Monday Micro – mind manipulating microbes revisited

By Siouxsie Wiles 23/09/2013

One of my favourite microbes is Toxoplasma gondii, the little protozoa famous for making mice less afraid of cats.

cat and mouse

T. gondii also causes toxoplasmosis which can be serious, especially for those with a compromised immune system. It’s the reason women are excused from cleaning cat litter trays during pregnancy as infection can result the baby being aborted or stillborn.

Found worldwide, this amazing parasite can infect virtually all warm-blooded animals, including humans. A study* published in 2009 suggested that up to a third of the global population has been exposed to and may be chronically infected with T. gondii, although infection rates differ significantly from country to country (1). They have a neat map in the paper showing prevalence rates of 40-60% for some countries in Europe and South America. North America sits at 10-20% and China at less than 10%.

In order to complete its life cycle, T. gondii requires the intestines of members of the cat family to sexually reproduce . And this is where the parasite employs a really neat trick – it seems to make mice lose their hard-wired fear for cats, giving the cats an easy meal and making sure the parasite gets to its primary host. Researchers showed this back in 2007, with the finding that chronically infected rodents no longer steered clear of cat pee (2).

A paper just out in PLOS One has investigated this lack of cat aversion using a few different strains of T. gondii, including one that is cleared by the immune system so cannot persist long term (3). Mice were infected with these different strains and then at different time points tested for their fear of cats. The way this is done is quite neat – mice are placed in a transparent enclosure which has either bobcat** or rabbit pee at various positions. The mice are then monitored to see if they avoid the region containing the pee.

What was interesting about the findings of this study is that the loss of aversion to cat pee was a general feature of infection with all the strains tested, including the strain that couldn’t persist. This suggests that T. gondii can interrupt the aversion response permanently, with no need for prolonged infection.

This is a scary prospect as there is a growing school of thought that T. gondii infection may be a risk factor for the development of schizophrenia, depression and suicide in people (4). Well, if they can affect the brains of mice, then why not humans?

*Alas it in an Eslevier journal and behind a paywall. Boo!

**So they don’t actually use pee from domestic cats instead getting the commercially available ‘predator pee’. The company in this case is called Leg Up Enterprises. Yes, seriously.

1. Pappas G, Roussos N, Falagas ME (2009) Toxoplasmosis snapshots: Global status of Toxoplasma gondii seroprevalence and implications for pregnancy and congenital toxoplasmosis. International Journal for Parasitology, 39 (12):1385–1394 

2. Vyas A, Kim S, Giacomini N, Boothroyd JC, Sapolsky RM (2007) Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors. Proceedings of the National Academy of Sciences, 104(15), 6442–6447.

3. Ingram WM, Goodrich LM, Robey EA, Eisen MB (2013) Mice Infected with Low-Virulence Strains of Toxoplasma gondii Lose Their Innate Aversion to Cat Urine, Even after Extensive Parasite Clearance. PLoS ONE 8(9): e75246. doi:10.1371/journal.pone.0075246

4. Henriquez SA, Brett R, Alexander J, Pratt J, Roberts CW (2009). Neuropsychiatric Disease and Toxoplasma gondii Infection. Neuroimmunomodulation 16:122–133 (DOI: 10.1159/000180267)

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