Observations leading to the investigation of common indigenous wild birds of the Nile Delta as potential circulating reservoirs of West Nile virus are outlined. Methods are described for accomplishing a serological survey of the six commonest avian species, namely: house sparrow, Passer domesticus; hooded crow, Corvus corone sardonicus; buff-backed heron, Bubulcus ibis; palm dove, Streptopelia senegalensis; kestrel, Falco tinnunculus; and rock pigeon, Columba livia. Neutralization testing of more than 500 specimens of avian sera showed a significant incidence of neutralizing antibodies in all six species collected in the West Nile virus-endemic area.
The hooded crow showed a remarkably high incidence of immunity—more than 80 per cent in several localities. In the Sindbis area, the positive incidence in this species diminished in the late spring collection, probably as a reflection of the new nonimmune fledgelings introduced during the nesting season, just prior to the most active mosquito transmission period.
To test the implications of this serological survey, experimental infection by feeding of West Nile virus-infected Culex pipiens and Culex univittatus mosquitoes on specimens of the common avian species was carried out. Susceptibility of all the species was demonstrated by measurement of the titer and duration of the postinoculation virus circulation. Similarity of susceptibility and virus circulating capacity of individuals in the same species was consistently demonstrated. The number of infective mosquito feedings had no relation to titer or duration of virus circulation, one bite being as effective as many. Mosquitoes were infected by feeding on the infected birds, and subsequently transmitted the virus.
Evidence for specificity of the neutralizing antibody against West Nile virus is given. Variable circulating virus titer and virulence relative to strain and passage level were verified.
The hooded crow proved to be the best virus circulator in titer as well as duration. The bird-mosquito-bird cycle of transmission was easily demonstrated with the hooded crow, house sparrow, and buff-backed heron, as an example of what may occur under natural conditions in the endemic environment of the Nile Delta of Egypt.
Staff member, The Rockefeller Foundation Virus Laboratories, New York, assigned to the Department of Virology, NAMRU-3, Cairo, Egypt.
CDR MSC USN, Department of Entomology.
Consultant, The Rockefeller Foundation Virus Laboratories, New York, assigned to the Department of Virology, NAMRU-3, Cairo, Egypt.