At one point, 25 premises were quarantined in five counties, starting with the original one in Manatee County, near Bradenton, and seven of them had positive horses. Quarantines are lifted when all horses on a particular property test negative, and after an extended period since their last exposure to EP.

Getty Images A series of cases of EP in Florida have been linked to two horses imported to the state from Mexico.

All are Quarter Horses that take part in unsanctioned racing on dirt tracks, the so-called local "brush track" circuit, officials say. Some 200 horses have been tested so far.

Spread of the disease is believed to be caused by poor management practices, such as shared needles and syringes, rather than through foreign or domestic ticks. No ticks of either variety in Florida have tested positive so far for Babesia caballi or B. equi, the two vectors for EP.