Eastern Equine Encephalitis Virus: A Case Report and Brief Literature Review of Current Therapeutic and Preventative Strategies

Background: Eastern equine encephalitis virus (EEEV) is a rare mosquito-borne illness exhibiting rapid neurological deterioration and permanent damage. Despite its >30% mortality and >60% long-term neurological damage, EEEV has no approved antiviral medication or vaccination. This report uniquely aims to describe a rare case of EEEV and provide a current literature review of therapeutic and preventative options from the clinical perspective to guide clinicians and public health workers, along with informing them about its impact and current knowledge gaps. Methods: A retrospective chart review of the electronic medical record was performed for a patient's 10-day hospital admission in July 2021. In addition, PubMed was searched using relevant keywords for a literature review of EEEV. Results: A 61-year-old woman presented with dysarthria and right-sided facial droop. Acute ischemic stroke was ruled out, and empiric intravenous (IV) antibiotics were initiated for possible infectious etiology. The patient developed worsening mental status and fever and was intubated, with antibiotics broadened with concern for meningitis along with tick-borne illness. The patient remained encephalopathic and febrile, and lumbar serologies were consistent with viral meningoencephalitis or acute disseminated encephalomyelitis. Several days after collection, quantitative antibody testing returned positive for EEEV. The patient was pronounced dead on hospital day 10. On review of the literature regarding EEEV, supportive care and prevention remain the cornerstone of management. Although early IV immunoglobulin and high-dose steroids have shown potential as treatments to reduce morbidity and mortality, no vaccines have been approved to date. Conclusion: Prospective trials and further investigations into treatment and preventative options may be useful in reducing the morbidity and mortality associated with EEEV.

Use of Cervid Serosurveys to Monitor Eastern Equine Encephalitis Virus Activity in Northern New England, United States, 2009-2017

Vertebrate surveillance for eastern equine encephalitis virus (EEEV) activity usually focuses on three types of vertebrates: horses, passerine birds, and sentinel chicken flocks. However, there is a variety of wild vertebrates that are exposed to EEEV infections and can be used to track EEEV activity. In 2009, we initiated a pilot study in northern New England, United States, to evaluate the effectiveness of using wild cervids (free-ranging white-tailed deer and moose) as spatial sentinels for EEEV activity. In Maine, New Hampshire, and Vermont during 2009-2017, we collected blood samples from hunter-harvested cervids at tagging stations and obtained harvest location information from hunters. U.S. Centers for Disease Control and Prevention processed the samples for EEEV antibodies using plaque reduction neutralization tests (PRNTs). We detected EEEV antibodies in 6 to 17% of cervid samples in the different states and mapped cervid EEEV seropositivity in northern New England. EEEV antibody-positive cervids were the first detections of EEEV activity in the state of Vermont, in northern Maine, and northern New Hampshire. Our key result was the detection of the antibodies in areas far outside the extent of documented wild bird, mosquito, human case, or veterinary case reports of EEEV activity in Maine, New Hampshire, and Vermont. These findings showed that cervid (deer and moose) serosurveys can be used to characterize the geographic extent of EEEV activity, especially in areas with low EEEV activity or with little or no EEEV surveillance. Cervid EEEV serosurveys can be a useful tool for mapping EEEV activity in areas of North America in addition to northern New England.

Ecology and Epidemiology of Eastern Equine Encephalitis Virus in the Northeastern United States: An Historical Perspective

In the current review, we examine the regional history, ecology, and epidemiology of eastern equine encephalitis virus (EEEV) to investigate the major drivers of disease outbreaks in the northeastern United States. EEEV was first recognized as a public health threat during an outbreak in eastern Massachusetts in 1938, but historical evidence for equine epizootics date back to the 1800s. Since then, sporadic disease outbreaks have reoccurred in the Northeast with increasing frequency and northward expansion of human cases during the last 20 yr. Culiseta melanura (Coquillett) (Diptera: Culicidae) serves as the main enzootic vector that drives EEEV transmission among wild birds, but this mosquito species will occasionally feed on mammals. Several species have been implicated as bridge vectors to horses and humans, with Coquilletstidia perturbans (Walker) as a leading suspect based on its opportunistic feeding behavior, vector competence, and high infection rates during recent disease outbreaks. A diversity of bird species are reservoir competent, exposed to EEEV, and serve as hosts for Cs. melanura, with a few species, including the wood thrush (Hlocichia mustelina) and the American robin (Turdus migratorius), contributing disproportionately to virus transmission based on available evidence. The major factors responsible for the sustained resurgence of EEEV are considered and may be linked to regional landscape and climate changes that support higher mosquito densities and more intense virus transmission.

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.

Resurgence of Interest in Eastern Equine Encephalitis Virus Vaccine Development

Eastern equine encephalitis virus (EEEV; Family Togaviridae), is an endemic pathogen first isolated in 1933 with distribution primarily in the eastern US and Canada. The virus has caused periodic outbreaks in both humans and equines along the eastern seaboard and through the southern coastal states. While the outbreaks caused by EEEV have been sporadic and varied geographically since the discovery of the virus, it has continued to expand its range moving into the Midwest states as well. Additionally, one of the largest outbreaks was recorded in 2019 prompting concerns that outbreaks were becoming larger and more frequent. Because the virus can cause serious disease and because it is transmissible by both mosquitoes and aerosol, there has been renewed interest in identifying potential options for vaccines. Currently, there are no licensed vaccines and control relies completely on the use of personal protective measures and integrated vector control which have limited effectiveness for the EEEV vectors. Several vaccine candidates are currently being developed; this review will describe the multiple options under consideration for future development and assess their relative advantages and disadvantages.

Enzootic mosquito vector species at equine encephalitis transmission foci in the República de Panamá

The identification of mosquito vector species present at arboviral enzootic transmission foci is important to understand transmission eco-epidemiology and to propose and implement prevention and control strategies that reduce vector-borne equine encephalitis transmission. The goal of this study was to identify mosquito species potentially involved in the transmission of enzootic equine encephalitis, in relation to their abundance and diversity at three endemic regions in the República de Panamá. We sampled adult mosquitoes during the dry and rainy season of Panamá. We employed CDC light traps with octanol, EV traps with CO₂ and Trinidad 17 traps baited with live hamsters. Traps were deployed in the peridomicile and extradomicile of houses from 18:00 to 6:00 h. We estimated the abundance and diversity of sampled species. We collected a total of 4868 mosquitoes, belonging to 45 species and 11 genera, over 216 sampling nights. Culex (Melanoconion) pedroi, a major Venezuelan equine encephalitis vector was relatively rare (< 2.0% of all sampled mosquitoes). We also found Cx. (Mel) adamesi, Cx. (Mel) crybda, Cx. (Mel) ocossa, Cx. (Mel) spissipes, Cx. (Mel) taeniopus, Cx. (Mel) vomerifer, Aedes scapularis, Ae. angustivittatus, Coquillettidia venezuelensis, Cx. nigripalpus, Cx. declarator, Mansonia titillans, M. pseudotitillans and Psorophora ferox all species known to be vectorially competent for the transmission of arboviruses. Abundance and diversity of mosquitoes in the sampled locations was high, when compared with similar surveys in temperate areas. Information from previous reports about vectorial competence / capacity of the sampled mosquito species suggest that sampled locations have all the elements to support enzootic outbreaks of Venezuelan and Eastern equine encephalitides.

