What is Potomac Horse Fever?
Potomac Horse Fever is a serious illness of horses that was first described in the area around the Potomac River in Maryland in 1979. The disease is now recognized 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. Much higher antibody levels are produced by the disease than from vaccination alone.
The University of Minnesota is collaborating with the University of California at Davis to investigate recent cases here. Please contact Dr. Julia Wilson at the College of Veterinary Medicine if you would like more information: 612 625-6700.
Download the entire document about Potomac Horse Fever to learn more about this serious disease.
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