3=Moesta|first3=Susanne|last4=Rikihisa|first4=Yasuko|date=2011-06-02|title=Neorickettsia risticii surface-exposed proteins: proteomics identification, recognition by naturally-infected horses, and strain variations|url=https://doi.org/10.1186/1297-9716-42-71|journal=Veterinary Research|language=en|volume=42|issue=1|pages=71|doi=10.1186/1297-9716-42-71|issn=1297-9716|pmc=PMC3127766|pmid=21635728}}</ref> The IFA serological test detects the presence of IgG and IgM antibodies against ''N. risticii'' in the blood, however does not differentiate whether an animal is actively infected or has had previous exposure.<ref name=":15">{{Cite journal|last=Mott|first=J.|last2=Rikihisa|first2=Y.|last3=Zhang|first3=Y.|last4=Reed|first4=S. M.|last5=Yu|first5=C. Y.|date=1997-09-01|title=Comparison of PCR and culture to the indirect fluorescent-antibody test for diagnosis of Potomac horse fever.|url=https://jcm.asm.org/content/35/9/2215|journal=Journal of Clinical Microbiology|language=en|volume=35|issue=9|pages=2215–2219|issn=0095-1137|pmid=9276390}}</ref> The use of IFA at a single point in time does not provide an accurate diagnosis and therefore requires IFA testing at multiple time points to detect increasing antibody titres. Increasing antibody titres in conjunction with clinical signs is suggestive of an active PHF infection.<ref name=":15" /> PCR assays can be performed on both the blood and stool of an infected animal and allows the detection of ''N. risticii'' in these samples. <ref name=":03">{{Cite journal|last=Xiong|first=Qingming|last2=Bekebrede|first2=Hannah|last3=Sharma|first3=Pratibha|last4=Arroyo|first4=Luis. G.|last5=Baird|first5=John. D.|last6=Rikihisa|first6=Yasuko|date=|editor-last=Dozois|editor-first=C. M.|title=An Ecotype of Neorickettsia risticii Causing Potomac Horse Fever in Canada|url=https://aem.asm.org/content/aem/82/19/6030.full.pdf|journal=Applied and Environmental Microbiology|volume=82(19)|pages=6030 - 6036|doi=10.1128/AEM.01366-16|issn=0099-2240|pmc=PMC5038023|pmid=27474720|via=American Society for Microbiology}}</ref> PCR is the test of choice in vaccinated animals because these animals will have antibodies against ''N. risticii ,'' which interferes with the IFA test. <ref name=":15" />

[[Cell culture]] isolation can also be used for the detection and isolation of ''Neorickettsia risticii'', although it is a more time-consuming technique that requires specialized equipment and is not routinely used as a diagnostic method.<ref name=":23">{{Cite journal|last=Teymournejad|first=Omid|last2=Lin|first2=Mingqun|last3=Bekebrede|first3=Hannah|last4=Kamr|first4=Ahmed|last5=Toribio|first5=Ramiro E.|last6=Arroyo|first6=Luis G.|last7=Baird|first7=John D.|last8=Rikihisa|first8=Yasuko|date=2020-02-25|title=Isolation and Molecular Analysis of a Novel Neorickettsia Species That Causes Potomac Horse Fever|url=https://mbio.asm.org/content/11/1/e03429-19|journal=mBio|language=en|volume=11|issue=1|doi=10.1128/mBio.03429-19|issn=2150-7511|pmid=32098825}}</ref> ''N. risticii'' is an obligate intracellular microorganism and as such is more technically difficult to culture, isolate, and ship to a diagnostic laboratory.<ref name=":03" /> Cell culture isolation detects the presence of live bacteria and can be used to isolate bacteria from an infected animal, the environment, or a parasitic host.<ref name=":23" /> Strain variation among isolates can be determined by identifying surface antigens or by performing whole-genome sequencing.<ref name=":03" /> Molecular analysis surface antigens has found significant genetic and phenotypic variation in strains of ''N. risticii'', these variations may contribute to vaccination failures.<ref>{{Cite journal|last=Biswas|first=Biswajit|last2=Vemulapalli|first2=Ramesh|last3=Dutta|first3=Sukanta K.|date=1998-08-01|title=Molecular Basis for Antigenic Variation of a Protective Strain-Specific Antigen of Ehrlichia risticii|url=https://iai.asm.org/content/66/8/3682|journal=Infection and Immunity|language=en|volume=66|issue=8|pages=3682–3688|doi=10.1128/IAI.66.8.3682-3688.1998|issn=0019-9567|pmid=9673249}}</ref>