Surveys for Antibodies Against Mosquitoborne Encephalitis Viruses in California Birds, 1996-2013

From 1996 through 2013, 54,546 individual birds comprising 152 species and 7 orders were banded, bled, and released at four study areas within California, from which 28,388 additional serum samples were collected at one or more recapture encounters. Of these, 142, 99, and 1929 birds from 41 species were positive for neutralizing antibodies against western equine encephalomyelitis virus (WEEV), St. Louis encephalitis virus (SLEV), or West Nile virus (WNV) at initial capture or recapture, respectively. Overall, 83% of the positive serum samples were collected from five species: House Finch, House Sparrow, Mourning Dove, California Quail, and Western Scrub-Jay. Temporal data supported concurrent arbovirus surveillance and documented the disappearance of birds positive for WEEV in 2008 and SLEV in 2003 and the appearance of birds positive for WNV after its invasion in 2003. Results of these serosurveys agreed well with the host selection patterns of the Culex vectors as described from bloodmeal sequencing data and indicated that transmission of WNV seemed most effective within urban areas where avian and mosquito host diversity was limited to relatively few competent species.

Dynamics of Vector-Host Interactions in Avian Communities in Four Eastern Equine Encephalitis Virus Foci in the Northeastern U.S

BACKGROUND

Eastern equine encephalitis (EEE) virus (Togaviridae, Alphavirus) is a highly pathogenic mosquito-borne zoonosis that is responsible for occasional outbreaks of severe disease in humans and equines, resulting in high mortality and neurological impairment in most survivors. In the past, human disease outbreaks in the northeastern U.S. have occurred intermittently with no apparent pattern; however, during the last decade we have witnessed recurring annual emergence where EEE virus activity had been historically rare, and expansion into northern New England where the virus had been previously unknown. In the northeastern U.S., EEE virus is maintained in an enzootic cycle involving the ornithophagic mosquito, Culiseta melanura, and wild passerine (perching) birds in freshwater hardwood swamps. However, the identity of key avian species that serve as principal virus reservoir and amplification hosts has not been established. The efficiency with which pathogen transmission occurs within an avian community is largely determined by the relative reservoir competence of each species and by ecological factors that influence contact rates between these avian hosts and mosquito vectors.

METHODOLOGY AND PRINCIPLE FINDINGS

Contacts between vector mosquitoes and potential avian hosts may be directly quantified by analyzing the blood meal contents of field-collected specimens. We used PCR-based molecular methods and direct sequencing of the mitochondrial cytochrome b gene for profiling of blood meals in Cs. melanura, in an effort to quantify its feeding behavior on specific vertebrate hosts, and to infer epidemiologic implications in four historic EEE virus foci in the northeastern U.S. Avian point count surveys were conducted to determine spatiotemporal host community composition. Of 1,127 blood meals successfully identified to species level, >99% of blood meals were from 65 avian hosts in 27 families and 11 orders, and only seven were from mammalian hosts representing three species. We developed an empirically informed mathematical model for EEE virus transmission using Cs. melanura abundance and preferred and non-preferred avian hosts. To our knowledge this is the first mathematical model for EEE virus, a pathogen with many potential hosts, in the northeastern U.S. We measured strong feeding preferences for a number of avian species based on the proportion of mosquito blood meals identified from these bird species in relation to their observed frequencies. These included: American Robin, Tufted Titmouse, Common Grackle, Wood Thrush, Chipping Sparrow, Black-capped Chickadee, Northern Cardinal, and Warbling Vireo. We found that these bird species, most notably Wood Thrush, play a dominant role in supporting EEE virus amplification. It is also noteworthy that the competence of some of the aforementioned avian species for EEE virus has not been established. Our findings indicate that heterogeneity induced by mosquito host preference, is a key mediator of the epizootic transmission of vector-borne pathogens.

CONCLUSION AND SIGNIFICANCE

Detailed knowledge of the vector-host interactions of mosquito populations in nature is essential for evaluating their vectorial capacity and for assessing the role of individual vertebrates as reservoir hosts involved in the maintenance and amplification of zoonotic agents of human diseases. Our study clarifies the host associations of Cs. melanura in four EEE virus foci in the northeastern U.S., identifies vector host preferences as the most important transmission parameter, and quantifies the contribution of preference-induced contact heterogeneity to enzootic transmission. Our study identifies Wood Thrush, American Robin and a few avian species that may serve as superspreaders of EEE virus. Our study elucidates spatiotemporal host species utilization by Cs. melanura in relation to avian host community. This research provides a basis to better understand the involvement of Cs. melanura and avian hosts in the transmission and ecology of EEE virus and the risk of human infection in virus foci.

Insights into the recent emergence and expansion of eastern equine encephalitis virus in a new focus in the Northern New England USA

BACKGROUND

Eastern equine encephalomyelitis virus (EEEV) causes a highly pathogenic zoonosis that circulates in an enzootic cycle involving the ornithophagic mosquito, Culiseta melanura, and wild passerine birds in freshwater hardwood swamps in the northeastern U.S. Epidemic/epizootic transmission to humans/equines typically occurs towards the end of the transmission season and is generally assumed to be mediated by locally abundant and contiguous mammalophagic "bridge vector" mosquitoes.

METHODS

Engorged mosquitoes were collected using CDC light, resting box, and gravid traps during epidemic transmission of EEEV in 2012 in Addison and Rutland counties, Vermont. Mosquitoes were identified to species and blood meal analysis performed by sequencing mitochondrial cytochrome b gene polymerase chain reaction products. Infection status with EEEV in mosquitoes was determined using cell culture and RT-PCR assays, and all viral isolates were sequenced and compared to other EEEV strains by phylogenetic analysis.

RESULTS

The host choices of 574 engorged mosquitoes were as follows: Cs. melanura (n = 331, 94.3 % avian-derived, 5.7 % mammalian-derived); Anopheles quadrimaculatus (n = 164, 3.0 % avian, 97.0 % mammalian); An. punctipennis (n = 56, 7.2 % avian, 92.8 % mammalian), Aedes vexans (n = 9, 22.2 % avian, 77.8 % mammalian); Culex pipiens s.l. n = 6, 100 % avian); Coquillettidia perturbans (n = 4, 25.0 % avian, 75.0 % mammalian); and Cs. morsitans (n = 4, 100 % avian). A seasonal shift in blood feeding by Cs. melanura from Green Heron towards other avian species was observed. EEEV was successfully isolated from blood-fed Cs. melanura and analyzed by phylogenetic analysis. Vermont strains from 2012 clustered with viral strains previously isolated in Virginia yet were genetically distinct from an earlier EEEV isolate from Vermont during 2011.

