Testing has also revealed the presence of two types of ticks known to be capable of transmitting the blood-borne protozoa, Theileria equi, which is behind the outbreak.
The announcement this week that two horses in New Jersey bought from the ranch in 2008 have also tested positive for the disease indicates it may have been present on the property for some months.
The US Department of Agriculture's latest report to the World Organisation for Animal Health, dated November 4, said a comprehensive investigation was being carried out by the department's Animal Plant Health Inspection Service (APHIS) and the Texas Animal Health Commission (TAHC).
It said all horses on the property, which is in Kleberg County, remain under quarantine.
"Anocentor nitens and Dermacentor variabilis, vectors known to be competent for transmitting equine piroplasmosis, were identified among ticks collected from multiple horses," said Dr John Clifford, a veterinarian who is deputy administrator for APHIS.
Additional testing of ticks and species identification work was ongoing, he said.
"Testing of all additional horses located on the index ranch, and of other epidemiologically linked horses, is ongoing."
The first confirmed case on the ranch was October 19.
Control measures being used involve control of ticks, quarantine, disinfection, and dipping and spraying.
Equine piroplasmosis is a notifiable disease in the US and horses entering the country are screened for it.
In June, the disease was detected in Missouri, with a related case found in Kansas. In 2008, the disease was detected in Florida - which was then the first occurrence of the disease in the US in 20 years. Those cases have been resolved.
The disease can affect horses, donkeys, mules or zebras. It causes clinical signs common to many diseases, including poor appetite and weight loss. Deaths can occur.
It is distributed worldwide, but appears to be absent from the Pacific region, where it has not been reported since 1976 (Australia).