== Treatment and Prevention ==
The main clinical presentation of ''Neorickettsia risticii'' is Potomac Horse Fever (PHF). Timely diagnosis and treatment of PHF is important for preventing the disease from progressing and causing clinical signs such as laminitis, endotoxemia and colic. Successfully preventing this progression can greatly increase an infected horse's chance of survival.<ref name=":3">{{Cite journal|last=Bertin|first=F. R.|last2=Reising|first2=A.|last3=Slovis|first3=N. M.|last4=Constable|first4=P. D.|last5=Taylor|first5=S. D.|date=2013|title=Clinical and Clinicopathological Factors Associated with Survival in 44 Horses with Equine Neorickettsiosis (Potomac Horse Fever)|url=https://www.onlinelibrary.wiley.com/doi/abs/10.1111/jvim.12209|journal=Journal of Veterinary Internal Medicine|language=en|volume=27|issue=6|pages=1528–1534|doi=10.1111/jvim.12209|issn=1939-1676}}</ref> Treatment of PHF utilizes a combination of antimicrobial and supportive therapy.<ref>{{Cite web|title=Potomac Horse Fever - Digestive System|url=https://www.merckvetmanual.com/digestive-system/intestinal-diseases-in-horses-and-foals/potomac-horse-fever|access-date=2020-11-10|website=Merck Veterinary Manual|language=en-US}}</ref> Oxytetracycline has been found to be the most successful antimicrobial treatment for PHF and has been correlated to increased survival time.<ref name=":3" /> When treatment is delayed until after the onset of clinical signs, additional therapy with doxycycline, demeclocycline or rifampin has been associated with higher antibody titer levels than those of untreated animals.<ref>{{Cite journal|last=Rikihisa|first=Yasuko|last2=Jiang|first2=Baoming M.|date=1989-03-01|title=Effect of antibiotics on clinical, pathologic and immunologic responses in murine Potomac horse fever: Protective effects of doxycycline|url=http://www.sciencedirect.com/science/article/pii/0378113589900710|journal=Veterinary Microbiology|language=en|volume=19|issue=3|pages=253–262|doi=10.1016/0378-1135(89)90071-0|issn=0378-1135}}</ref> In addition to antimicrobial treatment, supportive therapy for pain management, dehydration and gastrointestinal function should also be utilized to treat laminitis, endotoxemia and colic, respectively. Laminitis can be treated with a combination of cryotherapy and an opioid, such as butorphanol or morphine.<ref>{{Cite journal|last=Durán|first=María Carolina|last2=Marqués|first2=Fernando J.|date=2016-03-01|title=Detection of Neorickettsia risticii, the agent of Potomac horse fever, in a Gypsy Vanner stallion from Manitoba|url=https://pubmed.ncbi.nlm.nih.gov/26933267/|journal=The Canadian Veterinary Journal = La Revue Veterinaire Canadienne|volume=57|issue=3|pages=293–295|issn=0008-5286|pmc=4751770|pmid=26933267|via=}}</ref> Intravenous lactated Ringer's solution can be given for fluid and electrolyte replenishment and Flunixin Meglumine can be used to treat signs of colic, both of which can help offset signs of endotoxemia and prevent endotoxic shock.<ref name=":04">{{Cite journal|last=Xiong|first=Qingming|last2=Bekebrede|first2=Hannah|last3=Sharma|first3=Pratibha|last4=Arroyo|first4=Luis. G.|last5=Baird|first5=John. D.|last6=Rikihisa|first6=Yasuko|date=|editor-last=Dozois|editor-first=C. M.|title=An Ecotype of Neorickettsia risticii Causing Potomac Horse Fever in Canada|url=https://aem.asm.org/content/aem/82/19/6030.full.pdf|journal=Applied and Environmental Microbiology|volume=82(19)|pages=6030 - 6036|doi=10.1128/AEM.01366-16|issn=0099-2240|pmc=PMC5038023|pmid=27474720|via=American Society for Microbiology}}</ref>

