Cats or dogs? An eternal debate, with passionate advocates on both sides. Those in the pro dog camp have an unusual argument they could lobby: cats can infect you with mind controlling parasites.
Toxoplasma gondii is a single celled obligate parasite, which means it is unable to complete its life cycle without a suitable host. In this case, that would be members of the Felidae family (cats). Other warm blooded animals (including humans) can also be infected, but the parasite is unable to mature and reproduce. Eggs are excreted in cat faeces, but Toxoplasma can also be transmitted through undercooked meat, soil and from mother to baby if contracted during pregnancy (hence why pregnant women are told to avoid cat litter). Worldwide infection rates vary, but in some countries up to 95% of the population may be infected1. In the UK it is estimated up to a third of people will be infected at some point in their life. Someone you know may well be infected, and wouldn’t that just explain their odd behaviour?
Toxoplasma gondii has been dubbed the ‘zombie parasite’. Infection can persist throughout life in the central nervous system, modifying the structure and function of neurons from within, by such as silencing neurons and hindering apoptosis2. Over time this can lead to behavioural changes in the host, and gender differences have been shown on self-reported personality factor questionnaires3. Whilst infected people generally do not display obvious symptoms, they are 2.5x more likely to have a car accident4. This is potentially explained by long term infection leading to a prolonged reaction time to stimuli2.
(Yes this is me. Why do you think I hide in my lab all day?!)
Various studies have been conducted to unpick how this infection alters the host’s behaviour. In rodents, decreases have been seen in neophobia (fear of novelty), learning capability and motor performance2. One study found that rats infected with Toxoplasama gondii lost their innate fear of cats, even over the longer term, suggesting it may result in permanent structural changes in the brain5. Although infection occurs throughout the brain, high concentrations of infection has been found in the amygdala, a region that plays a key role in emotional processing, memory and decision making.
Additionally, infected rodents appear to display a “fatal attraction phenomenon”, essentially the conversion of the fear response to feline urine to something akin to attraction. Ordinarily rodents may fixate on the odour, the associated danger focusing their attention, but in the absence of fear that fixation may cause them to react in an abnormal way. This fear reduction may result from the parasite’s need to find their natural host, a cat, to complete their life cycle. Prey that lose fear of their predators are more likely to be eaten, transmitting the parasite from rat to cat. This demonstrates the “manipulation hypothesis”, which postulates that some parasites alter their host’s behaviour in a manner beneficial to themselves. This phenomenon has been studied in infected humans, again finding a gender difference, with infected men rating cat urine as more ‘attractive’, whilst infected women displayed a decreased rating6.
There is also a proposed link between Toxoplasama gondii infection and schizophrenia. Those diagnosed with schizophrenia are more likely than the general population to have been infected with Toxoplasama gondii7, although it is important to stress schizophrenia is an incredibly complicated disease with many factors increasing its risk. A proposed mechanism in schizophrenia is the slow growth of microscopic cysts in neurons, as persistent infection can increase the cell’s production of dopamine, potentially altering their function8,9. Alongside several other neurotransmitters, dopamine functioning appears to be abnormal in schizophrenia.
If you want to read more about ‘scientific zombies’ you should check out How to make a zombie, which covers Toxoplasama gondii, re-animation and all sorts of other interesting and disturbing scientific examples of ‘zombies’. If you’d rather read a more fictional version of a mind controlling parasite the Parasitology series by Mira Grant is also fantastic.
Undoubtedly cat lovers have not changed their minds after reading this, but then you wouldn’t would you? Your cat would not approve.
- Toxoplasma gondii Epidemiology & Risk Factors, (2015), CDC.
- Flegr, J. (2013). Influence of latent Toxoplasma infection on human personality, physiology and morphology: pros and cons of the Toxoplasma–human model in studying the manipulation hypothesis. Journal of Experimental Biology, 216.
- Lindova, J., et al (2006). Gender differences in behavioural changes induced by latent toxoplasmosis. International Journal for Parasitology, 36.
- Flegr, J., et al (2002). Increased risk of traffic accidents in subjects with latent toxoplasmosis: a retrospective case-control study. BMC Infect Dis, 2.
- Berdoy, M., Webster, J., & Macdonald, D. (2000). Fatal attraction in rats infected with Toxoplasma gondii. Proc Biol Sci, 267.
- Flegr, J., et al (2011). Fatal attraction phenomenon in humans: cat odour attractiveness increased for toxoplasma-infected men while decreased for infected women. PLoS Negl Trop Dis, 5.
- Torrey, E, et al (2007). Antibodies to Toxoplasma gondii in patients with schizophrenia: a meta-analysis. Schizophr. Bull. 33.
- Prandovsky, E., et al (2011). The Neurotropic Parasite Toxoplasma Gondii Increases Dopamine Metabolism. PLoS ONE, 6.
- Stanley Medical Research Institute (2014). http://www.stanleyresearch.org/patient-and-provider-resources/toxoplasmosis-schizophrenia-research/neurotransmitters-and-t-gondii/