West Nile encephalitis

Monitoring the epidemic of West Nile virus in equids in Spain, 2020-2021

The largest epidemic of West Nile virus (WNV) reported ever in Spain in both humans and equines occurred in 2020, affecting 77 humans and 139 equine herds. Here, we aimed to monitor the outbreaks detected in equid herds in Andalusia (southern Spain), the Spanish region where 89.9% of the outbreaks were reported, and to evaluate the virus circulation and risk factor associated with WNV exposure in the affected herds. The first WNV case was detected in mid-July 2020, the number of outbreaks peaked in mid-August and the last one was confirmed on 26th October 2020. WNV lineage 1 was detected in 12 clinically affected horses using real time RT-PCR. Molecular analysis evidenced high nucleotide identity with WNV sequences obtained from humans, birds and mosquitoes from Spain and Italy between 2020 and 2022. Between five and eight months after the WNV epidemic, a total of 724 equids (including 485 unvaccinated and 239 vaccinated animals) from 113 of the 125 affected herds in Andalusia were sampled. IgM and IgG antibodies against WNV were detected in 1.6% (8/485; 95%IC: 0.0-2.5) and 61.9% (300/485; 95%IC: 58.3-65.5) of the unvaccinated individuals, respectively. The seropositivity in vaccinated horses was 86.6% (207/239). The main risk factors associated with WNV exposure in unvaccinated equids were the breed (crossbreed), the location of animals in spring-summer (outside), and the presence of natural water ponds close to the surveyed herds. The high individual seroprevalence obtained in the affected herds indicates that WNV circulation was more widespread than the reported by passive surveillance during the WNV epidemic in 2020. The re-emergence of WNV in 2020 in southern Spain evidenced the needed to improve integrated surveillance systems, minimizing the impact of future cases in equids and humans in high-risk areas.

Comparison of West Nile Virus Disease in Humans and Horses: Exploiting Similarities for Enhancing Syndromic Surveillance

West Nile virus (WNV) neuroinvasive disease threatens the health and well-being of horses and humans worldwide. Disease in horses and humans is remarkably similar. The occurrence of WNV disease in these mammalian hosts has geographic overlap with shared macroscale and microscale drivers of risk. Importantly, intrahost virus dynamics, the evolution of the antibody response, and clinicopathology are similar. The goal of this review is to provide a comparison of WNV infection in humans and horses and to identify similarities that can be exploited to enhance surveillance methods for the early detection of WNV neuroinvasive disease.

Horses as Sentinels for the Circulation of Flaviviruses in Eastern-Central Germany

Since 2018, autochthonous West Nile virus (WNV) infections have been regularly reported in eastern-central Germany. While clinically apparent infections in humans and horses are not frequent, seroprevalence studies in horses may allow the tracing of WNV and related flaviviruses transmission, such as tick-borne encephalitis virus (TBEV) and Usutu virus (USUV), and consequently help to estimate the risk of human infections. Hence, the aim of our study was to follow the seropositive ratio against these three viruses in horses in Saxony, Saxony Anhalt, and Brandenburg and to describe their geographic distribution for the year 2021. In early 2022, i.e., before the virus transmission season, sera from 1232 unvaccinated horses were tested using a competitive pan-flavivirus ELISA (cELISA). In order to estimate the true seropositive ratio of infection with WNV, TBEV, and USUV for 2021, positive and equivocal results were confirmed by a virus neutralization test (VNT). In addition, possible risk factors for seropositivity using questionnaires were analyzed using logistic regression based on questionnaires similar to our previous study from 2020. In total, 125 horse sera reacted positive in the cELISA. Based on the VNT, 40 sera showed neutralizing antibodies against WNV, 69 against TBEV, and 5 against USUV. Three sera showed antibodies against more than one virus, and eight were negative based on the VNT. The overall seropositive ratio was 3.3% (95% CI: 2.38-4.40) for WNV, 5.6% (95% CI: 4.44-7.04) for TBEV, and 0.4% (95% CI: 0.14-0.98) for USUV infections. While age and number of horses on the holding were factors predicting TBEV seropositivity, no risk factors were discovered for WNV seropositivity. We conclude that horses are useful sentinels to determine the flavivirus circulation in eastern-central Germany, as long as they are not vaccinated against WNV.

West-Nil-Virus-Infektion bei 12 Pferden in Mitteldeutschland

Gegenstand und Ziel Die Vorstellungsgründe, klinische Symptomatik, Diagnostik, Therapie und Ergebnis der Therapie von 12 Pferden mit klinisch apparenter West-Nil-Virus (WNV)-Infektion werden beschrieben.

