It may seem like the result of just one tiny bite, but dengue transmission is in fact, a complex issue. Many factors affect the dynamics of dengue transmission: local vector control efforts, socio-economic conditions, cultural behaviors, local infrastructure, and climate change, to name a few.
A multi-disciplinary team from the National Center for Atmospheric Research (NCAR) and the University of Arizona has been investigating dengue transmission patterns in cities with high vector populations along the US-Mexico border. Dr. Kacey Ernst, an epidemiologist and Associate Professor in the Epidemiology and Biostatistics Department at the University of Arizona, and Dr. Mary Hayden, a behavioral scientist at the Research Applications Laboratory at NCAR, are leading the research at their respective institutions.
Scientists have been aware for some time that the dengue-transmitting Aedes aegypti mosquito has been moving north from Mexico into the US. Why are some areas of the US seeing sporadic local dengue transmission while others – some with large Aedes aegypti populations – are seeing none?
Climate change, dengue and the tales of two US cities
Past research had concluded that social factors prevented dengue emergence in the US, while lower standards of living were responsible for the high levels of dengue transmission in Mexico. The research team was not convinced this was entirely true.
[image courtesy of the University of Arizona and NCAR]
So, in 2012, to determine if other factors were coming into play, they compared two cities in the US – Key West, Florida, and Tucson, Arizona. Both cities had abundant Aedes aegypti populations, the first with dengue transmission and the second without. Could environmental aspects or human behaviors explain the emergence of dengue in some cities but not others?
“So why didn’t we see dengue in Tucson?” asks Dr. Hayden. “The potential for it to emerge was clearly there because of the city’s proximity to the US-Mexico border. Approximately 13,000 people a day cross the border from Mexico into the US – many from dengue-endemic countries.”
Although, both Tucson and Key West have the vector and are in locations where there is frequent travel from dengue endemic countries (Key West from Caribbean travel), Tucson had yet to see locally acquired cases of dengue. This led the team to draw the hypothesis that the climate was unsuitable for mosquitoes in the Arizona city. “If mosquitoes are not surviving long enough to acquire and then develop the virus to the point where they can infect someone, the transmission cycle will not be completed,” Dr. Ernst comments.
A similar story in Mexico
The hypothesis needed to be confirmed. More recently, the team compared two Mexican cities south of the border. Again, both had the vector in abundance, but only one had local dengue transmission.
“Nogales, a Mexican city just south of the border, has few of the infrastructural amenities found in the US yet still has no local dengue transmission,” reveals Dr. Ernst. “Hermosillo, a larger Mexican city around 200 kilometers further south and with fewer Aedes aegypti mosquitoes and somewhat better infrastructure, is endemic.”
Convinced that social factors were not behind the absence of dengue transmission in Nogales, the study team set about determining whether their earlier hypothesis was correct.
The study team has collected wild adult Aedes aegypti mosquitoes across six sites near the US-Mexico border. “The entomology lab at the University of Arizona, [run by Drs. Mike Riehle and Kathleen Walker] is aging the mosquitoes we catch to see whether they live long enough to survive the extrinsic incubation period for the virus (the time it takes for a mosquito that bites an infectious person to be able to transmit dengue virus to another person),” comments Dr. Hayden.
By aging the mosquitoes, the lab will be able to determine whether climate is one factor limiting dengue transmission cycles. They expect to find that the mosquitoes collected from sites with higher humidity and warmer temperatures – but not too hot — will be older than those collected from sites with cooler, drier conditions.
By aging the mosquitoes, the lab will be able to determine whether climate is limiting dengue transmission cycles. They expect to find that the mosquitoes collected from sites with higher humidity and warmer temperatures – but not too hot — will be older than those collected from sites with cooler, drier conditions.
Dr. Ernst explains how temperature might limit the cycles. “The critical survival length is a range. The incubation period the virus needs to mature is temperature-dependent. So the hotter it is the shorter the time they need to live. But if you reach really high temperatures, then it is detrimental to the mosquitoes’ survival.”
They also expect to find mosquitoes surviving to an older age in households with more vegetation, more people and indoor resting sites, more water storage, no competing vector species, and no control methods.
New models could help predict dengue transmission patterns
Another member of the NCAR team, meteorologist Dr. Andrew Monaghan, has worked with models demonstrating that areas with cooler and drier temperatures will result in fewer mosquitoes surviving past the incubation periods. The models also demonstrate the climatic thresholds at which this will occur. The team hopes this will also help prove their hypothesis.
“We can use models for surveillance and to better understand dengue transmission,” he explains. “Models can help us answer some of those questions where field work is too costly.”
A previous surveillance project, for example, strung together multiple models to produce a map of dengue risk. The first, a sophisticated weather model, generated fine-scale hourly weather fields. The second took data from a container distribution survey, along with the weather fields, to simulate the water height and temperature in those containers where mosquitoes spend their immature stages. The final two models simulated the Aedes aegypti lifecycles and dengue transmission, resulting in a dengue risk map.
When the team investigated the differences between Nogales and Hermosillo, Dr. Monaghan and a colleague from NASA, Dr. Cory Morin, then used a model to simulate some of the larger dengue outbreaks in Hermosillo over recent years. “We then used the climate data from Nogales to drive the exact same model and found we were not able to simulate dengue transmission,” he adds. The team then dug into the model to find out why.
The results are currently undergoing peer review, so we’ll have to wait to find out the answer. But if the hypothesis is correct, increased temperatures driven by climate change could have serious implications for transmission in regions on the margins of dengue epidemics.
Could this mean even faster growth of the dengue issue in the coming years?
Learn more about the lasting effects of dengue below