The outbreak has occurred at a Kleberg County ranch, which is under quarantine.
Horses which contract the disease in the United States are usually euthanized to eliminate the risk of spread.
Texas animal health officials said more than 32 horses at the ranch tested positive in the first round of testing for the disease on October 13.
Texas authorities had quarantined the ranch after a seven-year-old quarter horse mare became ill and tested positive for the disease the day before. All of the horses have been stabled at the ranch for years.
After the initial round of testing, 97 more horses were tested, with 69 showing positive for the disease.
Testing of horses on the ranch for the disease is continuing, but thus far a total of 101 positive horses have been disclosed.
"As many as 15 tick species are capable of carrying and transmitting the blood parasite responsible for causing equine piroplasmosis," says Texas state veterinarian Dr Bob Hillman.
"At this time, we do not know which species of tick is responsible for transmitting infection on the South Texas ranch."
Blood and tick samples have been collected and submitted for analysis to the National Veterinary Services Laboratory in Ames, Iowa.
Hillman said there is no vaccine for equine piroplasmosis, and treatment generally is not effective against the tick-borne infection.
To avoid spread of the disease, it is important to eliminate contact with ticks and to prevent the transfer of blood from one equine animal to another.
"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," he said.
Meanwhile, Lousiana's agriculture commissioner has warned the state's horse owners to watch for the disease.
Commissioner Mike Strain said horse owners and veterinarians should carefully monitor equines for signs of the disease, caused by microscopic parasites of the red bloods cells.
The parasites responsible are Babesia caballi and Theileria equi. Ticks ingest blood from infected equines and then bite uninfected equines, spreading the disease through the blood-feeding process.
Ticks carrying the parasites can be moved from one place to another via hay, bedding, feed and animal to animal.
The disease may also be passed from pregnant mares to foals in the uterus and by contaminated needles and other skin-penetrating instruments.
Symptoms may become apparent seven to 22 days after infection. Clinical symptoms include weakness, loss of appetite, fever, anemia, jaundiced mucous membranes, swollen abdomen and labored breathing. Horses that survive the acute infection phase may carry the parasites for an undetermined period of time in their bloodstreams.
Since the Kleberg County ranch has distributed brood stock nationally and internationally, some horses originating from that ranch may now be in Louisiana, Strain said.
Those horses will have to be tested to be certain that they are not carrying the disease.
Strain advised horse owners to continue practicing good biosecurity measures and contact their local veterinarians if any of their horses originated from that ranch or exhibit any of clinical signs consistent with those of the disease.