El Niño and Vector Borne Disease - What do we know about mosquitos, disease, and climate?

We are usually able to enjoy the patio by October, but the mosquitoes were still biting. What does the science tell us about El Niño and mosquito-borne disease? Short answer: it’s complicated.

With respect to mosquito-borne disease, we tend to think about a mosquito season; the period of the year during which mosquitoes are active. For diseases like West Nile virus, the cycle starts when mosquitoes bite birds, infecting them with the disease. Eventually, often in late summer and early fall, enough mosquitoes and birds are infected that the disease spills over into human and horse populations. This requires that there are enough infected mosquitoes and that these mosquitoes survive long enough to become infectious and bite humans or horses.

That gets us back to the mosquito-season. Immature mosquitoes—egg, larval, and pupal stages—require water to compete their life cycle. This water is provided both by precipitation and by human behavior, collecting in green pools, unprotected water storage, clogged gutters, or saucers under plants. Temperature drives how quickly an immature mosquito becomes an adult; typically, warmer temperatures speed up development. Another important piece is adult mosquito survival and host-seeking activity, both of which are mediated by humidity. Thus, the duration and intensity of any year’s mosquito season is going to be strongly influenced by temperature and precipitation.

This year, with respect to precipitation, we had a wet June but then a normal July–September rainfall. The rains are expected to remain above-average through the winter. Despite the mosquitoes we experienced this summer and fall, the implications of the summer lull in precipitation are unknown. Because of the ramping up of West Nile virus in the bird-mosquito cycle, we might actually escape a bad year and hopefully, the bird-mosquito transmission cycle was broken and won’t have enough time (enter the effect of cooler winter temperatures) for it to build up again and spill over to humans.

Research results are mixed, being highly dependent on the disease of concern and the region of study. Intuitively we know there must be an association between climate and vector life cycles, as the life cycle of invertebrates are often tightly associated with weather. However, with urban mosquitoes like the Culex species that transmit West Nile in the United States and the Aedes species that transmit dengue and chikungunya, human behavior plays a role in mediating exposure to mosquitoes, the availability of breeding sites, and even mosquito survival. Moreover, the diseases themselves have an intrinsic intra-annual cycle in which the proportion of a given population that is susceptible to infection fluctuates between years, confounding the climate-driven associations with temperature and precipitation.

That leaves us with a dissatisfying ‘we don’t know’ and ‘it all depends.’ Our experience will depend on the previous years’ mosquito abundance, previous years’ human or other host disease occurrence, current human or other host susceptibility, and the duration of the anomalous weather. It does leave us with a positive message, though: what we do as individuals does matter. Protect yourself from mosquitoes, empty breeding sites, cover your water storage, repair your window screens, and use mosquito repellent when you go out of doors.