Imagine that pesky tabby cat has been pooing in your backyard again. Unbeknown to you, it has transferred some of the parasite spores it was carrying onto your herb garden. Unintentionally, while preparing a tasty salad, you forget to wash your hands and infect yourself with the Toxoplasma gondii spores. For months you display no symptoms, then after six months you are driving your car more aggressively, taking chances in road junctions and generally filled with more road rage as you angrily gesticulate with fellow drivers. Could all this be linked to that tasty salad?
T. gondii is a fascinating protozoan parasite which, like many similar organisms, needs to move between several different host species in order to fully develop and reproduce. As such, it appears to have evolved clever methods to make transmission between hosts more likely. For example, studies have found that once rats – intermediate hosts – are infected they display less caution towards cats – the final stage hosts – and so the parasite is more likely to be passed on.
An increasing number of studies suggest humans known to be infected with these parasites could be more susceptible to schizophrenia, bipolar disorder, aggression and evenincreased suicide. Studies have even suggested you are two to three times more likely tohave a car crash if your blood tests positive for the parasite. This is particularly striking when it has been predicted that 30%-50% of the worldwide population may carry the parasite.
Not so cute when you know what they’re carrying. Shutterstock
Chicken or egg?
Very often criticisms of these studies come down to a chicken and egg question. Correlation doesn’t necessary mean causality. Are those aggressive, fast-driving people or those with behavioural conditions more likely to catch the parasites, or does the parasite cause these behavioural traits? Many of the studies were done retrospectively rather than looking at someone’s behaviour before and after they became infected with the parasites. So for now, we can’t say for sure whether your road rage really was linked to your salad.
What we do know is that there are plenty of examples in wildlife where parasites can manipulate the sex, growth, maturation, habitat and behaviour of their hosts. Hair worms, for instance, complete their lifecycle in a river or stream and appear to make their hosts – crickets – attracted to water.