Potomac Horse Fever Update
In the past several years, many horse owners in the Champlain Valley have become more aware of a disease called Potomac Horse Fever (PHF). First described around the Potomac River in Maryland in 1979 (hence, the name), PHF has now been identified throughout the United States and in other countries. Potomac Horse Fever is caused by bacteria now called Neorickettsia risticii (formerly known as Ehrlichia risticii). Because .the bacteria infect specific white blood cells early in the infection, the disease is also called Equine Monocytic Ehrlichiosis. The typical infection in horses is as follows: 2 to 4 days after infection, there is a mild fever that can go undetected. At 10 to 14 days, there is depression, fever, loss of interest in food, and a range of gastrointestinal signs from mild colic with or without soft manure, to severe and watery diarrhea. About 25 to 30% of cases will also develop laminitis, an inflammation of the laminae in the feet. Some of these cases can progress to founder. Depending on when diagnosis and treatment is made in relation to the onset of illness can have a big impact on how the horse will respond to treatment. The horses’ vaccination status can also make a big difference in how sick the animal will become. In this article, specific facts that pertain to early identification of PHF will be described, along with prevention strategies. These are the horse owner’s best tools for hopefully getting a jump on a potentially fatal disease for horses. A recent case in “Simon”, a Morgan gelding owned by Linda Corey, will be discussed to help highlight some of the key facts you should know about Potomac Horse Fever.
A few weeks ago, Simon’s call came in to my office. Simon had been running a high fever (up to 105 degrees F). He was very depressed and only partially interested in eating and drinking. Simon was well vaccinated against Influenza, Rhinopneumonitis, Tetanus, Eastern and Western Encephalitis, West Nile Disease, Rabies, and also Potomac Horse Fever. Simon’s fever had responded well to treatment with flunixin meglumine (this drug is better known by the trade name Banamine®, a non-steroidal anti-inflammatory drug that is part of the treatment for many cases of colic). When I examined Simon, he appeared depressed but well hydrated, and he had increased gut sounds (the gurgling and gas sounds made as food moves through the small intestine and colon). The rest of Simon’s physical examination was not especially abnormal, and the digital pulses in his feet were normal, which suggested that he did not have laminitis at that time. There were several findings in Simon that made us especially concerned about the possibility of Potomac Horse Fever. First, Simon had a very high fever. When a horse has a temperature above 101.5 degrees, it is considered a fever. While any infection has the potential to cause a fever in a horse, only a few will cause such a high fever. Infections by viruses (influenza, for example) can do this, but so will Neorickettsia, the agent of PHF. Secondly, Simon was very depressed. Depression is one of the hallmark clinical signs of PHF. These horses will stop eating, hang their heads, and lose their energy. Oftentimes the owner will find them just standing in the pasture, while they normally would be moving around and grazing. Because Simon had both depression and fever, and because it was summertime, PHF needed special consideration. After reviewing Simon’s history and his physical examination findings, and considering the time of year, a decision was made to start treating Simon for PHF. A catheter was placed in Simon’s left jugular vein and he was given the first of 5 treatments with oxytetracycline, an antibiotic that has been around for many years. Linda was given a crash course in catheter care, and instructions were left for the next 4 treatments with oxytetracycline. Blood was collected to check Simon’s Potomac Titer and also his blood protein. In many cases of PHF, blood protein will be below normal. In some cases, it can get so low that equine plasma from donor horses needs to be given as part of the treatment. Fortunately for Simon, his protein was low, but not too low. As long as he didn’t start developing laminitis or severe diarrhea, I told Linda that because he was vaccinated, he should not get too sick. Like many vaccines, PHF vaccine does not stop the horse from getting infected. Instead, it increases the chance that if it does get infected, it will not get severe signs that include laminitis and severe diarrhea. The blood test would not be back until the end of the next week, so it was important to start Simon’s treatment early, based only on his clinical signs, physical exam findings, and the time of year. One of the clues that a horse has PHF is that they will show a dramatic improvement in attitude within 24 to 36 hours of starting oxytetracycline treatment. Simon rubbed his catheter out within an hour! That was okay, though. He was given each treatment each day through a temporary catheter. Linda also monitored his fever twice daily, and Simon was continued on twice daily Banamine®. Each day, Simon was more like his normal self, his appetite improved, and by day 5, it was clear that Simon was done his treatments.
Towards the end of the following week, Simon’s titer came back. It was negative! The titer he had was low enough that it could just be from vaccination. A second titer was taken 2 weeks after his fever started. By this time, Simon’s body had made more antibodies to PHF, and the second titer was positive. Some newer tests that use DNA detection methods are also available for PHF, but if treatment has been started or if it is too early in the disease, the test may not show up positive.
Simon’s illness was diagnosed early and he had a good outcome, but how did he get the infection in the first place? Fortunately, while we don’t have all the answers for PHF, we do have some. Researchers at the University of California have done much of the work pertaining to how the disease is transmitted. It turns out that freshwater snails and aquatic insects play an important role. The PHF organism is inside of a parasite that is inside the snail. During certain times of the life cycle, the snails release larval stages into the water. These cercariae also contain the bacteria. From here, the water can be ingested by a horse and cause infection. Recent evidence also shows that aquatic insects, especially Caddisflies and Mayflies also can harbor the bacteria. They can hatch and then fly into pastures where they may be accidentally ingested. This may help explain why the disease can occur even in horses not having direct access to ponds, lakes, or streams. There is definitely an association between water and PHF, however.
One of the misconceptions about PHF is that the vaccinated horse cannot get infected. This is definitely not true, as most cases treated in this area have a history of good vaccination. Part of the problem with the vaccine is that currently all vaccine comes from a single strain of Neorickettsia that was isolated in Illinois in 1984, and there are numerous strains of the bacteria that have been identified. Hopefully, as time goes on, vaccine companies will try to incorporate other strains into the vaccine that may result in fewer horses developing active infection. One large vaccine producer is currently trying to develop a multi-strain recombinant vaccine. For now, it is definitely important to booster vaccinate horses for PHF this time of year, as most spring vaccine titers may be less protective than they were early in the summer. Another misconception about PHF is that a previously infected horse cannot get the infection again. Unfortunately, this is not the case. I have personally treated one horse that has been infected every summer for the past three years. This horse’s illness has been mild, fortunately, and he still gets vaccinated twice a year.
In summary, the take-home messages for PHF are these: early diagnosis and treatment can make a big difference in preventing severe disease, vaccination will not completely prevent infection, but will hopefully result in a milder form of the disease, and that it may be beneficial to prevent direct access to running or stagnant water. However, because flying insects may play a role in transmission, booster vaccination this time of year may be the best way to help get through the summer and fall without a problem.
Stephen Angelos, DVM, DACVIM
Large Animal Medical Associates
129 Main Street
Essex Junction, VT
802-879-4288