The disease is spread by two types of mosquitoes. The first is a species called The melanura slidesor the black-tailed mosquito. This mosquito tends to live in hardwood bogs and feeds on birds such as robins, herons, and wrens, spreading the virus to them. But the melanura mosquito does not often bite mammals. A different kind of mosquito, Coquillettidia perturbansis primarily responsible for the majority of human disease cases reported in the US. perturbans mosquito picks up the EEE virus when it feeds on birds and then infects the humans and horses it bites. Toward the end of summer, when mosquitoes reach their peak numbers and begin jostling for any available blood meals, human cases begin to increase.
Andreadis, who published a historical retrospective on the development of triple E in the northeastern US in 2021, said that climate change has emerged as a major driver of the disease.
“We have milder winters, we have hotter summers, and we have extreme rains and droughts,” he said. “Its impact on mosquito populations is likely to be profound.”
Warmer global average temperatures generally produce more mosquitoes, regardless of species.
Studies have shown that warmer air temperatures up to a certain threshold, around 90 degrees Fahrenheit, shorten the amount of time it takes for C. melanura eggs to hatch. Higher temperatures in spring and fall extend the number of days mosquitoes have to breed and feed. And they’ll feed more times during the summer when it’s hotter—mosquitoes are ectothermic, meaning their metabolism speeds up at higher temperatures.
Rainfall also plays a role in mosquito breeding and activity, as mosquito eggs need water to hatch. A warmer atmosphere holds more moisture, which means that even small rain events dump more water today than they did in the last century. The more standing water in roadside ditches, abandoned car tires, ponds, potholes, and potholes, the more opportunities mosquitoes have to breed. And warmer water reduces the incubation period for C. melanura eggs, leading one study to conclude that warmer-than-average water temperatures “increase the likelihood for amplification of EEE.”
