Researchers in Florida have pinpointed the local overwintering habitats of Culiseta melanura mosquitoes that host eastern equine encephalitis virus. They hope targeted control efforts can cull those mosquitoes before they transmit the virus to birds that carry it north to other parts of the country. (Photo by U.S. Centers for Disease Control and the Connecticut Agricultural Experiment Station, via CDC Public Health Image Library)

By Leslie Mertz, Ph.D.

Leslie Mertz, Ph.D.

Researchers have uncovered new details about the dangerous eastern equine encephalitis virus (EEEV) and the mosquitoes that transmit it. And, although that information was just published in the Journal of Medical Entomology in late April, mosquito-control teams are already putting it to work in the field with the hope of curbing future outbreaks of EEEV in the northeastern United States.

“I’ve been studying EEEV for probably 20 years, and it is the most pathogenic mosquito-borne virus that we have in this country,” says vector ecologist Thomas Unnasch, Ph.D., strategic area lead for global and planetary health at University of South Florida (USF). “It doesn’t affect a lot of people, but it is fatal in about 50 percent of those who are infected, and half of the survivors have severe neurological problems that leave them wheelchair-bound and requiring 24-hour care.”

Through an investigation of the transmission pathway, the research group has found a potential vulnerability in the life cycle of the primary EEEV-carrying mosquito species: Culiseta melanura, sometimes known as the black-tailed mosquito. The project was led by Kristi Miley, Ph.D., recently a graduate student in Unnasch’s lab who earned her doctorate in December 2020.

University of South Florida’s Kristi Miley, Ph.D., carries a mosquito-collecting aspirator vacuum while treading through a collection site in Citrus County. She spearheaded a new study that revealed the winter haunts of Culiseta melanura mosquitoes, providing a possible target and timing for insecticide treatments. Miley led the study during her graduate-student work and earned her doctorate in December 2020. (Photo courtesy of Thomas Unnasch, Ph.D.)

Vector ecologist Thomas Unnasch, Ph.D., of the University of South Florida calls eastern equine encephalitis virus (EEEV) “the most pathogenic mosquito-borne virus” in the United States. He oversaw a new research study that investigated Florida populations of the primary EEEV-carrying species, Culiseta melanura, which is ultimately responsible for EEEV infections in other parts of the country. The Centers for Disease Control and Prevention’s Center for Excellence in Vector-Borne Diseases funded the study. (Photo courtesy of USF Health)

The Florida Connection

In most of the United States, the cycle of EEEV transmission looks like this: Virus-carrying Cs. melanura mosquitoes become numerous in late summer and amplify the virus, which can then get passed on to people (and horses, which is how the virus gets its name). Transmission then stops when the mosquitoes die off when colder temperatures set in. In Florida, however, the mosquitoes can survive the winter, albeit in far fewer numbers than in the summer.

“Some phylogenetic studies that have been done over the last decade have shown that Florida is the reservoir for EEEV for the rest of the country and is constantly feeding the virus back to the northeastern United States,” Unnasch says. Previous studies also showed that birds are a primary host of Cs. melanura mosquitoes, so he believes migrating birds probably facilitate that northward movement of the virus.

Unnasch and the research group put their focus on the winter whereabouts of the mosquitoes. To do so, they worked with mosquito control teams in three Florida counties—Walton, Hernando, and Citrus—which have experienced considerable EEEV transmission in recent years. In those counties, control teams were already monitoring for various mosquito-borne diseases, including St. Louis encephalitis and EEEV, by placing chicken coops throughout the area and periodically taking blood samples from the chickens to see if they had been infected. The USF researchers used that EEEV data, along with reports about birds and other potential host animals near the chicken coops, and combined it with habitat maps to home in on the biological and ecological factors at play.

