- Potomac Horse Fever
Potomac Horse Fever (PHF) is a febrile disease affecting horses. It is also known as Shasta River Crud and Equine Monocytic Ehrlichiosis. It was first described in areas surrounding the
Potomac River northwest ofWashington, D.C. , in the 1980's, but cases have been described in many other parts of the United States, such asMinnesota ,California , andPennsylvania . Currently, it is found in more than 40 states within the US, as well as in CanadaPascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] .Cause
The causative agent of PHF is "Neorickettsia risticii" (formerly "Ehrlichia risticii"), an intracellular
rickettsia lbacterium [cite web | title = Potomac Horse Fever | work = The Merck Veterinary Manual | date = 2006 | url = http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/22204.htm | accessdate = 2007-06-14 ] Pascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] .Transmission
The vector of "Neorickettsia risticii" is believed to be a
trematode (fluke). The life cycle of the fluke takes it through freshwatersnail s and back into water, where it is ingested by the larval stages of several aquatic insects, includingcaddis flies andmayflies . It is thought that the main mode of infection is by accidental ingestion of infected adult insects, who may fly into barns and die in stalls or on pastures after eclosurePascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] . Experimental infection has been produced with oral administration of infected insects and subcutaneous inoculation of "N. risticii". All attempts to transmit the disease using ticks have failed.Several outbreaks of PHF have been found to coincide with mass emergences of burrowing mayflies of the genus "Hexagenia "; these insects hatch en masse and may be found littering the ground in nearby stables, where they are attracted by light.The entire natural history and life cycle of "N. risticii" has yet to be elucidated, but bats and birds may be wild reservoirs of infection.Unlike other causes of acutecolitis in horses, such as "Salmonella " and "Clostridium ", PHF is not spread directly from horse to horse.ymptoms
Symptoms of PHF include fever; depression (sometimes profound); inappetance; mild, colic-like symptoms; decreased manure production; profuse, watery, non-fetid diarrhea;
endotoxemia ;edema due to protein imbalances; abortion by pregnant mares; and, in 20-40% of cases, acutelaminitis and founder usually within 3 days of the initial symptoms, thought to be secondary to the endotoxemiaPascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] . Death may occur and is usually due to severe laminitis leading to founderPascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] .Horses may not always display any other symptoms beyond a feverPascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] .
Diagnosis
Diagnosis of PHF is accomplished by measuring
antibody titer s or PCR testing to look for the bacterium in the blood and feces. However, mostveterinarian s opt to initiate treatment right away, as the disease can progress quite quickly. Veterinarians may also runcomplete blood count s and chemistry andelectrolyte panels to determine the course of care.Radiograph s may be taken to track the progress of laminitic horses.Treatment
"N. risticii" responds well to
tetracycline antibiotics . Mild cases may be treated with oraldoxycycline , while severe cases are usually treated with intravenousoxytetracycline . Supportive care for severe cases is aimed at minimizing the effects of endotoxemia and preventing laminitis. This may include intravenous fluids and electrolytes to counteract the diarrheaPascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] ;NSAID s such as Banamine (flunixin meglumine); intravenousdimethyl sulfoxide ; administration of products such asBiosponge oractivated charcoal vianasogastric tube to bindendotoxin s;polymyxin B or plasma for endotoxemia; supportive shoeing; low doses of intramuscularacepromazine ; andpentoxifylline .Prevention
While a
vaccine is available for PHF, it does not cover all strains of the bacterium, and recent vaccine failures seem to be on the rise. Additionally, the vaccine usually produces a very weak immune response, which may only lessen the severity of the disease rather than prevent itPascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] .The vaccine is administered twice a year, in early spring and in early summer, with the first one innoculation given before the mayflies emerge and the second administered as a boosterPascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] .Some veterinarians have started making recommendations for farm management to try to prevent this disease:
* Maintaining riparian barriers along bodies of water may encourage aquatic insects to stay near their places of originPascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] .
* Turning off outside lights around the barn will prevent insects from being attractedPascoa, Elaine. "PHF Prevention." "Practical Horseman" July 2008: 60-66.] .
* Cleaning water buckets and feed areas frequently and keeping food covered will reduce the chance that the horse will accidentally ingest infected insects.References
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