THE EFFECT OF TOXOPLASMA-GONDII AND OTHER PARASITES ON ACTIVITY LEVELS IN WILD AND HYBRID RATTUS-NORVEGICUS

Using both correlational and experimental evidence, the relationship b etween parasite load and host activity was assessed in brown rats, Rat tus norvegicus. Two hypotheses were tested -(1) that parasites with in direct life-cycles, involving transmission between a prey and its pred ator, will alter the activity of the intermediate host so as to increa se its susceptibility to predation by the definitive host and (2) that activity levels in parasitized rats would be increased rather than de creased. Four groups of rats (n = 140) were examined. One group (n = 5 0) were wild brown rats trapped from 3 UK farmsteads, with naturally o ccurring parasites. The others were purpose-bred wild/laboratory hybri d rats with experimentally induced parasitic infections of either (n = 15) adult-acquired or (n = 15) congenitally-acquired Toxoplasma gondi i (an indirect life-cycle parasite), or (n = 15) Syphacia muris (a dir ect life-cycle parasite). Uninfected hybrid rats (n = 45), matched for sex, age and weight, served as controls. Rats were housed individuall y in outdoor cages, and their activities were recorded on video-tapes for 6 non-consecutive 10 h nights. Exercise wheels were also available for the hybrid rats. Out of 6 parasite species detected in the wild r ats, T. gondii was the only one which required predation by a definiti ve host to complete its life-cycle, and was also the only parasite to be associated with higher activity levels in infected than uninfected rats. Hybrid rats infected with T. gondii were also more active than t hose uninfected, whereas there were no differences in activity levels between S. muris infected and uninfected rats. This study shows that t he indirect life-cycle parasite T. gondii can influence the activity o f its intermediate host the rat. I suggest that this may facilitate it s transmission to the cat definitive host.