Variation in Infectivity of Saint Louis Encephalitis Viral Strains for Culex Pipiens Quinquefasciatus (Diptera: Culicidae)

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Abstract
Only 6 isolations of Saint Louis encephalitis (SLE) virus have been made from Argentina. These include 2 from patients with undifferentiated fever, 2 from rodents (CorAn-9124 and CorAn-9275), and 2 from mosquitoes (78V-6507 and 79V-2533). The infectivity of the last 4 strains for colonized Culex pipiens quinquefasciatus from Argentina was determined following intrathoracic inoculation of virus or by feeding mosquitoes on virus-laden suspensions. Comparison of the replication and dissemination of viral strains CorAn-9275 and 79V-2533 was made by serially dissecting orally infected mosquitoes and by testing heads, alimentary tracts, and body remnants separately. Each of the 4 viral strains replicated to high titer in Cx. p. quinquefasciatus following intrathoracic inoculation; however, day-20 infection rates never exceeded 5.1% in groups of mosquitoes fed on suspensions of CorAn-9124 and CorAn-9275. In contrast, infection rates were 90.9 and 87.5% in mosquitoes fed on suspensions of 78V-6507 and 79V-2533, and oral ID50's were 103.5 and 102.9 Vero cell PFU, respectively. Further, CorAn-9275 did not replicate well in Cx. p. quinquefasciatus alimentary tracts (avg. titers 102.1 and 102.5 PFU/ml on days 10 and 13) and rarely passed the mesenteronal barrier. In contrast, 79V-2533 readily infected mosquitoes by the oral route, replicated to high titer (avg. 104.8 PFU/ml) in the alimentary tract, and began disseminating by day 5. Widespread dissemination of virus occurred by day 10 but virus spread and multiplication continued at least through day 13, by which time viral antigen was detectable in head squashes of practically all mosquitoes tested. The infectivity of the 4 viral strains for Cx. p. quinquefasciatus correlates well with published reports on the virulence characteristics of these viral strains in vertebrates.

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