Material und Methoden Fallserie

Ergebnisse Die adulten Pferde (Alter 6–18 Jahre, 7 Stuten, 5 Wallache) aus Sachsen oder Sachsen-Anhalt wurden zwischen September 2018 und September 2020 mit unterschiedlichen Vorberichten vorgestellt. Alle Pferde wurden im August oder September vorgestellt und keines der Pferde war gegen das WNV geimpft. Fieber war das häufigste Allgemeinsymptom und trat bei 8/12 Pferden auf. An neurologischen Symptomen traten Muskelfaszikulationen (11/12 Pferde), Ataxie (8/12 Pferde) sowie Hyperästhesie und Kopfschiefhaltung (je 6/12 Pferde) am häufigsten auf. Bei allen Pferden wurde die Infektion mittels Nachweises von IgM sowie neutralisierenden Antikörpern gegen das WNV diagnostiziert, zwei euthanasierte Pferde waren zudem PCR-positiv. Die symptomatische Therapie beinhaltete vor allem nicht-steroidale Antiphlogistika oder Dexamethason sowie Infusionstherapie. Die Dauer des Klinikaufenthaltes betrug im Durchschnitt 7,5 Tage. Sieben Pferde erholten sich laut Besitzerangaben vollständig, für 2 Pferde war keine Information erhältlich.

Schlussfolgerungen und klinische Relevanz Die WNV-Enzephalomyelitis muss in Mitteldeutschland seit 2018 als Differentialdiagnose von im Sommer und Spätsommer auftretenden akuten neurologischen Erkrankungen ungeimpfter Pferdes in Betracht gezogen werden. Die beschriebenen Symptome sowie das Ergebnis der Therapie sind weitgehend deckungsgleich mit Berichten aus Nordamerika und anderen europäischen Ländern.

Arboviral Equine Encephalitides

A number of viruses transmitted by biological vectors or through direct contact, air, or ingestion cause neurologic disease in equids. Of interest are viruses of the Togaviridae, Flaviviridae, Rhabdoviridae, Herpesviridae, Bornaviridae, and Bunyaviridae families. Many are classified as arboviruses because they use arthropod vectors, whereas others are transmitted directly via ingestion, inhalation, or integument damage. The goal of this article is to provide an overview on pathophysiologic and clinical aspects of arboviruses of equine importance, including alphaviruses (Togaviridae) and flaviviruses (Flaviviridae).

Seroprevalence and Risk Factors for Equine West Nile Virus Infections in Eastern Germany, 2020

West Nile virus (WNV) infections were first detected in Germany in 2018, but information about WNV seroprevalence in horses is limited. The study’s overall goal was to gather information that would help veterinarians, horse owners, and veterinary-, and public health- authorities understand the spread of WNV in Germany and direct protective measures. For this purpose, WNV seroprevalence was determined in counties with and without previously registered WNV infections in horses, and risk factors for seropositivity were estimated. The cohort consisted of privately owned horses from nine counties in Eastern Germany. A total of 940 serum samples was tested by competitive panflavivirus ELISA (cELISA), and reactive samples were further tested by WNV IgM capture ELISA and confirmed by virus neutralization test (VNT). Information about potential risk factors was recorded by questionnaire and analyzed by logistic regression. A total of 106 serum samples showed antibodies against flaviviruses by cELISA, of which six tested positive for WNV IgM. The VNT verified a WNV infection for 54 samples (50.9%), while 35 sera neutralized tick-borne encephalitis virus (33.0%), and eight sera neutralized Usutu virus (7.5%). Hence, seroprevalence for WNV infection was 5.8% on average and was significantly higher in counties with previously registered infections (p = 0.005). The risk factor analysis showed breed type (pony), housing in counties with previously registered infections, housing type (24 h turn-out), and presence of outdoor shelter as the main significant risk factors for seropositivity. In conclusion, we estimated the extent of WNV infection in the resident horse population in Eastern Germany and showed that seroprevalence was higher in counties with previously registered equine WNV infections.

A Pilot Serosurvey for Selected Pathogens in Feral Donkeys (Equus asinus)

Simple Summary

This study aimed to assess the pathogen exposure status of recently captured feral donkeys from Death Valley National Park, California. Assays to detect the presence of antibodies to equine herpesvirus 1, equine influenza virus, West Nile virus, and Borrelia burgdorferi (the causative agent of Lyme disease) were performed on serum samples from these feral donkeys. The results indicate that this population is mostly naïve and likely susceptible to these common equid pathogens upon removal from the wild.

Abstract

Recent removal and relocation of feral donkeys from vast public lands to more concentrated holding pens, training facilities, and offsite adoption locations raises several health and welfare concerns. Very little is known regarding the common equid pathogens that are circulating within the feral donkey population in and around Death Valley National Park, California, USA. The aim of this study was to utilize serologic assays to assess previous exposure of these donkeys to equine herpesvirus 1 (EHV-1), equine influenza (EIV), West Nile virus (WNV), and Borrelia burgdorferi (the causative agent of Lyme disease). The results of this study indicate that this feral equid population is mostly naïve and likely susceptible to these common equid pathogens upon removal from the wild.

West Nile Virus: Veterinary Health and Vaccine Development

West Nile virus (WNV) (Flaviviridae: Flavivirus) was discovered in Africa more than 80 yr ago and became recognized as an avian pathogen and a cause of neurologic disease in horses largely during periodic incursions into Europe. Introduction of WNV into North America stimulated great anxiety, particularly in the equine industry, but also for pet owners and livestock producers concerned about the effect of WNV on other domestic animals. Numerous subsequent studies of naturally occurring and experimentally induced disease greatly expanded our understanding of the host range and clinical consequences of WNV infection in diverse species and led to rapid development and deployment of efficacious vaccines for horses. In addition to humans, horses are clearly the animals most frequently affected by serious, sometimes lethal disease following infection with WNV, but are dead-end hosts due to the low-magnitude viremia they develop. Dogs, cats, and livestock species including chickens are readily infected with WNV, but only occasionally develop clinical disease and are considered dead-end hosts for the virus.

