Nora Grenager, VMD, DACVIM

We have recently seen several cases of Potomac Horse Fever in our area and therefore wanted to provide a brief update on the disease.

What is Potomac Horse Fever? 

Potomac Horse Fever is a bacterial disease caused by the bacteria Neorickettsia risticii, which causes variable degrees (from mild to very severe) of fever, lethargy, poor appetite, diarrhea, mild colic, and laminitis in horses. It can also infrequently cause abortion in pregnant mares.

Potomac Horse Fever is NOT a contagious disease—so one horse cannot give it to another and it cannot be spread from horse to horse. The bacteria has been identified in freshwater snails, trematodes (flatworms) that parasitize those snails, and a variety of aquatic insects (e.g., mayflies, caddisflies, damselflies, stoneflies, and dragonflies). It is thought that horses contract the disease by accidentally eating or drinking the infected aquatic insects. Therefore horses that live around bodies of water or in barns that use night-time lighting (which attract these types of insects) are at increased risk of becoming infected. Signs of the disease can appear 10 to 18 days after ingestion.

Most cases of Potomac Horse Fever occur between July and September. Diagnosis is based on typical clinical signs in the right geographic area at the right time of year, combined with fecal and blood tests. Treatment involves intravenous antibiotics (oxytetracycline) and supportive care (i.e. intravenous fluids, anti-inflammatories, ice on the feet, etc., depending on the severity of the horse’s illness).

Vaccine Information

The vaccine provides protection and minimizes the severity of disease if a horse is infected, but does not usually completely prevent a horse from getting Potomac Horse Fever. It is recommended to try to minimize the exposure to infected insects by turning off stable lights at night in high risk areas. Usually the vaccine is given once or twice a year, based on individual risk (if once a year, usually in the spring so that antibody levels are highest throughout the summer, when the risk is greater). For horses at very high risk of infection, it can be administered up to 4 times a year.

If you have questions or concerns, please contact the team at Woodside Equine Clinic at (804) 798-3281.