Toxoplasma gondii’s life cycle is one of the most interesting fixtures of parasitology and biology as a whole. The definitive or reproductive host for the parasite is the common housecat, but the life cycle necessitates transmission through the barn mouse. Research shows that gondii’s eggs are present in a large proportion of cat’s litter boxes, which the barn mouse then interacts with via foraging behavior. The oocysts themselves are highly resistant to weather, temperature, rupture, etc, so they spread to anything that can be contaminated, namely soil, water, food, trash, etc. When the mouse ingests the egg, the parasite grows, and studies dictate an attraction to cat urine is promoted in the infected rodent. The definitive host, the cat, then ingests the mouse, infecting its intestinal track with the parasite, where Toxoplasma can reproduce. Some evolutionary theorists believe this demonstrates an “extended phenotype” which creates a linkage among these species as an adaptation by the Toxoplasma parasite which does not typically transfer directly from cat to cat.
Some human studies indicate a correlative relationship between Toxoplasma infection and mental illness or behavioral changes, but these are both controversial and purely correlative. No causal mechanism has been discovered for the behavioral changes in humans or mice.
Pregnant women are directed by the CDC to avoid cleaning cat litter boxes because Toxoplasma gondii infection is easily transmissible through the prenatal membrane, and prenatal infection is highly correlated with blindness and mental illness in the baby later in life.
What a sweet story, right?
Four intracellular Toxoplasma gondii parasites are shown undergoing cellular division by an internal budding process known as endodyogeny. Staining with a T. gondii surface antigen provided heart-shaped images (shot on Valentine’s Day). The definitive host of these parasites is the cat, but they infect many warm-blooded animals, including humans. While toxoplasmosis is typically a minor disease, T. gondii can cause severe central nervous system disorders of immunocompromised individuals—such as those with AIDS, organ transplants, and lymphoma—as well as birth defects in congenitally infected neonates. Eating undercooked meat and ingesting food or water contaminated with cat feces are the most common routes of infection for humans.
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