Officials believe two horses that entered Florida from Mexico are behind the state's outbreak of piroplasmosis.
It is the first time the blood-borne parastic disease has occurred in the United States in 20 years.
At its height, 25 properties were under quarantine and about 200 horses were undergoing testing. At one stage, seven properties had horses testing positive for the disease.
There are now 13 premises under state quarantine, with only one farm still having positive horses on the premises.
Florida agriculture officials said premises were being released from quarantine only after negative testing and an extended time since exposure to the disease.
The disease can be spread by ticks but extensive surveillance has found no evidence of the presence of the types of exotic tick which carry the disease. Domestic ticks with the potential to carry the disease have also been tested for piroplasmosis, but the results have been negative.
The evidence still indicates that transmission of the disease has been by management practices, possibly involving the use of needles, and not by a natural tick vector.
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