There are currently two types of vaccines for Potomac Horse Fever available. One is an inactive monovalent vaccine that protects against PHF only, and the other is an inactive multivalent vaccine that is combined with a rabies vaccination. There is some evidence to show that the multivalent PHF/rabies vaccine provides lower immunogenicity in contrast to the monovalent PHF vaccine when it is given with a separate rabies vaccine.<ref>{{Cite journal|last=McKenzie|first=H. C.|last2=Funk|first2=R. A.|last3=Trager|first3=L.|last4=Werre|first4=S. R.|last5=Crisman|first5=M.|date=2019-11-01|title=Immunogenicity of Potomac horse fever vaccine when simultaneously co‐administered with rabies vaccine in a multivalent vaccine or as two monovalent vaccines at separate sites|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/evj.13096|journal=Equine Veterinary Journal|language=en|volume=51|issue=6|pages=774–778|doi=10.1111/evj.13096|issn=0425-1644|pmc=PMC6850380|pmid=30859618|via=}}</ref> The efficacy of these vaccines has been reported to be quite poor due to the fact that they contain only one of several strains of ''N. risticii,'' but studies have also shown that vaccinated horses that do become infected tend to show less severe clinical signs when compared to unvaccinated horses.<ref>{{Cite journal|last=Dutta|first=S. K.|last2=Vemulapalli|first2=R.|last3=Biswas|first3=B.|date=1998-02-01|title=Association of deficiency in antibody response to vaccine and heterogeneity of Ehrlichia risticii strains with Potomac horse fever vaccine failure in horses|url=https://pubmed.ncbi.nlm.nih.gov/9466767|journal=Journal of Clinical Microbiology|volume=36|issue=2|pages=506–512|doi=10.1128/JCM.36.2.506-512.1998|issn=0095-1137|pmc=PMC104568|pmid=9466767|via=}}</ref> The vaccines are recommended to be given annually, but horses in endemic regions can be given a second dose 3-4 weeks following the initial vaccine to help increase the effectiveness. Pregnant mares should also be given an additional vaccine 4-6 weeks prior to foaling.<ref>{{Cite web|title=Potomac Horse Fever {{!}} AAEP|url=https://aaep.org/guidelines/vaccination-guidelines/risk-based-vaccination-guidelines/potomac-horse-fever|access-date=2020-11-10|website=aaep.org}}</ref>


==See also==
==See also==

Revision as of 02:19, 19 November 2020

Neorickettsia risticii
Neorickettsia risticii phylogenetic tree based on 16S rRNA gene sequences
Scientific classification
Kingdom:
Bacteria
Phylum:
Proteobacteria
Class:
Alphaproteobacteria
Order:
Rickettsiales
Family:
Anaplasmataceae
Genus:
Neorickettsia
Species:
N. risticii
Binomial name
Neorickettsia risticii (Dumler et al., 2001)
Synonyms

Ehrlichia risticii

Neorickettsia risticii (formerly Ehrlichia risticii)[1] is an obligate intracellular gram negative bacteria that typically lives as an endosymbiont to parasitic flatworms, specifically flukes.[2] N. risticii is the known causative agent of equine neorickettsiosis (also known as Potomac horse fever (PHF)),[3] which gets its name from its discovery near the Potomac River in Maryland and Virginia[4]. N. risticii was first recovered from horses in this region in 1984 but was not recognized as the causative agent of PHF until 1979.[4] Potomac horse fever is currently endemic in the United States but has also been reported with lower frequency in other regions, including Canada, Brazil, Uruguay, and Europe.[5] N. risticii is typically acquired in the middle to late summer near freshwater streams or rivers, as well as on irrigated pastures.[6] Although N. risticii is a well known causative agent for PHF in horses, it may act as a potential pathogen in cats and dogs as well.[7] Not only has Neorickettsia risticii been successfully cultured from monocytes of dogs and cats,[6] but cats have become clinically ill after experimental infection with the bacteria. [8] In addition, N. risticii has been isolated and cultured from human histiocytic lymphoma cells. [6]

