Cayenne Tick Responsible for Equine Piroplasmosis in Horses

Equine piroplasmosis (EP) is a tickborne disease of horses that has been largely absent from the United States for decades, thanks to cooperative federal and state efforts at eradication. Since 1978, the disease has been kept out of the country by testing horses for infection prior to importation and not allowing infected animals to enter.

While the United States has been considered free from the disease since 1978, sporadic cases have occurred in recent years. The largest of these was discovered on October 2, 2009, in Kleberg County, Texas, when a mare was presented for veterinary care with clinical signs of infection. Clinical signs of EP can include poor appetite and weight loss, and eventually the disease can cause death. EP can also affect all other equines, including donkeys, mules, and zebras. Subsequent investigation and testing by the Texas Animal Health Commission (TAHC) and the U.S. Deparment of Agriculture’s Animal and Plant Health Inspection Service (APHIS) confirmed the original case and identified more than 290 additional infected animals on the ranch.

EP is caused by the tick-transmitted microbe Theileria equi (also known as “Babesia equi”). Several tick species are capable of transmitting T. equi, so the first step to controlling the outbreak was to find out which tick species transmitted the disease to the mare.

Only two U.S. tick species—Dermacentor variabilis and Rhipicephalus (Boophilus) microplus—have previously been shown experimentally to be vectors of T. equi. Agricultural Research Service research leader Donald Knowles and his team of scientists at the Animal Diseases Research Unit in Pullman, Washington, worked with APHIS and TAHC to assess and prevent the spread of the Texas outbreak, which could have serious international trade implications if it is found to have spread beyond the original outbreak ranch. Part of their initiative was to identify the tick species responsible for the new outbreak.

“Our group identified the cayenne tick, Amblyomma cajennense, as the predominant tick species found on horses at the ranch,” says Knowles. “Although this species had not previously been shown to be a competent vector, adult cayenne ticks were collected from positive horses and allowed to attach and feed on a noninfected horse, and the ticks successfully transmitted T. equi.”

But how did the horses in a geographical area that was free of the disease become infected?

“One of the diagnostic tests previously used widely to screen horses being moved internationally has likely allowed for the entrance of infected horses into countries considered free of infection,” says Knowles. That test, says Knowles, is called the “complement fixation test.”

“A more recently developed test, cELISA, has enhanced detection of clinically silent, persistently infected horses and could have prevented the spread of the disease.”

Knowles and his team are treating some of the South Texas horses with imidocarb dipropionate. Knowles and his team have shown in laboratory tests that this drug not only cures the infected animal, but also renders it incapable of being a carrier that can infect other horses. So far, 14 horses have been successfully treated with the drug, but trials are still ongoing.

"Cayenne Tick Responsible for Equine Piroplasmosis in Horses" was published in the October 2011 issue of Agricultural Research magazine.