It has been recently discovered that the Cayenne tick, Amblyomma cajennense, can transmit equine piroplasmosis. This tick is common in southeast Texas, but not known to be present in other parts of the U.S.
Tracking the disease
“Equine piroplasmosis is reportable to the TAHC,” says Andy Schwartz, Texas Animal Health Commission assistant ex- ecutive director and epidemiologist. “When positive tests are reported, we try to fi nd out where the horse might have been exposed, how it became infected, where the disease may have spread,” he says.
Positive horses are discovered in several ways. “In addition to investigations there is some testing done for interstate move- ment and movement to racetracks. TAHC requires a nega- tive test within the past year for horses entering racetracks in Texas. If those tests reveal a positive animal we begin an investigation,” Schwartz says.
“Two years ago in Kennedy County, an investigation led us to a pasture of horses belonging to 5 owners. Nine of the 49 horses were test positive. The horses had been used on local ranches. We felt there might be other exposed horses in the area so we tested all horses in Kennedy County. A total of 17 positives were found, out of 182 horses tested.”
TAHC recently completed a countywide test of horses in Kleburg County. “We found 19 positives out of 987 tested. The positive horses were on 6 different premises, with 14 in 1 pasture. The other 5 were single positives among horses that were negative. Tick transmission was happening in just 1 pasture,” Schwartz says.
“APHIS covers the cost of testing as TAHC collects blood in each county. This is a way to fi nd horses that may have … stayed in South Texas where we have competent tick vectors. Horses that were positive that left and went anywhere else can’t spread EP if they went into an environment without
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competent ticks. We did not fi nd any tick transmission in outbreaks outside of South Texas. The other outbreaks were due to shared needles and use of blood products,” says Pelzel-McCluskey.
“The EP spread in counties along the coast may be due to the high population of Cayenne ticks,” says Schwartz. “The pastures where we found multiple positive horses had heavy infestation of these ticks. We haven’t seen tick transmission in other parts of the state. Our surveillance leads us to believe that these ticks are primarily south of San Antonio. We plan to do additional countywide testing along the coast, as risk designation warrants and agency resources allow. The next round of testing will likely start in early 2014,” says Schwartz.
Another tick, Dermacentor variabilis, inhabits the southeast- ern U.S. and is a competent vector, but not very effi cient.
The TAHC has been asked by individuals in the racing in- dustry how long the test requirements for horses coming to racetracks will be continued.
“There are no immediate plans to stop,” says Schwartz. “We’re still fi nding racehorses that are positive. Since 2009, a total of 68 positive Quarter Horse racehorses have been found, unrelated to the ranch horses. Several trainers have had multiple positive horses,” he says.
“We’ve also seen 3 racehorses that were test negative one year and positive the next, indicating an ongoing problem. We told the racing industry initially that we would test for a few years and then re-evaluate the requirement,” he says.
January 2014 The Cattleman 61
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