There are many names for Potomac Horse Fever including Equine Ehrlichial Colitis, Equine Monocytic Ehrlichiosis and Acute Equine Diarrhea Syndrome or more simply PHF. Whatever you want to call the condition, the symptoms are always the same. Not all horses that are exposed to Potomac fever get the same severe signs, some seem to recover without assistance while others can become fatally ill.
Horses develop Potomac Fever in the summer and it was originally believed that the bacteria (Neorickettsia risticii) lives in rodents or other small mammals during the rest of the year. It is now more known that the bacteria actually are found in snails and flukes found in fresh water streams and ponds. It is thought that horses drinking or walking through these areas contract the bacteria resulting in the transmission. In addition the biting insects that are hatched in the water are likely to feed on the tiny flukes in the water, thereby infecting themselves. When these mosquitoes and other biting insects mature, so does the Neorickettsia risticii bacteria. When the infected mosquito or insect bites a horse, the bacteria is passed to the horse. The virus then takes about 12 days to incubate or develop in the horse before the first symptoms are seen.
In many cases the first signs of Potomac fever are so mild that owners may just assume the horse is tired or has a slight respiratory problem. Often a mild fever, lack of energy and depression and a random high fever that comes and goes are all that are initially noted in this time period. Typically after about 10 days of the mild symptoms the more obvious and serious conditions such as laminitis or pain in the hoofs, extreme diarrhea that is almost liquid in constancy and colic will be prevalent. In some horses the legs will swell as may the abdomen, and pregnant mares will often abort.
Diagnosis of Potomac Fever is completed through fecal samples that test for the DNA of the bacteria as well as blood tests that indicate a higher than normal white blood cell count known as a monocyte. This blood test only works in early symptoms of the disease so the fecal sample is very important in diagnosis.
Treatment includes oxytetracyline, an antibiotic that is given intravenously. In addition to the antibiotic, horses are treated for dehydration if the diarrhea is severe or has been present for longer periods of time. The toxins produced by the bacteria are treated with anti-inflammatories that also help reduce the inflammation in the gut that causes the colic symptoms. Early anti-inflammatory treatment can also help prevent laminitis that can have long term debilitation effects on the horse. In very extreme cases or horses that do not respond well to traditional therapies, blood transfusions may be required.