At present, there is no vaccine to prevent human infection with Toxoplasma, which has just the one species, gondii.
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Explore this issueOctober/November 2018
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Food Safety Issues and Attention
In a peer-reviewed article titled “Toxoplasma gondii as a Parasite in Food: Analysis and Control,” published in the March 2016 issue of Microbiology Spectrum, Drs. Hill and Dubey provide a comprehensive literature review of statistics and studies of the parasite conducted by scientists throughout the world.
Despite being recognized as a significant foodborne pathogen, the knowledge related to food safety and public health significance T. gondii is typically limited, these scientists point out. “In particular, information on the wide range of food animals that can harbor this protozoan pathogen and thus serve as potential reservoirs for human infections is needed,” they mention.
T. gondii infects food animals and many game animal species, Drs. Hill and Dubey note. “Infected animals harbor tissue cysts, and human consumers can be infected by ingestion of these cysts in raw or undercooked meat,” they elaborate, adding that just one (potentially infective) tissue cyst may be present in 100 grams of meat. “A single T. gondii–infected pig can be a source of infection for many people since one market-weight hog (220 pounds or more) can yield over 600 individual servings of meat. Virtually all edible portions of an animal can harbor viable T. gondii tissue cysts, and tissue cysts can survive in live food animals for years.”
Dr. Hill collaborates with H. Ray Gamble, PhD, director of the National Academy of Science’s Fellowships Office, in authoring a Pork Safety Fact Sheet sponsored by the National Pork Board.
Drs. Hill and Gamble point out that human toxoplasmosis in the U.S. is estimated to cost $5.26 billion annually in medical costs, losses in personal productivity, and costs of special education and residential care. “An additional $100 million is attributed to medical costs of toxoplasmic encephalitis in AIDS cases,” these scientists say.
Drs. Hill and Gamble emphasize that animals exposed to this parasite rarely show signs of infection. “Animals are infected by ingestion of oocysts from the environment; or by predation of infected animals such as mice, birds, and other wildlife; by consumption of undercooked meat scraps; and in some species, through in utero transmission,” they relate. “Sheep and goats are important hosts of Toxoplasma in some countries and pose a major risk for human exposure, but are minor species in the U.S. Similarly, free-range chickens are known to be infected, often at high rates, but have not been indicated as a source of human infection in the U.S.”
Currently, no regulations require that meat be inspected for T. gondii and no further processing is mandated to inactivate the parasite, Drs. Hill and Gamble mention. However, they explain that many of the methods that have been followed for processing pork for inactivation of Trichinella spiralis (trichinae) are also effective for the inactivation of T. gondii. “For that reason, processed pork products should be safe for human consumption without further treatment,” Drs. Hill and Gamble say.
Inactivating T. Gondii
Thermal death curves for the interaction of temperatures and times required to kill T. gondii in meat have been generated in closely controlled scientific studies. “From these data, we know that T. gondii is killed in 336 seconds at 120 degrees Fahrenheit, in 44 seconds at 131 degrees Fahrenheit, and in six seconds at 142 degrees Fahrenheit,” Drs. Hill and Gamble note. “These times and temperatures apply only when the product reaches and maintains temperatures evenly distributed throughout the meat. The use of microwaves is not effective in killing T. gondii, probably because of uneven heating throughout the meat.”