A parasite that increases host lifespan.

Tenebrio molitor is an intermediate host for the rat tapeworm, Hymenolepis diminuta. Parasite oncospheres hatch in the beetle midgut and burrow through into the haemocoel, where they rapidly grow and mature into metacestodes. Repair of damage incurred during invasion and the nutritional demands of the parasites are likely to impose costs on the host. Despite these costs, there is an overall very highly significant difference in survival time (p < 0.001) between infected and control populations of beetles, with a hazard ratio of 2.35 (control versus infected). Infected females showed a 40% increase in survival time to 50% mortality and males showed a 25% increase in survival time to 50% mortality. This parasite-induced increase in host longevity is discussed in the light of changes in resource allocation that may occur in infected beetles. Previous findings have demonstrated that reproductive success is significantly reduced in infected females. The outcome of changes in the reproductive effort made by male beetles is less clear. We suggest that the optimum trade-off between reproduction and longevity may be altered to favour longer host survivorship, which is likely to enhance parasite transmission.