Modelling the effects of adult emergence on the surveillance and age distribution of medically important mosquitoes

Entomological surveillance is an important component of mosquito-borne disease control. Mosquito abundance, infection prevalence and the entomological inoculation rate are the most widely reported entomological metrics, although these data are notoriously noisy and difficult to interpret. For many infections, only older mosquitoes are infectious, which is why, in part, vector control tools that reduce mosquito life expectancy have been so successful. The age structure of wild mosquitoes has been proposed as a metric to assess the effectiveness of interventions that kill adult mosquitoes, and age grading tools are becoming increasingly advanced. Mosquito populations show seasonal dynamics with temporal fluctuations. How seasonal changes in adult mosquito emergence and vector control could affect the mosquito age distribution or other important metrics is unclear. We develop stochastic mathematical models of mosquito population dynamics to show how variability in mosquito emergence causes substantial heterogeneity in the mosquito age distribution, with low frequency, positively autocorrelated changes in emergence being the most important driver of this variability. Fitting a population model to mosquito abundance data collected in experimental hut trials indicates these dynamics are likely to exist in wild Anopheles gambiae populations. Incorporating age structuring into an established compartmental model of mosquito dynamics and vector control, indicates that the use of mosquito age as a metric to assess the efficacy of vector-control tools will require an understanding of underlying variability in mosquito ages, with the mean age and other entomological metrics affected by short-term and seasonal fluctuations in mosquito emergence.Author summary: Mosquito-borne diseases, such as malaria and dengue, remain a significant cause of mortality and morbidity. Predicting changes in the spread of these diseases from mosquito data remains a challenge because only older mosquitoes can transmit important mosquito-borne diseases and mosquito abundance measurements can be highly variable. Trials that test the efficacy of vector control tools using measures in both mosquitoes and people have found the metrics are not always consistent. Methods to measure the mosquito age quickly and accurately are in development, and changes in the mosquito age distribution have therefore been proposed as a metric to measure the efficacy of novel vector control tools. We develop mathematical models of mosquito reproduction, development and ageing to show that seasonality in adult mosquito emergence can result in substantial seasonality in the mosquito age distribution and complicate the interpretation of mean mosquito age and other widely used mosquito metrics. Using data from the dominant malaria vector Anopheles gambiae we predict that even over short time-scales there is likely to be substantial variability in the abundance and age of wild mosquito populations.