Researchers at the University of Wisconsin-Madison are developing a vaccine against Toxoplasma gondii, a parasite that is the third leading cause of foodborne deaths in the United States.
The parasite is found in warm-blooded animals as either a dormant cyst in muscle or brain tissue, or as an actively dividing form. Meat animals, such as hogs and sheep, harbor the dormant cyst. In cats, the actively dividing form can develop into an oocyst that is shed in its feces. Humans contract toxoplasmosis when they eat undercooked meat, drink contaminated water or contact contaminated cat feces. Once an animal (or person) is exposed to the parasite, it remains a host for life.
Microbiologist Laura Knoll and her colleagues are developing a strain of Toxoplasma that can’t form cysts. Knoll believes this strain could be used in a meat-animal vaccine, which would greatly reduce human exposure.
Although most exposures produce mild, flulike symptoms, some people are at risk for severe complications. “There are 3,000 congenital cases of toxoplasmosis in the United States every year,” says Knoll, who is an affiliate faculty member of the Food Research Institute and assistant professor in the medical microbiology and immunology department at the UW-Madison.
“Women first exposed to the parasite during pregnancy can transmit it to their fetuses through the placenta. Babies with toxoplasmosis are born with ocular problems, hydrocephalus and mental retardation. For immunocompromised people-AIDS patients, bone marrow and organ transplant recipients-encephalitis and retinitis are the main concerns,” she says.
The parasite is hardy and productive, according to Knoll. “Three days after a cat is infected (usually by eating an infected mouse or bird), it can excrete between two million and 20 million cysts in its feces,” she says. “The oocyst form is environmentally stable for up to 18 months.” Pregnant women are cautioned to avoid potential contact with infected cat feces by staying away from cat litter boxes throughout the duration of their pregnancies.
People who do not own cats can still be at risk for exposure to the parasite. “In the San Francisco Bay Area, 42 percent of people tested positive for exposure to the parasite,” says Knoll. “Cultural practices, such as eating undercooked meat or tasting meat while cooking it can cause infection. In France, where raw meat such as steak tartare is popular, we see even higher figures.”
In her lab, Knoll is developing a strain of Toxoplasma that cannot form cysts. “This non-cyst-forming strain would be an ideal animal vaccine and might also prevent the majority of transmissions to humans by blocking cyst development in animal meat,” says Knoll. So far she and fellow researchers Mary Pat Craver and Dana Mordue have isolated eight Toxoplasma mutants that cannot form cysts.
In a parallel project, Knoll and her team have screened 6,000 Toxoplasma gene sequences, looking for less virulent strains. “We have identified approximately 60 STMs (signature-tagged mutants) that cannot establish a chronic infection in mice,” says Knoll.
Initial vaccines will be tested on mice, to see if they are protected from potentially lethal Toxoplasma infections. Knoll anticipates developing a vaccine within the next five years that could be used for meat animals. “In this country, the vaccine could be used extensively in the pork industry. In Scotland and Australia, where Toxoplasma infection leads to a high number of spontaneous abortions in ewes, the vaccine could be used to vaccinate domestic sheep.”
As for large-scale vaccination of domestic cats, the primary host of the sexual phase of the parasite, Knoll is uncertain of the effectiveness of a vaccine. Pregnant with her second child, she is taking no chances, though. “When my two-year-old son Sam recently asked me if we could get a kitty, I said, ”Let”s wait.””