Washington State/USDA veterinary scientist creates test, treatment for malaria-like sickness in horses

When Washington State University (WSU) and U.S. Department of Agriculture (USDA) veterinary scientist Don Knowles, DVM, PhD, DACVP, got word two years ago that a rare but deadly infection was discovered among a group of horses in south Texas, he felt a jolt of adrenaline. Not only were the horses infected with a parasitic disease similar to malaria in humans, but the epicenter of the outbreak was found at no ordinary ranch.

It was the King Ranch, legendary for its world-class quarter horses, including former winners of the Triple Crown and Kentucky Derby.

Scientist and sleuth: Dr. Don Knowles and his team successfully identified the culprit of the widespread disease at King Ranch-the cayenne tick. (Photos courtesy of Washington State University)

One King Ranch horse had tested positive for the disease when the federal government first alerted Knowles. A few days later, it was a dozen; and then four dozen.

And so, at the request of federal agriculture officials, Knowles boarded a plane and headed south to investigate.

Parasitic storm

Equine piroplasmosis is so feared in the U.S. that the government bans horses that test positive from entering the country. Until the outbreak in Texas, only a few sporadic cases had ever been reported.

"We had regarded piroplasmosis as a foreign animal disease and suddenly here it was on U.S. soil, with not one or two cases but nearly 300—all concentrated at a ranch recognized for exemplary management practices," said Dudley Hoskins, an attorney with the American Horse Council in Washington, D.C., at that time. "To say we were concerned would be an understatement."

Piroplasmosis, also called equine tick fever, is transmitted to horses through the bite of a tick that carries either the Babesia caballi or Theileria equi parasites in its saliva. Similar to malarial parasites that infect humans, these creatures travel through the horse's circulatory system, multiplying and drilling through red blood cells.

No treatment, painful options

Many infected horses exhibit little more than cold-like symptoms, but in the U.S. horses have no natural resistance to the dangerous disease. Unimpeded, the parasites proliferate and destroy blood cells, triggering fever, anorexia and anemia.

"If a horse dies of piroplasmosis, anemia is often the cause," said Knowles. "It's a progressive process and a miserable way to die."

Before the outbreak in 2009, no standard treatment existed. If a horse tested positive for piroplasmosis, the owner had three government-mandated options to keep the disease from spreading: euthanize, quarantine or ship the horse out of country.

On the trail

Long before Knowles boarded that Texas-bound plane in 2009, he knew a lot about piroplasmosis. The periodic clusters that surfaced in temperate-climate states such as Florida proved the parasites sometimes slipped across the U.S. border in horses that had tested negative for the disease when, in fact, they were positive.

Because the test sometimes gave false negatives, Knowles was charged with developing a more reliable diagnostic test. He also was instructed to create a standardized treatment to kill the deadly parasites.

Hungry vectors, vulnerable hosts

Armed with two decades of piroplasmosis research and a team of scientists from his USDA unit and WSU, Knowles not only contained the outbreak but he and colleague Glen Scoles also identified a new blood-sucking culprit that had spread it.

"Prior to that outbreak, we knew of two tick species capable of transmitting the disease. There, we discovered a third," said Knowles.

He and his team identified the cayenne tick as the predominant carrier, a finding so important that the group later published a paper about it in the journal Emerging Infectious Diseases.

Eradicating the parasite

All said and done, Knowles and his team did more than identify a new eight-legged transmitter of piroplasmosis and develop an internationally accepted test to diagnosis it.

With high doses of imidocarb dipropionate, a drug used to treat certain diseases in cattle, they appeared to have eradicated the parasite.

The outcome of administering the drug was so successful that, after subsequent trials, it is now being evaluated as a standard treatment protocol in the United States.

"If approved for use, the treatment would offer a way to clear horses of infection," said Hoskins.