The course of St. Louis encephalitis virus infection of Culex pipiens pipiens Linn, mosquitoes was followed sequentially by electron microscopy. At the site of initial viral invasion of mosquito parenchyma in the midgut, epithelial infection involved a rather constant proportion of cells that yielded only moderate numbers of virus particles. Virus was observed in the midgut at locations where spread via the hemolymph could occur. Tissues in intimate contact with hemolymph (abdominal muscles, malpighian tubules, ovarian sheath) became infected, but only modest numbers of virus particles were ever produced. In sharp contrast, an ever increasing number of of virus particles were formed in the epithelial cells of the salivary glands. Virus was primarily yielded into the cisternae of endoplasmic reticulum and then shed from the apical end of cells into the lumen of the glands. Very few particles were associated with lateral or basal margins of salivary gland epithelium, indicating a directional "preference" of virus for shedding through apical plasma membrane. So much virus was shed into the limited space of the glandular lumen and its diverticula that dispersed particles formed into crystalline arrays from day 25 onward; one of the larger of these crystals was estimated to contain more than 50,000 virus particles. Changes in the infected cells of all the mosquito organs examined were interpreted as physiologic variances relative to feeding time and not as specifically due to viral cytopathology.
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