Mind Manipulation by Toxoplasma gondii
Written by Erica Wang
Edited by Rachel Larson
July 31st 2021
Edited by Rachel Larson
July 31st 2021
Can a parasite control your mind? The manipulation hypothesis suggests that a parasite can alter the behavior of its host when it is favorable for the parasite’s reproduction or transmission (Webster, 2001). Although this parasite can live in a wide variety of vertebrates, it is most well known for its association with rats, cats, and humans. Alteration in behavior due to toxoplasmosis, the disease associated with T. gondii, has been well documented in rats and mice, but for humans it is still unclear.
T. gondii has an indirect life cycle, meaning it has two kinds of hosts: a secondary host for asexual reproduction and a definitive host for sexual reproduction. The eggs, or oocysts, of T. gondii sit in the environment and wait to be ingested by a host. The oocysts can only mature and sexually reproduce within members of the cat family. When consumed by a non-cat host, the oocysts divide aesexually and form thin cysts in host brain cells (Webster, 2001). When an uninfected cat consumes a prey animal infected with T. gondii, the life cycle is completed. However, to increase the chances of completing their life cycle, T. gondii have cleverly evolved to change the behavior of their hosts.
Rats are some of the most wary animals; they instinctively avoid encounters with unfamiliar objects (Barnett & Cowan, 1976). However, rats afflicted with toxoplasmosis were found to be less wary and were more likely to approach a novel object than uninfected ones (Berdoy et al., 1995). This is advantageous for T. gondii because rats that are less afraid are easier for cats to spot and catch. What is even more interesting is that infected rats have shown not only a reduction in aversion, but a preference for areas with the scent of cat urine (Berdoy et al., 2000)! Since T. gondii requires a specific host to fully mature, it benefits from altering the intermediate host’s behavior in such a way that increases its predation risk. Parasites with direct life cycles, which only require one host type, are not expected to exhibit any manipulation of behavior. Researchers confirmed this by infecting rats with direct life cycle parasites like Coxiella burnetti. None of these rats showed changes in activity level or fearlessness (Moore & Gotelli, 1990; Webster, 1994).
As for humans, toxoplasmosis is usually due to accidental infection. For the vast majority of human infections, toxoplasmosis is in a latent form. T. gondii remains dormant and asymptomatic, but still viable for the host’s lifetime (Remington & Cavanaugh, 1965). It is estimated that 22-84% of human populations are infected with some form of toxoplasmosis (Desmonts & Couvreur, 1974), but since latent toxoplasmosis is largely asymptomatic it holds little clinical interest. Past studies looking into behavioral or mental conditions associated with latent toxoplasmosis have reported personality changes and higher prevalence of mental disorders. These results are under fire due to the lack of differentiating between the four forms of toxoplasmosis (Paul, 1956; Elias & Porsche, 1960), not having an appropriate sample of participants (Burkinshaw et al., 1953), or not considering confounding variables (Langset, 1975). More research is needed on this prevalent parasite to make any conclusions on its effects on human behavior.
Works Cited
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Berdoy, M., Webster, J.P. & Macdonald, D.W. (1995). Parasite-altered behaviour: is the effect of Toxoplasma gondii on Rattus norvegicus specific? Parasitology, 111(4), 403-409. https://doi.org/10.1017/S0031182000065902
Berdoy, M., Webster, J.P. & Macdonald, D.W. (2000). Fatal attraction in Toxoplasma-infected rats: a case of parasite manipulation of its mammalian host. Proc. R. Soc. B, 267, 267. https://doi.org/10.1098/rspb.2000.1182
Burkinshaw, J., Kiman, B.H. & Sorsby, A. (1953). Toxoplasmosis in relation to mental deficience British Medical Journal, 1(4812), 702. 10.1136/bmj.1.4812.702
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Langset, M. (1975). Toxoplasma-Serofarbtest bei lernbehinderten Kindern in 70% positiv. Medical Tribune, 16, 245-248.
Moore, J. & Gotelli, N.J. (1990). A phylogenetic perspective on the evolution of altered host behaviours: a critical look at the manipulation hypothesis. Parasitism and host behaviour, 193-233. Retrieved April 12, 2021, from https://www.cabdirect.org/cabdirect/abstract/19910882206
Paul, J. (1956). Untersuchungen mit dem Serofarbtest auf Toxoplasmose bei Kindern, I.-III. Teil. Monatsschr. Kinderheilkd, 10, 228-235.
Remington J.S. & Cavanaugh E.N. (1965). Isolation of the encysted form of Toxoplasma gondii from human skeletal muscle and brain. N Engl J Med, 273, 1308-1310. 10.1056/NEJM196512092732404
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