Q&A: The Intersection of Mosquito-borne Disease and Climate Change

By Erin Frick 

ALBANY, N.Y. (Sept. 23, 2024) — Eastern equine encephalitis (EEE) is a mosquito-borne disease that can induce severe fever, seizures, gastrointestinal issues and changes in behavior. About a third of EEE cases are lethal, and those who recover commonly experience lingering neurological problems. 

Considered rare in the United States, EEE is on the rise. The illness was identified in New York for the first time in nearly a decade last week and the person who contracted it has since died from the virus, Gov. Kathy Hochul announced Monday, making them the second person to die from the infection this season.

Climate change is an important driver of mosquito-borne disease. Warmer, wetter summers in the northeastern U.S. mean conditions are favorable for mosquitoes that transmit harmful diseases to people. Plus, the season of mosquito-borne disease risk is becoming longer, with the onset of warmer weather happening earlier and lasting later into the year. 

EEE virus is primarily transmitted in forested swampy areas, the preferred habitat of the black-tailed mosquito (Culiseta melanura). These landscapes play a vital role in shaping virus prevalence. Early EEE outbreaks tend to occur during unusually warm, wet summers, similar to the warm, humid periods experienced in the Capital Region earlier this summer.

University at Albany’s Alexander T. Ciota and Oliver Timm share insights on the rise of mosquito-borne diseases, how climate change is affecting mosquito populations and recommendations on how to protect against mosquito-borne disease.

Ciota is an associate professor in the Department of Biomedical Sciences at the College of Integrated Health Sciences. He is also director of the Arbovirus Laboratory at the New York State Department of Health’s Wadsworth Center. Timm is an associate professor in UAlbany’s Department of Atmospheric and Environmental Sciences.

Is the season of mosquito-borne disease risk becoming longer due to climate change? 

Alexander Ciota: When we evaluate the risk of mosquito-borne pathogens, it is important to consider a range of factors including land use, host dynamics, viral genetics and climate. How climate variation affects different mosquito-borne pathogens depends on the unique ecological characteristics of each pathogen and its primary mosquito vectors. 

While precipitation patterns are volatile and can have differential effects on mosquito populations, the consistent increases in temperature regionally are generally increasing the duration, intensity and distribution of mosquito-borne virus activity. Both spring mosquito emergence and cessation of activity in the fall are triggered by environmental cues, primarily temperature, so warming will generally correspond to longer mosquito seasons. 

Temperature also increases disease amplification and prevalence within the transmission season. This is because mosquito development accelerates at warmer temperatures, which will increase population size. 

Likelihood of virus transmission increases with higher temperatures as well. Because mosquitoes are ectotherms, increases in outside temperature increase internal temperatures, causing viruses to replicate faster and to higher levels. Since these viruses frequently mutate, this could also contribute to the emergence of new variants with altered virulence and transmissibility. 

An important exception to these trends: episodes of extreme heat, which can decrease mosquito longevity.

How is climate change affecting emerging arboviruses? 

Ciota: We are seeing shifts in the magnitude and distribution of endemic pathogens and an increased threat from exotic pathogens. This is partly due to increased global trade and travel, but range expansion of vectors due to climate change is also certainly contributing. For instance, both the Asian tiger mosquito (Aedes albopictus) and the yellow fever mosquito (Aedes aegypti) are steadily expanding their range as the climate becomes more suitable for these mosquitoes in the northern hemisphere. These are the primary vectors of numerous exotic pathogens such as dengue virus, Zika virus, chikungunya virus and yellow fever virus. 

As a result, we are beginning to see more cases of exotic viruses being acquired here in the U.S. For example, over the last five years, the first locally acquired dengue virus infections were reported in California, Arizona, West Virginia and North Carolina. Notably, there was also a locally acquired case of dengue in New York State in 2016.

Beyond EEE, are other mosquito-borne diseases on the rise in the eastern U.S.? 

