Following my recent post about behavioural modification of insects by Fungi such as Entomophthora, I was sufficiently intrigued to digress from my usual ecological and entomological subject matter and look at work on related processes in humans. Rather than a fungus, the organism I am particularly interested in is the widespread parasitic protozoan Toxoplasma gondii.
Spread to humans (and many, many other species) by various mechanisms including handling and consumption of raw meat, transplacentally, faecal contamination and via members of the Felidae, this organism causes toxoplamosis, the main symptoms of which are widely described (e.g. here). However, less well understood is the possible link between T. gondii infection and changes in human behaviour. Some studies (e.g. Henriquez et al. 2009) have indicated a link (causative or contributory) between infection and several psychiatric disorders such as depression and schizophrenia.
Other work has shown behavioural effects on rodents, and subsequently on humans (Flegr 2007). Along with some quite complex changes in personality traits, the effects of which appear to differ between men and women, some clear practical impacts were seen such as reduced reaction time (i.e. reduced psychomotor function). Now, in insects, behavioural changes by parasites have clearly evolved to improve the reproductive success of the parasite (such as the posture adopted by Entomophthora-infected flies which aids spore dispersal), but what about in vertebrates? Flegr (2007) helps here too. Reduced reaction time in rodents coupled with a felid host makes intuitive sense – slower mice and rats are more likely to be eaten by cats. In humans, the link is less obvious until you consider the ‘point of view’ of the parasite i.e. that it doesn’t know it is in a dead-end host – one that is highly unlikely to be eaten by a cat. So, the behavioural change still takes place, and impacts may be felt, or at least detected by targeted research. This logic holds true for another known effect on rodents – risk-taking behaviour which manifests as reduced fear of cats (and even active seeking of locations with cat urine) – which some studies such as Henriquez et al. (2009) have noted in humans (as increased rule-breaking rather than urine-seeking...). Again, this makes clear sense – unwary rats get eaten by cats.
The precise mechanism causing such changes is still not fully understood, but recent research (Webster & McConkey 2010) at the University of Leeds has found that T. gondii produces an enzyme with tyrosine hydroxylase and phenylalanine hydroxylase activity which may alter dopamine production and so affect mood, sociability, attention, motivation and sleep patterns. Linking back to the psychiatric conditions noted above, schizophrenia has long been linked to dopamine dysregulation. Ongoing research seeks to elucidate the mechanism of T. gondii effects at the molecular level. Personally, I find this fascinating - complex human behaviours being modified by a protozoan we often don't know is there (even if maybe a third of us have it), but back to the ecology...
Flegr, J. (2007). Effects of Toxoplasma on human behavior. Schizophrenia Bulletin 33(3): 757-760.
Henriquez, S.A., Brett, R., Alexander, J., Pratt, J., & Roberts, C.W. (2009). Neuropsychiatric disease and Toxoplasma gondii infection. Neuroimmunomodulation, 16 (2), 122-133 PMID: 19212132
Webster, J.P., & McConkey, G.A. (2010). Toxoplasma gondii-altered host behaviour: clues as to mechanism of action. Folia parasitologica, 57 (2), 95-104 PMID: 20608471
of research that grab my attention. As well as blogging, I undertake professional ecological & wildlife surveys covering
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