Meeting Abstract
Many parasite life histories incorporate a cryptic stage when parasites persist as covert (asymptomatic) infections when parasites are hidden in immunologically privileged sites or otherwise tolerated (e.g. latent periods of malaria and tuberculosis). Horizontal transmission occurs when cues trigger a transformation from cryptic to infectious stages, resulting in overt infection and disease. Covert infections are a common parasite strategy with improved detection methods demonstrating cryptic stages in a range of animal hosts and parasites, yet their significance is little explored. How might environmental change influence covert infection dynamics? This question is addressed by focusing on a colonial host (freshwater bryozoan)-endoparasite (myxozoan) system to examine how environmental and host conditions interact to influence infection dynamics and, in turn, cause an emerging fish disease. Our research reveals host condition-dependent cycling between covert and overt infections. When hosts are growing vigorously (warm temperatures/abundant food) virulent overt infections temporarily castrate bryozoans, producing stages infectious to fish and disease outbreaks. Stressful host conditions cause regression to avirulent cryptic stages. The parasite also exploits host clonal replication, undergoing ‘vertical transmission’ to asexual dispersive propagules. Such environmentally-conditioned persistence strategies are linked with high infection prevalences over space and time and facilitate parasite dispersal, establish disease reservoirs and trigger disease emergence. As environmental change (increasing temperatures and productivity) frequently drives parasite proliferation, disease outbreaks and spread mediated by covert/overt infection dynamics in permissive hosts may become increasingly common.
