Implication of sexual transmission of Zika on dengue and Zika outbreaks.

Dengue and Zika viruses belong to the same Flavivirus family and usually cocirculate within the same area. Both the viruses can be transmitted by a common mosquito species Aedes aegypti. However, non-vector-borne transmission of Zika virus, such as sexual transmission and vertical transmission, has been reported in recent studies. In this study, we develop a dengue-Zika coinfection model with a particular focus on the impact of Zika sexual transmission to the transmission dynamics of both dengue and Zika. Our sensitivity analysis shows that Zika sexual transmission has a significant influence on the Zika basic reproduction number. Consequently, Zika sexual transmission can lead Zika to be endemic within an area where vector-borne transmission only cannot. Theoretically, we prove that the disease-free equilibrium for dengue only model is always globally stable if the dengue basic reproduction number is less than 1. However, our cascade analysis and numerical simulations show that increasing the sexual transmission coefficient of Zika can also result in the persistence of dengue even though the dengue basic reproduction number is less than 1, due to the cocirculation of dengue and Zika and the antibody-dependent enhancement of Zika infection for dengue infection. Our numerical analyses also show that the endemic levels of Zika increase as the Zika sexual transmission probability increases.