Assays to detect West Nile virus in dead birds

Lack of identification of Flaviviruses in oral and cloacal swabs from long- and short-distance migratory birds in Trentino-Alto Adige (North-eastern Italy)

Background

West Nile virus (WNV) and Usutu virus (USUV), both belonging to the genus Flavivirus, are emerging in Italy as important human and animal pathogens. Migratory birds are involved in the spread of Flaviviruses over long distances, particularly from Africa to Europe. Once introduced, these viruses can be further be dispersed by short-distance migratory and resident bird species. Thus far, there is still a considerable knowledge gap on the role played by different bird species in the ecology and transmission mechanisms of these viruses. The Region of Trentino-Alto Adige (north-eastern Italy) is located on the migratory route of many of the short- and long-distance migratory birds that cross the Alps, connecting northern Europe and western Asia with southern Europe and Africa. Until now, only a silent circulation of WNV and USUV within the territory of the Province of Trento has been confirmed by serological screening, whilst no cases of infected humans or animals have so far been reported. However, continuous spillover events of both viruses have been reported in neighbouring Regions. The aim of this study was to monitor the circulation of WNV and USUV in Trentino-Alto Adige, in order to detect if active virus shedding occurs in migratory birds captured during their seasonal movements and to evaluate the role that different bird species could play in the spreading of these viruses.

Methods

We carried out a biomolecular survey on oral and cloacal swabs collected from migratory birds during seasonal migrations. Birds belonging to 18 transaharian and 21 intrapaleartic species were examined during spring (n = 176) and autumn (n = 146), and were tested using a generic nested-PCR.

Results

All samples tested negative for Flaviviruses. The possible causes of unapparent shedding, along with ecological and epidemiological implications are discussed.

Conclusions

The lack of detection of active virus shedding in these bird species does not exclude the circulation of these viruses within the Trentino-Alto Adige region, as reported in previous studies. The possible ecological implications are discussed.

Detection of West Nile virus

West Nile virus (WNV; Flavivirus, Flaviviridae) is a spherical enveloped virion containing single-stranded, positive-sense RNA, approximately 11 kb in length. The virus is the most widely distributed flavivirus in the world. Genetic analysis reveals two major lineages of virus, I and II, and several possible newly recognized lineages. Lineage I strains are most commonly associated with outbreaks of neurologic disease, although lineage II virus has led to large epidemics of fever, as in South Africa in 1974. Infection with WNV leads to a wide range of diseases from mildly febrile to severely neurologic, but asymptomatic -infections occur most frequently. Approximately one in 140 infected individuals develop neurologic -disease. The virus is maintained in an enzootic cycle, where it is transmitted between ornithophilic mosquitoes of the Culex genus and predominantly passeriform birds. Equines and humans are considered incidental hosts since they do not mount high enough viremia for mosquitoes to become infected -following feeding. Laboratory diagnosis of WNV infection is predominantly serological, although -caution is advised because of the high degree of cross-reactivity among flaviviruses. Field specimens, especially mosquitoes and dead birds, collected as part of surveillance programs, are tested for the presence of viral nucleic acid, viral antigen, or infectious virus. Rapid test protocols have been developed in response to the expansion of WNV in the United States. Since WNV is classified as a Biosafety Level-3 (BSL-3) agent by CDC, it is recommended that once this virus is identified in a diagnostic specimen, all infectious virus should be handled in a BSL-3 laboratory in Class II biosafety cabinets by laboratory staff who are trained to work at this level of containment. Assay protocols are described and the necessary equipment and supplies listed.

On the effect of diagnostic misclassification bias on the observed spatial pattern in regional count data--a case study using West Nile virus mortality data from Ontario, 2005

Geographic epidemiology is concerned with the investigation of spatially referenced data to discover spatial patterns in the health status of populations. In this context it is generally assumed that a perfect diagnostic test is used to classify individuals as being positive or negative, meaning the health status is measured without error. In this work the effect of an imperfect diagnostic test on spatial patterns of disease in regional count data is investigated in a case study. Specifically the misclassification effect on the semivariogram, Moran's I statistic and the spatial scan test are evaluated for the situation of West Nile virus infections among dead birds sampled from the 30 public health units of southern Ontario in 2005. We illustrate that under large sample conditions no serious spatial bias is introduced by use of an imperfect diagnostic test as long as the imperfection itself is spatially unbiased.

Assessment of methods for prediction of human West Nile virus (WNV) disease from WNV-infected dead birds

Background

West Nile virus (WNV) is currently the leading cause of arboviral-associated encephalitis in the U.S., and can lead to long-term neurologic sequelae. Improvements in dead bird specimen processing time, including the availability of rapid field laboratory tests, allows reassessment of the effectiveness of using WNV-positive birds in forecasting human WNV disease.

