Most have only heard of the little parasite Toxoplasma gondii in the context of cats and pregnant women. If you’ve been pregnant, you may have heard your doctor tell you to steer clear of the kitty litter. This is because Toxoplasma, which infects cats, can pass to humans through cat feces and infect the developing fetus. That infection can lead to a host of problems like hearing loss, vision problems, jaundice and low birth weight.
But doctors and scientists have become interested in Toxoplasma for a different reason. The parasite needs to infect mice and birds to reproduce. When cats then eat these animals, they become infected and shed the parasite in their feces. The infection in mice has a fascinating effect on their brain to increase their chances of being eaten by a cat. Infected mice become more active at night when cats are hunting and become less afraid of them. Normally mice avoid the smell of cat urine, but infected mice seem to be drawn to the smell instead.
So if Toxoplasma can change the brains of infected mice, could it also change the brains of infected humans? For many years infection was thought to have no effect on those with normal immune systems, but a variety of studies have found increased activity, decreased reaction times, altered personality profiles and changes in sensory perception in those infected. Further studies have also uncovered an association between Toxoplasma infection and schizophrenia.
Many studies have confirmed the link and while correlating the two doesn’t lead to a causal link, the correlation is strong. Based on this correlation, previous studies have attempted to estimate how many cases of schizophrenia are likely due to toxoplasmosis that could potentially be prevented by treating infection with antibiotics.
These studies have pegged the number at around 13% of cases, but the formula they used to make that calculation made some big, unsupported assumptions both about schizophrenia and about toxoplasmosis. Prior research assumed we know how often those infected end up with schizophrenia and that age plays no role in how likely you are to be infected with Toxoplasma. Neither of those assumptions is true. The major contribution of a new study out this week is to revise those numbers and the formula to make it more accurate.
While the math is pretty complicated, the team essentially built a mathematical model that took into account both the different groups of people out there (those infected versus those not infected who have schizophrenia, for example) and the fact that age can change the equation. From that model, they came up with what’s called an odds ratio, which is a number that shows how likely someone with one thing is to have another thing.
The odds ratio between toxoplasmosis and schizophrenia was 2.71, meaning a person infected was almost three times as likely as someone not infected to end up with schizophrenia. This number could then be used to figure out how many of the cases diagnosed each year probably result from Toxoplasma infection, which came out to a staggering 21% or one in five cases.
So should you get rid of your cat or get someone else to take out the kitty litter? Unfortunately, you can still pick up the parasite from the environment and from certain meats even if you don’t have a cat, which is why more than 20% of us are already infected. Additionally, infection doesn’t cause symptoms, which makes knowing when to treat the infection very difficult. Instead, this study indicates that developing a vaccine against the parasite, both for ourselves and our pets, may be more pressing and important than many previously realized and could have a big effect on the devastating impact of mental illness.