Are food-borne parasites a major cause of car accidents?
There has been a fair amount of media buzz about the parasitic protozoan toxoplasma gondii. It’s a parasite whose primary lifecycle involves transmission between cats (who eat infected rats) and rats (that eat fæces from infected cats). The strange thing about this parasitic disease, which is called toxoplasmosis, is that it changes the behaviour of the rats, slowing their reaction times and making them less fearful — of things like cats.
It’s a neat trick for the parasite. But one wouldn’t want brain-altering protozoa to get into the wrong place!
Rats and cats are not the only homes for toxoplasma gondii. Human beings can also contract toxoplasmosis, and indeed it is endemic around the world. It can be contracted either by eating undercooked meat or by ingesting trace quantities of cat fæces (yuck, but it happens). The prevalence of the latent form of the infection ranges widely — 4.3% in South Korea; 22% in Britain and the U.S.; 80% in France. These rates (and the fact that the high risk window for transmission by cats is only a couple of weeks) suggest very strongly that meat is the primary vector for human infection. Toxoplasmosis rarely causes obvious symptoms, except in people who are immuno-suppressed or pregnant, where acute infection can occur and is extremely serious. This is why pregnant women are told to aovid cats and undercooked meat.
Latent toxoplasmosis rarely causes obvious symptoms. But what might those tricks aimed at altering rat brains do to us? A rash of papers on the subject have appeared in the last few years, many of them produced by a single research group in the Czech Republic. These results correlate toxoplasmosis with — and sometimes accuse the parasite of causing — the most extraordinary things: making women more promiscuous; making them bear more male children; making men less intelligent and more jealous and anti-social; causing schizophrenia; slowing reaction times. Some of the results about personality are a little tenuous. But one of the most striking results was not at all tenuous: a 2002 study published in BMC Infectious Diseases finding that toxoplasmosis infection rates are very high among drivers who have been involved in car accidents, around three times those in the general population. This study was originally conducted in the Czech Republic, but was succesfully replicated in Turkey [PDF]. The correlation is real.
It’s difficult to believe that an obscure parasite could be causing such sin and misery, and we still don’t know that it is. The question is whether there are any plausible causes that could be common to both toxoplasmosis and the slow reaction times (or something else) that result in car accidents. The most obvious variable one would reach for is social class. Often, bad things — like diseases and car accidents — are more likely to happen to the poor. But toxoplasmosis may be an exception. Perhaps wealthy people eat more French food, and are more likely to drive in countries like the Czech Republic and Turkey (we’d like to see this tested). Another variable that has been suggested is “risk taking behaviour”, but it seems incredible that risk takers would really eat that much more rare meat.
We here at Science That Matters have another variable we propose testing: immune system strength. A weak immune system could lead to both latent toxoplasmosis and to other symptomatic conditions that increase the risk of car accidents. If some data becomes available to refute that possibility… we will be persuaded that parasites are in fact a major cause of car accidents.