Clinical and epidemiological features of West Nile virus equine encephalitis in New South Wales, Australia, 2011

BACKGROUND

Between February and June 2011, more than 300 horses with unexplained neurological disease were observed in New South Wales, Australia. A virulent strain of West Nile virus (WNV_NSW2011 ), of Australian origin, was shown to be the cause of many of these cases.

METHODS

We reviewed the clinical descriptions provided by veterinary practitioners and the associated laboratory results. Although there was a range of clinical signs described, ataxia was the only sign that was consistently described in laboratory-confirmed cases.

RESULTS

WNV was detected in brain samples by real-time reverse transcription PCR assay and virus isolation. For serological confirmation of clinical cases, an equine IgM ELISA specific for WNV was shown to be the most effective tool.

CONCLUSION

A state-wide serological survey undertaken after the outbreak indicated that, contrary to expectation, although infection had been widespread, the seroprevalence of antibodies to WNV was very low, suggesting that there could be a significant risk of future disease outbreaks.

West Nile virus infections are here! Are we prepared to face another flavivirus epidemic?

Emerging arthropod-borne viruses (arboviruses), such as chikungunya and Zika viruses, are a major threat to public health in countries like Brazil where biodiversity is high and medical care is sometimes precarious. West Nile fever is a disease caused by the West Nile Virus (WNV), an RNA virus belonging to the Flaviviridae family. It is transmitted by infected mosquitoes to numerous animals like birds, reptiles and mammals, including human and non-human primates. In the last decade, the number of reported cases of WNV infection in humans and animals has increased in the Americas. Circulation of WNV in forests and rural areas in Brazil has been detected based on serological surveys and, in 2014, the first case of West Nile fever was confirmed in a patient from Piauí State. In 2018, the virus was isolated for the first time from a horse from a rural area in the state of Espírito Santo presenting with a neurological disorder; this raises the possibility that other cases of WNV encephalitis may have occurred without clinical recognition and without laboratory diagnosis by specific assays. The imminent WNV outbreak poses a challenge for Brazilian clinicians and researchers. In this review, we summarize the basic biological and ecological characteristics of this virus and the clinical presentation and treatment of febrile illnesses caused by WNV. We also discuss the epidemiological aspects, prophylaxis of WNV infections, and monitoring strategies that could be applied in the possibility of a WNV outbreak in Brazil.

First isolation of West Nile virus in Brazil

BACKGROUND

Serological evidence of West Nile virus (WNV) infection has been reported in different regions of Brazil from equine and human hosts but the virus had never been isolated in the country.

OBJECTIVES

We sought to identify the viral etiology of equine encephalitis in Espírito Santo state.

METHODS

We performed viral culture in C6/36 cells, molecular detection of WNV genome, histopathology and immunohistochemistry from horse cerebral tissue. We also carried out sequencing, phylogenetic analysis and molecular clock.

FINDINGS

Histopathologic analysis from horse cerebral tissue showed injury related to encephalitis and WNV infection was confirmed by immunohistochemistry. The virus was detected by reverse transcription quantitative polymerase chain reaction (RT-qPCR) from brain tissue and subsequently isolated in C6/36 cells. WNV full-length genome was sequenced showing the isolated strain belongs to lineage 1a. The molecular clock indicated that Brazilian WNV strain share the same common ancestor that were circulating in US during 2002-2005.

MAIN CONCLUSIONS

Here we report the first isolation of WNV in Brazil from a horse with neurologic disease, which was clustered into lineage 1a with others US WNV strains isolated in beginning of 2000’s decade.

West Nile virus in horses during the summer and autumn seasons of 2015 and 2016, Portugal

West Nile fever (WNF) is an emergent disease in Europe, under surveillance in the European Union. Following a 5-year period of apparent silence (autumn 2010 to summer 2015), West Nile virus (WNV) reemerged in the South of Portugal, in July 2015. Here we present data from the onset, geographic location within mainland Portugal, and outcome of clinical cases of WNV infection in horses in 2015 and 2016. During the transmission seasons of 2015 and 2016, twenty-seven horses, most symptomatic (n=20) were found positive to IgM, pr-E immunoglobulins and VNT, leading to the subsequent report to Animal Disease Notification System of the European Commission (ADNS) by the Portuguese National Authority for Animal Health. Outbreaks occurred in the middle summer (August) and early/mid autumn (October/November) of 2015 and 2016, in the southern regions of the country (Alentejo and Algarve). Compared with the previous WNV transmission seasons of 2004 and 2010, a higher number of cases were reported in 2015 and 2016. The results of our study contribute to increase information concerning the geographic areas affected and time period for WNV transmission risk in Portugal.

