By Eleanor Lenher, DVM
Potomac Horse Fever (PHF) is a seasonal, sporadic, but acute and potentially fatal typhlocolitis of horses caused by infection with an obligate intracellular gram-negative bacteria, Neorickettsia risticii (formerly known as Ehrlichia risticii). A novel species of Neorickettsia, N. findlayensis, has recently been isolated from PHF cases and shown experimentally to be capable of causing disease.
PHF has been reported from most states in the United States, five provinces in Canada, South America (Brazil, Uruguay), Europe (France, The Netherlands), and India.
Horses of all breeds and ages may be affected, but PHF is not common in younger horses (<1 year). Clinical cases of PHF occur sporadically and outbreaks are uncommon.
The clinical manifestations of PHF can vary from case to case, however, depression, anorexia and fever are among the most common signs. The majority of clinical disease appears to be mild or subclinical. Diarrhea, which ranges from mild to severe, watery, pipe-stream feces occurs in 45%–60% of cases. Some cases may show colic. Laminitis develops in about 20%–25% of PHF cases. Abortions have been reported months after the infection. The reported mortality rate ranges from PHF 5% to 30%.
N. risticii has a complex life cycle. N. risticii infects trematodes and mammals and the reservoir for the organism is most commonly a trematode species with 2-life stages in intermediate hosts. Neorickettsia-infected trematodes (virgulate cercariae) parasitizes freshwater snails and during periods of warm water temperatures infected cercariae are released from the snails. Cercarieae then infect and develops into metacercariae in the second intermediate host, which are aquatic insects such as caddis flies, mayflies, damselflies, dragonflies, and stoneflies. Aquatic insects are abundant in the natural environment and may represent a major source of infection during the summer and fall. Horses grazing near rivers or creeks could be exposed to N. risticii-infected cercariae in water, or ingest metacercariae in a second intermediate host such as an aquatic insect along with grass, consume adult insects trapped on the water surface, or consume adult insects that are attracted by stable lights and accumulate in feed and water. N. risticii has been detected by PCR in adult trematodes in the intestines of bats, birds, and amphibians.
In endemic areas, clinical cases are strongly associated with rivers, lakes, or other aquatic habitats. Increased risk of PHF is associated with horses grazing pastures bordering waterways (freshwater rivers, streams, ponds, irrigation ditches, etc.); horses coming from an area with a high PHF prevalence or a farm with history of PHF; or travel to an area with a high incidence of PHF. The majority of clinical cases of PHF occur in mid-to-late summer. In the USA and Canada, most clinical PHF cases occur between late June to early September.
Other causes of enterocolitis, including salmonellosis, clostridial colitis, cyathostomes (small strongyles), antibiotic-induced colitis among others should be ruled out.
Hematological analysis is highly variable but leukopenia with a neutropenia and lymphopenia may be initially present in some cases. Other common abnormalities are an elevated packed cell volume, total proteins, with hypoproteinemia in more severe cases. The most common biochemical abnormalities include hyponatremia, hypochloremia, hypokalemia, metabolic acidosis, and pre-renal azotemia.
Confirmatory diagnosis should be based on the molecular detection of the organism in peripheral blood and/or feces. PCR is a sensitive, rapid diagnostic test widely used in most veterinary diagnostic laboratories. Isolation of N. risticii from blood culture is also confirmatory of infection, however, this requires specialized equipment, highly trained laboratory personnel, is time-consuming and is only available for research purposes.
Serological tests are of limited diagnostic value in a clinical case, however a ≥4-fold increase or decrease in titers between acute and convalescent serum samples is confirmatory of infection. The acute sample should be collected as soon as first clinical signs are observed and the convalescent sample should be collected 1-2 weeks later. False-positives have been reported with some serological tests.
The treatment of choice consists of intravenous oxytetracycline for 3-5 days and supportive care with IV isotonic crystalloid fluid replacement therapy. A rapid recovery and decrease in fatality rate is observed when oxytetracycline therapy is rapidly instituted. If relapses occur following the cessation of IV oxytetracycline therapy a second course of IV oxytetracycline is recommended. NSAIDs, such as flunixin meglumine may be useful to treat endotoxemia and fever. Stall rest during the course of therapy is recommended and a grass-based hay diet is recommended until fecal consistency is normal.
PHF is not contagious therefore contact with recovered or currently ill animals is not associated with the development of PHF. Limiting access to freshwater streams and ponds during the months of summer may help reduce exposure to the organism.
The effectiveness of vaccination in preventing clinical disease is questionable but horses in endemic areas may be vaccinated with an inactivated monovalent vaccine. There have been anecdotal reports of reduced severity of clinical signs in vaccinated horses. Incomplete protection from vaccination has been attributed to extensive variability in the major surface antigens and lack of cross-protection between strains.
AAEP Forum article courtesy of The Horse magazine, an AAEP Media Partner.
Reviewed and updated by Drs. Luis G. Arroyo and John Baird in 2020.