CONCLUSIONS

Culiseta melanura acquired blood meals primarily from birds and focused feeding activity on several competent species capable of supporting EEEV transmission. Culiseta melanura also occasionally obtained blood meals from mammalian hosts including humans. This mosquito species serves as the primary vector of EEEV among wild bird species, but also is capable of occasionally contributing to epidemic/epizootic transmission of EEEV to humans/equines. Other mosquito species including Cq. perturbans that feed more opportunistically on both avian and mammalian hosts may be important in epidemic/epizootic transmission under certain conditions. Phylogenetic analyses suggest that EEEV was independently introduced into Vermont on at least two separate occasions.

[EPIDEMIOLOGIC ANALYSIS OF OUTBREAKS OF DISEASES CAUSED BY AMERICAN EQUINE ENCEPHALITIS CAUSATIVE AGENTS IN ENDEMIC REGIONS]

Epidemiologic analysis of epidemic outbreaks caused by American equine encephalitis causative agents is carried out in the review. Eastern equine encephalomyelitis (EEE), Western equine encephalomyelitis (WEE) and Venezuela equine encephalomyelitis (VEE) viruses are etiologic agents of dangerous transmissive diseases that are usually accompanied by fever and neurologic symptoms. Among the New World alphaviruses, VEE virus has the most potential danger for humans and domestic animals. Currently, enzootic strains of VEE play an increasing role as etiologic agents of human diseases. Most of the VEE cases in humans in endemic regions during inter-epidemic period are caused by infection with VEE subtype ID virus. A possibility of emergence of novel epidemic outbreaks of VEE is determined by mutations of ID subtype strains into IC subtype, and those currently pose a potential threat as an etiologic agent of the disease. Despite low morbidity, EEE and WEE are a problem for healthcare due to a relatively high frequency of lethal outcomes of the disease.

A duplex real-time reverse transcriptase polymerase chain reaction assay for the detection of St. Louis encephalitis and eastern equine encephalitis viruses

A duplex TaqMan real-time reverse transcriptase polymerase chain reaction (PCR) assay was developed for the detection of St. Louis encephalitis virus (SLEV) and eastern equine encephalitis virus (EEEV), for use in human and vector surveillance. The respective targets selected for the assay were the conserved NS5 and E1 genes of the 2 viruses. Because of the insufficient number of NS5 sequences from SLEV strains in the GenBank database, we determined the sequence of an approximately 1-kb region for each of 25 strains of SLEV to select primers and probes in a conserved region. Our assay has a sensitivity of 5 gene copies (gc)/reaction for EEEV and 10 gc/reaction for SLEV, and its performance is linear for at least 6 log(10) gc. The assay is specific and detected all strains of SLEV (69) and EEEV (12) that were tested. An internal control ensures detection of efficient nucleic acid extraction and possible PCR inhibition.

Comparison of immune responses of brown-headed cowbird and related blackbirds to west Nile and other mosquito-borne encephalitis viruses

The rapid geographic spread of West Nile virus (family Flaviviridae, genus Flavivirus, WNV) across the United States has stimulated interest in comparative host infection studies to delineate competent avian hosts critical for viral amplification. We compared the host competence of four taxonomically related blackbird species (Icteridae) after experimental infection with WNV and with two endemic, mosquito-borne encephalitis viruses, western equine encephalomyelitis virus (family Togaviridae, genus Alphavirus, WEEV), and St. Louis encephalitis virus (family Flaviviridae, genus Flavivirus, SLEV). We predicted differences in disease resistance among the blackbird species based on differences in life history, because they differ in geographic range and life history traits that include mating and breeding systems. Differences were observed among the response of these hosts to all three viruses. Red-winged Blackbirds were more susceptible to SLEV than Brewer's Blackbirds, whereas Brewer's Blackbirds were more susceptible to WEEV than Red-winged Blackbirds. In response to WNV infection, cowbirds showed the lowest mean viremias, cleared their infections faster, and showed lower antibody levels than concurrently infected species. Brown-headed Cowbirds also exhibited significantly lower viremia responses after infection with SLEV and WEEV as well as coinfection with WEEV and WNV than concurrently infected icterids. We concluded that cowbirds may be more resistant to infection to both native and introduced viruses because they experience heightened exposure to a variety of pathogens of parenting birds during the course of their parasitic life style.

Role of California (Callipepla californica) and Gambel's (Callipepla gambelii) quail in the ecology of mosquito-borne encephalitis viruses in California, USA

Gambel's and California quail were infected repeatedly whenever western equine encephalomyelitis virus (WEEV), St. Louis encephalitis virus (SLEV), and (WNV) West Nile virus were active during summer in California. The timing of virus appearance and quail infection coincided well with the appearance of chicks in nature, leading us to hypothesize that large coveys containing these non-immune birds could be important in focal virus amplification in rural settings. However, experimental infection studies with chicks, juveniles, and adults of both quail species using sympatric strains of WEEV, SLEV, and WNV indicated that only immature birds were competent hosts for WEEV, producing viremias sufficiently elevated to efficiently infect Culex tarsalis mosquitoes. Quail were less competent hosts for WNV and were incompetent for SLEV. Large populations of quail that frequently are infected with SLEV or WNV, but produce low to moderate viremias, may serve as dead end hosts for these viruses. Due to their abundance and repeated infection, these birds may attenuate virus amplification in rural areas of California and possibly could be one reason why WNV epidemics seem to occur more frequently in urban and periurban than in rural landscapes.

Does feeding on infected mosquitoes (Diptera: Culicidae) enhance the role of song sparrows in the transmission of arboviruses in California?

Song sparrows, Melopiza melodia, inoculated subcutaneously with either western equine encephalomyelitis virus (family Togaviridae, genus Alphavirus, WEEV) or West Nile virus (family Flaviviridae, genus Flavivirus, WNV) developed elevated viremias, and they were considered to be competent experimental hosts for both viruses. However, birds that ingested from three to 20 mosquitoes containing comparable amounts of either WEEV or WNV failed to become infected, indicating limited oral susceptibility. Comparatively few field-collected birds had antibodies against either WEEV or WNV, indicating that this species was infrequently bitten by infectious mosquitoes in nature and probably was of limited importance in viral amplification.

Meningitis and Encephalomyelitis in Horses

This article provides an overview of meningitis and encephalomyelitis in horses, including diagnostic tests, treatment developments, and preventative measures reported in the equine and human medical literature of the past few years.