Diagnosis of West Nile Virus Infection in Horses

The North American West Nile virus (WNV) epizootic, which began in 1999, has caused significant morbidity and mortality in horses. Because experimental infection has failed to consistently produce encephalitis in inoculated horses, investigation of naturally occurring cases was used to optimize strategies for diagnosis of this disease. Although WNV RNA could be detected by reverse transcriptase—polymerase chain reaction (RT-PCR) performed on whole blood collected from both clinically affected horses and unaffected herdmates, the diagnostic sensitivity of this approach was low compared with IgM-capture enzyme-linked immunosorbent assay. In addition, it was observed that 18.5% of herdmates of clinically ill horses seroconverted to WNV yet exhibited no overt clinical signs of WNV encephalitis. West Nile viral RNA was detected in neural tissue of 46 of 64 dead horses that were suspected of having WNV encephalitis. Some of these animals were IgM negative or had not been tested serologically. A primary cause of death other than WNV encephalitis was identified in 15 of the 64 cases, whereas the final diagnosis for 3 of these cases remains unresolved. Quantitative RT-PCR analysis of neural tissue from WNV RNA—positive horses demonstrated that the medulla contained the highest mean concentration of viral RNA and that WNV RNA could be detected in samples extracted from formalin-fixed neural tissue. A comparison of WNV RT-PCR amplification strategies found that nested RT-PCR improved diagnostic sensitivity only slightly over a single round of amplification and that a quantitative (TaqMan) assay had sensitivity and specificity that were equivalent to those of nested amplification.

West Nile virus-specific immunoglobulin isotype responses in vaccinated and infected horses

OBJECTIVE

To compare antibody responses of horses naturally infected with West Nile virus (WNV) and those vaccinated against WNV, to identify whether vaccination interferes with the ability to diagnose WNV infection, and to determine the duration of antibody responses after vaccination.

SAMPLE

Sera from horses naturally infected with WNV (n = 10) and adult WNV-naïve horses before and after vaccination with a live canarypox virus-vectored vaccine (7) or a killed virus vaccine (8).

PROCEDURES

An established WNV IgM capture ELISA was used to measure IgM responses. Newly developed capture ELISAs were used to measure responses of 8 other WNV-specific immunoglobulin isotypes. A serum neutralization assay was used to determine anti-WNV antibody titers.

RESULTS

WNV-specific IgM responses were typically detected in the sera of WNV-infected horses but not in sera of horses vaccinated against WNV. Natural infection with and vaccination against WNV induced an immunoglobulin response that was primarily composed of IgG1. West Nile virus-specific IgG1 was detected in the sera of most horses 14 days after vaccination. Serum anti-WNV IgG1 and neutralizing antibody responses induced by the killed-virus vaccines were higher and lasted longer than did those induced by the live canarypox virus-vectored vaccine.

CONCLUSIONS AND CLINICAL RELEVANCE

On the basis of these findings, we recommend that horses be vaccinated against WNV annually near the beginning of mosquito season, that both IgM and IgG1 responses against WNV be measured to distinguish between natural infection and vaccination, and that a WNV IgG1 ELISA be used to monitor anti-WNV antibodies titers in vaccinated horses.

Application of West Nile virus diagnostic techniques

West Nile virus (WNV) is an enveloped RNA virus in the family Flaviviridae and belongs to Japanese encephalitis virus serocomplex group. The WNV has a wide geographic distribution that includes Africa, Europe, Asia, America and Australia. Recently, it has re-emerged as an important pathogenic organism, illustrated by the series of WNV outbreaks in North America and in Europe. Several hundred people are sacrificed by WNV infection every year. WNV can infect many mammals, birds, reptiles and amphibians. A variety of diagnoses for WNV infection have been developed, such as virus isolation, nucleotide amplification, antigen detection and serology. Flaviviruses, including WNV, share common nucleotide sequences and antigenic epitopes. Understanding these properties that can influence cross-reactivity is important for accurate diagnosis, especially because areas with multiple flaviviruses are currently expanding. Herein, the authors outline the different diagnostic methods for detecting WNV infection as well as important considerations in using these methods.

Retrospective space-time analysis methods to support West Nile virus surveillance activities

The steep increase in human West Nile virus (WNV) infections in 2011-2012 in north-eastern Italy prompted a refinement of the surveillance plan. Data from the 2010-2012 surveillance activities on mosquitoes, equines, and humans were analysed through Bernoulli space-time scan statistics, to detect the presence of recurrent WNV infection hotspots. Linear models were fit to detect the possible relationships between WNV occurrence in humans and its activity in mosquitoes. Clusters were detected for all of the hosts, defining a limited area on which to focus surveillance and promptly identify WNV reactivation. Positive relationships were identified between WNV in humans and in mosquitoes; although it was not possible to define precise spatial and temporal scales at which entomological surveillance could predict the increasing risk of human infections. This stresses the necessity to improve entomological surveillance by increasing both the density of trapping sites and the frequency of captures.

