What is Potomac Horse Fever?
Potomac Horse Fever is a serious illness of horses that was first recognized
in the area around the Potomac River in Maryland in 1979. The disease is
now found throughout the United States as well as in other countries. It is
caused by a type of tiny bacteria, Neorickettsia risticii, which is related to the
organism that causes salmon poisoning in dogs. Previously, the horse’s
organism was called Ehrlichia risticii, so the disease in horses is also known
as Equine Monocytic Ehrlichiosis. After the organism is ingested, it multiplies
in the intestinal tract, where it can cause marked inflammation (colitis). This
leads to the clinical signs of fever, depression, poor appetite, and in most
cases, diarrhea. A subset of horses will founder and pregnant mares can
abort. Some will also develop swelling of their lower limbs or body wall. In the
early stages of the disease, the bacteria may rarely be seen in a type of
white blood cell, called monocytes, which may be more numerous than
normal on a complete blood count. This abnormality is transient, so the
diagnosis most often relies on molecular diagnostics, such as PCR testing of
the blood or feces, which detects the DNA of the organism. These samples
need to be taken before antibiotics are given. If that is not possible, then
paired blood samples can be evaluated to measure the antibody levels
produced in response to active infection.
Although the fever is more common in the east it can also be found in areas
where there is standing water which provides habitat for aquatic insects. If
the disease is caught early, treatment with antibiotics can help fight off the
disease. Intense supportive care is mandatory, which includes plenty of
fluids, proper diet and anti-inflammatory drugs.
There is a preventative vaccine available through veterinarians which helps
protect horses against Potomac horse fever, but it is not 100% effective.
Horses that are vaccinated may need a booster. Vaccinated horses can still
get the disease; but they may have a less intense reaction and recover
quicker. Check with your veterinarian for a vaccination schedule. You can
also control where your horse drinks providing him with clean fresh water
and eliminate any standing water as an option if possible.
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