Host range, amplification and arboviral disease emergence

Etiologic agents of arboviral diseases are primarily zoonotic pathogens that are maintained in nature in cycles involving arthropod transmission among a variety of susceptible reservoir hosts. In the simplest form of human exposure, spillover occurs from the enzootic cycle when humans enter zoonotic foci and/or enzootic amplification increases circulation near humans. Examples include Eastern (EEEV) and Western equine encephalitis viruses (WEEV), as well as West Nile (WNV), St. Louis encephalitis (SLEV) and Yellow fever viruses. Spillover can involve direct transmission to humans by primary enzootic vectors (e.g. WNV, SLEV and WEEV) and/or bridge vectors with more catholic feeding preferences that include humans (e.g. EEEV). Some viruses, such as Rift Valley fever, Japanese encephalitis and Venezuelan equine encephalitis viruses (VEEV) undergo secondary amplification involving replication in livestock animals, resulting in greater levels of spillover to humans in rural settings. In the case of VEEV, secondary amplification involves equines and requires adaptive mutations in enzootic strains that allow for efficient viremia production. Two of the most important human arboviral pathogens, Yellow fever and dengue viruses (DENV), have gone one step further and adopted humans as their amplification hosts, allowing for urban disease. The ancestral forms of DENV, sylvatic viruses transmitted among nonhuman primate reservoir hosts by arboreal mosquitoes, adapted to efficiently infect the urban mosquito vectors Aedes aegypti and Ae. albopictus during the past few thousand years as civilizations arose. Comparative studies of the sylvatic and urban forms of DENV may elucidate the evolution of arboviral virulence and the prospects for DENV eradication should effective vaccines be implemented.

Immune responses to commercial equine vaccines against equine herpesvirus-1, equine influenza virus, eastern equine encephalomyelitis, and tetanus

Horses are commonly vaccinated to protect against pathogens which are responsible for diseases which are endemic within the general horse population, such as equine influenza virus (EIV) and equine herpesvirus-1 (EHV-1), and against a variety of diseases which are less common but which lead to greater morbidity and mortality, such as eastern equine encephalomyelitis virus (EEE) and tetanus. This study consisted of two trials which investigated the antigenicity of commercially available vaccines licensed in the USA to protect against EIV, EHV-1 respiratory disease, EHV-1 abortion, EEE and tetanus in horses. Trial I was conducted to compare serological responses to vaccines produced by three manufacturers against EIV, EHV-1 (respiratory disease), EEE, and tetanus given as multivalent preparations or as multiple vaccine courses. Trial II compared vaccines from two manufacturers licensed to protect against EHV-1 abortion, and measured EHV-1-specific interferon-gamma (IFN-gamma) mRNA production in addition to serological evidence of antigenicity. In Trial I significant differences were found between the antigenicity of different commercial vaccines that should be considered in product selection. It was difficult to identify vaccines that generate significant immune responses to respiratory viruses. The most dramatic differences in vaccine performance occurred in the case of the tetanus antigen. In Trial II both vaccines generated significant antibody responses and showed evidence of EHV-1-specific IFN-gamma mRNA responses. Overall there were wide variations in vaccine response, and the vaccines with the best responses were not produced by a single manufacturer. Differences in vaccine performance may have resulted from differences in antigen load and adjuvant formulation.

Eastern equine encephalitis in a flock of African penguins maintained at an aquarium

Eastern equine encephalitis (EEE) was diagnosed in a flock of African penguins. Diagnosis was based on history and clinical signs and confirmed via serologic testing, virus isolation, reverse transcriptase-polymerase chain reaction (RT-PCR) assay, and histologic examination. Clinical signs in penguins included anorexia, behavior changes, depression, regurgitation, ataxia, recumbency, and seizures, and some penguins did not have any clinical signs. Mean +/- SD number of days that affected penguins had clinical signs was 12 +/- 5 days. Abnormalities initially detected on CBC included heterophilic leukocytosis and anemia; lymphocytosis and monocytosis were detected later. Plasma biochemical abnormalities included high activities of aspartate amino-transferase and creatine kinase, hyponatremia, hypochloremia, hyperglycemia, and high concentrations of globulin, triglycerides, and cholesterol. Mean +/- SD number of days required for resolution of CBC and plasma biochemical abnormalities was 67 +/- 24 days after the onset of clinical signs. Treatment consisted of supportive therapy. All penguins survived with the exception of one that was euthanatized; histopathologic findings were consistent with encephalitis. Results of RT-PCR assays performed on tissue from the right cerebrum of the penguin that was euthanatized were positive for EEE viral RNA. An inability to isolate virus several weeks after illness suggested successful viral clearance in recovered penguins. To the authors' knowledge, EEE infection in any penguin species has not been reported.

Potential vectors of West Nile virus following an equine disease outbreak in Italy

In the late summer of 1998, an outbreak of equine encephalomyelitis due to West Nile virus (WNV) occurred in the Tuscany region of central Italy. The disease was detected in 14 race horses from nine localities in four Provinces: Firenze, Lucca, Pisa and Pistoia. The outbreak area included Fucecchio wetlands (1800 ha), the largest inland marsh in Italy, and the adjacent hilly Cerbaie woodlands with farms breeding horses. To detect potential vectors of WNV, entomological surveys of Fucecchio and Cerbaie were undertaken during 1999-2002 by collecting mosquito larvae from breeding sites and adult mosquitoes by several methods of sampling. Among 6023 mosquitoes (Diptera: Culicidae) collected, 11 species were identified: Aedes albopictus (Skuse), Ae. vexans (Meigen), Anopheles atroparvus Van Thiel, An. maculipennis Meigen s.s., An. plumbeus Stephens, Culex impudicus Ficalbi, Cx. pipiens L., Culiseta longiareolata Macquart), Ochlerotatus caspius (Pallas), Oc. detritus (Haliday) and Oc. geniculatus (Olivier). In Fucecchio marshes, Cx. impudicus predominated with seasonal peak densities in spring and autumn: its greatest abundance during early spring coincides with arrival of migratory birds from Africa. In Cerbaie hills, Cx. pipiens predominated with peak population density in late summer. No viruses were isolated from 665 mosquitoes processed. These findings, plus other data on Italian mosquito bionomics, suggest a possible mode of WNV transmission involving the most abundant Culex in the Fucecchio-Cerbaie areas. Culex impudicus, being partly ornithophilic, might transmit WNV from migratory to non-migratory birds during springtime; Cx. pipiens, having a broader host range, would be more likely to transmit WNV from birds to horses and, perhaps, to humans by late summer.

Natural infection of a great egret (Casmerodius albus) with eastern equine encephalitis virus

In July 2001, a great egret (Casmerodius albus) was found dead in Charlton County, Georgia (USA) and submitted to the Southeastern Cooperative Wildlife Disease Study (The University of Georgia, Athens, Georgia). Histopathologic findings included severe hepatic necrosis and necrosis of sheathed arterioles. Eastern equine encephalitis (EEE) virus was isolated from brain and heart using Vero cells and was identified using a standard micro-neutralization test and reverse transcription polymerase chain reaction (rtPCR). Streptavidin-biotin alkaline phosphatase immunohistochemistry using mouse anti-EEE virus monoclonal antibody demonstrated EEE antigen within cells of the sheathed arterioles and scattered mononuclear cells in the splenic parenchyma. To the authors' knowledge, this is the first description of natural infection and pathologic effects of EEE virus infection in a great egret.

