Reevaluating the evidence for Toxoplasma gondii-induced behavioural changes in rodents

The ubiquitous protozoan parasite Toxoplasma gondii has been associated with behavioural changes in various hosts, including humans. In rodents, these behavioural changes are thought to represent adaptive manipulation by T. gondii to enhance transmission from intermediate hosts to the feline definitive host. In this review, we have tabulated evidence of changes in motor coordination, learning, memory, locomotion, anxiety, response to novelty and aversion to feline odour in rodents experimentally infected with T. gondii. In general, there was no consistent indication of the direction or magnitude of behavioural changes in response to infection. This may be due to the use, in these experimental studies, of different T. gondii strains, different host species and sexes and/or different methodologies to measure behaviour. A particular problem with studies of behavioural manipulation is likely to be the validity of behavioural tests, that is, whether they are actually measuring the traits that they were designed to measure. We suggest that future studies can be improved in three major ways. First, they should use multiple tests of behaviour, followed by multivariate data analysis to identify behavioural constructs such as aversion, anxiety and response to novelty. Second, they should incorporate longitudinal measurements on the behaviour of individual hosts before and after infection, so that within-individual and between-individual variances and covariances in behavioural traits can be estimated. Finally, they should investigate how variables such as parasite strain, host species and host sex interact with parasite infection to alter host behaviour, in order to provide a sound foundation for research concerning the proximate and ultimate mechanism(s) responsible for behavioural changes.