Quadraplex qRT-PCR assay for the simultaneous detection of Eastern equine encephalitis virus and West Nile virus

In order to increase testing throughput and reduce cost, we developed a multiplex real-time assay that identifies both Eastern equine encephalitis virus and West Nile virus. The assay allows for the screening for the presence of both the nonstructural and envelope genes of both viruses simultaneously allowing for confirmatory testing to be done in a single assay. We utilized newly designed primers and probes, each labeled with a unique fluorescent label allowing for differentiation using an ABI 7500 real-time PCR machine. The use of Quanta Biosciences qScript XLT One-Step RT-qPCR® Toughmix allowed for a quadraplex assay without loss of sensitivity when compared to the previously run singleplex reaction as seen with viral RNA PFU control dilution series. There was no cross reactivity between the viruses within the reaction, and upon utilization of the assay during surveillance, there was no cross reactivity with other historically encountered arthropod-borne viruses. The results from the quantitative Reverse Transcriptase - Polymerase Chain Reaction were comparable to those achieved by cell culture which was performed on a subset of the field mosquito pools screened during the 2012 surveillance season. The multiplex assay resulted in savings in both time and resources for the lab and faster turn-around of results.

West Nile virus in north-eastern Italy, 2011: entomological and equine IgM-based surveillance to detect active virus circulation

Since 2008, West Nile Virus (WNV) has expanded its range in several Italian regions, and its yearly recurrence suggests the virus may have become endemic in some areas. In 2011, a new plan based also on the detection of IgM antibodies was implemented in the north-eastern Italian regions of Veneto and Friuli Venezia Giulia, aiming to early detect WNV infections in areas where the virus had already circulated during the previous summers, and in adjacent zones. From July to November 2011, 1880 sera from 521 equine premises were screened by a commercial IgM capture ELISA. Mosquitoes were captured by CDC-CO2 traps at 61 locations in the two regions. Collected mosquitoes were identified, pooled by species/date/location and examined by real-time RT-PCR and sequencing. Passive surveillance was carried out on clinically affected horses and non-migratory wild birds found dead. IgM sero-positive equines were detected in 19 holdings, five in the area with WNV circulation (AWC) and 14 in the surveillance area (SA); 10 more horse premises tested positive to further serological controls within 4 km of the positive holdings. A total of 85,398 mosquitoes of 15 species were collected and 2732 pools examined. Five Culex pipiens pools tested positive for the presence of WNV. Passive surveillance on non-migratory wild birds allowed detection of the virus only in one found dead collared dove (Streptopelia decaocto), of 82 birds sampled. The WNV belonged to the lineage 2, which had been isolated for the first time in Italy earlier in 2011. By the first week of October, nine human cases had been confirmed in the same area. The implementation of a protocol combining IgM screening of horses with surveillance on mosquito vectors proved to be valuable for early detecting WNV circulation.

Assessing the risks of West Nile virus-infected mosquitoes from transatlantic aircraft: implications for disease emergence in the United Kingdom

The number of West Nile virus (WNV)-infected mosquitoes aboard aircraft from the United States that arrive in the United Kingdom each summer was determined using a quantitative risk assessment. In the worst-case scenario, when WNV levels in mosquitoes are high (at epidemic levels) the probability of at least one WNV-infected mosquito being introduced into the United Kingdom was predicted to be 0.99. During these periods, a mean of 5.2 infected mosquitoes were estimated to be aboard flights from the United States to the United Kingdom during May to October, with 90% certainty that the exact value lies between one and ten mosquitoes. Heathrow airport was predicted to receive the majority of the infected mosquitoes (72.1%). Spatial analysis revealed the region surrounding Heathrow satisfies the criteria for potential WNV exposure as both WNV-competent mosquitoes and susceptible wild bird species are present. This region is, therefore, recommended for targeted, risk-based surveillance of WNV-infected mosquitoes in addition to an increased awareness of the risks to horses, birds and humans.

Seroprevalence of West Nile virus in feral horses on Sheldon National Wildlife Refuge, Nevada, United States

We screened 1,397 feral horses (Equus caballus) on Sheldon National Wildlife Refuge, Nevada, United States, for IgM and IgG against flavivirus during 2004-2006, 2008, and 2009. Positive serum samples were tested for neutralizing antibodies to West Nile virus (WNV) and St. Louis encephalitis virus (SLEV). One animal was positive for antibody against WNV in 2004, but all others tested in 2004-2006 were negative. In 2008 and 2009, we found evidence of increasing seropositive horses with age, whereas seroprevalence of WNV decreased from 19% in 2008 to 7.2% in 2009. No horses were positive for antibody against SLEV. Being unvaccinated, feral horses can be useful for WNV surveillance.

Environmental risk factors for equine West Nile virus disease cases in Texas

West Nile Virus (WNV) was first detected in the Texas equine population during June 2002. Infection has since spread rapidly across the state and become endemic in the equine population. Environmental risk factors associated with equine WNV attack rates in Texas counties during the period 2002 to 2004 were investigated. Equine WNV attack rates were smoothed using an empirical Bayesian model, because of the variability among county equine populations (range 46-9,517). Risk factors investigated included hydrological features (lakes, rivers, swamps, canals and river basins), land cover (tree, mosaic, shrub, herbaceous, cultivated and artificial), elevation, climate (rainfall and temperature), and reports of WNV-positive mosquito and wild bird samples. Estimated county equine WNV attack rate was best described by the number of lakes, presence of broadleaf deciduous forest, presence of cultivated areas, location within the Brazos River watershed, WNV-positive mosquito status and average temperature. An understanding of environmental factors that increase equine WNV disease risk can be used to design and target disease control programs.

