Potomac horse fever is a serious, potentially fatal disease caused by a bacteria called Neorickettsia risticii. First described in 1979 in Maryland near the Potomac River, the disease has since spread to numerous locations in the United States, Canada, and Europe.
Potomac horse fever is a seasonal disease, usually occurring between late spring and fall. This disease is one that needs to be recognized early and treated swiftly to avoid dire consequences, so knowing what you’re looking for is of utmost importance.
Etiology and Pathogenesis
Neorickettsia risticii (formerly Ehrlichia risticii) is a gram-negative obligate intracellular bacterium with an affinity for monocytes, meaning it must live within the cell. Unlike Anaplasma phagocytophilium, a bacterium that causes similar, milder clinical signs, N. risticii cannot be visibly seen within monocytes on a blood smear.
N. risticii is transmitted to horses by freshwater snails, trematodes (flukes) carried by snails, and aquatic insects such as caddisflies, mayflies, damselflies, dragonflies, and stoneflies. Infection usually occurs when a horse accidentally ingests an infected insect or snail in their grass or water.
The incubation period is approximately 10 to 18 days after infection. Although the organism can be isolated from the feces, clinically ill horses are not contagious and can be housed with healthy, non-infected horses. The disease is not considered zoonotic, which means it cannot be passed from horses to humans.
Clinical signs of Potomac horse fever can vary, but they generally start with mild depression and anorexia (not eating), followed by a fever ranging from 102-107 degrees Fahrenheit. Diarrhea can occur, but it develops in only about 60% of cases. Affected horses may show signs of mild colic or more severe signs such as sepsis and dehydration. Laminitis can occur as a complication of Potomac horse fever in 30% of affected horses. These clinical signs are indistinguishable from other causes of enterocolitis such as Salmonella or C. difficile infection.
Abortions can occur in mares that are infected with the disease while pregnant. If infected in early to mid-gestation, mares tend to abort approximately 2 1/2 to 3 months following clinical signs. The abortion is generally accompanied by placentitis and retained fetal membranes (the placenta/fetal membranes are considered retained if not completely passed within 3 hours of birth or abortion), which can lead to serious complications such as sepsis and laminitis if not treated quickly.
Bloodwork in infected horses in the early stages generally shows low white blood cells (leukopenia) with low neutrophils and lymphocytes (neutropenia and lymphopenia) and low platelets (thrombocytopenia), but bloodwork can also appear normal. Several days after onset, there can be a rebounding, marked increase in white blood cells (leukocytosis). Other abnormalities that can be seen with Potomac horse fever include decreased electrolytes, decreased protein levels, and increased packed cell volume.
A presumptive diagnosis of Potomac horse fever can be made based on the presence of the described clinical signs, the season, geographic location, vaccination history, and CBC/serum chemistry results. A definitive diagnosis is made by isolating the N. risticii organism using polymerase chain reaction (PCR). Indirect fluorescent antibody (IFA) testing can be done to measure the horse’s antibody titer to Potomac horse fever, but this must be interpreted carefully as some horses have not seroconverted at the time the sample is taken, or horses that live in endemic areas can maintain high titers without clinical disease.
Depending on the severity of the horse’s symptoms, the horse can be treated on the farm or may require hospitalization. Antibiotic therapy with oxytetracycline is the most common treatment and is usually started as soon as Potomac horse fever is suspected. Other treatments that may be necessary include anti-inflammatory medications such as Banamine®, IV fluids, electrolytes, and specific laminitis treatment/prevention.
The response to antibiotic therapy can be very dramatic and is considered diagnostic if the attitude and fever of the patient improve within 24-48 hours. Treatment is generally continued for 3 to 7 days, and the normal recovery time is approximately one week with no other complications. If not treated quickly, horses can decompensate rapidly and develop fatal complications from the disease.
Luckily, Potomac horse fever is not passed from horse-to-horse contact. There are vaccines available, one of which -- POTOMAVAC + IMRAB® -- is combined with a rabies vaccine. The efficacy of preventing all Potomac horse fever disease is questionable, due to decreased response of some horses to the vaccine, lack of antibody protection at the site of exposure (since the disease is transmitted by oral ingestion of the organism), and multiple (greater than 14!) different strains of N. risticii in the environment. Even though complete protection may not be achieved, vaccination is still recommended, especially in areas where the disease is known to exist, due to its ability to lessen the severity of the disease’s symptoms. Reducing potential insect ingestion by turning off barn lights at night, which can attract the insects and allow them to drop into feed or water buckets, is also recommended.
Recommended Vaccination Schedules
Adult horses that have been previously vaccinated: Vaccinate every 6 to 12 months. A 6-month vaccination interval is recommended for horses in areas known to contain the disease.
Adult horses previously unvaccinated or with an unknown vaccine history: Vaccinate with a primary series of two doses, 3 to 4 weeks apart, then vaccinate every 6 to 12 months.
Pregnant mares that have been previously vaccinated: Vaccinate every 6 to 12 months. Be sure to schedule one dose to be given as a pre-foaling vaccination 4 to 6 weeks before foaling. No studies have been published to examine the efficacy of the Potomac horse fever vaccine to prevent abortion due to N. risticii.
Pregnant mares previously unvaccinated or with an unknown vaccine history: Vaccinate with a primary series of two doses, 3 to 4 weeks apart. Schedule it so that the second dose is given 4 to 6 weeks prior to foaling, then continue to vaccinate every 6 to 12 months.
Foals: Due to maternal antibody interference and a low risk of disease, vaccination is recommended to begin after 5 months of age. Vaccinate with a primary series of two doses, 3 to 4 weeks apart. In high-risk areas, a third dose is recommended at 12 months of age. Then continue to vaccinate every 6 to 12 months.
Horses with a natural infection that have recovered: Administer a two-dose primary series, 3 to 4 months apart, or a booster (if previously vaccinated) 12 months following recovery from the disease.