Understanding Equine Monocytic Ehrlichiosis

Equine monocytic ehrlichiosi (EME) has case fatality rates ranging from 5 to 30%.
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Equine monocytic ehrlichiosi (EME) is also known as Potomac horse fever (PHF) and equine ehrlichial colitis. The disease has since been reported in most states in the United States, at least three Canadian provinces, and also in parts of South America, Europe, and India. The disease usually occurs near rivers, lakes, and wet pastures from mid- to late-summer.

The cause of EME is Neoricksettsia risticii, formerly Ehrlichia risticii. The reservoir for the causal agent is not clear but has been isolated from ticks, aquatic insects, flukes, and other helminths (parasitic worms). Snails act as an intermediate host in the fluke cycle. Horses are thought to be infected through the ingestion of insects, often mayflies, which may land in drinking water. Experimentally, the incubation period ranged from one to three weeks in horses.

In the early stage of the infection, horses might become anorexic, depressed, pyrexic (they have a fever), and have decreased gut sounds. This is usually followed by loose stools or watery diarrhea and colic. In the late stages of the disease, affected animals could have severe dehydration, ventral abdominal edema, and laminitis. Death is the consequence of cardiovascular compromise and toxemia. Case fatality rates range from 5 to 30%. Transplacental transmission is reported often leading to fetal resorption, abortion, or weak foals. Horse-to-horse transmission is not thought to occur.

Horses that recover from the disease could have protective immunity for up to two years. Available vaccines appear to have variable efficacy. Limiting proximity of horses to rivers, ponds, lakes, and low-lying pastures during the peak EME season and eliminating lighting at night in horse stables to minimize attraction of insects could reduce the risk of infection

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