Aspectos epidemiológicos da Febre do Oeste do Nilo

Desde sua introdução na América do Norte em 1999, mais de 27.500 casos humanos da infecção por West Nile virus (WNV) foram reportados nos Estados Unidos da América (EUA), resultando em mais de 1000 casos fatais. Recentemente, a disseminação do vírus para o hemisfério sul foi confirmada com a detecção de animais infectados pelo WNV em território sul-americano. A soropositividade para WNV em eqüídeos na Colômbia e Venezuela e o isolamento do vírus nestes animais na Argentina, reiteram a necessidade da manutenção do sistema de vigilância enzoótica para WNV em território brasileiro. Aspectos pertinentes à infecção, patogenia e epidemiologia do WNV são discutidos neste artigo.

Amperometric immunosensor for the detection of anti-West Nile virus IgG

An amperometric immunosensor for the detection of West Nile virus (WNV) IgG was developed. This device was based on the immobilization of T7 phages, which were modified by an additional peptide sequence taken from the virus and used as antigen. The electropolymerization of a phage-amphiphilic pyrrole ammonium mixture previously adsorbed on the electrode surface provided an efficient entrapment of phages in a polypyrrole film. After incubation with a secondary peroxidase-labeled antibody, the immunosensors were applied to the quantitative amperometric determination of WNV-antibody at 0 V vs Ag/AgCl via the reduction of the enzymically generated quinone in the presence of hydroquinone and H2O2. The optimum immunosensor configuration detected low WNV-antibody dilutions down to a titer of 1:10(7) with an excellent regeneration of the immunosensor response by glycine treatment.

Remote sensing based identification of environmental risk factors associated with West Nile disease in horses in Camargue, France

Geographic information system and remote sensing technologies were used to identify landscape features associated with risk of West Nile virus transmission as defined by the presence of confirmed horse cases. SPOT-4 images of Camargue area were used to generate a map of landscape categories of epidemic foci and the geographic information system was employed to determine the proportion of landscape components surrounding 10 horse case sites and 17 control sites. The spatio-temporal analysis of the cases outbreak gave the best results for a spatial window of 9 km and a temporal window of 18 days. Two clusters were identified (relative risk=3.35), both in the wet area of Camargue, near the town "Les Saintes Maries de la Mer". The first one was 4 km radius, the second 9 km. The relationships between the presence of cases and proportions of landscape categories were analyzed using generalized linear model. The best model indicated that rice fields and dry bushes, wet "sansouire" and open water were the major components of the landscape that were associated with the presence of West Nile virus cases.

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.

Prediction of human cases of West Nile virus by equine cases, Saskatchewan, Canada, 2003

In 2003, an outbreak of West Nile virus (WNV) occurred in Saskatchewan, Canada from July to September. One-hundred thirty-three horse cases and 947 human cases were recorded and data were analyzed retrospectively for evidence of clustering to determine if clinical infection in the horse population could be used to estimate human risk of infection with WNV. Kulldorff's scan statistic was used to identify spatial-temporal clusters in both the human and horse cases. In most areas, human clusters were not preceded by horse clusters. In one area, a significant cluster of horse cases preceded human cases by 1 week; however, 1 week does not provide sufficient time for human-health authorities to act and provide advance warning for the public.

Environmental and ecological determinants of West Nile virus occurrence in horses in North Dakota, 2002

West Nile virus (WNV) outbreak in North Dakota in 2002 included over 569 horse cases, clustered mainly in the eastern and northeastern parts of the state. The pattern of occurrence observed suggested existence of specific environmental and ecological factors that increased the risk for infection and illness in those locations. We developed a predictive model with factors that explained the pattern of WNV occurrence observed. Results indicated that surface elevation, temperature, precipitation, reported WNV-positive birds, reported WNV-positive humans, and reported WNV-positive mosquitoes were important predictors of occurrence in horses. However, case distance from water bodies was not significant in the model. Future predictive models of WNV occurrence in horses should take into account these factors in order to improve accuracy and reliability. Research into other potential determinants such as horse management factors are required to determine more differential risk factors associated with WNV occurrence in horses.

Chemokine receptor CCR5 promotes leukocyte trafficking to the brain and survival in West Nile virus infection

The molecular immunopathogenesis of West Nile virus (WNV) infection is poorly understood. Here, we characterize a mouse model for WNV using a subcutaneous route of infection and delineate leukocyte subsets and immunoregulatory factors present in the brains of infected mice. Central nervous system (CNS) expression of the chemokine receptor CCR5 and its ligand CCL5 was prominently up-regulated by WNV, and this was associated with CNS infiltration of CD4+ and CD8+ T cells, NK1.1+ cells and macrophages expressing the receptor. The significance of CCR5 in pathogenesis was established by mortality studies in which infection of CCR5-/- mice was rapidly and uniformly fatal. In the brain, WNV-infected CCR5-/- mice had increased viral burden but markedly reduced NK1.1+ cells, macrophages, and CD4+ and CD8+ T cells compared with WNV-infected CCR5+/+ mice. Adoptive transfer of splenocytes from WNV-infected CCR5+/+ mice into infected CCR5-/- mice increased leukocyte accumulation in the CNS compared with transfer of splenocytes from infected CCR5-/- mice into infected CCR5-/- mice, and increased survival to 60%, the same as in infected CCR5+/+ control mice. We conclude that CCR5 is a critical antiviral and survival determinant in WNV infection of mice that acts by regulating trafficking of leukocytes to the infected brain.

