Case Study: Potomac Horse Fever – Dr. Nadine Salomon
Patrick, a 6 year old Shire gelding, presented to AE with a four day history of lethargy, mild fever and decreased appetite. The morning he presented he had profuse watery diarrhea. Physical exam revealed an elevated heart rate, respiratory rate, temperature and a depressed attitude. Patrick’s mucous membranes were bright pink with a toxic line. Blood work revealed significant dehydration and mildly elevated kidney enzymes. Initial treatment included a large volume of fluid to help rehydrate Patrick, anti-inflammatories to help with his fever and inflammation, medications to counteract any toxins in his blood, anti-diarrheals, and antibiotics.
From Patrick’s history and presentation, Potomac Horse Fever (PHF) was strongly suspected. A fecal sample was sent off for PCR testing for PHF as well as other suspected causes for the diarrhea such as salmonella, clostridium and lawsonia. Salmonella is a zoonotic disease, meaning that humans can contract it from animals. It was important to rule out salmonella as it is a safety concern for the owners as well as the doctors and staff working with Patrick.
Because PHF was suspected, Patrick was started on oxytetracycline, the broad spectrum antibiotic of choice for PHF. By day two of treatment Patrick’s attitude and appetite started to improve. His elevated temperature was gone by day three and he started to have more normal bowel movements. The PCR tests come back that day confirming PHF. Patrick was discharged four days into his treatment and continued anti-diarrheals and antibiotics at home. By day 8 the owners reported that Patrick was back to his normal playful attitude.
What is it? Potomac Horse Fever is an acute enterocolitis caused by an intracellular bacterium, Neorickettsia risticii.
What signs do you see with PHF? The first prominent sign of PHF often observed by owners is a decrease in feed intake. In fact, some horses may not initially show any other clinical signs except not eating. Other typical clinical signs observed include: fever, colic, depression, diarrhea, and laminitis. Clinical signs and severity vary but the onset of colitis can result in depression and anorexia, and may or may not be accompanied by a fever and/or diarrhea. N. risticii can cause abortions, and it is related to placentitis and retained placenta in most cases. The aborted fetus will show signs of the infection as well.
When and where is this disease an issue? The disease is seasonal and is seen in the end of June, July, and August in Minnesota. However PHF’s occurrence correlates with the emergence of the aquatic insect, the mayfly. PHF tends to be seen in horses pastured near streams and rivers. Potomac Horse Fever is found throughout North America and has been reported in most regions of the United States and Canada. Studies have shown that 16 - 33 percent of horses with no history of having had the disease have evidence of having been exposed to N. risticii.
How does a horse get N. risticii? Several studies have been conducted to determine the route of infection and have revealed that a parasite, living in freshwater snails and aquatic insects, is a reservoir for N. risticii. The parasites and N. risticii have been found in adult and immature forms of aquatic insects, such as caddisflies, mayflies, damselflies, dragonflies, and stoneflies. Therefore, the seasonality of PHF is likely related to the seasonality of the flying insect. But how does an equid become infected? These types of insects do not feed on warm blooded mammals; so, it is likely that an equid may ingest an aquatic insect or snail while grazing, or consume a flying insect that has landed in drinking water. It has also been suggested that the insects might be attracted to the barn area by outdoor lights during the night.
Can it be transmitted from horse to horse? Direct transmission from horse to horse does not occur.
Is there a test to diagnose PHF? Yes. The most common is a PCR test and it can be performed on blood or feces to diagnose PHF.
What other diseases can look like PHF? Salmonella, clostridium, nonsteroidal antinflamatory toxicity (banamine, bute, equiox), antimicrobialassociated enteritis, fungal colitis, toxic enteritis, etc. have similar signs to PHF.
Is there a way to treat PHF? The drug of choice to treat N. risticii infection in horses is oxytetracycline. Oxyetracycline, is given intravenously and should be administered once a day for three to five days, and it is most effective when given in the early stage of the disease.
What can I do to control or prevent exposure? As aquatic insects appear to be a predisposing factor for PHF you should attempt to decrease your horse’s exposure. Turning off lights at night in the stable and barn area will decrease the amount of insects attracted to the area and contamination of feed and water.
Should I vaccinate? Several vaccines are commercially available in the United States, but their efficacy is questionable. There are multiple strains of PHF, however they are not all present in the vaccine. Research has shown that once a horse develops an infection with N. risticii, the horse can develop immunity to that particular strain for as long as 20 months. In contrast, the PHF vaccine has been associated with a poor immune response. If you are going to vaccinate, we recommend you vaccinate the end of May or early June.