Identification of Salivary Gland Escape Barriers to Western Equine Encephalitis Virus in the Natural Vector, Culex tarsalis

Abstract

Herein we describe a previously uninvestigated salivary gland escape barrier (SEB) in Culex tarsalis mosquitoes infected with two different strains of Western equine encephalitis virus (WEEV). The WEEV strains were originally isolated either from mosquitoes (IMP181) or a human patient (McMillan). Both IMP181 and McMillan viruses were fully able to infect the salivary glands of Culex tarsalis after intrathoracic injection as determined by expression of mCherry fluorescent protein. IMP181, however, was better adapted to transmission as measured by virus titer in saliva as well as transmission rates in infected mosquitoes. We used chimeric recombinant WEEV strains to show that inclusion of IMP181-derived structural genes partially circumvents the SEB. Author StatementDuring the first half of the previous century, WEEV was responsible for large outbreaks throughout the northern United States and Canada that caused severe disease in horses and people. Over the past 60 years, cases of WEEV have mysteriously faded and the pathogen is rarely encountered in the clinic today. Salivary gland escape barriers (SEB) are a relatively neglected field of study in arbovirology, and this study provides a valuable contribution to the field by describing a SEB found in otherwise vector competent Culex tarsalis mosquitoes. Although midgut barriers are well studied, less is known about barriers to transmission in the salivary glands. Although salivary gland infection occurs at a high rate following direct injection of virus into the hemocoel, we noticed that only [~]20-30% of infected mosquitoes transmit detectable infectious virus in their saliva. Additionally, although the more pathogenic patient-derived McMillan strain of WEEV infected salivary glands at a similar rate, its transmission was more severely restricted than the mosquito-derived but less pathogenic Imperial 181 strain. We were able to trace determinants of viral transmission to the 6K/E1 region of the gene encoding the viral structural polyprotein. WEEV is a valuable research model for the closely related Eastern equine encephalitis virus and Venezuelan equine encephalitis virus we believe that our findings are applicable to other members of Togaviridae.