Replication of West Nile virus in equine peripheral blood mononuclear cells

A cell model of primary monocytes and other mononuclear cells isolated from equine blood was used to study the kinetics of West Nile virus (WNV) replication in a natural host. West Nile virus has emerged on the North American continent as a significant cause of morbidity and mortality in a wide range of avian and mammalian species. While other flaviviruses are known to infect monocytes and lymphocytes, the ability of WNV to productively replicate in specific immune cells of peripheral blood has not been assessed. In this study, enriched populations of monocytes and lymphocytes as well as purified monocytes, CD4+, CD8+ and B lymphocytes were obtained from equine blood. Productive WNV replication was demonstrated by viral growth curves, quantitative RT-PCR for WNV RNA, and indirect immunofluorescence detection of a non-structural WNV protein. Enriched and purified monocytes consistently supported productive viral replication in blood from nine of nine horses tested while a minor subset of CD4+ lymphocytes supported productive replication in cells from three of the nine horses tested. Peak viral titers of 3.2-6.6 log10 PFU/ml were reached at 6 days post-inoculation (p.i.) and titers were maintained through 10-15 days p.i. Activation of monocytes with bacterial lipopolysaccharide, which resulted in activation of nuclear transcription factor kappaB (NF-kappaB) plus elevation of nitric oxide and type I interferon levels, reduced or eliminated WNV replication. These results suggest that immune cells of the peripheral blood may serve as target cells for initial replication of WNV and may play a role in subsequent viral dissemination. Furthermore, primary equine immune cell cultures represent a potentially useful model of a natural WNV host when testing compounds such as antivirals for use in WNV treatment.

Immunologic responses to West Nile virus in vaccinated and clinically affected horses

OBJECTIVE

To compare neutralizing antibody response between horses vaccinated against West Nile virus (WNV) and horses that survived naturally occurring infection.

DESIGN

Cross-sectional observational study.

ANIMALS

187 horses vaccinated with a killed WNV vaccine and 37 horses with confirmed clinical WNV infection.

PROCEDURE

Serum was collected from vaccinated horses prior to and 4 to 6 weeks after completion of an initial vaccination series (2 doses) and 5 to 7 months later. Serum was collected from affected horses 4 to 6 weeks after laboratory diagnosis of infection and 5 to 7 months after the first sample was obtained. The IgM capture ELISA, plaque reduction neutralization test (PRNT), and microtiter virus neutralization test were used.

RESULTS

All affected horses had PRNT titers > or = 1:100 at 4 to 6 weeks after onset of disease, and 90% (18/20) maintained this titer for 5 to 7 months. After the second vaccination, 67% of vaccinated horses had PRNT titers > or = 1:100 and 14% had titers < 1:10. Five to 7 months later, 33% (28/84) of vaccinated horses had PRNT titers > or = 1:100, whereas 29% (24/84) had titers < 1:10. Vaccinated and clinically affected horses' end point titers had decreased by 5 to 7 months after vaccination.

CONCLUSIONS AND CLINICAL RELEVANCE

A portion of horses vaccinated against WNV may respond poorly. Vaccination every 6 months may be indicated in certain horses and in areas of high vector activity. Other preventative methods such as mosquito control are warranted to prevent WNV infection in horses.

Evaluation of an outbreak of West Nile virus infection in horses: 569 cases (2002)

OBJECTIVE

To characterize an outbreak of West Nile virus (WNV) infection in horses in North Dakota in 2002, evaluate vaccine effectiveness, and determine horse characteristics and clinical signs associated with infection.

DESIGN

Retrospective study.

ANIMALS

569 horses.

PROCEDURE

Data were obtained from veterinary laboratory records, and a questionnaire was mailed to veterinarians of affected horses.

RESULTS

Affected horses were defined as horses with typical clinical signs and seroconversion or positive results of virus isolation; affected horses were detected in 52 of the 53 counties and concentrated in the eastern and northeastern regions of the state. Among affected horses, 27% (n = 152) were vaccinated against WNV, 54% (309) were not, and 19% (108) had unknown vaccination status; 61 % (345) recovered, 22% (126) died, and 17% (98) had unknown outcome. The odds of death among nonvaccinated horses were 3 and 16 times the odds among horses that received only 1 or 2 doses of vaccine and horses that were vaccinated according to manufacturer's recommendations, respectively. Horses with recumbency, caudal paresis, and age > 5 years had higher odds of death, whereas horses with incoordination had lower odds of death, compared with affected horses without these characteristics.

CONCLUSIONS AND CLINICAL RELEVANCE

Vaccination appears to have beneficial effects regarding infection and death caused by WNV.

