Outbreaks could be linked to climate
Mosquito experts say viruses like EEE more likely in future
This year’s outbreak of mosquito-borne Eastern equine encephalitis that has killed two Connecticut residents and three other people in New England fits a pattern that scientists have been predicting for years.
Climate researchers say global warming is resulting in hotter summers, milder winters, and more frequent and heavier rains. Mosquito experts believe those factors result in better survival and breeding rates for key disease-transmitting insects and can increase the prevalence of dangerous viruses like EEE.
But scientists also say the complex ways in which EEE is transmitted between infected migrating birds, other birds, multiple kinds of mosquitoes and humans make it very hard to pinpoint a direct link be--
tween this year’s virus outbreak and climate change.
“It’s difficult, if not impossible, to say it is cause and effect,” said Theodore Andreadis, head of the Connecticut Agricultural Experiment Station’s mosquito monitoring and research program. “But it’s all consistent with [climate change].”
There are other critical factors involved in the increasing likelihood of EEE outbreaks in Connecticut, according to Andreadis and other researchers, including changes in the state’s landscape and housing patterns. Those include the reforestation and expansion of mosquito-breeding swamps and the increased number of nature-loving people who now live near those once-rural areas.
Eastern equine encephalitis is a rare disease but one with a high fatality rate. Only seven human cases of EEE are reported in an average year in the U.S. But about three of every 10 people who contract the disease die.
The two Connecticut residents diagnosed with EEE so far in 2019 — one from East Lyme and one from Old Lyme — were the first people in the state to be infected with the virus since 2013. The Connecticut case six years ago resulted in death.
Connecticut experts have now identified EEE-infected mosquitoes in at least 15 communities, almost all in the eastern part of the state. They are warning residents to avoid outdoor activity around dawn and dusk, when mosquitoes are most active, and various towns and school systems are restricting or canceling late-afternoon and evening events to lessen the risk of infections.
Mosquito season in Connecticut generally runs through the summer and into October, ending when the first hard frost of autumn halts mosquito activity.
Climate experts have predicted that global warming will mean longer, hotter, wetter summers and milder winters in the Northeast. Scientists also warn that tropical mosquitoes capable of transmitting viruses like EEE, dengue, chikungunya, Zika, West Nile and other diseases are also expanding their range.
“The direct effects of temperature increase are an increase in immature mosquito development, virus development and mosquito biting rates, which increase contact rates … with humans,” Maria Diuk-Wasser wrote in a study in 2013, when she was a staff member with the Yale School of Public Health. Diuk-Wasser is an expert in mosquitoborne diseases, and is now an associate professor at Columbia University.
The hotter, longer summers that climate researchers blame on global warming is not only beneficial to mosquito breeding but “can translate to a more rapid buildup of a virus in mosquito populations,” Andreadis said.
The EEE virus exists year-round in Florida, according to disease experts. Birds that spend their winters in Florida, such as the snowy egret, often contract the disease when bitten by infected mosquitoes, then migrate in the spring to Connecticut and other New England states.
Many of those migrating birds tend to nest in wooded, swampy areas where mosquitoes like to breed — areas that for hundreds of years in Connecticut were often drained and cleared for agriculture. Over the past century, many of those parts of rural Connecticut were allowed to return to forested swamps perfect for mosquitoes.
The more frequent, often-heavy rains and warmer temperatures associated with climate change are also expected to improve breeding conditions for mosquitoes.
In a 2013 paper published with Philip Armstrong in the New England Journal of Medicine, Andreadis noted that “EEE virus transmission is highly seasonal and dependent on weather conditions, occurring within specific forested swamp habitats where the main mosquito vector … resides.”
Connecticut has had a very hot summer and many parts of the state have seen lots of rainfall, conditions that Andreadis said were ideal for many mosquito populations, including the six species in Connecticut known to be able to transmit EEE.
That paper six years ago also named the two key mosquito species involved in the EEE cycle: C. melanura, which mainly bites birds; and Coquillettidia perturbans, which is also known as the cattail mosquito. The first type attacks birds, including some carrying the EEE virus from Florida, and then transmits the virus to other bird species. The second type of mosquito bites birds, where it can become infected with EEE, and bites mammals like humans.
Many of the heavily wooded marsh and swamp areas where both species tend to breed happen to be in southeastern Connecticut, the section of the state where the two people who contracted EEE live.
The cattail mosquito, “is an aggressive human biter … and the most likely suspect for transmission to humans,” Andreadis said.
Andreadis said Massachusetts researchers began finding cattail mosquitoes infected with EEE “in large numbers” as early as July.
“That was a harbinger of what was to come,” he said.
“The environmental conditions and weather conditions … that come with climate change are all consistent with creating suitable conditions for increased mosquito populations,” Andreadis said. He added that an increase in those mosquito populations also increases the risks for mosquito-borne diseases like EEE.