Taxonomy, Morphology, and Identification

Neorickettsia risticii is a member of the Order Rickettsiae and Family Anaplasmataceae[9]. Members of this order are ą-Proteobacteria[9]. N. risticii are obligate intracellular organisms[9] of equine monocytes/macrophages and glandular intestinal epithelium[10]. Individual organisms are small, pleomorphic (coccoid to ellipsoidal) gram negative aerobes that lack LPS and peptidoglycan in their cell walls[11][12]. They have two cell forms: a reticular body type form that is large and light, and a smaller elementary body type form that is electron-dense[10][12][9]. They can exist as individual cells or as morulae (colonies similar in appearance to mulberries) and replicate in parasitophorous vacuoles via binary fission within the host cell[12][9]. Identification can be done based on cell culture isolation, serology, or PCR assay[13][10][14]. However, N. risticii is rarely identified in monocytes from peripheral blood smears[10]. PCR assays can be performed on blood and feces (antemortem identification) or on fresh or formalin fixed tissues (postmortem identification)[10]. N. risticii can be viewed using light microscopy with a variety of stains: Giesma and Romanowsky's will stain dark blue to purple[12], Machiavello will stain red, and hematoxylin eosin (H&E) will stain pale blue[10].

Transmission

Proposed life cycle and mode transmission of Neorickettsia risticii.

The exact mode of transmission of N. risticii has been undetermined for many years due to its complex life cycle. N. risticii is transmitted vertically inside a trematode (fluke), Acanthatrium oregonense, and the transmission of N. risticii is closely related to the life cycle of the trematode.[15][16] As the trematode develops and gets into the next host, N. risticii is passed on to the next host along with it.[16] The proposed mode of transmission involves two intermediate hosts (an aquatic snail and an aquatic insect such as caddisflies and mayflies.[15] The egg of the fluke is ingested by an aquatic snail and cercaria form of the fluke develops and emerges from the snail into the environment.[15] The cercaria then invades the second intermediate host, the larval or nymph stage of an aquatic insect. Inside the insect, the fluke develops into a metacercaria.[15] The adult insect is then ingested by an insectivore such as a bat or a swallow.[15][16] It is thought that bats and swallows serve as the definitive hosts for the fluke and natural reservoirs of N. risticii.[15]

It is proposed that horses can be exposed to N. risticii through two routes. Horses can be exposed through skin penetration by infected cercaria form of the fluke. This is commonly seen when horses are grazing near contaminated water sources.[17] Another route of exposure is through directly ingesting the second intermediate host, aquatic insects, containing the infected metacercariae.[15]

Pathogenesis

It is proposed that N. risticii is an obligate intracullular bacterium which prefer replicating in host monocytes.[18] Once phagocytized by the monocyte, the pathogen prevents lysosomal fusion with the phagosome thus escaping the host defense mechanism.[15] Once N. risticii establishes infection inside the host, the primary targets are the epithelial cells, mast cells, and macrophages of the host's large intestine.[15] The infected intestinal epithelial cells suffer from impaired electrolyte transport and loss of microvilli, which contribute to decreased electrolyte absorption and increased water loss in the large intestine.[19] Therefore, diarrhea is one of the common clinical signs of equine neorickettsiosis.

