Eastern equine encephalitis in Tennessee: 2002-2008

Structural constraints link differences in neutralization potency of human anti-Eastern equine encephalitis virus monoclonal antibodies

Selection and development of monoclonal antibody (mAb) therapeutics against pathogenic viruses depends on certain functional characteristics. Neutralization potency, or the half-maximal inhibitory concentration (IC50) values, is an important characteristic of candidate therapeutic antibodies. Structural insights into the bases of neutralization potency differences between antiviral neutralizing mAbs are lacking. In this report, we present cryo-electron microscopy (EM) reconstructions of three anti-Eastern equine encephalitis virus (EEEV) neutralizing human mAbs targeting overlapping epitopes on the E2 protein, with greater than 20-fold differences in their respective IC50 values. From our structural and biophysical analyses, we identify several constraints that contribute to the observed differences in the neutralization potencies. Cryo-EM reconstructions of EEEV in complex with these Fab fragments reveal structural constraints that dictate intravirion or intervirion cross-linking of glycoprotein spikes by their IgG counterparts as a mechanism of neutralization. Additionally, we describe critical features for the recognition of EEEV by these mAbs including the epitope-paratope interaction surface, occupancy, and kinetic differences in on-rate for binding to the E2 protein. Each constraint contributes to the extent of EEEV inhibition for blockade of virus entry, fusion, and/or egress. These findings provide structural and biophysical insights into the differences in mechanism and neutralization potencies of these antibodies, which help inform rational design principles for candidate vaccines and therapeutic antibodies for all icosahedral viruses.

Ecology of Eastern Equine Encephalitis Virus in the Southeastern United States: Incriminating Vector and Host Species Responsible for Virus Amplification, Persistence, and Dispersal

Eastern equine encephalitis virus (EEEV; family Togaviridae, genus Alphavirus) is a mosquito-borne pathogen found in eastern North America that causes severe disease in humans and horses. The mosquito Culiseta melanura (Coquillett) (Diptera: Culicidae) is the primary enzootic vector of EEEV throughout eastern North America while several mosquito species belonging to diverse genera serve as bridge vectors. The ecology of EEEV differs between northern and southern foci, with respect to phenology of outbreaks, important vertebrate hosts, and bridge vector species. Active transmission is limited to roughly half of the year in northern foci (New York, New Hampshire, Massachusetts, Connecticut), while year-round transmission occurs in the southeastern region (particularly Florida). Multiple phylogenetic analyses indicate that EEEV strains circulating in northern foci are likely transported from southern foci by migrating birds. Bird species that overwinter or migrate through Florida, are bitten by Cs. melanura in late spring, and arrive at northern breeding grounds in May are the most likely candidates to disperse EEEV northward. Available data indicate that common yellowthroat and green heron satisfy these criteria and could serve as virus dispersers. Understanding the factors that drive the phenology of Cs. melanura reproduction in the south and the timing of avian migration from southern foci could provide insight into how confluence of these biological phenomena shapes outbreaks of EEE throughout its range. This information could be used to develop models predicting the likelihood of outbreaks in a given year, allowing vector control districts to more efficiently marshal resources necessary to protect their stakeholders.

Field Analysis of Biological Factors Associated With Sites at High and Low to Moderate Risk for Eastern Equine Encephalitis Virus Winter Activity in Florida