Experimental infection of California birds with western equine encephalomyelitis and St. Louis encephalitis viruses

A total of 27 bird species from the San Joaquin and Coachella valleys of California were inoculated subcutaneously with sympatric strains of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses. Overall, 133 of 164 birds inoculated with WEE virus developed a viremia detected by plaque assay; significantly greater than 72 of 163 birds inoculated with SLE virus. Host competence was calculated as the average number of days that each avian species had a viremia > or = 2 log10 plaque-forming units per 0.1 ml, the threshold for infecting susceptible Culex tarsalis Coquillett, the primary vector of these viruses in California. Eleven of 20 species inoculated with WEE virus had a value > or = 1 and were considered to be competent hosts, whereas only six of 22 species inoculated with SLE virus had a value > or = 1. Overall, 133 of 164 birds inoculated with WEE virus and 105 of 163 inoculated with SLE virus produced antibody detectable by enzyme immunoassay and/or plaque reduction neutralization test. Six birds infected with WEE virus (one house finch, three mourning doves, one Brewer's sparrow, and one white-crowned sparrow) and nine birds infected with SLE virus (two house finches, three white-crowned sparrows, one song sparrow, two Western scrub-jays, and one orange crowned warbler) contained viral RNA detected by reverse transcription-polymerase chain reaction at necropsy > 6 wk postinoculation; infectious WEE and SLE viruses were only recovered from three mourning doves and an orange-crowned warbler, respectively, after blind passage in mosquito cells. Our study indicated that birds with elevated field antibody prevalence rates may not be the most competent hosts for encephalitis viruses and that relatively few birds developed chronic infections that could be important in virus persistence and dispersal.

Patterns of avian seroprevalence to western equine encephalomyelitis and Saint Louis encephalitis viruses in California, USA

Temporal and spatial changes in the enzootic activity of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were monitored at representative wetland study sites in the Coachella, San Joaquin, and Sacramento valleys of California from 1996 to 1998 using three methods: (1) virus isolation from pools of 50 host-seeking Culex tarsalis Coquillett females, (2) seroconversions in flocks of 10 sentinel chickens, and (3) seroprevalence in wild birds collected by mist nets and grain baited traps. Overall, 74 WEE and one SLE isolates were obtained from 222,455 Cx. tarsalis females tested in 4,988 pools. In addition, 133 and 40 seroconversions were detected in 28 chicken flocks, and 143 and 27 of 20,192 sera tested from 149 species of wild birds were positive for antibodies to WEE and SLE, respectively. WEE was active in all three valleys, whereas SLE only was detected in Coachella Valley. Seroconversions in sentinel chickens provided the most sensitive indication of enzootic activity and were correlated with seroprevalence rates in wild birds. Avian seroprevalence rates did not provide an early warning of pending enzootic activity in chickens, because positive sera from after hatching year birds collected during spring most probably were the result of infections acquired during the previous season. Few seroconversions were detected among banded recaptured birds collected during spring and early summer. Age and resident status, but not sex, were significant risk factors for wild bird infection, with the highest seroprevalence rates among after hatching year individuals of permanent resident species. Migrants (with the exception of mourning doves) and winter resident species rarely were positive. House finches, house sparrows, Gambel's quail, California quail, common ground doves, and mourning doves were most frequently positive for antibodies. The initial detection of enzootic activity each summer coincided closely with the appearance of hatching year birds of these species in our study areas, perhaps indicating their role in virus amplification. Bird species most frequently positive roosted or nested in elevated upland vegetation, sites where Cx. tarsalis host-seeking females hunt most frequently. These serosurveys provided important background information for planned host competence and chronic infection studies.

[Identification of the encephalitis equine virus, Parana, Brazil]

INTRODUCTION

In the period of 1996-1999 some virus associated with encephalitis have been reported in horses from different regions of Paraná State, Brazil. To identify the etiologic agent associated with this illness, mosquitoes and serum samples were collected in the endemic area.

METHODS

The study area corresponded to four municipalities of Paraná State, Brazil. Mosquitoes were captured in Shannon trap and human bait. After identification, they were processed for virus isolation. Blood of equines were collected in the municipalities of Querência do Norte and Colorado. Antibodies to different Alphavirus and Flavivirus were analyzed by hemagglutination inhibition test. Specific seroneutralization reactions were performed in those sera with a positive reaction in the hemagglutination test.

RESULTS

The mosquitoes genus collected were: Culex, Aedes, Mansonia, Coquillettidia, Psorophora, Sabethes, Wyeomyia, and Limatus. Even thought no virus was isolated, serologic analyses showed hemagglutinazing antibodies to Eastern equine encephalitis, Mucambo, Pixuna, Maguari, and St Luis encephalitis viruses. The neutralization test showed specific reaction to Eastern equine encephalitis virus in 12 tested sera.

CONCLUSIONS

Species of mosquitoes that could be potential vectors of encephalitis, buniavirus, and other arboviruses of epidemiological importance were collected. It is believed that Eastern equine encephalitis virus affected the equines populations in the study regions because of the symptoms and antibodies for the virus in the sera detected in these equines.

Development of reverse transcription-PCR assays specific for detection of equine encephalitis viruses

Specific and sensitive reverse transcription-PCR (RT-PCR) assays were developed for the detection of eastern, western, and Venezuelan equine encephalitis viruses (EEE, WEE, and VEE, respectively). Tests for specificity included all known alphavirus species. The EEE-specific RT-PCR amplified a 464-bp region of the E2 gene exclusively from 10 different EEE strains from South and North America with a sensitivity of about 3,000 RNA molecules. In a subsequent nested PCR, the specificity was confirmed by the amplification of a 262-bp fragment, increasing the sensitivity of this assay to approximately 30 RNA molecules. The RT-PCR for WEE amplified a fragment of 354 bp from as few as 2,000 RNA molecules. Babanki virus, as well as Mucambo and Pixuna viruses (VEE subtypes IIIA and IV), were also amplified. However, the latter viruses showed slightly smaller fragments of about 290 and 310 bp, respectively. A subsequent seminested PCR amplified a 195-bp fragment only from the 10 tested strains of WEE from North and South America, rendering this assay virus specific and increasing its sensitivity to approximately 20 RNA molecules. Because the 12 VEE subtypes showed too much divergence in their 26S RNA nucleotide sequences to detect all of them by the use of nondegenerate primers, this assay was confined to the medically important and closely related VEE subtypes IAB, IC, ID, IE, and II. The RT-PCR-seminested PCR combination specifically amplified 342- and 194-bp fragments of the region covering the 6K gene in VEE. The sensitivity was 20 RNA molecules for subtype IAB virus and 70 RNA molecules for subtype IE virus. In addition to the subtypes mentioned above, three of the enzootic VEE (subtypes IIIB, IIIC, and IV) showed the specific amplicon in the seminested PCR. The practicability of the latter assay was tested with human sera gathered as part of the febrile illness surveillance in the Amazon River Basin of Peru near the city of Iquitos. All of the nine tested VEE-positive sera showed the expected 194-bp amplicon of the VEE-specific RT-PCR-seminested PCR.

