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Potomac Horse Fever (PHF), also known as Equine monocytic ehrlichiosis, originated in the eastern United States near the Potomac River. This disease has since been found in other geographic locations in the U.S. and Canada. Potomac Horse Fever is caused by the Neorickettsia risticii (formerly Ehrlichia risticii) organism. PHF is a seasonal disease, occurring most often at the onset of hot weather in July, August, and September. It is relatively rare for horses in our practice range to contract PHF, but horses who live near ponds, streams, and wetland areas are at a higher risk.

The Neorickettsia risticii organism is found in nature and has a complex life cycle. The life cycle involves flukes, freshwater snails, slugs, and aquatic insects such as caddis flies, mayflies, damselflies, dragonflies, and stoneflies. Horses become infected with the organism after consuming the infected snails, slugs, or aquatic insects. Horses may ingest the organism through grass, hay, and/or water. Horses that become infected usually show clinical signs 10-15 days later.

Clinical signs are different for all horses, although the typical signs are an acute onset of mild depression and anorexia (not eating), followed by the horse becoming febrile (102-107 degrees Fahrenheit). The horse may then become colicky and often within 24-48 hours a moderate to severe diarrhea occurs. Some horses may develop severe toxemia and dehydration resulting in cardiovascular compromise. Swelling along the under side of the abdomen or in the distal extremities may occur as well. Laminitis is often a sequela, and even if the horse recovers from Potomac Horse Fever, the associated laminitis may not resolve.

Diagnosis is based on the horse’s clinical signs, along with the seasonal and geographical occurrence of the disease in endemic areas. A definitive diagnosis of PHF is the detection of the Neorickettsia risticii organism in the blood or the feces of the infected horse.

Prompt treatment with oxytetracycline (antibiotic) can reduce the morbidity and mortality rate of the disease. Dehydration can occur rapidly in infected horses and supportive treatment such as intravenous fluids may be necessary. Laminitis can arise during or shortly after a horse shows clinical signs of Potomac Horse Fever, and is often very detrimental to the horse in these cases.

There is a Potomac Horse Fever vaccine available which can be given to help protect your horse, although the vaccine has limited efficacy due to the following proposed explanations: lack of seroconversation (meaning the horse fails to develop antibodies to the organism after vaccination); and, there are multiple strains of the organism in nature–whereas only one strain is presently available in the PHF vaccine.

In areas where Potomac Horse Fever is an endemic problem, methods should be taken to reduce the number of snails, slugs, and aquatic insects that can potentially spread the Neorickettsia risticii organism. Bodies of water should be drained, and outside lights should be turned off at night to decrease the number of aquatic insects around your horse’s food and drinking water. Carefully inspect hay for dead insects and always provide fresh clean drinking water for your horses.

Always monitor your horse closely for any changes in attitude, behavior, eating or drinking habits, or changes in his/her manure output. If you have any questions or concerns about Potomac Horse Fever, please give us a call.