We examined deposition of eastern equine encephalomyelitis virus (EEEV) in the alimentary tract of its enzootic mosquito vector, Culiseta melanura, to detect potential sites of initial infection. Artificial viremias were created by injecting purified [3H]uridine-labeled EEEV intravenously into one-day-old chicks. Mosquitoes were allowed to engorge, incubated 1–2 hr, fixed, and whole abdomens and thoraces of 25 specimens embedded. Tagmata were sectioned, mounted on slides and coated with autoradiographic emulsion. Following exposures of 1–4 months, slides were developed and examined microscopically.
In the majority of mosquitoes, label (virus) was detected only within the midgut; most virus was concentrated in a band of expressed serum adjacent to the abdominal midgut epithelium. Small amounts of virus were also deposited within folds in the cardial thoracic midgut of 96% of mosquitoes. Viral penetration into epithelial cells was detected throughout the abdominal midgut in all mosquitoes, and in the thoracic midgut of 20%. A small number (4/25) of mosquitoes examined showed signs of leaky abdominal midguts, with virus detected in the abdominal hemocoel.
Concentration of EEEV in expressed serum adjacent to the abdominal midgut epithelium may enhance initial midgut infection. Leaky abdominal midguts exhibited by some mosquitoes may also facilitate rapid systemic infections of Cs. melanura. Deposition of virus in thoracic alimentary tissues suggests the possibility of early EEEV infection of the anterior midgut.