‘As common as Toxoplasma’ : fearless mice & brain parasites.
After nearly 6 months of keeping this research blog, I decided it was high time I wrote a post on an actual parasite – dish up the good stuff (historical and literary parasites excluded!) Accordingly, today’s blog is on one of my favourite unicellular parasites Toxoplasma gondii.
T. gondii is a parasite with a mouse intermediate host and cat definitive host, meaning that it is passed from mouse to cat in the food chain (where the adult stages reproduce). Occasionally the parasite will infest a human – accidentally – this is called an accidental or dead-end host, ‘dead-end’ because we don’t usually get eaten by cats!
A brief summary of the life cycle is as follows: a mouse ingests toxoplasma oocysts in the soil or water; these develop inside the mouse [tachyzoites and bradyzoites] and encyst in mouse muscle tissue. The infected mouse is then eaten by a cat. The parasites develop into adults inside the cat and reproduce. The offspring are then passed in the faeces, ready for ingestion by another mouse!
Other animals such as sheep and pigs may ingest the oocysts directly from the cat faeces or from oocysts in the environment (e.g. on grass) and themselves become infested with T. gondii. Humans can become infested if they consume food or water contaminated with the oocysts from the soil (vegetables grown in contaminated areas), directly from the cat faeces (e.g. changing the litter box) or from eating undercooked meat with muscle cysts (e.g. pork). If humans ingest T. gondii parasites they may encyst in muscle tissue: usually skeletal, myocardial, or even cerebral!
But the reason I am so fascinated by this parasite is because of its co-evolutionary success. Infesting > 50% of the human population worldwide (with 22.5% of the population of the United States above the age of 12 infested, and up to 95% of the population in some other parts of the world) they are very successful colonizers, and have a few tricks up their sleeves to help! The most impressive of these is the ability to change the fear response in their mouse intermediate host. An infested mouse displays fearlessness in place of the natural fear response and therefore they are more likely to be eaten by a cat. Several research studies suggest that they may have similar (but accidental) effects in humans – promoting ‘reckless behaviour’ such as dangerous driving and insecurity or neuroticism. The parasite has also been tentatively linked to schizophrenia and suicidal tendencies, inspiring dramatic headlines like: ‘Cat parasite that worms into humans’ brains can drive victims to suicide’ and ‘How your cat is making you crazy’.
My ‘favourite’ fact about Toxoplasma gondii is that it is one of the few parasites that can traverse the umbilical cord and induce abnormalities or even miscarriages in pregnant women – this provides a biological explanation for the old wives’ tale: ‘Pregnant women should avoid cats’.
And that concludes today’s whistle-stop introduction to one of our most common ‘guests’ Toxoplasma gondii.
For further reading see:
Flegr, J. ‘Effects of Toxoplasma on Human Behaviour’ Schizophrenia Bulletin 33(2007)3 pp757-760.
Flegr, J., Preiss, M., Klose, J. et al. ‘Decreased level of psychobiological factor novelty seeking and lower intelligence in men latently infected with the protozoan parasite Toxoplasma gondii Dopamine, a missing link between schizophrenia and toxoplasmosis?’ Biological Psychology 63(2003)3 pp253-268.
Lafferty, K. D. ‘Can the common brain parasites, Toxoplasma gondii, influence human culture?’ Proceedings of the Royal Society 273(2006)1602 pp2749-2755. Available at: http://rspb.royalsocietypublishing.org/content/273/1602/2749.short
Torrey, E. F., and Yolken, Y. H. ‘Toxoplasma gondii and Schizophrenia’ Emerging Infectious Diseases 9(2003)11 pp1375-1380. Available at: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3035534/pdf/03-0143.pdf
Webster, J. P., ‘Rats, Cats, People and Parasites: the impact of latant toxoplasmosis on behaviour’ Microbes and Infection 3(2001)12 pp1037-1045.
Yolken, R. H., Bachmann, S., Rouslanova, I. et al. ‘Antibodies to Toxoplasma gondii in Individuals with First-Episode Schizophrenia’ Clinical Infectious Diseases 32(2001)5 pp842-844.