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
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.
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.