A parasite that affects the mind

A colleague recently drew my attention to a study1 entitled ‘Is Toxoplasma gondii a potential risk for traffic accidents in Turkey?’; it appears that it is, which lends support to the idea that it is a more significant public health problem than it is often thought to be.

T. gondii is a protozoan parasite whose definitive hosts are cats and other felids; oocysts from cat faeces contaminate soil and so reach herbivorous animals in which, after the initial infection, dormant cysts are formed in the brain and muscles; cats are reinfected when they eat infected small mammals. Humans become infected either by exposure to contaminated soil (by hand-to-mouth transmission) or vegetables, or by eating undercooked meat from infected livestock.

The prevalence of infection in humans varies greatly between countries; for example, it is of the order of 20% in the UK but 80% in France. Such high prevalence levels even in wealthy countries persist with little concern because infection has been thought to be harmless in most cases; there may be mild symptoms at the time of infection, but severe symptoms usually occur only in people with depressed immune systems (who are at risk for many other infections), or in babies born to women who become infected for the first time during pregnancy.

The study by Yereli et al. found that among a sample of Turkish people involved in road accidents, the prevalence of latent infection, indicated by the presence of antibodies in the blood, was significantly higher than in a control group from the general population. This is in accordance with a series of studies by J. Flegr et. al. which found2 that people with latent infection had slower reaction times and3 were more likely to be involved in road accidents.

It is also known from studies by J. P. Webster et al. that latent T. gondii infection in rats can make them less cautious, and even in some cases cause them to be attracted to the scent of cats4; this benefits the parasite by increasing its chances of being transmitted to the next host. A recent study5 by A. Vyas et al. indicates that this effect is very precise, leaving other responses to potential danger unharmed, although the mechanism by which this is achieved remains unknown.

As humans are not often eaten by members of the cat family, it might be surprising if exactly the same effect occurred in humans, and further studies by Flegr et al. suggest that effects on human behaviour take the form of generalised pathology rather than targeted manipulation (even in mice, only those recently infected show significantly different behaviour), but they also show that the effects are not confined to reaction times and accident rates but include differences in personality. A review6 of a number of studies (a search of CAB Abstracts finds some more recent ones) indicates that toxoplasmosis may be linked to schizophrenia.

A search of CAB Abstracts for articles on Toxoplasma and human or animal behaviour does not find many records other than those by the authors mentioned above, so it seems that the subject is not a major research topic, but that there is enough evidence of effects to suggest that control of infection is more important than has usually been thought. An article7 in New Scientist a few years ago gives an overview of the subject and comes to a similar conclusion; the subject has also been reviewed8 by J.P. Webster.

What no-one appears to have investigated yet is whether countries with higher prevalences of T. gondii infection have higher road accident rates; there might well be too many variables to show an association, let alone a causal link, so such a study might not prove anything, but it is still an interesting idea to speculate about.

References:

1: Yereli, K. et al.: Is Toxoplasma gondii a potential risk for traffic accidents in Turkey? Forensic Science International (2006) vol. 163 no. 1-2, pp. 34-37.

2: Havlíček, J. et al.: Decrease of psychomotor performance in subjects with latent ‘asymptomatic’ toxoplasmosis. Parasitology (2001) vol. 122 no. 5, pp. 515-520.

3: Flegr, J. et al.: Increased risk of traffic accidents in subjects with latent toxoplasmosis: a retrospective case-control study. BMC Infectious Diseases (2002) vol. 2 no. 11.

4: Berdoy, M. et al.: Fatal attraction in rats infected with Toxoplasma gondii. Proceedings of the Royal Society of London. Series B, Biological Sciences (2000) vol. 267 no. 1452, pp. 1591-1594.

5: Vyas, A. et al.: Behavioral changes induced by Toxoplasma gondii infection of rodents are highly specific to aversion of cat odors. Proceedings of the National Academy of Sciences of the United States of America (2007) vol. 104 no. 15, pp. 6442-6447.

6: Torrey, E. F. and Yolken, R. H.: Toxoplasma gondii and schizophrenia. Emerging Infectious Diseases (2003) vol. 9 no. 11, pp. 1375-1380.

7: Randerson, J.: All in the mind? New Scientist (2002) issue 2366, 26 October, p. 41.

8: Webster, J. P.: Rats, cats, people and parasites: the impact of latent toxoplasmosis on behaviour. Microbes and Infection (2001) vol. 3 no. 12, pp. 1037-1045.

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