Arboviruses are an emerging threat in the Americas. In 2017, more than 400 symptomatic cases of Zika were observed in the United States, with more than 5000 being observed in pregnant women in Mexico. While locally transmitted dengue is not currently prevalent in the continental US, it is present in Mexico, which had thousands of confirmed dengue cases in 2017. The difference in viral presence is not entirely due to the presence of the vector, since both viruses are vectored by Aedes aegypti, which is present in part of the US. The difference in local transmission may be due to temperature-induced differences in the reproductive ratio resulting from temperature-induced differences in the extrinsic incubation period (EIP), as well as mosquito biting behavior and survival. We use an individual based model for mosquito infection and biting to find the number of infectious bites per infectious mosquito by temperature. We also use this model to compare the relative potential reproductive ratio of the three viruses in New Orleans, LA, Miami, FL, and Monterrey, Mexico.
Results/Conclusions
We find that a viral Allee effect occurs in cool temperatures, as the EIP of each virus becomes long relative to the mosquito life expectancy, and if the mosquito does not acquire the virus in its first bloodmeal (which is unlikely when the disease is scarce), it may not survive to transmit the virus. A similar effect occurs at excessively warm temperatures, as the reduction in EIP does not keep pace with the reduction in mosquito life expectancy. In Monterrey, we predict the peak of transmission to happen in the fall, when mosquitoes were relatively warm as larva and so bite relatively frequently, but are comparably cool as adults and so live longer than in summer. This corresponds to the time in the year when we see the peak of cases and the peak of biting activity. We also predict a wide window where each virus can invade in Miami, and a narrower window in New Orleans resulting from more temperature variation. These differences, coupled with the viral Allee effect, where the virus cannot invade from scarcity over a wide range of temperatures, help to explain the relative arboviral burden in these three cities.