Western equine encephalomyelitis virus: in vivo infection and morphogenesis in mosquito mesenteronal epithelial cells

The infection and morphogenetic events associated with the replication of Western equine encephalomyelitis (WEE) virus within the mesenterons of Aedes dorsalis and three strains of Culex tarsalis are compared and contrasted. WEE virus apparently penetrates mesenteronal epithelial cells in vivo through membrane fusion. Profiles of apparent membrane fusion events were observed between virus particles and the microvillar surface of the mesenteron and naked nucleocapsids are observed intracellularly along the apical margin of the mesenteronal epithelial cell within 3 h of ingestion of the bloodmeal. Further, no viral particles were found in association with endocytotic nor lysosomal vacuoles during the initial phases of infection. In those strains of Cx. tarsalis that supported viral replication and in Ae. dorsalis, accumulations of nucleocapsids and maturation of WEE virus were evident along basolateral membranes of the mesenteron by 22-24 h after ingestion of the blood-meal. Maximal extracellular nascent virus occurred between 30-36 hrs. The Knights Landing strain of Cx. tarsalis revealed no subcellular morphological alteration in response to infection throughout the period of study. However, distinct morphological structures associated with the infection were observed in strains or species with enhanced susceptibility compared to Knights Landing (i.e., Cx. tarsalis WS-3 and Ae. dorsalis). In both, apical accumulations of nucleocapsids were apparent by 29 h post infection. These nucleocapsids were most often embedded in a rather amorphous matrix and occasionally in association with membrane profiles; presumably endoplasmic reticulum. Ae. dorsalis also demonstrated some alterations in response to WEE viral infection that were unique relative to Cx. tarsalis and some of these may be considered cytopathological. First, progeny virions were observed repeatedly within lysosomal figures. Second, extensive cytoplasmic vacuolization was noted and occasionally it appeared that these vacuolated cells were being sloughed off into the lumen of the mesenteron.