Temperature and intraspecific competition are important factors influencing the growth of all organisms, including parasites. The temperature increase is suggested to stimulate the development of parasites within poikilothermic hosts. However, at high parasite densities, this effect could be diminished, due to stronger intraspecific competition. Our study, for the first time, addressed the joint effects of warming and parasite abundances on parasite growth in poikilothermic hosts. The growth of the common fish parasite larvae (trematode Diplostomum pseudospathaceum) within the rainbow trout at different infection intensities and temperatures (15°C and 18°C) was experimentally investigated. The results showed that temperature was positively correlated with both parasite infection success and growth rates. The growth rates increased much more compared to those in many free-living poikilothermic animals. Atypically for a majority of parasites, D. pseudospathaceum larvae grow faster when abundant (Allee effect). The possible causes for this phenomenon (manipulation cost sharing, etc.) are discussed in this study. Importantly, limited evidence of the interaction between temperature and population density was found. It is likely that temperature did not change the magnitude of the Allee effect but affected its timing. The impact of these effects is supposed to become more pronounced in freshwater ecosystems under current climate changes.

Trypanosoma rangeli is a protozoan parasite, which does not cause disease in humans, although it can produce different levels of pathogenicity to triatomines, their invertebrate hosts. We tested whether infection imposed a temperature-dependent cost on triatomine fitness using T. rangeli with different life histories. Parasites cultured only in liver infusion tryptose medium (cultured) and parasites exposed to cyclical passages through mice and triatomines (passaged) were used. We held infected insects at four temperatures between 21 and 30 °C and measured T. rangeli growth in vitro at the same temperatures in parallel. Overall, T. rangeli infection induced negative effects on insect fitness. In the case of cultured infection, parasite effects were temperature-dependent. Intermoult period, mortality rates and ecdysis success were affected in those insects exposed to lower temperatures (21 and 24 °C). For passaged-infected insects, the effects were independent of temperature, intermoult period being prolonged in all infected groups. Trypanosoma rangeli seem to be less tolerant to higher temperatures since cultured-infected insects showed a reduction in the infection rates and passaged-infected insects decreased the salivary gland infection rates in those insects submitted to 30 °C. In vitro growth of T. rangeli was consistent with these results.