Everybody knows she’s a femme fatale: What are the ecological conditions that produce good disease vectors?

Background/Question/Methods

Theory on vector-borne disease predicts that long-lived mosquitoes have the greatest potential to transmit disease.  All other things being equal small female mosquitoes, emerging from crowded larval conditions, would be shortest-lived and therefore would be predicted to be least likely to be vectors.   Recent laboratory investigations with Aedes mosquitoes have shown, however, that crowded larval environments, with strong competition among larvae, yield small adults that are more susceptible to arthropod-borne viruses, and are more likely to become competent vectors than larger adults  from uncrowded larval environments. Thus, there are two alternative hypotheses for how crowded larval conditions affect frequency of infection:   The hypothesis that crowded conditions shorten adult life predicts that probability of virus infection increases with adult size.  In contrast, the hypothesis that crowded conditions raise adult susceptibility to infection predicts that probability of virus infection decreases with adult size.  We tested these hypotheses by assessing the relationship of adult female body size for Aedes aegypti to probability of being infected with Dengue in the field in Rio de Janiero, Brazil, during a Dengue outbreak.  We measured wings of 386 field collected females from 24 sites with active Dengue transmission and then determined whether individuals were infected with Dengue using RT-PCR.

Results/Conclusions

Logistic regression of infection vs. date of collection and wing length showed that wing length was significantly and positively related to Dengue infection.  Collection date was not significantly related to Dengue infection.  Among sites, mean wing length of females was not significantly related to proportion of females infected with Dengue.  Thus, field data are inconsistent with the hypothesis that crowded larval conditions raise adult susceptibility to Dengue, and are consistent with the hypothesis that crowded larval conditions render adults less likely to be vectors.  In a laboratory study we tested whether size was related to longevity using two strains of Aedes aegypti raised in uncrowded conditions and offered Dengue-infected or uninfected blood meals.  Proportional hazards analysis showed that Dengue infected blood meals significantly reduced female longevity.  Longevity had a quadratic relationship to size, first increasing, then decreasing with size.  These results are consistent with the smallest adults being shortest-lived, and thus less important as vectors of Dengue. Our results indicate that Aedes aegypti adults derived from uncrowded, well-fed larval rearing environments are likely to be most important as vectors of disease.