Identification of super-infected Aedes triseriatus mosquitoes collected as eggs from the field and partial characterization of the infecting La Crosse viruses

Abstract

BackgroundLa Crosse virus (LACV) is a pathogenic arbovirus that is transovarially transmitted by Aedes triseriatus mosquitoes and overwinters in diapausing eggs. However, previous models predicted transovarial transmission (TOT) to be insufficient to maintain LACV in nature.ResultsTo investigate this issue, we reared mosquitoes from field-collected eggs and assayed adults individually for LACV antigen, viral RNA by RT-PCR, and infectious virus. The mosquitoes had three distinct infection phenotypes: 1) super infected (SI+) mosquitoes contained infectious virus, large accumulations of viral antigen and RNA and comprised 17 of 17,825 (0.09%) of assayed mosquitoes, 2) infected mosquitoes (I+) contained no detectable infectious virus, lesser amounts of viral antigen and RNA, and comprised 3.7% of mosquitoes, and 3) non-infected mosquitoes (I-) contained no detectable viral antigen, RNA, or infectious virus and comprised 96.21% of mosquitoes. SI+ mosquitoes were recovered in consecutive years at one field site, suggesting that lineages of TOT stably-infected and geographically isolated Ae. triseriatus exist in nature. Analyses of LACV genomes showed that SI+ isolates are not monophyletic nor phylogenetically distinct and that synonymous substitution rates exceed replacement rates in all genes and isolates. Analysis of singleton versus shared mutations (Fu and Li's F*) revealed that the SI+ LACV M segment, with a large and significant excess of intermediate-frequency alleles, evolves through disruptive selection that maintains SI+ alleles at higher frequencies than the average mutation rate. A QTN in the LACV NSm gene was detected in SI+ mosquitoes, but not in I+ mosquitoes. Four amino acid changes were detected in the LACV NSm gene from SI+ but not I+ mosquitoes from one site, and may condition vector super infection. In contrast to NSm, the NSs sequences of LACV from SI+ and I+ mosquitoes were identical.ConclusionsSI+ mosquitoes may represent stabilized infections of Ae. triseriatus mosquitoes, which could maintain LACV in nature. A gene-for-gene interaction involving the viral NSm gene and a vector innate immune response gene may condition stabilized infection.

Dissecting vectorial capacity for mosquito-borne viruses

“…Although this review only will address peroral transmission from a vector to a vertebrate, other modes of transmission exist, e.g., vertical transmission, both transovarial (TOT; virus in egg; [21] and vertical (virus on surface of egg as for certain flaviviruses; [22]), co-feeding transmission from an infected arthropod to uninfected arthropods feeding in close proximity and without evidence of a substantial viremia in the host [23], first noted with tick-borne pathogens [24], and non-biologic transmission such as by mechanical transmission, as with Rift Valley Fever virus [25]. Peroral transmission, as with other aspects of vector competence, is virus- and vector species-specific (see references for infection differences).…”

Detection of Sindbis and Inkoo Virus RNA in Genetically Typed Mosquito Larvae Sampled in Northern Sweden

“…aegypti eggs may produce infected adult mosquitoes (Khin and Than 1983 ). Arboviruses have also been found in field-collected mosquito larva that has been reared to adult stage (Dhileepan et al 1996 , Reese et al 2010 ).…”

“…As far as we know, only one study from Australia has suggested vertical transmission of SINV and the related Ross River virus, family Togaviridae , genus Alphavirus (Dhileepan et al 1996 ). INKV has not been studied regarding vertical transmission, but the La Crosse virus (family Bunyaviridae , genus Orthobunyaviridae ) in the California serogroup has been found in field collected larvae and mosquito eggs in North America (Reese et al 2010 ).…”

Bunyavirus-Vector Interactions

“…triseriatus , which plays a key role in amplification and maintenance of the virus by producing large numbers of LACV infected progeny and eggs in which the virus may overwinter to emerge in the spring [ 52 , 55 ]. Rates of TOT and filial infection can exceed 70%, although infection rates in field-collected materials are often significantly lower [ 56 , 57 ]. LACV is also transmitted horizontally from infected Ae.…”