Eastern equine encephalitis virus (EEEV) is the most pathogenic arbovirus endemic to the United States. Studies have demonstrated Florida's role as a regional reservoir for the virus and its ability to support year-round transmission. Previous research has developed risk index models for mapping locations most at risk for EEEV transmission. We compared vector abundance, vector feeding behavior, potential host species, and fauna presence at high versus low-moderate risk sites during the winter and spring. Predicted high-risk sites had a significantly greater abundance of mosquitoes overall, including Culiseta melanura (Coquillett) (Diptera: Culicidae), the primary enzootic vector of EEEV. Twenty host species were identified from Cs. melanura bloodmeals, with the majority taken from avian species. Culiseta melanura largely fed upon the Northern Cardinal (Cardinalis cardinalis (Passeriformes: Cardinalidae)), which accounted for 20-24.4% of the bloodmeals obtained from this species in years 1 and 2, respectively. One EEEV-positive mosquito pool (Cs. melanura) and nine EEEV seropositive sentinel chickens were confirmed during winter-spring collections from high-risk sites; no seropositive chickens nor mosquito pools were found at the low-moderate risk sites. These results suggest that high-risk sites for EEEV activity are characterized by habitats that support populations of Cs. melanura and which may also provide ample opportunities to feed upon Northern Cardinals. The overall low level of mosquito populations during the winter also suggests that control of Cs. melanura populations in winter at high-risk sites may prove effective in reducing EEEV transmission during the peak summer season.

Eastern Equine Encephalitis Virus: A Scoping Review of the Global Evidence

Background: Eastern equine encephalitis virus (EEEV) is a mosquito-borne virus that is primarily found in North America and the Caribbean. Over the past decade there has been an increase in virus activity, including large outbreaks in human and horse populations. Predicted climate change is expected to affect the range of mosquitoes including vectors of EEEV, which may alter disease risk posing a public health concern. Methods: A scoping review (ScR) was conducted to identify and characterize the global evidence on EEEV. A thorough search was conducted in relevant bibliographic databases and government websites. Two reviewers screened titles and abstracts for relevance and the characteristics of relevant articles were extracted using a uniformly implemented data collection form. The study protocol was developed a priori and described the methods and tools used and this article follows the PRISMA-ScR guidelines for reporting ScRs. Results: The ScR included 718 relevant research articles. The majority of the articles originated from North America (97%) between 1933 and 2019. EEEV has been identified in 35 species of mosquitoes, over 200 species of birds, various domestic animals, wild mammals, reptiles, and amphibians. Articles identified in this ScR primarily covered three topic areas: epidemiology of hosts and vectors (344 articles) including surveillance results (138), pathogenesis of EEEV in hosts (193), and in vitro studies characterizing EEEV (111). Fewer articles evaluated the accuracy of diagnostic tests (63), the efficacy of mitigation strategies (62), transmission dynamics (56), treatment of EEEV in hosts (10), societal knowledge, attitudes, and perceptions (4), and economic burden (2). Conclusion: With the projected impact of climate change on mosquito populations, it is expected that the risk of EEEV could change resulting in higher disease burden or spread into previously unaffected areas. Future research efforts should focus on closing some of the important knowledge gaps identified in this ScR.

Sindbis virus neutralising antibodies detected in Swedish horses

A number of viruses transmitted by mosquitoes are well known to cause disease in both humans and horses, ranging from mild fevers to mortal neurological disease. A recently discovered connection between the alphavirus Sindbis virus (SINV) and neurological disease in horses in South Africa initiated this serological study in northern Europe, where the same genotype of SINV (SINV-I) is also highly endemic. We tested 171 serum samples, originally obtained from horses for other reasons from April to October 2019, for presence of SINV neutralising antibodies using a plaque reduction neutralisation test (PRNT). The serum from six horses reduced the plaque count more than 80%, and two out of these reduced the plaque count more than 90%. These horses were sampled in six different regions of Sweden, and included individuals sampled from April to August. This study shows that horses in Sweden have become infected with SINV and developed neutralising antibodies. Potential connections between infection and development of disease are important questions for future studies.

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SINV is a arbovirus that has been associated with disease in horses in South Africa.

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SINV is common in Sweden but any connection to equine infection is unknown.

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In this study, neutralising antibodies against SINV were detected in Swedish horses.

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This is the first study to indicate SINV infection in European horses.

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The clinical impact of SINV on horses will be an important focus for future studies.

A molecular understanding of alphavirus entry

Alphaviruses cause severe human illnesses including persistent arthritis and fatal encephalitis. As alphavirus entry into target cells is the first step in infection, intensive research efforts have focused on elucidating aspects of this pathway, including attachment, internalization, and fusion. Herein, we review recent developments in the molecular understanding of alphavirus entry both in vitro and in vivo and how these advances might enable the design of therapeutics targeting this critical step in the alphavirus life cycle.

