The location of midgut bacteria relative to meconial peritrophic membranes (MPMs) and changes in bacterial numbers during midgut metamorphosis were studied in Anopheles punctipennis (Say), Culex pipiens (L.), and Aedes aegypti (L.) pupae and newly emerged adults. After adult emergence in Aedes, Anopheles, and most Culex, there were few to no bacteria in the midgut. In most newly emerged adult mosquitoes, few bacteria were found in either the lumen or within the MPMs/meconia. In a few Culex specimens, high numbers of bacteria were found in the MPMs/meconia and low numbers in the lumen. In all three species bacterial counts were high in fourth instars, decreased after final larval defecation, increased in young pupae, and increased further in old pupae. A very effective gut sterilization mechanism is operating during mosquito metamorphosis and adult emergence. This mechanism appears to involve the sequestration of remaining larval gut bacteria within the confines of the meconium and one or two MPMs and the possible bactericidal effect of the exuvial (molting) fluid, which is ingested during the process of adult emergence.

The mechanism by which arboviruses bypass the basal lamina of mosquito midgut cells and enter the body cavity has been unclear. Experiments using Venezuelan equine encephalitis viral replicon particles, which express the green fluorescent protein gene in cells, indicate the operation of tissue conduits, possibly involving tracheae and visceral muscles, that facilitate virus movement through the basal lamina. Ultrastructural studies of the midgut reveal evidence for possible complete penetration of the basal lamina by tracheal cells and regions of modified basal lamina associated with visceral muscle. The modified basal lamina closely resembles proventricular matrix material known to allow virus passage.

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The dissemination of Rift Valley fever (RVF) virus in females of the epidemic vector Culex pipiens was studied immunocytochemically. Among orally infected specimens, viral antigen was detected in all major midgut regions, although individuals varied with respect to which regions were infected. Among specimens with disseminated infections (infections beyond the midgut epithelium), antigen was detected in most tissues, including those of the nervous and endocrine systems. However, no antigen was found in the ovarian follicular epithelia, oocytes/nurse cells, or female accessory gland. A viral dissemination index (DI) based on several tissues was determined for each orally infected specimen and used to estimate the extent of viral dissemination in the hemocoel. Plots of DI values as a function of time after ingestion of an infectious blood meal indicated that dissemination was sporadic, and that once virus escapes from the midgut, its spread to other tissues is rapid. The most common sequence of tissue/organ infection following escape of virus from the midgut epithelium appears to be as follows: intussuscepted foregut, fat body, salivary glands and thoracic ganglia, epidermis, and ommatidia of the compound eyes. Reduced fecundity and survival identified in other studies of RVF virus-infected Cx. pipiens are probably explainable on the basis of extensive tissue and organ infection resulting in an overall energy drain. Infection of regulatory tissues and organs such as the ganglia of the central nervous system, neurosecretory cells, and the corpora allata may also have detrimental effects on the overall functioning of a mosquito, even in the absence of widespread infection.

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