The disease, which can be carried by certain species of ticks, has been confirmed in 334 horses, according to the US Department of Agriculture's latest report to the World Organisation for Animal Health.
"To date," said spokesman Dr John Clifford, "even though Theileria equi positive horses have been identified in 12 states, all positive horses are directly epidemiologically linked to the index premises.
"All positive equine piroplasmosis horses identified so far in the investigation are either currently living on the index premises or previously lived on the index premises."
He said 289 positive horses are located on the index ranch in Texas, 22 are on other premises in Texas, one is in Alabama, two are in California, five are in Florida, one is in Georgia, five are in Louisiana, one is in Minnesota, two are in North Carolina, three are in New Jersey, one is in Tennessee, one is in Utah, and one is in Wisconsin.
He said all known positive horses remain under quarantine.
Dr Clifford said ticks which have been shown to be experimentally capable of transmitting the disease have been found on the Texas property.
"Additional tick transmission studies are pending and the tick investigations are continuing," he said.
Theileria equi is a protozoa, which can also be spread either by ticks or through use of shared, contaminated needles.