Ciota: The general trend for all endemic vector-borne pathogens is increased activity. This year is one of the highest on record for West Nile virus activity in NYS mosquitoes. We are also seeing increases in lesser-known mosquito-borne viruses such as Jamestown Canyon virus and Cache Valley virus. Because there are so many dynamic factors that influence these pathogens, we will continue to have relatively high and low years, but the increasingly favorable climactic conditions, together with the emergence of new variants, means the probability of high levels of activity is increasing.

Is EEE prevalence changing? What particular health risks does it pose? 

Ciota: This year has actually not been an uncharacteristically high year for EEE in terms of prevalence in NYS mosquitoes, but has been remarkable in that we are seeing activity in nontraditional locations with an unprecedented number of equine cases. This, together with the reported human cases in neighboring states, has led to the elevated concern. 

There is a lot to be learned about the ecology of EEE including how it is maintained between seasons, the relative importance of different bird species, and the potential for different mosquitoes to transmit the virus. However, it is likely that the warm and wet spring conditions this year created more suitable habitat for expansion of the black-tailed mosquito, the main EEE vector, which breeds in wooded swamps. Fortunately, this mosquito rarely feeds on humans. When humans do become infected, the disease is often quite devastating. Unlike most mosquito-borne viruses for which severe disease is rare, EEE is thought to have a fatality rate greater than 30%, with nonfatal infections frequently being associated with long-term neurological complications. 

What are your top recommendations for people to stay safe during mosquito season? 

Ciota: Although there are equine vaccines for EEE and West Nile virus, there are no commercially available prophylaxis or approved human vaccines for endemic arboviruses. Personal protective measures to avoid mosquito bites remain the best defense. These include wearing long sleeves and pants outdoors, particularly at dusk and dawn, using insect repellents containing DEET, eliminating standing water from your property, and making sure you have intact screens in doors and windows in your home. Avoiding wooded swampy areas (i.e., black-tailed mosquito habitat) is also a good protective step. 

How do seasonal changes impact vector-borne disease risk broadly? 

Oliver Timm: For vector-borne diseases, it is important to remember both the vector that carries the virus and the host that gets infected with the virus are part of an ecosystem that follows a seasonal cycle. Temperature regulates the onset of biological activity in spring including new plant growth, return of migrating birds and the first reappearance of mosquitoes or ticks. These activities depend strongly on average climatic temperatures. 

An early warm spell in March can kick off an early start of tick activity and faster growth of mosquito populations. New York’s transition seasons are very short and tend to see highly variable weather conditions. While we aren’t seeing a strong shift in the onset mosquito-borne diseases locally yet, over the coming decades, we expect an earlier onset of the prevalent diseases transmitted by ticks and mosquitoes such as West Nile Virus and Lyme disease.

In the Northeast, are we seeing vector-borne diseases in new places due to changing climatic conditions? 

Timm: Disease vectors are moving into regions with previously unfavorable climate conditions. Ticks are generally expanding northward, and mosquitoes are encroaching into higher-elevation mountain regions. These changes are mainly driven by rising temperature. Such ecosystem changes are the first ingredient for introducing new vector-borne diseases. However, the abundance of vectors is not all that matters for the transmission and reproduction of a particular virus. Several other factors are important. 

For example, there are temperature ranges in which the vector is most active, and the virus reproduction and incubation time are optimal. For West Nile Virus to spread in nature, the optimal temperature is between 73-78 degrees Fahrenheit. Recent studies have made it possible to work out the suitable temperature ranges for various mosquitoes and viruses (Socket et al. 2020). 

Together with information from future climate change scenarios, our research group developed regional maps which include predictions as to which regions in New York will begin to see temperature regimes that support West Nile Virus transmission. Overall, we expect to see increasingly favorable conditions for West Nile transmission in New York, especially in the Northwestern regions of the state (Keyel et al., 2021). 

A big question scientists are tackling now is: How will rainfall patterns change into the future, and how will this, paired with rising summer temperatures, impact vectors and the diseases they carry? The general increase in rainfall that we’re seeing in the northeastern U.S. could lead to more favorable breeding grounds for mosquitoes and associated diseases.