Why do female Aedes aegypti (Diptera : Culicidae) feed preferentially and frequently on human blood?

Adult female Aedes aegypti (L.), the vector of dengue and yellow fever viru ses. have an affinity for feeding on human blood and a tendency to forego f eeding on sugar. This observation challenges two tenets of mosquito biology : ii) mosquitoes imbibe plant carbohydrates for synthesis of energy reserve s and blood for reproduction and (2) egg production is reduced when mosquit oes feed on human blood compared with blood from other species. Sub-optimal amounts of the amino acid isoleucine in human blood (particularly free iso leucine in plasma) are thought to be responsible for lowered egg production when human blood is ingested. We tested the hypothesis that feeding on hum an blood is associated with a selective advantage for Ae. aegypti and is an underlying reason for this mosquito's intimate and epidemiologically impor tant relationship with human beings. Our five experiments examined the effe cts of different isoleucine concentrations on accumulated energy reserves, frequency of host contact, survival, and egg production. When mosquitoes im bibed blood meals over a 7- to 10-d period and were not fed sugar. increase d isoleucine concentration decreased energy reserves and did not increase e gg production. Aedes aegypti took smaller but more frequent blood meals whe n feeding on a low-isoleucine human host daily compared with a high-isoleuc ine mouse host. Previous reports that isoleucine enhances egg production we re confirmed only when females were fed sugar, an unusual behavior for most domestic Ae. aegypti populations. Females fed human blood and water had gr eater age-specific survival (l(x)), reproductive output (m(x)), and cumulat ive net replacement (R-0) than cohorts fed human blood plus sugar or isoleu cine-rich mouse blood with or without access to sugar. The unique isoleucin e concentration of human blood is associated with Ae, aegypti's unusual pro pensity to feed preferentially and frequently on humans-a behavior that inc reases this mosquito's fitness. synthesis of energy reserves, and contact w ith human hosts, making it an especially effective disseminator of human pa thogens.