Potomac Horse Fever (PHF) is a potentially-fatal febrile illness affecting horses caused by the intracellular bacteriumNeorickettsia risticii. PHF is also known asShasta River Crud andEquine MonocyticEhrlichiosis. It was first described in areas surrounding thePotomac River northwest ofWashington, D.C., in the 1980s, 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 U.S. states andCanada.[1]
The causative agent of PHF isNeorickettsia risticii (formerlyEhrlichia risticii), an intracellularrickettsialbacterium.[1][2]
The vector ofNeorickettsia risticii is believed to be atrematode (fluke). The life cycle of the fluke takes it throughfreshwatersnails and back into water, where it is ingested by thelarval 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 enclosure.[1] Experimental infection has been produced with oral administration of infected insects and subcutaneous inoculation ofN. 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 genusHexagenia; 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 ofN. risticii has yet to be elucidated, but bats and birds may be wild reservoirs of infection.Unlike other causes of acutecolitis in horses, such asSalmonella andClostridium, PHF is not spread directly from horse to horse.
Signs and symptoms of PHF include acute-onset fever, depression (sometimes profound),inappetence, mildcolic-like symptoms, decreased manure production, profuse watery non-fetid diarrheaendotoxemia,edema due to protein imbalances, abortion by pregnant mares, and acutelaminitis (20 to 40 percent of cases). Infected horses founder usually within three days of the initial symptoms, thought to be secondary to endotoxemia.[1] Death may occur and is usually due to severe laminitis leading to founder.[1]
Horses may not always display any other symptoms beyond a fever.[1]
Diagnosis of PHF is accomplished by measuringantibody titers orPCR testing to look for the bacterium in the blood and feces. However, mostveterinarians opt to initiate treatment right away, as the disease can progress quite quickly. Veterinarians may also runcomplete blood counts and chemistry andelectrolyte panels to determine the course of care.Radiographs may be taken to track the progress of laminitic horses.
N. risticii responds well totetracycline 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 includeintravenous fluids and electrolytes to counteract the diarrhea;[1]NSAIDs such as Banamine (flunixin meglumine); intravenousdimethyl sulfoxide; administration of products such asBiosponge oractivated charcoal vianasogastric tube to bindendotoxins;polymyxin B or plasma for endotoxemia; supportive shoeing; low doses of intramuscularacepromazine; andpentoxifylline.
While avaccine 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 it.[1] The vaccine is administered twice a year, in early spring and in early summer, with the first one inoculation given before the mayflies emerge and the second administered as a booster.[1]
Some veterinarians have started making recommendations for farm management to try to prevent this disease: