"Equine piroplasmosis may be carried and transmitted by as many as 15 species of ticks," said Dr Bob Hillman, executive director of the Texas Animal Health Commission.
"Although ticks have been collected from the South Texas ranch for testing, final results are not complete, and it is not known if any of the ticks can serve as a host for the disease."
To date, 101 horses have tested positive to the protozoal disease, the outbreak of which is centred on Kleberg County.
While ticks are able to transmit the disease, needle-sharing can also be responsible for its spread.
Canada and several US states have imposed movement restrictions or additional entry requirements on horses from Texas since the outbreak was notified in mid-October.
"We are continuing the equine piroplasmosis disease investigation," said Hillman, who is the Texas state veterinarian. "No horse movement is being allowed from or to the ranch where the infection was detected.
"While this tick-borne disease has not been considered endemic in the US, cases of the disease, scientifically known as Theileria equi, and previously called Babesia equi, have been detected in the US.
"Our epidemiologists are tracing the movement of specific equine animals. Blood tests will be conducted, and the animals will be examined for ticks.
"Individual equine owners will be contacted, if their horse needs to be tested by animal health officials."
Hillman would not be drawn on how many horses will be tested or how many may be exposed or infected.
"Until the epidemiological work and testing of potentially exposed horses is completed, there is no way to predict how many horses may be affected with this tick-borne illness," he said.
Hillman said horses infected with equine piroplasmosis may appear well, while others may exhibit a host of non-specific clinical signs, such as fever or anaemia.
These clinical signs also could be attributed to a variety of other diseases or causes. Blood tests are needed to diagnosis the disease.
"Equine owners should talk with their private veterinary practitioners about complying with interstate movement requirements, testing recommendations and protecting their horses from ticks.
"If a horse appears to be ill, it should be evaluated by an accredited private veterinary practitioner," he said.
"Before moving horses from Texas, we urge you and your veterinarian to check with animal health officials for any state of destination, to ensure the animals have met all entry requirements.
"Regulatory requirements can be fluid as disease situations evolve, so it is essential to call each state each time you haul."
Hillman urged equine owners and veterinarians to call state animal health officials directly before hauling, as many states have not yet distributed entry requirement information.
Entry requirements and restrictions can be viewed at http://www.tahc.state.tx.us. Contact information for state veterinarians may be obtained from the TAHC at 800-550-8242, ext. 710.