While a study published last year may have given cat owners a jolt by suggesting that there’s a link between cat ownership and brain tumors, a new study says that the connection may have been significantly oversold.
Last year, a team of French researchers published a paper in the scientific journal Biology Letters alleging that brain cancers are more common in adults who live in countries with higher incidences of infection by Toxoplasmosis gondii, a single-celled parasite that is passed to humans via exposure to cat feces. This week, the team has published a second paper in Biology Letters saying that cat ownership is not, in fact, a predictor of brain cancers or even of infection by Toxoplasmosis itself.
Study author Frederic Thomas of France’s Institut de recherche pour le développement (Institute of Developmental Research, or IRD), a scientist specializing in the genetic development of infectious diseases, wrote that the original project had researched Toxo and brain cancers in 37 countries and found that in countries with higher infection rates, brain tumors were more common. The team produced a second paper showing that in France, regional deaths from brain cancer “correlated positively with the local seroprevalence of T. gondii.” However, in the end the researchers found that, “These results do not demonstrate causation but suggest that T. gondii should be investigated further as a possible oncogenic pathogen of humans.”
A counter-study mounted in the U.K. compared a cohort of middle aged women, 18 percent of whom owned one cat or more. Comparing long-term health outcomes, scientists were unable to find any increased incidence of brain cancer among cat owners. Surprisingly, the study didn’t find a higher incidence of Toxoplasmosis, either, in spite of the working theory is that humans who get Toxo catch it while cleaning their pets’ litter boxes, inhaling the parasites in oocyst, or egg form.
Biology Letters claims that multiple studies have shown that cat ownership is not a strong predictor of Toxoplasmosis gondii infection. While a gestation period inside a cat is necessary for the organism’s life-cycle, most humans who contract the disease catch it through contact with contaminated soil or vegetables, or consumption of undercooked meat.
There have been recent studies speculating that Toxo infection can cause distinct changes in human behavior, just as it does in infected rats, who lose their fear of cats and become drawn to the smell of cat urine once infected, making them easier prey for cats and more useful to the parasites, who need to be ingested by felines to reach the next stage of their development.
The newest paper refuting the Toxo-brain tumor link concludes that no evidence has been isolated to suggest that one disease causes the other, but qualifies that point slightly by saying that more research could be done, “Clearly, what is now needed to assess the role of T. gondii in brain cancer risk are studies that compare the seroprevalence of T. gondii in individuals with brain cancer with matched controls.”