Horses infected with the tick-borne disease equine piroplasmosis have been located in 11 US states.
There are now 317 horses which have tested positive for Theileria equi, the protozoa that causes the disease.
The figures are contained are the latest report from the US Department of Agriculture to the World Organisation for Animal Health (OIE).
The report, from Dr John Clifford, deputy administrator of the Animal and Plant Health Inspection Service (APHIS), said his service and individual state agriculture departments were conducting comprehensive investigations.
As of 11 November 2009, testing had confirmed equine piroplasmosis in 317 horses.
Of these, 288 positive horses are located on the Texas ranch in Kleberg County where the outbreak was first identified.
Seven horses 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, and one is in Wisconsin.
"All known Theileria equi positive horses are currently under quarantine," Clifford said.
He said additional testing of ticks, including identifing species, is continuing.
While there are 15 species of ticks known to spread the disease, it can also be spread by contaminated needles.
Testing has so far revealed the presence of two types of ticks known to be capable of transmitting the blood-borne protozoa behind the outbreak.
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).
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