Clinical Significance

Neorickettsia risticii is the infectious cause of Equine neorickettsiosis, or the colloquially termed Potomac Horse Fever (PHF) [20]. This disease is acquired when horses ingest a trematode host that is infected with the bacteria [20]. The trematodes are inside intermediate hosts, such as aquatic insects and molluscs, that can be located on pasture and are accidentally grazed by the horses [20]. Because of the presence of the arthropod vector, it seems that this disease is of concern in the summer, when warmer weather permits these vectors to be present on pasture [21]. N. risticii is able to stay inside these trematodes through their development stages, and can also be transmitted to future generations through a transovarial transmission route [22]. Once infected, the horse is not contagious to other horses, as the infection must be spread by the intermediate host ingestion [20].

This disease can cause horses to become feverish, experience liquid diarrhea, they can show a quiet demeanor, go off their food, which can lead to colic and laminitis [20]. Intestinal lesions previously seen with PHF include pronounced enterocolitis with ulcerative erosions and evident reduction in villus projections[23]. The lesions will result in the horse having severe gestrointestinal disease, and reduced ability to absorb nutrients. Lesions that were linked to the presence of the bacteria in the blood include hepatitis, kidney tubule inflammation, loss of renal arterial blood supply, and inflammation of the adrenal glands[23]. There are also changes to the blood in response to infection, and early PHF can result in reduced white blood cells, including low neutrophils and low lymphocytes, while progression of PHF can result in an inflammatory response and increased white blood cells [23]. Replication of N. risticii happens within the epithelial lining of the intestines, and within myeloid cells such as macrophages, monocytes, and mast cells [22]. The condition can be fatal in 3 out of 10 horses that are infected, but up to 1/3 of horses infected have been shown to be asymptomatic, so there is variability in the disease presentation [20]. Due to the severe symptoms associated with the disease, and fatality rate, this condition is of concern for horse owners.

If a pregnant mare is infected with N. risticii and has symptoms of PHF, there is a chance that it may cause abortion and lesions in the fetus. Studies have been done on mares that were infected either naturally or artificially with N. risticii and experienced abortion. The fetuses had lesions of liver inflammation, heart inflammation, enterocolitis, and inflammation of the mesenteric lymph nodes [24]. Abortion seems to be a notable complication of PHF, as was showcased in a study that infected pregnant mares with N. risticii resulting in abortion in just over half of the subjects [24]. This infection does not cause inflammation of the placenta, and lesions are generally absent from the placenta or very minor, yet it does cause lesions in the fetus itself, which is quite unique [24]. PHF is not a well documented cause for equine abortion, and is often not chosen as a differential diagnosis [24]. However, it could be argued that N. risticii requires more research in this area, and could be a more common cause of equine abortion than was previously thought

Diagnostics

Diagnosis of Potomac Horse Fever (PHF) is most commonly based on serological techniques and therefore requires collecting a blood sample from an infected horse. Neorickettsia risticii is detected in the blood using an indirect immunofluorescent antibody (IFA) test or by PCR identification.[25] The IFA serological test detects the presence of IgG and IgM antibodies against N. risticii in the blood, however does not differentiate whether an animal is actively infected or has had previous exposure.[26] The use of IFA at a single point in time does not provide an accurate diagnosis and therefore requires IFA testing at multiple time points to detect increasing antibody titres. Increasing antibody titres in conjunction with clinical signs is suggestive of an active PHF infection.[26] PCR assays can be performed on both the blood and stool of an infected animal and allows the detection of N. risticii in these samples. [27] PCR is the test of choice in vaccinated animals because these animals will have antibodies against N. risticii , which interferes with the IFA test. [26]

Cell culture isolation can also be used for the detection and isolation of Neorickettsia risticii, although it is a more time-consuming technique that requires specialized equipment and is not routinely used as a diagnostic method.[28] N. risticii is an obligate intracellular microorganism and as such is more technically difficult to culture, isolate, and ship to a diagnostic laboratory.[27] Cell culture isolation detects the presence of live bacteria and can be used to isolate bacteria from an infected animal, the environment, or a parasitic host.[28] Strain variation among isolates can be determined by identifying surface antigens or by performing whole-genome sequencing.[27] Molecular analysis surface antigens has found significant genetic and phenotypic variation in strains of N. risticii, these variations may contribute to vaccination failures.[29]

