Effects of Initial Dose on Eastern Equine Encephalomyelitis Virus Dependent Mortality in Intrathoracically Inoculated <I>Culiseta melanura</I> (Diptera: Culicidae)

Abstract

Viral growth characteristics that favor rapid and prodigious virion production may increase virus transmission but be detrimental to infected hosts. Several arboviruses, including eastern equine encephalomyelitis (EEE) virus, negatively affect the survival of their infected mosquito vectors. To test the hypothesis that the mosquito virulent properties of EEE virus are caused by the presence of intrinsic viral growth properties, we investigated the effects of infecting dose on the survival of intrathoracically inoculated Culiseta melanura (Coquillett). Daily survival of age-matched females inoculated with either a low initial dose of 10(1.5) plaque-forming units (PFUs) per mosquito or a high initial dose of 10(5.5) PFUs per mosquito was monitored for 8 wk. Compared with diluent inoculated controls, mosquitoes from both dosage groups displayed highly significant decreases in survival. No significant differences in daily survival were detected between the two infected groups. Virus production within inoculated mosquitoes was assessed by sampling mosquitoes every 12 h for 96 h after inoculation. Rapid virus amplification occurred in both dosage groups, and by 24 h after exposure the mean viral loads in mosquitoes inoculated with the low dose were comparable to those inoculated with the high dose. Likewise, although detectable virions appeared sooner in the saliva of high dosage mosquitoes, by 72 h after inoculation no significant differences in virus transmission were detected between the two exposure groups. These results indicate that the virulence of EEE virus for its enzootic North American mosquito vector is not dosage dependent and likely reflects the inherent growth properties of this virus within infected mosquitoes.