Response of house finches to infection with sympatric and allopatric strains of western equine encephalomyelitis and St. Louis encephalitis viruses from California

Adult house finches from Kern County were inoculated subcutaneously with recent sympatric and allopatric isolates of western equine encephalomyelitis and St. Louis encephalitis (SLE) viruses made from Culex tarsalis Coquillett collected in Kern County and Coachella Valley, CA, respectively. Virulence, as measured by the amplitude of the viremia response during days 1 and 2 postinfection, varied significantly among strains, but independently of geographic origin. The intensity of the immune response, as measured by an enzyme immunoassay and a plaque reduction neutralization test, seemed to be independent of virulence, especially for SLE where some strains failed to produce a detectable viremia but elicited a strong antibody response. Our preliminary data indicated that strain virulence may be associated with the level of enzootic activity during the year of isolation.

Method of infection does not alter response of chicks and house finches to western equine encephalomyelitis and St. Louis encephalitis viruses

The effects of method of infection and virus dose on the viremia and antibody responses of 1-wk-old chicks and after-hatching-year house finches to infection with western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were studied under laboratory conditions. Using a capillary tube technique, females from 2 strains of Culex tarsalis Coquillett mosquitoes were estimated to expectorate from 1.0 to 1.7 log10 plaque forming units (PFU) of WEE and from 1.9 to 2.2 log10 PFU of SLE. Based on the proportion of parenterally infected females that transmitted and the number that blood fed during each experiment, virus doses per bird were estimated to be 1.0-1.9 log10 PFU for WEE and 1.4-2.3 log10 PFU for SLE. When infected with comparable doses of WEE by subcutaneous inoculation, there was no significant difference in the duration or magnitude of the viremia response between birds infected by mosquito bite or syringe; few birds developed a viremia response after infection with SLE, precluding analysis. In chickens, increasing the syringe dose of WEE from 0.3 to 1.7 log10 PFU/0.1 ml shortened the time when viremia first appeared from 3 to 1 d postinfection and increased the duration of the viremia period from 1 to 3 d, but did not alter the maximum viremia titer. In house finches, increasing the syringe dose of WEE from 2.6 to 3.3 log10 PFU/0.1 ml did not alter markedly the viremia response. Most birds developed antibody detected by enzyme immunoassay (EIA) or plaque reduction neutralization test (PRNT). In chickens, WEE EIA levels and PRNT titers were higher for birds infected by syringe than by mosquito bite, whereas in house finches the pattern was reversed. For birds infected with SLE, there was overlap among groups infected by mosquito bite or syringe. These results indicate that subcutaneous syringe inoculation provides a biologically sound mode of infection that did not alter viremia and antibody responses when compared with infection by mosquito bite.

Genetic variation among isolates of western equine encephalomyelitis virus from California

The mechanism for long-term maintenance of western equine encephalomyelitis (WEE) virus in California was investigated by studying genetic variation in the E2 portion of the genome of 55 strains of WEE virus isolated since 1938 from different locations in California. Four major lineages were evident: virus strains isolated from the Central Valley since 1993 and Los Angeles in 1991 formed lineage A; southern California strains isolated since 1978 and isolates from the Central Valley from 1978 to 1987 formed lineage B; northern California isolates from 1968 to 1971 formed lineage C; and early isolates from 1938 to 1961 formed a fourth lineage, D. The separation of strains from north and south of the Tehachapi and San Bernardino Mountains (i.e., the Central Valley and southern California, respectively) since 1991 indicates that there has been little recent movement of virus between the two regions and recent strains from these two locations appear to be evolving independently. However, within the Central Valley and within southern California, virus appears to circulate freely, perhaps by movement of birds or mosquito vectors. Although the current virus lineage in the Central Valley may have been introduced from an unknown source in 1991, introduction and establishment of new viral genotypes from outside California do not seem to occur regularly. It appears most likely that virus is maintained in separate geographic areas of California through local persistence in enzootic foci.

Molecular epidemiological studies of veterinary arboviral encephalitides

Recent studies using molecular genetic approaches have made important contributions to our understanding of the epidemiology of veterinary arboviral encephalitides. Viruses utilizing avian enzootic hosts, such as Western equine encephalitis virus (WEEV) and North American Eastern equine encephalitis virus (EEEV), evolve as relatively few, highly conserved genotypes that extend over wide geographic regions; viruses utilizing mammalian hosts with more limited dispersal evolve within multiple genotypes, each geographically restricted. Similar findings have been reported for Australian alphaviruses. This difference may be related to vertebrate host relationships and the relative mobility of mammals and avians. Whereas EEEV and Venezualan equine encephalitis virus (VEEV) utilize small mammalian hosts in the tropics, most WEEV genotypes probably utilize avian hosts in both North and South America. The ability of mobile, infected avian hosts to disperse alphaviruses may result in continual mixing of virus populations, and thus limit diversification. This high degree of genetic conservation is also exhibited by EEE and Highlands J viruses in North America, where passerine birds serve as amplifying hosts in enzootic transmission foci. Most equine arboviral pathogens, including EEEV, WEEV and Japanese encephalitis virus (JEV), occur in a naturally virulent enzootic state and require only appropriate ecological conditions to cause epizootics and epidemics. However, VEE epizootics apparently require genetic changes to convert avirulent enzootic strains into distinct epizootic serotypes. All of these arboviruses have the potential to cause severe disease of veterinary and human health importance, and further molecular epidemiological studies will undoubtedly improve our ability to understand and control future emergence.

Vector competence of Aedes dorsalis (Diptera: Culicidae) from Morro Bay, California, for western equine encephalomyelitis virus

In laboratory vector competence studies, Aedes dorsalis (Meigen) collected from Morro Bay, CA, did not vertically transmit sympatric strains of western equine encephalomyelitis virus (WEE). This population of Ae. dorsalis was highly susceptible to oral infection and was a competent horizontal vector of WEE. The E2 region of the viral genome of the 3 virus strains isolated from Ae. dorsalis in Morro Bay were closely related genetically to a strain of WEE isolated in 1953 from a geographically separate location that is used regularly in the laboratory. These laboratory findings support recent field research and indicate that Ae. dorsalis probably does not play a significant role in WEE persistence in coastal California.

Experimental infection of turkey poults with western equine encephalitis virus

The pathogenicity of a field isolate of western equine encephalitis (WEE) virus, which was recovered from a breeder hen during investigations of egg production drops in California turkey flocks, was tested in 2-wk-old turkey poults. No symptoms or mortality were observed in poults inoculated intramuscularly with 4.2 log10 50% embryo lethal doses of virus; however, the infection did result in mild to moderate lymphoid necrosis in the bursa of Fabricius and thymus glands beginning on the first day postinoculation. In addition, WEE virus could be isolated from the blood of infected poults for up to 3 days postinoculation.

Efficacy of eastern equine encephalitis immunization in whooping cranes

An epizootic of eastern equine encephalitis (EEE) at the Patuxent Wildlife Research Center (PWRC), Laurel, Maryland (USA), in 1989 provided an opportunity to determine if EEE immunization protected whooping cranes (Grus americana). Based on seroconversion of 31% of sympatric hatch-year sandhill cranes, Grus canadensis, and a previous 35% case fatality rate in whooping cranes, 17 (37%) of the 46 susceptible whooping cranes should have been exposed to virus and six should have died. As there were no deaths in these birds, the EEE vaccination program appeared to be efficacious in this whooping crane population.

