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Potomac Horse Fever (PHF)

Etiology

Potomac Horse Fever is caused by the rickettsia Neorickettsia risticii. The disease is more commonly seen in late summer and early fall and is more frequent in the Northeast part of the country, like Vermont and New York states. The infectious agent is carried by trematodes which live in aquatic snails. This particularity explains why the disease is more frequently observed near rivers or ponds.

Clinical signs

Potomac Horse Fever often starts with non specific clinical signs of lethargy, decreased appetite and fever (>101.5oF). If the disease goes untreated and progresses, most horses will develop diarrhea which can often times lead to dehydration and protein loss. The inflammation of the intestinal wall that accompanies the diarrhea can lead to release of toxins and bacteria into the bloodstream, which is called “endotoxemia”. Endotoxemia is responsible for the onset of laminitis that occurs in 20-30% of horses affected by the disease. Laminitis is unfortunately often severe with PHF and highlights the importance of treating rapidly.

Diagnosis

PHF can be diagnosed using blood or manure in a PCR assay. Serological testing (antibody measurement) is also possible. Because the disease progresses rapidly, it is often recommended to start treatment while waiting for the test results. Complete blood work (complete blood count and chemistry) is often recommended to verify the severity of the disease and its impact on the main organs.

Prevention

Reducing exposure to the agent is difficult. However, a vaccine exists and can help reduce the clinical signs of the disease. Indeed, it is important to know that this vaccine doesn’t prevent infection, but does decrease the chance of developing a clinical illness. Because different strains exist, the vaccine may be less effective in different regions.

Treatment

The rickettsial organism is sensitive to antibiotics from the tetracycline family. Because it can be a severe disease, it is usually recommended to treat with intravenous oxytetracycline for 3-5 days. Horses that don’t tolerate oxytetracycline can be treated with oral medication from the same family, such as minocycline. Part of the treatment includes supportive care, such as banamine, fluid therapy, icing the feet, rest, etc.