The Texas Animal Health Commission said yesterday that additional testing was being conducted to determine the extent of infection.
Horses on the property are under quarantine and a thorough disease investigation is under way.
Equine piroplasmosis can affect horses, donkeys, mules or zebras and cause clinical signs common to many diseases, including poor appetite and weight loss. Deaths can occur.
Some infected equine animals may exhibit few or no signs of disease. Those animals that survive the acute phase of infection may continue to carry the parasite, Theileria equi for long periods of time.
"Although equine piroplasmosis is not considered to be endemic in the US, cases have been detected on occasion," said Dr Bob Hillman, state veterinarian for Texas and head of its animal health commission.
In June, equine piroplasmosis was detected in Missouri, with a related case found in Kansas. In 2008, the infection was detected in Florida - which was then the first occurrence of the disease in the US in 20 years. These cases have been resolved.
"As many as 15 tick species are capable of carrying and transmitting the blood parasite responsible for causing equine piroplasmosis," Dr. Hillman said. "At this time, we do not know which species of tick is responsible for transmitting infection on the South Texas ranch.
"Additional blood and tick samples are being collected and will be submitted for analysis to the National Veterinary Services Laboratory in Ames, Iowa."
Dr Hillman said there is no vaccine for equine piroplasmosis, and treatment generally is not effective against it. To avoid spread of the disease, it is important to eliminate contact with ticks and to prevent the transfer of blood from one horse to another.
"Equine owners may want to consult with their veterinarians for recommendations on preventing tick infestation," said Dr Hillman. "If equines exhibit signs of illness, a veterinarian should be contacted, so appropriate samples may be collected for laboratory testing.
"Currently, we have no indication that this tick-transmitted disease has occurred at other sites in Texas, but maintaining vigilance for this blood parasite is necessary in determining the extent of this disease situation."
Horses confirmed with the disease in the US are euthanised.