Toxoplasma gondii is a parasite transmitted through rodents and sometimes found shed in cat feces. Historically it has been a concern among pet owners because human infection is possible. While in most cases humans only have flu-like symptoms, it can cause problems with the fetus when pregnant women become infected.
In recent years this parasite has become a new concern because evidence came out that the parasite causes behavioral changes in rodents and could do so in humans. To quote the Wikipedia entry on the disease:
A number of studies have suggested that subtle behavioral or personality changes may occur in infected humans, and infection with the parasite has recently been associated with a number of neurological disorders, particularly schizophrenia. A 2015 study also found cognitive deficits in adults to be associated with joint infection by both T. gondii and Helicobacter pylori
However, a recent study by Karen Sugden et al. at Duke University** has given some evidence that psychological and behavioral changes previously described may not exist after all.
Our results suggest that a positive test for T. gondiiantibodies does not result in increased susceptibility to neuropsychiatric disorders, poor impulse control or impaired neurocognitive ability… this is, to our knowledge, the most comprehensive assessment of the possible link between T.gondii infection and a variety of impairments in a single cohort.
This is, to our knowledge, the most comprehensive assessment of the possible link between T. gondii infection and a variety of impairments in a single cohort. Previous positive associations have been reported across different studies, often in selected or clinical samples; for example, one study will examine the link to violence, another the link to schizophrenia, and yet another the link to self-injury, and so forth.
Is this definitive? I'm not sure about that. Several other studies have shown some degree of correlation so I'm not completely ready to rely solely on this more recent paper. However, the results are interesting and certainly call into question some of those previous research.
The main point to take away is that toxoplasmosis is very uncommon from pet cats, and few cat owners are at risk from any kind of disease due to this parasite. There is reason to be cautious, but people shouldn't get rid of their cats or be overly concerned.
** Sugden K, Moffitt TE, Pinto L, Poulton R, Williams BS, & Caspi A (2016). Is Toxoplasma Gondii Infection Related to Brain and Behavior Impairments in Humans? Evidence from a Population-Representative Birth Cohort. PloS one, 11 (2) PMID