Vector-Host Interactions of Culiseta melanura in a Focus of Eastern Equine Encephalitis Virus Activity in Southeastern Virginia

Abstract

Eastern equine encephalitis virus (EEEV) causes a highly pathogenic mosquito-borne zoonosis that is responsible for sporadic outbreaks of severe illness in humans and equines in the eastern USA. Culiseta (Cs.) melanura is the primary vector of EEEV in most geographic regions but its feeding patterns on specific avian and mammalian hosts are largely unknown in the mid-Atlantic region. The objectives of our study were to: 1) identify avian hosts of Cs. melanura and evaluate their potential role in enzootic amplification of EEEV, 2) assess spatial and temporal patterns of virus activity during a season of intense virus transmission, and 3) investigate the potential role of Cs. melanura in epidemic/epizootic transmission of EEEV to humans and equines. Accordingly, we collected mosquitoes at 55 sites in Suffolk, Virginia in 2013, and identified the source of blood meals in engorged mosquitoes by nucleotide sequencing PCR products of the mitochondrial cytochrome b gene. We also examined field-collected mosquitoes for evidence of infection with EEEV using Vector Test, cell culture, and PCR. Analysis of 188 engorged Cs. melanura sampled from April through October 2013 indicated that 95.2%, 4.3%, and 0.5% obtained blood meals from avian, mammalian, and reptilian hosts, respectively. American Robin was the most frequently identified host for Cs. melanura (42.6% of blood meals) followed by Northern Cardinal (16.0%), European Starling (11.2%), Carolina Wren (4.3%), and Common Grackle (4.3%). EEEV was detected in 106 mosquito pools of Cs. melanura, and the number of virus positive pools peaked in late July with 22 positive pools and a Maximum Likelihood Estimation (MLE) infection rate of 4.46 per 1,000 mosquitoes. Our findings highlight the importance of Cs. melanura as a regional EEEV vector based on frequent feeding on virus-competent bird species. A small proportion of blood meals acquired from mammalian hosts suggests the possibility that this species may occasionally contribute to epidemic/epizootic transmission of EEEV.

Fig 1. Map of the study area and mosquito collection sites in Suffolk, VA, 2013.

(A) Close-up of Hampton Roads in southeastern Virginia with the City of Suffolk and Great Dismal Swamp National Wildlife Refuge shaded in. (B) Average number of Cs. melanura captured at each trap site per trap night. (C) Total number of blood-fed mosquitoes collected at each trap site. (D) Total number of eastern equine encephalitis virus-positive Cs. melanura pools at each trap site.

Fig 2. Avian-derived blood meals in Culiseta melanura and avian frequencies.

(A) Percentage of avian-derived blood meals in Cs. melanura compared with the average avian frequencies in the City of Suffolk, VA and surrounding cities/counties (City of Portsmouth, City of Chesapeake, Southampton County, and Isle of Wight County), May through October 2013. (B) Monthly frequencies of avian species based on point count data in the City of Suffolk, VA and surrounding cities/counties (City of Portsmouth, City of Chesapeake, Southampton County, and Isle of Wight County), May through October 2013.

Fig 3. Eastern equine encephalitis virus (EEEV) isolations and Maximum Likelihood Estimations (MLEs) in Culiseta melanura.

Weekly EEEV isolations and MLEs compared to average Cs. melanura collected per trap night in Suffolk VA, 2013. The line graph represents the weekly average Cs. melanura captured per trap citywide. The bar graph represents the total number of EEEV-positive mosquito pools from each week with a corresponding MLE above the bar (calculated with both Vector Test and PCR positives of Cs. melanura pools).