Like many successful predators, Aedes aegypti’s hunting patterns closely follow the daily rhythms of its prey.
“One of the most interesting things about them is how closely they are tied to humans and human activity,” said Kacey Ernst, an assistant professor of epidimiology in the University of Arizona’s Mel and Enid Zuckerman College of Public Health.
Ernst heads a team of UA scientists that recently received $2.3 million from the National Institutes of Health to study populations of A. aegypti that inhabit the region between Guaymas, Mexico and Southern Arizona.
Co-principal investigators with Ernst include Yves Carrière, Kathleen Walker and Michael Riehle of the UA College of Agriculture and Life Sciences, and Andrew Comrie, senior vice president for academic afairs and provost and also of the UA School of Geography and Development.
“They prefer to bite the first three hours after sunup and three hours before sundown,” said Ernst, talking about the mosquito species that carries the virus that causes dengue fever, which affects as many as 100 million people worldwide each year.
Very few cases of dengue typically are seen in the U.S., Ernst said. The most recent outbreaks occurred in Key West and Miami, Fla., and in Brownsville, Texas, but they subsided after control efforts were implemented to reduce the amount of contact people had with the disease-carrying mosquitoes.
Across the southern border, however, is a different story. “There is actually a lot of dengue transmission in Mexico even as far north as Hermosillo, only about 200 miles south of the U.S.-Mexico border,” said Ernst.
It has been suggested that the disparity in dengue transmission on either side of the U.S.-Mexico border is a result of different social factors, Ernst said.
“In Mexico, there may be fewer people that have air conditioners than in the U.S., and so their houses and workplaces may not be sealed as tightly. Also there may be areas where there is not as good of sanitation, so you may have detritus and containers that can house mosquito larvae.”
Containers such as trash cans are a favorite breeding ground of A. aegypti, Ernst said: “They can exploit even very small manmade containers that are discarded and can hold water.”
It doesn’t help that many people collect water and store it in their backyards, effectively creating little mosquito breeding ponds. “If you looked inside there could be mosquito larvae swimming around,” Ernst said.
But a lot of people in Tucson collect water as well, Ernst said, which if left out and not properly treated or covered could be a new home to mosquito larvae. “It’s not just in Mexico that you have these issues,” she said.
Ernst and her colleagues believe there are too many similarities in social and environmental conditions between areas of Tucson and parts of northern Mexico to fully explain the comparative lack of dengue transmission in the U.S.
Also, just south of the Arizona-Sonora, Mexico border in Nogales, Sonora there has been no local transmission of dengue despite the similar social conditions in Sonora and other areas of Mexico.
“Our thought is that it’s not just the social factors, particularly because Nogales, Sonora has not had reported dengue transmission within the city,” said Ernst. If social factors were the only reason for there to be more dengue outbreaks in Mexico, Ernst said, there would be cases in Nogales.
The NIH grant will allow Ernst and her colleagues to study populations of A. aegypti for four years. The team will collect mosquito larvae and adults along a geographic transect from Guaymas, Sonora, to Tucson, including Hermosillo, Magdalena and Nogales.
“We’re looking across that gradient at different factors that may influence the emergence of dengue,” Ernst said. “Transmission starts to get less and less as you go north. Why is that? What kind of climatic influences might there be that could influence the survival of A. aegypti?”
One of the team’s main hypotheses is that in the hotter and drier northern climate, the mosquitoes may not survive long enough to become infected and transmit the virus, Ernst said.
The team will collect mosquito adults and larvae to find out how many of the mosquito populations in each area of its study actually are A. aegypti, how many mosquito species are present, and their relative population sizes.
“Mike Riehle’s lab has developed a technique that allows us to determine the age of the mosquitoes caught in the field as too young to transmit, possibly able to transmit and old enough to transmit. We can compare the ages of mosquitoes across the transect,” Ernst said. The team also will investigate how easily the mosquitoes in the study area are able to transmit the virus.
The team also is investigating how climate influences survival of the mosquitoes, Ernst said. “If we can understand the relationship between climate and A. aegypti survival now, we can model how dengue risk might change as the climate changes.”
The study will involve interviews and inspections of people’s homes, Ernst said, considering environmental, climatic and social factors that could have an impact on the rate of dengue transmission. “Maybe social factors do play a role in dengue transmission, so we will be looking at them as well,” she said.
The UA team will work with the National Center for Atmospheric Research, or NCAR, and with the Universidad de Sonora in Hermosillo, Sonora, Mexico, for the research.
Diseases such as dengue, Ernst said, can “geographically creep across a landscape. Places in closer proximity are generally more similar, so if you have a place that can sustain transmission right next to a place that is on the edge then it’s more likely you will have emergence of the disease.”
Understanding the factors that influence dengue transmission is especially important in Southern Arizona considering the proximity to areas where severe outbreaks sometimes occur, Ernst said. “Because of that proximity, we have a greater potential for the A. aegypti to act as disease vectors here. We have a lot of people coming to and from areas with greater rates of transmission. Border crossings are in the thousands per day where you have people crossing from Arizona into Mexico and vice versa, so the chance of transmission of dengue here is greater.”
“There is no vaccine and no treatment for dengue,” said Ernst. The only real control measures are reducing the number of mosquitoes and the amount of contact people have with them, she said.
Removal of standing water that could become a mosquito breeding ground is one of the first control measures to prevent an outbreak, Ernst said. “The most important thing is to remove containers that hold water or to make sure that items like rain barrels are covered so that the mosquitoes cannot lay their eggs in them, or to treat barrels with mosquito dunks.”
Mosquito dunks can be found at most hardware stores, Ernst said, and contain a bacteria species that is toxic only to mosquito larvae. “This allows you to still have your water harvesting containers without producing more mosquitoes,” she said.
“Understanding whether or not there is a potential for emergence here in Tucson is incredibly important for public health planning and for control strategies that might need to be put in place, and for community awareness,” Ernst said. “Having that awareness is essential to community preparedness in the event that the disease emerges here.”