Researchers Estimate Optimal Temperature for Aedes Arbovirus Transmission

Researchers at Stanford University are mapping how changes in temperature can impact the length of mosquito season geographically, and the optimal temperature at which mosquitoes contract and transmit viruses.

In a study published in PLOS Neglected Tropical Diseases, the researchers investigated how temperature influences the spread of Zika, Dengue, and Chikungunya. Specifically, the researchers analyzed the “seasonal and geographic range of transmission” of diseases spread by Aedes aegypti and Ae. albopictus mosquitoes. They also calculated how transmission of the arboviruses spread by these mosquitoes would change based on temperature. In addition, the researchers used human case data from regions with differing climates to back their findings.

Commenting on the aim of the study, lead author, Erin Mordecai, PhD, assistant professor of Biology at Stanford University, said in a press release, “Dengue epidemics have been on the rise in the past couple decades, so, there’s been a growing effort trying to understand why we’re seeing more Dengue, and what the relationship is between Dengue transmission and climate.” In fact, Euronews reports that the World Health Organization recently declared a Dengue Fever epidemic in Peru, after more than 3,000 cases were confirmed and 19 deaths were linked to the virus on May 16, 2017.

However, that is not to say that other arboviruses are not causing panic. The recent Zika epidemic has also caused much concern. Last year, the virus traveled northward from South America, where it is endemic, and was found to be locally transmitted in two US states. In addition, a news outlet recently reported that Zika cases in Cuba are on the rise this year.

According to the PLOS study, mosquito season lasts for only 3 months in temperate climates, whereas it can last even longer in tropical and subtropical areas. For this reason, the researchers argue, areas in temperate climate zones are less likely to experience major epidemics of mosquito-borne viruses. This is because temperature can dictate the length of time it takes for a mosquito to acquire a virus from one feeding, and transmit it in another. Temperature can also control mosquito life-cycle, as well as how often a mosquito feeds, which the researchers outline in the study. Dr. Mordecai notes, however, that although temperature can affect these traits, they “tend to be nonlinear.”

The research findings revealed that mosquitoes were more likely to acquire or transmit a disease in temperatures of 29.1°C, or 84.38°F, than in colder or warmer temperatures. Therefore, in areas where temperatures are lower or higher than the optimal transmission temperature, it is likely that the transmission rate will decrease.

The researchers looked at the “human transmission occurence and incidence data, respectively, by country-week in the Americas and the Caribbean” between 2014 and 2016. When comparing their estimates of how temperature would impact transmission rate with actual data regarding human transmission rates from these regions, the researchers found that the results matched up. They wrote, “Although predicted [relationship between temperature and transmission] correlated with the observed occurrence and magnitude of human incidince for all three viruses, these observed indicdence metrics were higher for [Dengue] than for [Chikungunya] and [Zika].”

Dr. Mordecai and her team believe that their research can help estimate potential outbreaks of arboviral diseases. In addition, the authors concluded, “Mechanistic models like the one presented here are useful for extrapolating the potential geographic range of transmission beyond the current envelope of environmental conditions in which transmission occurs (eg, under climate change and for newly invading pathogens).”

Dr. Mordecai and her research team hope that these findings can help prevent future epidemics like that caused by Zika. “There's lots of discussion about what's going to be the next thing. What's the next Zika?” she said. “We really want to build more predictive models that take climate information and make predictions about when and where we can invest in vector control to try to prevent epidemics.”

Building these models may provide an avenue to predicting epidemics of mosquito-borne diseases before they happen. For regions with low socioeconomic standing, where Dr. Mordecai notes that there are increased rates of Zika, Dengue, and Chikungunya, these predictions may incite advanced vector-control efforts, thus hindering future epidemics.