Methods

Using New York State integrated WNV surveillance data from transmissions seasons in 2001–2003, this study determined which factors associated with WNV-positive dead birds are most closely associated with human disease. The study also addressed the 'delay' period between the distribution of the dead bird variable and the distribution of the human cases. In the last step, the study assessed the relative risk of contracting WNV disease for people who lived in counties with a 'signal' value of the predictor variable versus people who lived in counties with no 'signal' value of the predictor variable.

Results

The variable based on WNV-positive dead birds [(Positive/Tested)*(Population/Area)] was identified as the optimum variable for predicting WNV human disease at a county level. The delay period between distribution of the variable and human cases was determined to be approximately two weeks. For all 3 years combined, the risk of becoming a WNV case for people who lived in 'exposed' counties (those with levels of the positive dead bird variable above the signal value) was about 2 times higher than the risk for people who lived in 'unexposed' counties, but risk varied by year.

Conclusion

This analysis develops a new variable based on WNV-positive dead birds, [(Positive/Tested)*(Population/Area)] to be assessed in future real-time studies for forecasting the number of human cases in a county. A delay period of approximately two weeks between increases in this variable and the human case onset was identified. Several threshold 'signal' values were assessed and found effective at indicating human case risk, although specific thresholds are likely to vary by region and surveillance system differences.

Crow deaths caused by West Nile virus during winter

In New York, an epizootic of American crow (Corvus brachyrhynchos) deaths from West Nile virus (WNV) infection occurred during winter 2004–2005, a cold season when mosquitoes are not active. Detection of WNV in feces collected at the roost suggests lateral transmission through contact or fecal contamination.

Zoonotic viral diseases and the frontier of early diagnosis, control and prevention

Public awareness of the human health risks of zoonotic infections has grown in recent years. Currently, concern of H5N1 flu transmission from migratory bird populations has increased with foci of fatal human cases. This comes on the heels of other major zoonotic viral epidemics in the last decade. These include other acute emerging or re-emerging viral diseases such as severe acute respiratory syndrome (SARS), West-Nile virus, Ebola virus, monkeypox, as well as the more inapparent insidious slow viral and prion diseases. Virus infections with zoonotic potential can become serious killers once they are able to establish the necessary adaptations for efficient human-to-human transmission under circumstances sufficient to reach epidemic proportions. The monitoring and early diagnosis of these potential risks are overlapping frontiers of human and veterinary medicine. Here, current viral zoonotics and evolving threats are reviewed.

Detection of seroconversion to West Nile virus, Usutu virus and Sindbis virus in UK sentinel chickens

We previously reported evidence of West Nile virus (WNV) circulation in UK birds, probably introduced by migratory birds from overseas. We now demonstrate WNV-specific seroconversion in sentinel chickens raised on an English farm. Maternal neutralizing antibodies to WNV in hatchlings declined within three weeks. During the following months, healthy chickens developed WNV neutralizing antibodies that were confirmed by immunoblotting and indirect immunofluorescence tests using WNV antigens. The proportion of seropositive chickens was higher for WNV than for Usutu virus or Sindbis virus. Attempts to isolate infectious virus or to detect viral RNA in the sera, failed.

Field and Laboratory Evaluation of Diagnostic Assays for Detecting West Nile Virus in Oropharyngeal Swabs from California Wild Birds

Three diagnostic assays for detecting West Nile virus (WNV) in avian oral swabs were evaluated in California in 2004 and 2005: two commercial antigen-capture assays, VecTest and Rapid Analyte Measurement Platform (RAMP), and reverse transcriptase-polymerase chain reaction (RT-PCR) of oral swabs in a specialized viral transport medium (VTM). Results from this study demonstrated that VTM was excellent for transportation and maintenance of WNV in avian oral swab samples and allowed for detection by RT-PCR and subsequent confirmation by virus isolation. Oral swabs and kidney tissue in VTM tested by RT-PCR were found to have similar accuracy in detecting WNV in corvids. The two antigen-capture assays, VecTest and RAMP, provided few false positives for corvids, with over 95% specificity. When performed by multiple local agencies throughout the state, VecTest and RAMP were similarly sensitive for oral swabs of American Crows (Corvus brachyrhynchos) (70% and 64%, respectively). Data from known WNV positive corvid oral swabs in VTM tested by antigen-capture assays at a diagnostic laboratory suggested that RAMP was more sensitive than VecTest. Due to high probability of false negatives, neither test is recommended for use on non-corvids. While WNV antigen-capture assays were effective screening tools for corvids, they were markedly less sensitive for Western Scrub Jays (Aphelocoma californica).