West Nile virus: epidemiology, clinical presentation, diagnosis, and prevention

West Nile virus was recognized in the United States for the first time in 1999, when it caused an epidemic of encephalitis and meningitis in New York City, NY. Since then, the disease has been steadily moving westward, and human cases were recognized in 39 states and the District of Columbia in 2002. The infection is caused by a flavivirus that is transmitted from birds to humans through the bite of culicine mosquitoes. Most infections are mild, with symptoms primarily being fever, headache, and myalgias. People older than 50 years are at highest risk of severe disease, which may include encephalomyelitis. In 2002, 5 new modes of transmission were recognized: blood product transfusion, organ transplantation, breast-feeding, transplacental transmission, and occupational exposure in laboratory workers. The transmission season was long, with cases occurring into December in some parts of the United States. Currently, there is no specific drug treatment or vaccine against the infection, and avoiding mosquito bites is the best way to protect against the disease.

Serological, reverse transcriptase-polymerase chain reaction, and immunohistochemical detection of West Nile virus in a clinically affected dog

Necropsy of an older dog submitted for evaluation of renal and central nervous system disease revealed histologic lesions compatible with West Nile viral encephalitis and myocarditis, as seen in other species. Using reverse transcriptase-polymerase chain reaction detection of envelope sequences, viral RNA was detected in most organs, and quantitative polymerase chain reaction revealed that at least 1,000 times more RNA was present in kidney than in brain, heart, spleen, or lung. Immunohistochemical evaluation of the kidney revealed intense staining of West Nile viral antigens in renal tubular epithelium and casts located within multifocal granulomatous interstitial inflammation. A canine immunoglobulin M (IgM)-capture enzyme-linked immunosorbent assay was developed, and patient serum was strongly positive for viral antibody. Retrospective and ongoing evaluation of sera from dogs with neurological disease and of those submitted for heartworm testing detected 4 dogs that were subclinically infected but without additional sickness. Judged by this experience, the kidney of West Nile virus-infected dogs may be an important target organ, one that might be suitable for antemortem biopsy. The presence of virus-specific IgM was demonstrated in the serum of this dog, and finding 4 positives among 169 additional canine sera received since late July 2002 suggests that seroconversion appears to be relatively uncommon in dogs during the outbreak in Missouri.

Findings in cerebrospinal fluids of horses infected with West Nile virus: 30 cases (2001)

OBJECTIVE

To evaluate CSF in horses with confirmed West Nile virus encephalomyelitis.

DESIGN

Retrospective study.

ANIMALS

30 horses.

PROCEDURE

Results of CSF analyses from horses with acute neurologic signs attributed to West Nile virus infection that was confirmed by immunoglobulin M antibody capture ELISA were reviewed and analyzed.

RESULTS

Among 30 CSF samples, findings in 8 (27%) were within reference ranges and in 22 (73%) were abnormal. Among the 22 abnormal samples, mononuclear pleocytosis was found in 16 (73%) and high protein concentration with nucleated cell count within reference range was found in 6 (27%) samples. A predominance of lymphocytes was found in 11 of 16 samples with mononuclear pleocytosis, and a predominance of large mononuclear cells was found in 5 of 16 samples. Sensitivities of analyses of CSF obtained from the lumbosacral and atlanto-occipital regions of the spinal cord were 89 and 50%, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that in horses with acute onset of neurologic signs caused by West Nile virus encephalomyelitis, findings in the CSF are likely to be abnormal, mononuclear pleocytosis with lymphocytic predominance may be most commonly observed, and CSF collected from the lumbosacral region may be abnormal more commonly than CSF collected from the atlanto-occipital region.

West Nile virus activity in the United States, 2001

West Nile virus (WNV) first appeared in the naive environment of the Western Hemisphere in 1999 in New York. Genetic analysis determined that the virus was introduced into the United States from the Mediterranean Basin. This review discusses the spread of the virus in 2001 from the initial focus in Queens, New York, to widespread activity in the eastern and midwestern United States. It concentrates on viral ecology, epizootiology, pathology, prediction, and prevention. Research questions to further our understanding of the transmission cycle of WNV are discussed, including host-preference studies, molecular confirmation of implicated mosquito vectors, and survival of WNV in the temperate environment of the United States. Comparisons are drawn with two other arboviruses enzootic in the United States, eastern equine encephalitis, and St. Louis encephalitis viruses. Although not recently introduced, these two viruses also demonstrated increased activity in the United States in 2001.

West Nile virus outbreak in horses, southern France, 2000: results of a serosurvey

During late summer and autumn 2000, a West Nile fever outbreak in southern France resulted in 76 equine clinical cases; 21 horses died. We report the results of a large serosurvey of all equines within a 10-km radius of laboratory-confirmed cases. Blood samples were obtained from 5,107 equines, distributed in groups of 1 to 91 animals. West Nile virus immunoglobulin (Ig) G antibodies were found in 8.5% of animals (n=432). Forty-two percent of the IgG-positive animals were also IgM positive. Horses living in small groups were more affected than those in large groups. The results suggest that West Nile virus is not endemic in the affected area, the Camargue; rather, sporadic outbreaks are separated by long silent periods.

Equine West Nile encephalitis, United States

After the 1999 outbreak of West Nile (WN) encephalitis in New York horses, a case definition was developed that specified the clinical signs, coupled with laboratory test results, required to classify cases of WN encephalitis in equines as either probable or confirmed. In 2000, 60 horses from seven states met the criteria for a confirmed case. The cumulative experience from clinical observations and diagnostic testing during the 1999 and 2000 outbreaks of WN encephalitis in horses will contribute to further refinement of diagnostic criteria.