Given their global distribution and abilities to persist in the host, helminths can play a crucial role in affecting risk of infections by increasing individual variation in infection. Helminth co-infections are of particular interest because by altering host immune responses, they can modify host susceptibility and thus intensity and transmission of other parasites/pathogens. The dynamics of co-infection were examined using two helminths of the European rabbit. Individuals were simultaneously challenged with a primary dose of both parasites, and changes in intensity were examined in relation to local and systemic immune responses. Both helminths persisted in co-infected rabbits; however, contrasting dynamics and immune responses were observed. Graphidium strigosum intensity was high throughout the co-infection, while Trichostrongylus retortaeformis intensity decreased but was not completely cleared. A Th2 response was observed against G. strigosum, while a mixed Th1/Th2 profile was found to T. retortaeformis. A comparison with our previous work on single infections showed that G. strigosum intensity was higher in co-infected than single infected hosts, while T. retortaeformis showed no significant changes. Differences were also observed in the cytokine profiles, blood cell concentrations and antibody trends. Overall, host variability during helminth co-infections can be generated by significant differences in immune responses and/or parasite dynamics.
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