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Equine - Equine Piroplasmosis: Frequently Asked Questions

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Anne Lichtenwalner DVM PhD: University of Maine Extension
Elizabeth McEvoy DVM and Donald E. Hoenig VMD: Maine State Veterinarians

Equine Piroplasmosis (EP) is a tick-borne disease of horses that may, in some cases, be fatal. Unfortunately, it is not preventable by vaccination. There are two protozoa (Babesia caballi and Theileria equi [formerly known as Babesia equi]) that can cause EP. The protozoa must be transported between horses via injection, such as a tick bite. Recently, EP has been reported to be passed from mares to foals, possibly during pregnancy, and to be detectable in the bone marrow of infected, but clinically healthy horses. A safe, effective, reliable prevention or treatment for this reportable disease has not been found. Unlike Eastern Equine Encephalitis (EEE) virus, EP does not affect human beings.

EP causes anemia, but is not necessarily fatal to horses. Three ticks are known to be experimentally capable of transmitting the protozoa to horses: Dermacentor albipictus, the winter tick, Dermacentor variabilis, the dog tick, and Boophilus microplus, the tropical cattle tick. Both the winter tick and the dog tick are found in Maine. However, the most likely tick to pass on the disease, Dermacentor nitens, the tropical horse tick, is not found in Maine. Another method of transmitting the disease is by reusing needles to vaccinate or treat horses. Therefore, it is critical to avoid re-using injection needles on livestock. New, sterile, disposable hypodermic needles should be used for every injection.

During June 2010, two seropositive horses, imported from India, were reported in Massachusetts. Other cases have been reported in the US since positive cases were detected on a Texas ranch during 2008. Due to the presence of potential tick vectors for the disease, and the presence of these potentially infected horses in the Northeast region, owners and veterinarians are urged to become aware of this disease and its clinical signs, diagnosis, and prevention.

Frequently asked questions

What are the signs of EP in horses?

Clinically affected horses may show lethargy, may be feverish, and may have swelling of the lower legs (edema). They may be pale and very anemic. However, horses can carry these protozoan parasites in the blood or bone marrow without showing clinical signs. Because the signs of the disease are non-specific and similar to many other diseases and conditions, it can be difficult to diagnose. EP can be confirmed with laboratory testing. If you suspect that your horse may have been exposed to EP, contact your veterinarian, the local extension office or the state veterinarian (Dr. Don Hoenig, (207) 287-3701). This is a reportable disease in Maine.

How is EP transmitted?

By ticks, and by contaminated medical equipment.

Is EP preventable?

This disease is preventable by avoiding tick bites, use of properly sterilized needles and surgical instruments, and by careful quarantine and testing of horses being shipped from locations known to be EP-positive. Consultation with your equine veterinarian and with the state veterinarians is suggested when transporting horses. If you show horses, consider careful pest control to avoid the vectors of the disease.

Can a horse with EP be treated?

There is no clinically proven, safe and effective treatment. High dose imidocarb is an effective treatment, but is expensive and not necessarily safe in horses.

Should we be concerned about EP in humans?

There is no evidence that this particular protozoa affects humans, or that the disease spreads from horses to humans. However, ticks can spread other diseases to humans, including Lyme disease, so tick prevention is always a good plan.

Are other animals affected?

Horses, mules, donkeys and zebras are known to be affected.

Will EP simply go away?

The risk of disease in the northeast is considered to be low, given that the most effective vector, the tropical horse tick, is not known to survive here. However, with increased importation of horses from other countries and movement of horses into and out of the southeast regions of the US, there is an increased risk for the disease.

More information:

Animal Health: Equine Piroplasmosis, USDA


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