A new study led by Wendy Ingram from the University of California, Berkeley, and published in the journal PLOS One suggests that Toxoplasma gondii infection in mice, which makes them lose their fear of cats, may lead to permanent changes in the fear mechanisms in the rodents as their loss of fear of their predators continues to persist even after parasite is no longer detectable in their brains.
Even after infection with Toxoplasma has been removed from rodents' brains, they continue to behave as if unafraid of the smell of cat urine, suggesting that the infection causes long-term changes in the brain. Previous studies have suggested this persistent behavioral change may be due to brain inflammation or parasite cysts within brain cells. In this study, the researchers found that this lack of fear persisted even when inflammation markers or cysts could not be detected in mice. Based on experiments with low-virulence strains of Toxoplasma and exposing mice to the parasites over a long period of time, the authors suggest that the lack of fear occurs within the first three weeks of infection. The behavioral change persists even in the absence of parasite cysts and brain inflammation, suggesting a different mechanism may be responsible, such as proteins injected by the parasite into host cells. Ingram elaborates, "It is remarkable that even after the infection has been largely or completely cleared, a profound behavioral change persists. Simply having a transient infection resulting in what is potentially a permanent change in host biology may have huge implications for infectious disease medicine."