Wet tree holes can persist even in Florida’s dry winter season, and they serve as an enticing habitat for mosquitoes that harbor the eastern equine encephalitis virus. “Those tree holes are apparently the key that allows Culiseta melanura to really get going and why there are much more of them at high-risk sites than low-risk sites,” says Thomas Unnasch, Ph.D., of the University of South Florida. Shown here is a larval sampling tool being used to collect Cs. melanura mosquito larvae living in standing water near tree roots in Connecticut. (Photo by U.S. Centers for Disease Control and the Connecticut Agricultural Experiment Station, via CDC Public Health Image Library)

Kristi Miley, Ph.D., uses a “vacuum aspirator,” a modified handheld vacuum fitted with soft bath scrubbers to snugly fit and help extract mosquitoes from a resting shelter (black tube). The resting shelter attracts female mosquitoes that have blood-fed and are seeking a place for egg maturation. She credits Nathan Burkett-Cadena, Ph.D., from the the University of Florida’s Florida Medical Entomology Laboratory for sharing the vacuum aspirator design. (Photo courtesy of Thomas Unnasch, Ph.D.)

Kristi Miley, Ph.D., uses a “vacuum aspirator,” a modified handheld vacuum fitted with soft bath scrubbers to snugly fit and help extract mosquitoes from a resting shelter (black tube). The resting shelter attracts female mosquitoes that have blood-fed and are seeking a place for egg maturation. She credits Nathan Burkett-Cadena, Ph.D., from the the University of Florida’s Florida Medical Entomology Laboratory for sharing the vacuum aspirator design. (Photo courtesy of Thomas Unnasch, Ph.D.)

The researchers were able to confirm not only that Cs. melanura mosquitoes were clearly the primary vector of the virus but also that their major bird host was the Northern Cardinal. “In addition, we found that two habitats were really high risk for EEEV infestation: hardwood outland swamps, so basically wetlands with hardwoods and not cypress; and tree plantations,” Unnasch says.

By taking a closer look at the two habitats, the researchers identified a similar mosquito-enticing feature: wet tree holes that persist even in Florida’s dry winter season. “Those tree holes are apparently the key that allows Culiseta melanura to really get going and why there are much more of them at high-risk sites than low-risk sites,” he says.

Putting Findings to the Test

The findings present a promising control opportunity. Instead of mosquito-control teams beginning to apply insecticides when mosquito numbers escalate and EEEV infection starts to appear in the sentinel chickens, which is usually in late spring, the researchers suggested in the paper that preemptive spot treatments in wintering tree holes might make sense. “Our question was: Can we knock down that Culiseta melanura population, interrupt transmission at those hotspots, and then prevent the population from peaking in May or June, which is also about time when a lot of birds are migrating through and heading up to New York and Massachusetts?” Unnasch says. “If so, we might be able to block the movement of the virus north and eliminate it throughout all of the northeastern United States as well.”

The researchers centered their investigation on three Florida counties, as shown in the top image: Citrus (a), Hernando (b) and Walton (c), all of which had experienced EEEV transmission in recent years. Through a comparison of sites with a high risk (black dots) of infestation with EEEV-carrying mosquitoes and those with a low risk (stars), they were able to identify the preferred habitat of the mosquitoes, especially during the winter when mosquito numbers are low and the insects are concentrated in smaller areas. (Image originally published in Miley et al 2021, Journal of Medical Entomology)

The researchers also looked at the variety of Culiseta melanura mosquitoes’ hosts and found they fed mainly on birds overall, preferentially on northern cardinals. Note: Chicken data is skewed because researchers place chicken coops in the field to determine infection rates. All other listed animals are naturally occurring. (Image originally published in Miley et al 2021, Journal of Medical Entomology)

Usually, ideas suggested in newly published studies are not put to the test immediately, but when it comes to mosquitoes in Florida, “many of the mosquito-control programs will jump right on it and are more than happy to try things out,” Unnasch says. In fact, Hernando County mosquito-control teams have done just that with the new findings, and they just finished winter spot insecticide applications in tree holes at a high-risk site.

“It’s very rewarding to be able to figure out a little bit more about the biology and ecology of the mosquitoes and also have a lever to use the new knowledge hopefully to improve our control measures and curb EEEV,” Unnasch says. “And, thanks to the mosquito-control people here, who are really great, collaborative and open to new ideas, we’ll know not very long from now whether this idea actually works.”

Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.