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.
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.
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.
Diagnosis & Treatment
Obviously, diagnosis and treatment need to be rapid, but unfortunately diagnosis of PHF is not easy. The fever, depression, and loss of appetite may appear many hours before the diarrhea. There may be mild colic symptoms, but this doesn't happen consistently. In this early stage, a "sick horse" may be all that's obvious to you as the owner. (Note: This is a good time to emphasize that any sick horse should always have his temperature taken, and all horses with a fever who are off their feed should be examined by your veterinarian.) These "sick horses" will often be assumed to "have a virus," but don't stop there. If you get your veterinarian out at this early stage, he/she will be able to detect abnormal intestinal activity and often changes in the color of the gums that will tip him/her off to the fact that there is an intestinal problem.
When the diarrhea appears, PHF will definitely be high on the list of possible problems afflicting your horse, but other infectious causes of diarrhea-such as Salmonella and/or Clostridia-can cause the same symptoms. Your veterinarian will want to run blood tests to check for dehydration, electrolyte problems, and white blood cell changes that will sometimes help establish and confirm the diagnosis. A definite diagnosis can be made by PCR testing of your horse's blood or manure. This type of test detects the genetic material of the organism. Antibody testing is also available, but there can be false positives if your horse has been exposed to PHF before. Both PCR testing and antibody determination also take time, and time is something the PHF horse does not have!
After considering all the factors, including where you live and how common PHF is in your area, your veterinarian may decide to treat before any test results are back. Horses treated with intravenous oxytetracycline usually respond very well, especially if the problem is caught in its early stages. Flunixin meglumine (Banamine®) is given to help control pain and counteract the effects of the toxins. Your horse's feet may be iced to help block damaging toxin effects on the laminae. Many horses also require intravenous fluids and electrolytes to replace losses from severe diarrhea.
There is a vaccination for PHF, but it is used sporadically, depending on risk. The vaccine works against only one strain of the organism, while there are multiple different strains that can infect your horse. Disease development in vaccinated horses has been reported several times. A study in New York showed that it's more cost effective to recognize clinical cases of PHF and treat them promptly than it is to keep all the horses on your farm vaccinated against the disease. You should consult with your own veterinarian, however, prior to making any vaccination decisions for your horse. If you do decide to vaccinate, your horse must receive an initial vaccine and then a booster vaccine in two to four weeks for first-time use.
Your best defenses against Potomac Horse Fever are:
• Be observant. If your horse seems "off," check it out.
• Always alert your veterinarian if your horse has a fever and is off his feed.
• Keep horses away from natural water sources.
• Keep your water troughs free of dead flies.
• Avoid leaving on lights around your barn at night.