Modeling the Environmental Suitability for Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus (Diptera: Culicidae) in the Contiguous United States

Abstract

The mosquitoes Aedes (Stegomyia) aegypti (L.)(Diptera:Culicidae) and Ae. (Stegomyia) albopictus (Skuse) (Diptera:Culicidae) transmit dengue, chikungunya, and Zika viruses and represent a growing public health threat in parts of the United States where they are established. To complement existing mosquito presence records based on discontinuous, non-systematic surveillance efforts, we developed county-scale environmental suitability maps for both species using maximum entropy modeling to fit climatic variables to county presence records from 1960-2016 in the contiguous United States. The predictive models for Ae. aegypti and Ae. albopictus had an overall accuracy of 0.84 and 0.85, respectively. Cumulative growing degree days (GDDs) during the winter months, an indicator of overall warmth, was the most important predictive variable for both species and was positively associated with environmental suitability. The number (percentage) of counties classified as environmentally suitable, based on models with 90 or 99% sensitivity, ranged from 1,443 (46%) to 2,209 (71%) for Ae. aegypti and from 1,726 (55%) to 2,329 (75%) for Ae. albopictus. Increasing model sensitivity results in more counties classified as suitable, at least for summer survival, from which there are no mosquito records. We anticipate that Ae. aegypti and Ae. albopictus will be found more commonly in counties classified as suitable based on the lower 90% sensitivity threshold compared with the higher 99% threshold. Counties predicted suitable with 90% sensitivity should therefore be a top priority for expanded mosquito surveillance efforts while still keeping in mind that Ae. aegypti and Ae. albopictus may be introduced, via accidental transport of eggs or immatures, and potentially proliferate during the warmest part of the year anywhere within the geographic areas delineated by the 99% sensitivity model.

Keywords: Aedes aegypti; Aedes albopictus; MaxEnt; environmental suitability; risk model.

Fig. 1

Modeled suitability maps for (a) Ae. aegypti and (b) Ae. albopictus. The red points show the presence records from 1960 to 2016 used to build the models. Darker shading indicates a higher probability of suitability, as described in the text. Cut-off probabilities for Ae. aegypti are 0.36, 0.22, and 0.057 and for Ae. albopictus are 0.33, 0.25, and 0.03, for the 90, 95, and 99% sensitivity thresholds, respectively.

Fig. 2

Model response curves for Ae. aegypti suitability in relation to (a) winter GDDs and (b) maximum temperature during the warmest month; and for Ae. albopictus suitability in relation to (c) winter GDDs and (d) precipitation during the driest month. Each curve shows the mean response of the ten replicate MaxEnt runs (black line) and the mean ± 1 standard deviation (dark shading) and represents how the logistic probability changes as each variable is changed while keeping all other variables at their average sample value.

Fig. 3

Environmental suitability models based on 99% sensitivity and presence records from 1995 to 2016 (Hahn et al. 2017) for (a) Ae. aegypti and (b) Ae. albopictus.