Toxoplasma gondii
Parasite that alters host behavior to favor feline predation.
Toxoplasma gondii is a parasitic alveolate responsible for the disease toxoplasmosis. It exists worldwide and can infect nearly any warm-blooded animal, but only members of the cat family (Felidae) serve as definitive hosts, meaning the parasite can only undergo sexual reproduction within them.
In rodents, the parasite alters behavior in ways that make them more likely to be caught by cats. Infected rats show less aversion to cat urine, while infected mice become less anxious, more exploratory, and lose their general fear of predators. Since cats are one of the few hosts where *T. gondii* can reproduce sexually, these behavioral changes are thought to be evolutionary adaptations that boost the parasite’s reproductive success—rodents that don’t avoid cat areas are more likely to become prey. The main mechanism behind these changes involves epigenetic remodeling in the neurons that control the relevant behaviors.
In humans, infection is usually asymptomatic, but it can cause serious toxoplasmosis in infants and people with weakened immune systems. During the first few weeks after exposure, mild flu-like symptoms may appear, but otherwise healthy adults typically show no symptoms. This latent infection has been linked to subtle behavioral, psychiatric, and personality changes. Compared to uninfected people, infected individuals may have a reduced aversion to cat urine (though this differs by gender) and an increased risk of schizophrenia and suicidal thoughts. Early evidence suggests *T. gondii* may cause some of the same brain changes in humans as in rodents. However, many of these associations are debated, and a 2016 study found them to be weak.
*T. gondii* is one of the most common parasites in developed countries. Serological studies estimate that up to 50% of the global population has been exposed to and may be chronically infected, though rates vary widely by country. As of 2018, the highest IgG seroprevalence was in Ethiopia, at 64.2%.
**Structure**
*T. gondii* has organelles called rhoptries and micronemes, which contain proteins for invading host cells and manipulating the host’s immune response. To inject these proteins, the parasite uses an apical complex at the tip of its cell to puncture the host membrane and release the organelle contents.
Quick Facts
- Genus
- Toxoplasma
- Species
- gondii
Facts from the source article.
Lore & Background
Toxoplasma gondii has a life cycle broadly summarized into two components: a sexual component that occurs only within cats (felids, wild or domestic), and an asexual component that can occur within virtually all warm-blooded animals, including humans, cats, and birds. Because T. gondii can sexually reproduce only within cats, cats are the definitive host. All other hosts are intermediate hosts. Sexual reproduction in the feline definitive host occurs when a feline is infected, for example by consuming an infected mouse carrying the parasite's tissue cysts. The parasite survives passage through the stomach, eventually infecting epithelial cells of the cat's small intestine, where it undergoes sexual development and produces millions of oocysts.
Reader's Guide
Toxoplasma gondii is significant for its widespread prevalence and its ability to manipulate host behavior, particularly in rodents, to increase the likelihood of predation by cats, its definitive host. This manipulation, supported by studies showing decreased aversion to cat urine in infected rats and lowered anxiety in infected mice, is thought to be an evolutionary adaptation that enhances the parasite's reproductive success. In humans, infection is generally asymptomatic in healthy adults, but latent infection has been associated with subtle behavioral, psychiatric, and personality alterations, including a decreased aversion to cat urine and an increased risk of schizophrenia and suicidal ideation. However, many of these associations have been debated, and a 2016 study found them to be weak. The parasite's ability to infect a wide range of warm-blooded animals and its resilience in the environment make it a persistent public health concern, especially for immunocompromised individuals and infants. Its presence in urban runoff, as noted by the San Diego Natural History Museum, can also affect marine life such as sea otters.
Did You Know?
- T. gondii can only undergo sexual reproduction within members of the cat family (Felidae).
- Infected rats show a decreased aversion to cat urine, which is thought to increase their chances of being preyed upon by cats.
- Tissue cysts can form in virtually any organ but predominantly persist in the brain, eyes, and striated muscle.
- The highest IgG seroprevalence of T. gondii was estimated in Ethiopia at 64.2% as of 2018.
Frequently Asked Questions
What is Toxoplasma gondii?
Toxoplasma gondii is a parasitic alveolate responsible for the disease called toxoplasmosis. It is found worldwide and can infect virtually any warm-blooded animal.
What is Toxoplasma gondii's most famous ability?
The parasite is best known for rewiring rodent behavior to make them more vulnerable to feline predation. Infected mice lose their natural fear of predators and become more exploratory, while infected rats show reduced aversion to cat urine.
How does Toxoplasma gondii complete its lifecycle?
Only members of the cat family (Felidae) serve as definitive hosts, meaning sexual reproduction of the parasite occurs exclusively within them. Every other warm-blooded animal functions as an intermediate host where the parasite persists but cannot undergo sexual reproduction.
How widespread is Toxoplasma gondii exposure?
Up to half of the global human population is estimated to have encountered this parasite at some point in their lives. Ethiopia records the highest seroprevalence of any country at 64.2% as of 2018.
Why is Toxoplasma gondii considered a landmark pathogen?
It stands as a textbook example of a microbe that manipulates host behavior to guarantee its own transmission. By making rodents less cautious around cats, the parasite ensures its return to the only host where it can complete sexual reproduction.
More in Infectious Agents And Pathogens 1-21
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced