What’s Potomac horse fever?

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Potomac horse fever is caused by the bacterium Neorickettsia risticii and can cause a range of symptoms including high fever, diarrhea, colitis, laminitis, and depression. The disease is transmitted through ingestion of infected water or insects and is most contagious in late summer and early fall. A vaccine is available but may not be very effective. Preventative measures include keeping horses away from natural water sources and covering hay and grain. Treatment involves a combination of tetracycline and banamine.

Neorickettsia risticii (N. risticii), also known as equine monocytic ehrlichiosis, is a bacterium that causes Potomac horse fever if ingested. It attacks the gastrointestinal tract and causes a wide range of symptoms, especially high fever, sweating, projectile diarrhea, acute colic, colitis (inflammation), dehydration, loss of appetite, and depression. Laminitis and founder of all four hooves has been reported in the worst cases. Pregnant mares have also been known to miscarry. Edemas (swelling) of the legs and body have been reported due to a protein imbalance occurring.

Potomac horse fever first gained national attention in the United States during a sudden outbreak in the summer of 1979. This initial outbreak occurred in the Potomac River basin, northwest of Washington, DC, and of the nearly 100 horses that got sick, a third of them died. The disease was quickly recognized in at least 32 other states. Considerable research was done, but it was not until 1984 that the first discovery was made and the N. risticii bacterium was isolated in the blood of an infected pony. The researchers then focused on the vector of the disease and, after eliminating several insects, such as ticks, the carriers were suspected to be birds, bats and flying insects.

In 1998, the main vectors were established as caddisflies, mayflies, damselflies, dragonflies and damselflies. These flies are recognizable by their moth or butterfly shapes and transparent wings. It also became apparent that the horses at highest risk of disease were those that were near rivers, lakes, or large bodies of water. Through various developmental stages, a flatworm called a fluke transports the N. risticii organism via freshwater snails, insect larvae, and adult aquatic insects.

Horses can become infected by drinking directly from infected water or by ingesting adult flying insects. Horses that ingest as few as five infected bugs, dead or alive, can contract the bacteria. The large numbers of mayflies during the late summer and early fall could easily allow for accidental ingestion into their forage or grain.

The first vaccine for this disease was approved in 1988, but it hasn’t been found to be very effective in preventing the disease. However, it reduces the severity of symptoms. It is important for horse owners and horse caretakers to follow up the first vaccination with a repeat booster within two to four weeks. So an annual vaccination is a valuable safeguard for horses living in regions close to water.

Diagnostic testing for Potomac horse fever takes two to three days. Samples for blood or stool testing should be collected before antibiotics are given. Antibodies from the vaccine can also interfere with test results. People who suspect a horse may be infected should isolate the animal immediately. Electrolytes can be given to help with dehydration which can occur quickly.
There are a number of preventative measures that will help keep this disease from infecting a barn. The most important is to prevent horses from drinking from natural water sources such as ponds and streams. Buckets of water should be kept clean and free of dead bugs. Night lighting attracts mayflies and other insect vectors, so lights should not be left on, especially on evenings in July and August when aquatic insects are more common. It’s also best for cyclists not to ride in outdoor arenas at night during this time. All hay and grain that is exposed to lighted areas should be covered and should be inspected before being fed to any horse.

Potomac horse fever is most contagious in late summer and early fall when mayflies and other species are at their peak. The most successful treatment appears to be a combination of tetracycline and banamine, given intravenously for three days. This disease is not a diagnosis to be taken lightly; however, it no longer carries the high risk of mortality it once did.

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