Potomac Horse Fever (PHF) got its nickname because it was first described as a specific syndrome in horses living close to the Potomac River in Maryland. The disease is now found throughout the United States and in many foreign countries, as well. PHF is a bacterial disease caused by the organism Neorickettsia (a.k.a. Ehrlichia) risticii. It's also known as Equine Monocytic Ehrlichiosis.
Carriers
The major carrier of the organism is a parasite called a fluke that lives on water snails. When immature flukes hatch in warm waters, they carry the organism with them into the water. One way horses can become infected is by drinking water that contains the immature flukes (called cercaria) and the organism. There are also some flying insects that-as immature larvae-will feed on the cercaria, then carry the organism greater distances from the water when they mature to flying adult insects.
The organism that causes PHF has been found all over the world. Anywhere you find water and water snails, you're likely to find N. risticii. Horses who are kept close to natural bodies of water are at highest risk. However, even a ditch that tends to collect water and never completely dries out can be a source. In fact, another nickname for PHF is "ditch fever." It's also believed that the flying insects harboring the bacteria can be carried by the wind as far as several miles away from their wet breeding grounds. Horses then become infected if they eat these insects along with pasture.
PHF outbreaks tend to occur in two patterns, one in the spring and one later in the summer/fall. The early cases are usually diagnosed in horses who live close to natural water sources, who are using natural water sources, or who are on pasture during a rainy spring. These early cases most likely are caused by horses taking in the infected flukes. Later cases often occur at greater distances from any water source, and these infections are believed to be caused by the infected flies, who migrate away from the water sources. PHF is not seen over the winter, leading researchers to conclude that hay does not pose a risk, even if infected insects were baled along with the hay. This may be because the heat of the hay-curing process destroys the organism.
Symptoms
The consequences of a horse ingesting the N. risticii organism range from no symptoms at all to death so rapid that the owner never observed the horse to be ill. The typical PHF case is a horse with severe depression, loss of appetite, and fever, followed by diarrhea. The organism multiplies inside the intestinal tract, causing a severe inflammation of the horse's colon, or colitis. This in turn allows bacteria and/or bacterial toxins to gain access to the blood stream through the damaged bowel wall. These toxins are normally present in the intestinal "soup" but do not pass through to the blood when the intestine is healthy.
The severe diarrhea can rapidly lead to dehydration and electrolyte abnormalities. Circulating bacterial toxins put the horse at high risk for developing laminitis, even collapse of the circulation, and death. Pregnant mares can abort even up to a few months following infection. Loss of blood protein through the damaged intestinal wall can result in edema of the legs and/or abdomen.