Mosquito Community and West Nile Virus Risk on a National Guard Training Base in Oklahoma

Military bases are important areas for mosquito surveillance to maintain active duty combat readiness and protect training exercises. The aim of this study was to assist Camp Gruber National Guard training facility personnel to assess their mosquito community and West Nile virus (WNV) risk using biweekly sampling of 50 sites. Between May and October 2018, 10,259 adult female mosquitoes consisting of 6 genera and 26 species were collected over 662 trap-nights using 2 trap types. The most commonly collected genus was Culex (72.2% of total), followed by Psorophora (13.3%) and Aedes (10.2%). Of note, most of the medically important species were collected in the area containing troop living quarters, including 1 WNV-positive pool of Culex tarsalis. Two specimens of Aedes aegypti were collected around a vehicle storage area. While smaller in land mass size than many other active military bases in Oklahoma, the diversity of species at Camp Gruber was comparable to collections from 4 larger bases in Oklahoma. These data demonstrate the need for regular season-long mosquito monitoring of training bases to protect the health of active duty and reserve military personnel.

Eastern Equine Encephalitis Virus in the United States, 2003-2016

Eastern equine encephalitis virus (EEEV) is a mosquito-borne alphavirus found in the eastern United States. Eastern equine encephalitis virus disease in humans is rare but can result in severe, often fatal, illness. This report summarizes the national EEEV surveillance data for 2003 through 2016, including human disease cases and nonhuman infections. Over the 14-year period, 633 counties from 33 states reported EEEV activity; 88% of those counties reported EEEV activity only in nonhuman species. A total of 121 human cases of EEEV disease were reported, with a median of eight cases reported annually. The national average annual incidence of EEEV neuroinvasive disease was 0.03 cases per million population. States with the highest average annual incidence included New Hampshire, Massachusetts, Vermont, Maine, and Alabama. Eastern equine encephalitis virus neuroinvasive disease incidence was highest among males and among persons aged < 5 and > 60 years. Overall, 118 (98%) case patients were hospitalized and 50 (41%) died. The case fatality ratio was highest among case patients aged ≥ 70 years. Nonhuman surveillance data indicate that the geographic range of EEEV is much greater than human cases alone might suggest. In areas where the virus circulates, health-care providers should consider EEEV infection in the differential diagnosis for meningitis and encephalitis. Providers are encouraged to report suspected cases to their public health department to facilitate diagnosis and consider interventions to mitigate the risk of further transmission. Because human vaccines against EEEV are not available, prevention depends on community efforts to reduce mosquito populations and personal protective measures to decrease exposure to mosquitoes.

Vector Competence and Capacity of Culex erraticus (Diptera: Culicidae) for Eastern Equine Encephalitis Virus in the Southeastern United States

Field studies of the ecology of eastern equine encephalitis virus (EEEV; family Togaviridae, genus Alphavirus) in the southeastern United States have demonstrated that Culex erraticus (Dyar and Knab) is the most common mosquito at many enzootic sites and is often infected with the virus. However, the competence of Cx. erraticus for EEEV has not been explored in detail. Culex erraticus females were collected from the field and fed upon EEEV-infected chicks. The infected mosquitoes were provided honey for nutrition and to monitor for time to infectiveness. Of the mosquitoes that survived the 14-d postfeeding period, 89% were infected and 84% had evidence of a disseminated infection, though titers were generally low. EEEV was first detected in honey 6 d postinfection and was detected in samples collected from 94% of the mosquitoes with a disseminated infection overall. These data and others were then employed to estimate the relative vectorial capacity of Cx. erraticus at an EEEV enzootic site in Alabama. The vectorial capacity of Cx. erraticus at this site was 44% of Culiseta melanura (Coquillett), the accepted enzootic vector, suggesting Cx. erraticus may play a role in transmitting EEEV in areas where it is abundant and Cs. melanura rare.