Transmission of the human malaria Plasmodium falciparum is maximum at much lower temperatures than previously predicted

Generally, malaria transmission intensity has been thought to increase with temperature, which has raised concerns about how global climate change may further affect currently endemic areas. However, evidence from rodent malaria models suggests that warmer temperatures actually reduce transmission potential through increasing mosquito mortality and reducing maximum infectious parasite prevalence. We used Anopheles stephensi mosquitoes infected with the human malaria parasite Plasmodium falciparum to assess whether these predictions may hold true for natural vector-parasite combinations. After mosquitoes took an infectious blood meal, females were allocated across six different constant temperature regimes ranging from 21°C to 34°C. Salivary glands were dissected daily to estimate the change in proportion of infectious mosquitoes over time. Mortality was also assessed daily. Predictably, we observed reduced time to infectiousness at hotter temperatures. However, mortality increased with temperature, and we observed a marked decrease in the proportion of infectious mosquitoes over time in the warmer temperatures, suggesting that there are lethal effects of P. falciparum to mosquitoes at temperatures above 27°C, which has never before been documented. Interestingly, when combining the daily probability of survival and infectiousness, we observe that the greatest transmission potential for P. falciparum occurs at 24°C, about 3-9 degrees cooler than theoretically predicted. This suggests that although malaria transmission in some endemic areas may decrease as temperatures warm, optimal transmission areas may shift geographically, prompting a need to reconsider our predictions of climate-mediated changes in transmission in order to make optimum use of vector and parasite control resources.