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Potomac Horse Fever

(Equine Monocytic Ehrlichiosis, Ditch Fever, Shasta River Crud, Equine Ehrlichial Colitis)

By

Allison J. Stewart

, BVSc (Hons), PhD, DACVIM-LAIM, DACVECC, School of Veterinary Science, University of Queensland

Medically Reviewed Oct 2022

Potomac horse fever (PHF) is an acute enterocolitis syndrome producing mild colic, fever, and diarrhea in horses of all ages, as well as abortion in pregnant mares. The causative agent is Neorickettsia risticii. The infection of enterocytes of the small and large intestine results in acute colitis, which is one of the principal clinical signs of PHF. The disease occurs in spring, summer, and early fall and is associated with pastures bordering creeks or rivers.

The epidemiology of PHF has been shown to involve a trematode vector. Sporadic disease due to N risticii has been reported in dogs and cats; cattle appear to be resistant to infection. PHF has been reported in many areas of the US and Canada using an indirect fluorescent antibody test as evidence of exposure; however, recent studies indicate a high rate of false-positive titers with this test, and the true geographic range of distribution is not known. Isolation or detection of the causative agent from clinical cases of PHF using conventional cell culture or PCR assay has been reported only from California, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, New York, Ohio, Oregon, Pennsylvania, Texas, and Virginia.

Etiology and Pathogenesis of Potomac Horse Fever

The causative agent of Potomac horse fever, N risticii, is a gram-negative obligate intracellular bacterium with a tropism for monocytes. Initial morphologic studies of this organism isolated from cell culture, as well as the serologic responses of N risticii, caused this bacterium to be assigned to the genus Ehrlichia. However, DNA analyses have shown N risticii is most closely related to N helminthoeca, the agent of salmon poisoning in dogs, and Ehrlichia sennetsu, which causes a rare infection in humans in Japan. N risticii is not visible in monocytes in blood films from clinical cases, in contrast to Anaplasma phagocytophilum, which is readily identifiable in granulocytes of infected horses.

N risticii has been identified in freshwater snails and isolated from trematodes released from the snails. N risticii DNA was detected in 13 species of immature and adult caddisflies (Trichoptera), mayflies (Ephemeroptera), damselflies (Odonata, Zygoptera), dragonflies (Odonata, Anisoptera), and stoneflies (Plecoptera).

Transmission studies using N risticii–infected caddisflies have reproduced the clinical disease. One route of exposure is believed to be inadvertent ingestion of hatched aquatic insects that carry N risticii in the metacercarial stage of a trematode. The incubation period is ~10–18 days. The causative organism is present in the feces of experimentally infected horses; however, the biologic significance of this is unknown. Clinically ill horses are not infectious and can be housed with susceptible horses. Additional studies are needed to determine the exact role of the vector and helminth hosts in the complex life cycle of N risticii.

Clinical Findings and Lesions of Potomac Horse Fever

The initial clinical features of Potomac horse fever are mild depression and anorexia, followed by a fever of 38.9°–41.7°C (102°–107°F). At this stage, intestinal sounds may be decreased. Within 24–48 hours, a moderate to severe diarrhea, with feces ranging in consistency from soft piles to watery, develops in ~60% of affected horses. The onset of diarrhea is often accompanied by mild abdominal discomfort.

Some horses develop severe clinical signs of sepsis and dehydration. Clinical signs can be indistinguishable from those of Salmonella and other infectious causes of enterocolitis. Laminitis can occur as a severe complication of PHF in 20%–30% of affected horses. PHF may present with all or any combination of these clinical signs:

  • Hematologic findings vary in the early stage of PHF from leukopenia (characterized by neutropenia and lymphopenia) and thrombocytopenia to a normal hemogram, despite evidence of systemic illness. A common finding in cases of PHF is a marked leukocytosis, which is normally seen within a few days of onset.

  • Several months after clinical disease in pregnant mares, abortion due to fetal infection with N risticii may occur. Experimentally, pregnant mares infected at 100–160 days of gestation abort at 190–250 days of gestation. The abortion is accompanied by placentitis and retained placenta.

  • Necropsy findings are nonspecific and reveal diffuse inflammation, mainly in the large intestines. Fetal lesions include colitis, periportal hepatitis, and lymphoid hyperplasia of mesenteric lymph nodes and spleen.

Diagnosis of Potomac Horse Fever

  • Cell culture of N risticii

  • Rising paired titers

  • PCR assay

A provisional diagnosis of Potomac horse fever often is based on the presence of typical clinical signs and on the seasonal and geographic occurrence of the disease. A definitive diagnosis of Potomac horse fever should be based on isolation or identification of N risticii from the blood or feces of infected horses by cell culture or PCR assay. Serologic testing is of limited value as a diagnostic tool, although many infected horses have high antibody titers at the time of infection. Because of the high prevalence of false-positive titers, interpretation of the indirect fluorescent antibody test in individual horses is difficult. Rising paired titers can be helpful.

Isolation of the agent in cell culture, although possible, is time-consuming and not routinely available in many diagnostic laboratories. A real-time PCR assay that allows detection of N risticii DNA within 2 hours is a much more feasible test for routine diagnostic examination. To enhance the chances of detection of N risticii, the assay should be performed both on blood and fecal samples, because the presence of the organism in blood and feces may not necessarily coincide.

Treatment of Potomac Horse Fever

  • Oxytetracycline

  • Fluids and NSAIDs

  • Prophylactic cryotherapy

Horses with Potomac horse fever can be treated successfully with oxytetracycline (6.6 mg/kg, IV, every 12 hours), if given early in the clinical course of the disease. A response to treatment is usually seen within 12 hours. This is associated with a drop in rectal temperature, followed by an improvement in demeanor, appetite, and borborygmal sounds. If treatment is begun early, clinical signs frequently resolve by the third day. Generally, antimicrobial treatment is for no more than 5 days.

In animals that exhibit clinical signs of enterocolitis, fluids and NSAIDs should be administered (see treatment for salmonellosis Treatment Salmonellosis is one of the most commonly diagnosed infectious causes of diarrhea in adult horses. Clinical manifestations range from no abnormal clinical signs (subclinical carrier) to acute... read more Treatment ). Laminitis is more common in PHF than in other causes of enterocolitis and, if it develops, is usually severe and often refractory to treatment. Prophylactic use of cryotherapy of the feet should be instigated immediately. The overall case fatality rate is 5%–30%.

Prevention of Potomac Horse Fever

For prevention of Potomac horse fever, several inactivated whole-cell vaccines based on the same strain of N risticii are commercially available. Although vaccination has been reported to protect 78% of experimentally infected ponies, it has been only marginally protective in the field. Vaccine failure has been attributed to antigenic and genomic heterogeneity among the > 14 different strains of N risticii isolated from naturally occurring cases. Vaccine failure may also be due to lack of antibody protection at the site of exposure, because the natural route of transmission has been determined to be oral ingestion of the agent. Minimizing insect ingestion in stabled horses by turning off barn lights at night, which normally attract the insects, has been suggested.

No zoonotic risk is known.

Key Points

  • Potomac horse fever is a severe form of enterocolitis that is recognized in North America.

  • Treatment involves supportive care and oxytetracycline.

  • Prophylactic cryotherapy of the feet can help prevent laminitis.

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