Equine monocytic ehrlichiosis (Potomac horse fever): a review.

This study provides an overview of Equine monocytic ehrlichiosis, also known as Potomac horse fever, tracing its discovery in the late 1970s in Maryland, USA, its causes, symptoms, diagnostic methods, and treatment, as well as the development of preventive measures such as vaccination.

Overview of the Disease

Equine monocytic ehrlichiosis (EME), often referred to as Potomac horse fever (PHF), is an infectious disease in horses first recognized in the late 1970s. It is characterized by acute enteritis – inflammation of the intestine. The disease has been traced to a rickettsial organism known as Ehrlichia risticii. Its presence has been reported in several states beyond Maryland, where it was first discovered. In areas where the disease is endemic, vaccination is usually administered to prevent its occurrence.

Clinical Manifestations

• Symptoms associated with PHF range from depression, fever, anorexia (loss of appetite), decreased or no intestinal sounds, severe watery diarrhoea, and laminitis (inflammation of the sensitive layers of tissue inside the horse’s hoof).
• However, the clinical signs of the disease have shown to vary significantly between naturally occurring cases and those developed for research experiments.

Diagnostic Methods

• The diagnosis of PHF heavily depends on serological testing – a test that measures the concentration of antibodies in the blood serum.
• The widely-used method for diagnosing PHF is the immunofluorescent antibody test (IFA).
• More recently, a new diagnostic method has been developed – the enzyme-linked immunosorbent assay (ELISA).

Treatment and Mortality

• If left untreated, PHF can be fatal, with a mortality rate ranging from 15% to 35%.
• The most effective treatment against PHF appears to be antibiotics from the tetracycline series. These antibiotics have not only demonstrated efficacy against the disease-causing organism in in vitro experiments but also in an in vivo murine model, suggesting they may be useful in treating clinical cases of PHF.