The tissue form of T. gondii (a microscopic cyst consisting of bradyzoites, the asexual third stage of the parasite) can be transmitted to humans by food. CDC explains that people become infected by eating undercooked, contaminated meat, especially pork, lamb, and venison; accidental ingestion of undercooked, contaminated meat after handling it and not washing hands thoroughly; or eating food that has been contaminated by knives, utensils, cutting boards, or other foods that had contact with raw, contaminated meat. (Of note, per CDC: Toxoplasma cannot be absorbed through intact skin.)
Moreover, people can accidentally swallow the oocyst form of the parasite. CDC points out that this can occur after cleaning a cat’s litter box when the cat has shed Toxoplasma in its feces; after touching or ingesting anything that has come into contact with a cat’s feces that contain Toxoplasma; by ingestion of oocysts in contaminated soil, such as by not washing hands after gardening, or eating unwashed fruits or vegetables from a garden; or drinking water contaminated with T. gondii.
A serological test that can distinguish oocyst transmitted toxoplasmosis from tissue cyst transmitted toxoplasmosis in humans has been developed by ARS APDL. “Thousands of serum samples representing Toxoplasma infected people from the U.S. and other countries were tested,” says Dolores Hill, PhD, an ARS APDL research parasitologist. “Results of this investigation indicate that more than 70 percent of infected humans acquired their infection by exposure to oocysts,” she relates.
“Given recent evidence that many people become infected by ingesting oocysts, we must ascertain which foods confer greatest risk, and devise ways to remove or inactivate those oocysts without eroding such foods’ nutritional quality, flavor, or palatability,” notes Benjamin Rosenthal, SD, also an ARS APDL research parasitologist.