Potomac Horse Fever
By: Chauncey Smith, DVM
Potomac Horse Fever (PHF) is caused by the bacteria, Neorickettsia risticii. Equids acquire the bacteria by consuming infected aquatic insects on pasture. The common culprit is the mayfly. The disease often occurs in late summer and throughout fall, when the insect load is highest. Cases have been reported in the winter and spring seasons. Potomac Horse Fever can affect any horse.
Owners and trainers might note lethargy and inappetence prior to other clinical signs. Abnormal parameters, such as an elevated heart rate and/or respiratory rate are described. These findings are caused by an elevated body temperature. A fever is any temperature above 101.5F. Potomac Horse Fever will cause diarrhea in most patients. In some cases, horses will develop laminitis. Pregnant mares are at risk of abortion.
Potomac Horse Fever is a medical emergency. Early treatment can reduce or eliminate symptoms. Veterinarians will use nonsteroidal anti-inflammatory therapy to manage the fever. Specific antibiotic protocols have been described for the treatment of this disease.
Care providers should note any change in the attitude or appetite of a horse. Having a thermometer and stethoscope is recommended. Any fever should be reported to your veterinarian. A horse with an elevated temperature should be actively cooled via cold water horsing or an alcohol bath. Water intake should be monitored because hydration status can greatly affect the disease process.
A vaccine for Potomac Horse Fever is available. The vaccine does not prevent the disease but can reduce its severity. Horses should be vaccinated prior to the peak seasons of exposure. The clinical signs of Potomac Horse Fever are similar to those of many infectious diseases. Isolation and biosecurity protocols should be implemented if a fever or diarrhea are noted in any horse. Potomac Horse Fever, however, is not contagious so these practices will not be warranted once a diagnosis is confirmed.
