Parasites in your brain, why it may matter.

T. gondii is an intracellular protozoan (Beverley 1976) capable of infecting all mammals. Its associate disease, toxoplasmosis, is of significant economic, veterinary and medical importance (Luft & Remington 1986; Schmidt & Roberts 1989) and has sparked renewed interest due to its debilitating reactivation in AIDS and other immunosuppressed patients (Luft & Remington 1986). T. gondii has an indirect life cycle, where members of the cat family are the definitive hosts of the parasites and the only mammals known to shed T. gondii oocysts with their faeces (Hutchinson et al. 1969). If the oocysts are ingested by another mammal such as a wild rodent (the intermediate host) small thin-walled cysts form in various tissues, most commonly the brain.
Indeed, there are several reasons to predict that the T. gondii parasite may be able to achieve this. Principally, the formation of parasitic cysts in the brain of its host places T. gondii in a privileged position to manipulate behaviour (Werner et al. 1981). Accordingly, recent studies on both wild and wild laboratory hybrid rats have demonstrated that T. gondii causes an increase in activity (Webster 1994b) and a decrease in neophobic (fear of novelty) behaviour (Webster et al. 1994; Berdoy et al. 1995b), both of which can be argued to facilitate transmission to the felid [i.e. cat] definitive host. In contrast, other costly behavioural patterns such as competition for mates and social status (Berdoy et al. 1995a), which do not have any obvious impact upon cat predation rate, are left unaltered by the parasite (Berdoy et al. 1995b).

(Fatal Attraction in Rats Infected with Toxoplasma gondii
By M. Berdoy; J. P. Webster; D. W. Macdonald
Proceedings: Biological Sciences, Vol. 267, No. 1452. (Aug. 7, 2000), :1591)

To make a long story short rats with parasites have been observed to lack a fear of cat scented-areas, which may indicate that parasites are manipulating the brains of the rats in ways that increase their transmission rate through predation, thus completing their life-cycle. This may matter to us more than the way that some parasites control the brains of ants and the like given some of the similarities of the rat and human brain. It is estimated that about 50% of people are infected with the same parasite, which could impact how they feeel. E.g.

Finally, we believe that these results may also provide a functional explanation of the altered brain function in infected humans, where T. gondii prevalence has been found to range from 22% in the UK to 84% in France ( Desmonts & Couvreur 1974). Although humans represent a dead-end host for the parasite, our results could suggest that the reports of altered personality and IQ levels in T. gondii-infected patients (Burkinshaw et al. 1953; Flegr & Hrdy 1994) represent the outcome of a parasite evolved to manipulate the behaviour of another mammal. It is noteworthy that rat behaviour is often viewed as the outcome of a conflict between pronounced neophobic reactions and strong exploration tendencies characteristic of opportunistic omnivores. The uneasy balance between these conflicting motivations, very pronounced in rats but also visible in humans (‘the omnivores paradox’, Rozin 1976), may thus provide a particularly fertile ground for manipulation by T. gondii.

(Ib. :1593-1594)

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