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Photo: Colt in field of tall plants.
ARS researchers have found that a drug commonly used to treat cattle and sometimes dogs for a blood parasite can eliminate transmission risk for one of the parasites that cause babesiosis in horses. Photo courtesy of Kim Kaplan.

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Drug Eliminates Parasite that Causes Babesiosis in Horses

By Sharon Durham
September 28, 2009

A drug commonly used to treat cattle and sometimes dogs for a blood parasite can, at a relatively high dose, completely eliminate the parasite Babesia caballi from horses, Agricultural Research Service (ARS) scientists have discovered.

B. caballi, a blood parasite transmitted by ticks, is one of the culprits behind the disease babesiosis in horses. Equine babesiosis is also caused by another blood parasite called Babesia (Theileria) equi. The drug imidocarb dipropionate has been used in the United States for many years to treat diseases like Texas fever, also referred to as cattle fever or babesiosis in cattle.

In response to the needs of U.S. veterinarians, research leader Don Knowles and his colleagues at the ARS Animal Disease Research Unit in Pullman, Wash., studied the effectiveness of the drug in horses. They found that a relatively high dose of the drug not only eliminated B. caballi, but also left the horses incapable of transmitting babesiosis.

Though the high dose of the drug is generally well-tolerated by horses, some side effects include stomach upset and diarrhea. Similar collaborative research is being conducted concerning the effectiveness of imidocarb and other potential drugs on B. (Theileria) equi.

The ARS researchers collaborated with scientists at Washington State University in Pullman and with USDA's Animal and Plant Health Inspection Service (APHIS).

In the United States, babesiosis is considered a foreign disease in horses, though it is common in nearby locales including the U.S. territory of Puerto Rico. It is important to assure complete parasite elimination because infected horses can appear healthy, but can still transmit the disease.

Horses presented for import into the United States are tested at the border. Those that test "positive" are either destroyed or returned to their place of origin. However, infected horses occasionally escape detection and enter the United States. Since such horses are often retested for subsequent international movement, they are then discovered to be infected and placed under quarantine at great expense to the state and the owner. Therefore, methods to eliminate the parasite from such horses and eliminate transmission risk were sought.

If approved for use in the United States, imidocarb dipropionate would offer a humane way to clear horses of B. caballi and allow them to enter or remain in the country.

This work was published in the journal Antimicrobial Agents and Chemotherapy.

ARS is the principal intramural scientific research agency of the U.S. Department of Agriculture.