Toxoplasmosis poses a particular threat to people with weakened immune systems and can be contracted from undercooked meat and through contact with cat faeces.
Toxoplasma gondii is a microscopic, single-celled parasite that must pass through two hosts to complete its life cycle. It begins in the digestive tract of a member of the cat family before seeking out an intermediate host – almost any warm-blooded animal – where it lodges in tissue, ready to be transmitted to another host cat.
Humans are most commonly infected by consuming undercooked meat, unwashed vegetables and contaminated drinking water, but also from through exposure to contaminated cat faeces via litter boxes or contaminated soil when gardening, which highlights the importance of wearing gardening gloves and regular hand washing.
People cannot catch toxoplasmosis by simply petting or living with a cat. The parasite, meanwhile, can be transmitted from mother to child during pregnancy.
The vast majority of human infections are asymptomatic, but in some cases the parasite can cause a serious flu-like illness. In the United States, toxoplasmosis causes several hundred deaths each year, making it the second most common cause of food-related deaths after salmonella.
Severe cases can lead to complications such as toxoplasmic encephalitis (inflammation of the brain) or widespread organ damage affecting the lungs, liver, and heart. If a woman contracts toxoplasmosis for the first time during pregnancy, the parasite can cross the placenta and infect the fetus, potentially leading to miscarriage, stillbirth or severe congenital conditions such as hydrocephalus (fluid on the brain) and encephalopathy.
There is also growing evidence that even asymptomatic T. gondii infections may carry risks. Some studies suggest that infected individuals are more likely to die from any cause over a given period compared with uninfected people. Infection has also been linked to higher rates of schizophrenia, bipolar disorder, risk-taking behaviour and suicide.
However, research in this area remains at an early stage, and no clear causal relationship has been established. It is possible that individuals with existing health conditions are more susceptible to infection, rather than the parasite directly causing these outcomes.
To complete its life cycle, the parasite must return to a cat’s digestive system. It may achieve this by altering the behaviour of infected animals. In rodents, for example, T. gondii appears to affect the nervous system, reducing fear of predators. Infected mice and rats become bolder and more active, spend less time grooming, and are more likely to venture into open areas. They are also slower to react to danger, increasing the likelihood of being caught and eaten by a cat.
Similarly, scientists in the Gabon found that infected chimpanzees were attracted to the urine of leopards, which prey on the apes. “This suggests that it could be real parasite manipulation,” said the leader of the research Clémence Poirotte, of France’s Centre d’Écologie Fonctionnelle et Évolutive.
In the United Kingdom, an estimated 10% to 30% of the human population carries the parasite. Some infected people report concentration problems and slower reaction times.
“The behavioural changes reported in humans are generally assumed to be side effects of a manipulation that evolved in appropriate hosts, like rodents, to increase parasite transmission to cats,” Poirotte told BBC Wildlife.
