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To determine whether a Sindbis virus (family Togaviridae, genus Alphavirus, SINV) infection in Aedes aegypti (L.) (Diptera: Culicidae) affected its response to the repellent DEET, we orally exposed Ae. aegypti to an artificial bloodmeal containing SINV or diluent and then allowed to feed on a 10% sucrose suspension containing 3% DEET. When tested seven or more days after the initial bloodmeal, although none of the diluent-exposed mosquitoes fed on the DEET-sucrose suspension, at least 60% of the SINV-exposed mosquitoes fed on the suspension. When legs from the SINV-exposed mosquitoes were tested to determine dissemination status, 89% of those that fed on the DEET-sucrose suspension contained virus. In contrast, only 34% of the nonfeeders had a disseminated infection. When offered a choice between sucrose with or without DEET, a significantly higher percentage of the SINV-exposed mosquitoes than the control mosquitoes fed on the sucrose containing 3% DEET. Together, these results indicate that mosquitoes with a disseminated SINV infection may be less responsive to DEET than uninfected mosquitoes. Therefore, repellent use may be less effective in deterring infected mosquitoes from biting than previously believed.

Differential host cell responses to the alphavirus Sindbis were observed in visceral muscles of the adult female mosquito Aedes albopictus. Following intrathoracic inoculation with SIN, muscles associated with the midgut, hindgut, and ovary resulted in clearance, persistence, and refractoriness to virus, respectively. Prominent sarcomeres characteristic of myofilaments were identified in muscles associated with these three organs by phalloidin labeling of actin, confirming these cells as muscle. The location of virus antigen mimicked the distribution of actin in both mid- and hindgut-associated muscles. Furthermore, these myofilaments remained intact following virus clearance from midgut muscles and during virus persistence in hindgut muscles. Changes in the temporal onset of virus antigen following high titer inoculum compared with standard titer inoculum was observed in anterior midgut muscles, but not in muscles associated with the posterior midgut or hindgut. Muscle bundles closely approximated the gut surface, while a wispy association was displayed at the ovary surface. Prominent ultrastructural differences were observed in the basal lamina attached to the gut compared with the ovary. Additionally, ultrastructural evidence for virus-associated pathology was observed in gut-associated muscles and gut epithelium. Visceral muscles, all composed of the same tissue type, but associated to three different organs in the insect abdomen, responded differentially to Sindbis. We speculate that variations in structure, function or physiology and ultrastructure inherent to insect host cells or organs interactions reflect the complicated milieu of the organism and contribute to differential virus phenotypic expression in muscle cells.

Virus dissemination and associated pathology were examined in Aedes albopictus after intrathoracic inoculation of Sindbis virus (SIN), the prototypic Alphavirus. At 10 days postinfection, virus RNA was detected in all three-body segments of the insect. Colocalization of virus antigen with structural pathology was observed in mosquito salivary glands and midgut-associated visceral muscles, representing yet another example of arbovirus-associated pathology in a mosquito host. SIN antigen and gross pathology were detected in lateral lobes, but not the medial lobe of salivary glands, whereas virus antigen, vacuolated cytoplasm, and myofilament misalignment were detected in the visceral muscles at the midgut exterior surface. Early in the midgut infection, virus antigen was localized in small foci on the organ surface that progressed to a grate work-like banding pattern that eventually cleared. Both the salivary glands and the midgut are essential to insect survival and reproduction. Additionally, these organs provide a pathway for virus transmission in nature. Although SIN infection may not shorten the mosquito life span, persistent coexistence could permit survival of both host and microbe as well as contribute to alterations in insect behavior.