Treatment and Prevention

The main clinical presentation of Neorickettsia risticii is Potomac Horse Fever (PHF). Timely diagnosis and treatment of PHF is important for preventing the disease from progressing and causing clinical signs such as laminitis, endotoxemia and colic. Successfully preventing this progression can greatly increase an infected horse's chance of survival.[30] Treatment of PHF utilizes a combination of antimicrobial and supportive therapy.[31] Oxytetracycline has been found to be the most successful antimicrobial treatment for PHF and has been correlated to increased survival time.[30] When treatment is delayed until after the onset of clinical signs, additional therapy with doxycycline, demeclocycline or rifampin has been associated with higher antibody titer levels than those of untreated animals.[32] In addition to antimicrobial treatment, supportive therapy for pain management, dehydration and gastrointestinal function should also be utilized to treat laminitis, endotoxemia and colic, respectively. Laminitis can be treated with a combination of cryotherapy and an opioid, such as butorphanol or morphine.[33] Intravenous lactated Ringer's solution can be given for fluid and electrolyte replenishment and Flunixin Meglumine can be used to treat signs of colic, both of which can help offset signs of endotoxemia and prevent endotoxic shock.[34]

There are currently two types of vaccines for Potomac Horse Fever available. One is an inactive monovalent vaccine that protects against PHF only, and the other is an inactive multivalent vaccine that is combined with a rabies vaccination. There is some evidence to show that the multivalent PHF/rabies vaccine provides lower immunogenicity in contrast to the monovalent PHF vaccine when it is given with a separate rabies vaccine.[35] The efficacy of these vaccines has been reported to be quite poor due to the fact that they contain only one of several strains of N. risticii, but studies have also shown that vaccinated horses that do become infected tend to show less severe clinical signs when compared to unvaccinated horses.[36] The vaccines are recommended to be given annually, but horses in endemic regions can be given a second dose 3-4 weeks following the initial vaccine to help increase the effectiveness. Pregnant mares should also be given an additional vaccine 4-6 weeks prior to foaling.[37]

See also

References

  1. ^ Dumler JS, Barbet AF, Bekker CP, et al. (2001). "Reorganization of genera in the families Rickettsiaceae and Anaplasmataceae in the order Rickettsiales: unification of some species of Ehrlichia with Anaplasma, Cowdria with Ehrlichia and Ehrlichia with Neorickettsia, descriptions of six new species combinations and designation of Ehrlichia equi and 'HGE agent' as subjective synonyms of Ehrlichia phagocytophila". Int. J. Syst. Evol. Microbiol. 51 (Pt 6): 2145–65. doi:10.1099/00207713-51-6-2145. PMID 11760958.
  2. ^ Teymournejad, Omid; Lin, Mingqun; Bekebrede, Hannah; Kamr, Ahmed; Toribio, Ramiro E.; Arroyo, Luis G.; Baird, John D.; Rikihisa, Yasuko (2020-02-25). "Isolation and Molecular Analysis of a Novel Neorickettsia Species That Causes Potomac Horse Fever". mBio. 11 (1). doi:10.1128/mBio.03429-19. ISSN 2150-7511. PMID 32098825.
  3. ^ Gibson, Kathryn E.; Rikihisa, Yasuko; Zhang, Chunbin; Martin, Carole (2005). "Neorickettsia risticii is vertically transmitted in the trematode Acanthatrium oregonense and horizontally transmitted to bats". Environmental Microbiology. 7 (2): 203–212. doi:10.1111/j.1462-2920.2004.00683.x. ISSN 1462-2920.
  4. ^ a b Chaichanasiriwithaya, W.; Rikihisa, Y.; Yamamoto, S.; Reed, S.; Crawford, T. B.; Perryman, L. E.; Palmer, G. H. (1994-12-01). "Antigenic, morphologic, and molecular characterization of new Ehrlichia risticii isolates". Journal of Clinical Microbiology. 32 (12): 3026–3033. ISSN 0095-1137. PMID 7533780.
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