Malaria disease transmission models are important tools for controlling and eliminating disease spread. However, a model is only as good as the assumptions about the various variables. Dr. Courtney Murdock, a member of the UGA’s Center for Tropical and Emerging Global Diseases and professor at Cornell University, has been studying how various biological and environmental factors influence mosquito survival. In a study recently published in the Proceedings of the Royal Society B led by graduate student Kerri Miazgowicz, Murdock and her colleagues examined several life traits, such as biting, feeding, and egg production, over the course of the life span of the mosquito Anopheles stephensi in hopes of providing better data for the models.
“Due to a lack of high-quality entomological data in general, researchers are often forced to input data from multiple disease systems to inform models in a given system or use approximations of key model components,” said Murdock.
An. stephensi is the primary mosquito species that transmits malaria in India. While most of the focus on malaria is most often associated with sub-Saharan Africa, it is widespread in the Indian subcontinent and throughout southeast Asia. Several Plasmodium species cause malaria, but P. falciparum is the deadliest of them. It has also shown drug resistance to current treatments. Control of the mosquito population is an important component in malaria control and elimination programs. Researchers need to be able to more accurately predict where mosquito populations will occur as climate changes and current territories become unsuitable living and breeding grounds. Program managers need to be prepared to incorporate more northern regions in their control efforts.
Current models rely on data that are only snapshots in time and often from multiple mosquito species, particularly the African mosquitoes, which are vectors for different malaria species. Miazgowicz, Murdock, and colleagues wanted to determine if the data from a single species of mosquito and parasite, over the course of its entire lifespan, significantly influenced current models in determining disease transmission in hopes of creating more accurate models.
The single most important factor driving current models is temperature. Mosquitoes are cold-blooded animals and therefore rely on their environment to regulate their body temperature. However, temperature is not the only factor influencing life traits. Currently, data are only available as snapshots in time. These incomplete data do not take into account for changes in mosquito behavior and life traits that occur over the course of the mosquito’s life. Murdock and her colleagues have recorded changes in biological function as the mosquito ages. Just as people slow down biologically as they age – metabolism slows, reproduction ability declines, etc. – the same is true for mosquitos. They also found that various traits peak at different times depending on temperature. Importantly they found that temperature and age significantly affected the number of females taking a blood meal (this is the means in which malaria parasites are transmitted to humans) on a given day, average daily egg production, and ultimately survival.
The findings in this study indicated that the addition of An. stephensi data yielded qualitatively different temperature-transmission suitability relationships compared to models that included multiple malaria vectors. With An. stephensi data, the model predicted a broader geographical range of temperature suitability.
“Accounting for these age and species effects in models of transmission potential alters how much of South Asia is predicted to be suitable for malaria relative to models that do not account for these factors,” said Murdock.
These findings can lead to improved malaria transmission models. However, more study outside the laboratory is needed to truly understand the impact mosquito age has on life traits and thermal tolerance.
“This study highlights a critical need for more research in natural settings characterizing the effects of age on mosquito biology to improve predictions of current and future risk,” concluded Murdock.