Biological characteristics of an enzootic subtype of western equine encephalomyelitis virus from Argentina

In order to expand our knowledge on the biological characteristics of an enzootic South American subtype of western equine encephalomyelitis (WEE) virus, strain AG80-646, we inoculated guinea pigs, rabbits, newborn chickens and Vero and chick embryo cell cultures with this and other WEE and Wee-related viruses. AG80-646 was found apathogenic for guinea pigs even when inoculated intracranially (i.e.) or intraperitoneally (i.p.), and the animals did not develop viraemia. AG80-646 killed rabbits and the animals developed high viraemia (peak titer was 7.0 log PFU/0.1 ml). These data and previous serological evidence led us to look for a mammal as a natural host. AG80-646 was found lethal for newborn chickens inoculated subcutaneously (s.c.) (peak viraemia titer was 6.6 log PFU/0.1 ml). AG80-646 produced plaques (diameter 0.8-1.0 mm) in Vero and chick embryo cells 3-4 days post infection (p.i.) A comparison of AG80-646 with other WEE complex virus strains led to the following observations: (1) The lethality for guinea pigs was high for the two epizootic Argentinian strains, Cba 87 and Cba CIV 180, zero for the two enzootic strains, AG80-646 and BeAr 10315 (virus Aura), and intermediate for the Russian strain Y62-33 (low by i.c. route and zero by i.p. route); (2) AG80-646 was more virulent for rabbits inoculated i.p. than the three epizootic strains Cba 87, Cba CIV 180 and McMillan; (3) AG80-646 was less virulent for new-born chickens than the Argentinian epizootic strain Cba CIV 180; (4) The viraemia level correlated always with the strain virulence in each animal host. This study provides tools for the differentiation of WEE complex viruses and strains in the future ecological work on WEE in South America.

Eastern equine encephalitis transmission to emus (Dromaius novaehollandiae) in Volusia County, Florida: 1992 through 1994

From May 1992 through October 1994, sera were collected from 204 domestic emus (Dromaius novaehollandiae) at a ranch in Volusia County, FL, and tested for antibody evidence of arboviral infection. Hemagglutination-inhibition (HI) and neutralizing (NT) antibodies to eastern equine encephalitis (EEE) virus were identified in sera collected during each year. In addition, HI and NT antibodies to St. Louis encephalitis virus were detected in 3 naturally infected emus. Isolations of EEE virus were made from emu blood and tissues collected in 1992 and 1994, when EEE-related mortality in emus was 14% and 1%, respectively. A total of 259 mosquito pools was collected and tested for arbo-viruses during the 3-year study. The EEE virus was isolated from 4 of 140 mosquito pools (2 Anopheles crucians and 2 Culex erraticus pools) in 1992 and 3 of 10 pools (all Culex nigripalpus) in 1994. Emus vaccinated against EEE virus showed evidence of short-term HI antibody acquisition. Evidence of EEE antibody transfer from naturally infected hens to their offspring is reported.

Protection of pigs by vaccination of pregnant sows against eastern equine encephalomyelitis virus

Serum-virus neutralizing antibodies were detected in serum and colostrum of sows vaccinated during pregnancy with commercially available vaccines against eastern equine encephalomyelitis virus (EEEV), and antibodies were detected in serum from nearly all pigs from vaccinated sows following colostrum uptake. Serum-virus neutralizing antibody (SVN) test titers were measured in colostrum and pigs at the next farrowing, and additional vaccination of sows prior to the third farrowing led to elevated SVN titers in serum, colostrum and all pigs. Six pigs from vaccinated sows challenged at 8 to 9 days of age with 1 x 10(6) TCID50 EEEV did not develop the high temperatures or signs of central nervous system disease that 6 pigs from non-vaccinated sows developed. Virus was isolate from blood and oropharyngeal swabs from all pigs from non-vaccinated sows with blood virus titers as high as 9.3 x 10(4) TCID50, while only low levels of virus were detected in blood and oropharyngeal swabs from pigs from vaccinated sows. Virus was also isolated from tonsils collected at necropsy from 3 pigs from non-vaccinated and 1 pig from vaccinated sows. Vaccination of pregnant sows leads to development of maternal antibodies that are transmitted via colostrum to pigs and are protective against clinical EEEV related disease after experimental challenge with EEEV. In addition, vaccination prevents amplification of virus in infected pigs and could result in protection of animals and farm labor in the environment of infected pigs.

Antibodies to arthropod-borne encephalitis viruses in small mammals from southern Florida

From 1987 through 1991, blood samples were collected from 10 species of small mammals in Indian River Country, Florida (USA). Sera from 1,347 animals were analyzed for hemagglutination-inhibition (HI) antibody to St. Louis encephalitis (SLE) and eastern equine encephalitis (EEE) viruses. Of these, 75 (5.6%) were positive for HI antibody to SLE virus and 121 (9.0%) were positive for EEE antibody. Sera from five mammalian species were tested for neutralizing (NT) antibody to SLE, EEE, Highlands J (HJ a member of the western equine encephalitis virus complex), or Everglades (EVE, a member of the Venezuelan equine encephalitis complex) viruses. By serum neutralization tests, 26 (46%) of 57 had SLE antibodies, 14 (24%) of 58 had EEE antibodies, two (3.2%) of 63 had HJ antibodies, and 9 (14%) of 63 had EVE antibodies. One Sigmodon hispidus and one Peromyscus gossypinus had NT antibodies both to EEE and HJ viruses. Blood samples from 512 mammals were tested for virus. Isolations of one EVE virus and two unidentified arenaviruses were made from P. gossypinus and one EVE virus isolate was made from a S. hispidus.

Evaluation of outbreaks of disease attributable to eastern equine encephalitis virus in horses

OBJECTIVES

To evaluate outbreaks of disease attributable to eastern equine encephalitis virus (EEEV) in horses in Michigan, and the associated environmental patterns and weather conditions, so that factors could be identified that may have predisposed horses in specific areas of the state to infections with EEEV.

DESIGN

Epidemiologic retrospective records analysis.

ANIMALS

Data on EEEV vectors, wild-bird reservoir hosts, and incidental hosts, including horses and human beings, obtained from census reports and medical records compiled between 1942 and 1991.

PROCEDURE

Patterns detected during outbreaks of disease attributable to EEEV infections in horses were compared to associated water drainage patterns, distributions of EEEV vectors, wild-bird reservoir and incidental hosts, and weather conditions.

RESULTS

Michigan has all of the elements required to sustain EEEV on a state-wide basis. Outbreaks of disease attributable to EEEV in horses have recurred in a similar regional distribution in Michigan. Regions of Michigan that have specific patterns for water drainage, specific mosquito species, and areas with higher than expected amounts of precipitation have been associated with outbreaks of disease attributable to EEEV in horses.

CLINICAL IMPLICATIONS

Evaluation of environmental patterns, weather conditions, and vector and reservoir host distributions may be useful to identify areas in Michigan and elsewhere in which horses and human beings are at increased risk for an outbreak of disease attributable to EEEV.

