Persistence of encysted Toxoplasma gondii in tissues of equids fed oocysts.

Thirteen 6-month- to 13-year-old equids were each inoculated orally with 10,000 infective Toxoplasma gondii oocysts. The equids remained clinically normal. Equids were euthanatized on postinoculation days 33, 45, 90, 104, 120, 140, 162, 183, 197, 204, 211, 294, and 476. Their tissues were bioassayed for viable oocysts by feeding muscles to Toxoplasma-free cats and by inoculation of pepsin digests of 13 organs into mice. Using mouse inoculation, T gondii was recovered from tissues of the 9 equids euthanatized on each of postinoculation days 33, 45, 120, 140, 183, 197, 211, 294, and 476. Toxoplasma gondii was isolated from the tongue and intestines of 4 equids, from the heart of 3 equids, from thigh muscles, brain, and heart of 2 equids, and from the eye, lungs, and spinal cord of 1 equid. Muscles from 9 of 13 equids were infectious to cats, as evidenced by feline shedding of oocysts. Results indicate that T gondii can persist in edible tissues of living equids up to 476 days and that non-cooked horsemeat should not be fed to cats or eaten by human beings.