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Potomac horse fever (PHF), also known as equine monocytic ehrlichiosis, has been reported in the U.S. since the early 1980s. It was first identified in the area around the Potomac River in Maryland and has since been found in most areas of the U.S. as well as in Canada, Europe, India, and South America. Horses are most commonly affected in mid to late summer and early fall.

Signs of the disease may include high fever, inappetence, colic, diarrhea, and laminitis. Not all signs are present in every horse, and severity of the signs varies. Dehydration, colitis, and abdominal edema develop in later stages, with as many as 30% of affected horses dying.

PHF is not thought to spread from horse to horse, but in pregnant mares, the infection can cross the placenta to affect the fetus. Mares may have resorption of the fetus in early pregnancies or abortion in longer-established pregnancies. If an affected mare delivers a live foal, it is likely to be weak or show signs of oxygen deprivation (dummy foal syndrome). Affected mares also have a higher than average chance of placentitis or a retained placenta.

Broodmares and other horses can get some protection from commercially available vaccines, but no completely effective vaccine has been developed.

Because PHF apparently affects horses that ingest infected insects, minimizing contact with insects is a preventive management strategy. Aquatic insects and snails are major carriers, and it may be safer for broodmares to drink from waterers than from natural water sources such as streams or ponds. Horses grazing in well-drained pastures may be at less risk than those turned out in wet, marshy, or streamside pastures. Avoiding the use of insect-attracting barn lights at night may also decrease risk.

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