Larval Exposure to the Bacterial Insecticide Bti Enhances Dengue Virus Susceptibility of Adult Aedes aegypti Mosquitoes

Abstract

Understanding the interactions between pathogens sharing the same host can be complicated for holometabolous animals when larval and adult stages are exposed to distinct pathogens. In medically important insect vectors, the effect of pathogen exposure at the larval stage may influence susceptibility to human pathogens at the adult stage. We addressed this hypothesis in the mosquito <i>Aedes aegypti</i>, a major vector of arthropod-borne viruses (arboviruses), such as the dengue virus (DENV) and the chikungunya virus (CHIKV). We experimentally assessed the consequences of sub-lethal exposure to the bacterial pathogen <i>Bacillus thuringiensis</i> subsp. <i>israelensis</i> (<i>Bti</i>), during larval development, on arbovirus susceptibility at the adult stage in three <i>Ae. aegypti</i> strains that differ in their genetic resistance to <i>Bti</i>. We found that larval exposure to <i>Bti</i> significantly increased DENV susceptibility, but not CHIKV susceptibility, in the <i>Bti</i>-resistant strains. However, there was no major difference in the baseline arbovirus susceptibility between the <i>Bti</i>-resistant strains and their <i>Bti</i>-susceptible parental strain. Although the generality of our results remains to be tested with additional arbovirus strains, this study supports the idea that the outcome of an infection by a pathogen depends on other pathogens sharing the same host even when they do not affect the same life stage of the host. Our findings may also have implications for <i>Bti</i> as a mosquito biocontrol agent, indicating that the sub-optimal <i>Bti</i> efficacy may have counter-productive effects by increasing vector competence, at least for some combinations of arbovirus and mosquito strains.