Evidence for multiple foci of eastern equine encephalitis virus (Togaviridae:Alphavirus) in central New York State

A regional surveillance system for eastern equine encephalitis (EEE) virus was established in central New York in 1984 after the 2nd human EEE fatality occurred in 1983. Extensive mosquito surveillance activities were coordinated with the rapid laboratory processing of mosquito specimens for EEE virus. Active surveillance for EEE infections in humans and equines also was initiated. Results of long-term surveillance detected the presence of multiple Culiseta breeding swamps. A 6-yr interepizootic period (1984-1989) was followed by 2 yr of equine EEE. In 1990, there were 7 equine cases and a record number of EEE virus isolations from mosquitoes (n = 86), wild birds (n = 27), and sentinel pheasants (n = 7). In 1991, 7 equine cases also occurred, although there were fewer isolations from mosquitoes (n = 40). The sequence to the appearance of EEE virus at swamps and upland sites and at individual swam complexes, and the spatial and temporal distribution of equine cases provide evidence for multiple foci of EEE virus in central New York. The role of infected Culiseta melanura (Coquillett) in the transfer of EEE virus between swamp and upland areas and among swamp complexes is advanced.

Clinical and pathological features of Nigerian equine encephalitis

Thirteen cases of a disease with a low morbidity and very high mortality in horses in Nigeria are described; the disease is characterised by fever (rectal temperature > or = 40 degrees C), generalised muscle spasms, ataxia, increased respiratory and heart rates and terminal lateral recumbency. The illness generally lasts three to five days but durations of 12 to 30 hours have been observed. Laboratory investigations, including histopathology and serology suggest a viral aetiology, possibly an alphavirus of the equine encephalitis group.

Detection of eastern equine encephalomyelitis virus RNA in formalin-fixed, paraffin-embedded tissues using DNA in situ hybridization

Eastern equine encephalomyelitis (F.EE) virus was detected in infected formalin-fixed horse and emu tissues and in infected chicken embryo fibroblasts. Results of in situ hybridization using a digoxigenin-labeled 40-base DNA probe complementary to a conserved region of the EEE virus RNA compared favorably with results of both virus isolation and serum neutralization tests. This technique may be useful for diagnosis of EEE virus infection in various animal species, especially when fresh tissues are not available for analysis, and also will provide a means for studying the involvement of alphaviruses in pathogenesis studies.

Immunohistochemical diagnosis of eastern equine encephalomyelitis

An immunohistochemical (IHC) assay was developed for the detection of eastern equine encephalomyelitis (EEE) virus antigen in formalin-fixed, paraffin-embedded tissues. All cases of EEE diagnosed at the Michigan State University Animal Health Diagnostic Laboratory from 1991 through 1994 were evaluated. The diagnosis was based on histopathologic examination of the brain and confirmatory virus, isolation. Sections of cerebrum from 26 equids and 5 birds were assessed by IHC. Histologically normal brain tissues from 2 horses and 1 pheasant and brain tissues from 2 cases of equine neurologic disease with diagnoses other than EEE served as negative controls. The IHC assay was based on standard streptavidin-biotin technology, using a commercially available kit and a monospecific polyclonal primary antibody preparation derived from murine ascitic fluid. Nineteen of 20 equids and all 5 birds positive by histopathology and virus isolation were positive for EEE virus antigen by IHC. Three equids with histologic lesions compatible with a diagnosis of EEE but negative by virus isolation also were negative for virus antigen by IHC. In 3 other equids, histopathology and IHC were positive for EEE, but virus isolation was not attempted because of contamination of the brain specimen. The IHC assay of formalin-fixed, paraffin-embedded brain tissues for EEE virus antigen is a rapid, effective test for confirming a histopathologic diagnosis of EEE, and assay results correlate well with virus isolation results.

Population size, parity structure, and wing length of Coquillettidia perturbans in an Ohio focus of eastern equine encephalitis

Adult female density, parity status, and wing length were determined weekly for a population of Coquillettidia perturbans in an area enzootic for eastern equine encephalitis virus in central Ohio. Samples were collected in CO2-baited CDC miniature light traps from the first week in June through the 2nd week of September 1992. Population density indicated a single emergence peak during the 2nd week in July. However, parity rates showed 2 peaks, occurring in the first week of August (70.9% parous) and the 2nd week of September (55.3% parous), which suggested that there was a relatively small 2nd generation. Average wing length declined significantly over the season. The decline in size was negatively correlated with average air temperature occurring at least 6 wk before the time of emergence. Despite the seasonal decline in wing length, the low coefficient of variation for the average wing length (5.5) indicated relatively little variation in size. Comparison of parous and nulliparous female wing lengths each week suggested that there was no association between size and survival in this species.

Experimental infection of wading birds with eastern equine encephalitis virus

To study the susceptibility of wading birds to eastern equine encephalitis (EEE) virus and to determine their potential as reservoir or amplifying hosts, fledgling glossy ibises (Plegadis falcinellus) and snowy egrets (Egretta thula) were captured in New Jersey (USA) and shipped to Colorado (USA) where they were experimentally inoculated with EEE virus. All 16 snowy egrets and 14 (93%) of 15 of the glossy ibises inoculated became viremic with moderate titers, and all survivors developed neutralizing antibody. Six ibises and two egrets died during the first week after inoculation, and EEE virus was isolated from the tissues of three birds. Our experimental results support field evidence about the relative involvement of glossy ibises and snowy egrets in the epizootiology of EEE virus in New Jersey.

Centers for Disease Control and Prevention (CDC). Arboviral disease--United States, 1994

Arboviruses are mosquitoborne and tickborne agents that persist in nature in complex cycles involving birds and mammals, including humans. Characteristics of arboviral infection include fever, headache, encephalitis, and sometimes death. In 1994, health departments in 20 states reported 100 presumptive or confirmed human cases of arboviral disease to CDC. Of these, 76 were California (CAL) serogroup encephalitis; 20, St. Louis encephalitis (SLE); two, western equine encephalomyelitis (WEE); one, eastern equine encephalomyelitis (EEE); and one, Powassan encephalitis (POW). This report summarizes information about arboviral disease in the United States during 1994.

Landscape ecology of arboviruses in southern California: patterns in the epizootic dissemination of western equine encephalomyelitis and St. Louis encephalitis viruses in Coachella Valley, 1991-1992

Temporal and spatial patterns in the initiation and dissemination of western equine encephalomyelitis and St. Louis encephalitis virus activity in Coachella Valley during 1991 and 1992 were detected by testing pools of host-seeking Culex tarsalis Coquillett for virus infection and sentinel chickens for seroconversions. Both viruses repeatedly were detected first at a salt marsh adjacent to the Salton Sea in the southeastern corner of the study area and then disseminated to the northwest to freshwater marsh, agricultural, and residential habitats. Virus dissemination was relatively slow (< 1 km/d) and may have been accomplished by dispersive host-seeking mosquitoes. Repeated early-season recovery of virus activity indicated that both viruses may persist interseasonally in salt marsh habitat.