BELMONT, New York (CBS SF) — Horse racing Triple Crown contender California Chrome had some small but very dangerous company on the race track last week.

An opossum was seen wandering around the Belmont Park racetrack on New York’s Long Island during a workout.

The critter crossed paths with the Kentucky Derby and Preakness Stakes winner as the colt completed a practice run.

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The innocent-looking opossum isn’t so innocent, however.

The marsupial carries the parasite Sarcocystis neurona which can be found it its feces. The U.S. Department of Agriculture characterizes the parasite as the most common cause of equine protozoal myeloencephalitis (EPM) in horses in America.

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The illness attacks horses’ brain and nervous system and the results can be devastating.

“They’ll be wobbly, uncoordinated, they’ll misplace their legs into space and these will be recognized as neurological deficit,” said Dr. Nicola Pusterla with UC Davis Veterinary Medicine.

The disease can be fatal.

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California Chrome is in training for the third leg of the Triple Crown, the Belmont Stakes on June 7th. Chrome could be first horse to sweep the Kentucky Derby, Preakness and Belmont in 36 years.