Statewide Surveillance and Forecasting of West Nile Virus Activity in Iowa . Abstract. Acknowledgements. Conclusion. Introduction. Materials & Methods. Discussion. Results. Figure 3-Local public health sanitarians draw blood from a sentinel chicken for detection of WNV antibodies.
Statewide Surveillance and Forecasting of West Nile Virus Activity in Iowa
Materials & Methods
Figure 3-Local public health sanitarians draw blood from a sentinel chicken for detection of WNV antibodies.
Figure 6-New Jersey Light traps were used to trap mosquitoes for WNV detection by RT-PCR.
Bradley G. Changstrom,1 Thomas F. Gahan,1 Mary J.R. Gilchrist, PhD,1 and James S. Gill, MD, PhD1,2
1University of Iowa Hygienic Laboratory, Department of Serology, University of Iowa, Iowa City, IA
2Center for Emerging Infectious Diseases, Department of Epidemiology, University of Iowa College of Public Health, Iowa City, IA
Surveillance methods that are able to predict future disease outbreaks for seasonal diseases are ideal in order to slow yearly disease transmission by alerting public health officials to a pending disease threat. According to the CDC, in 2002 dead bird submissions were the first positive surveillance event in 61% of counties nationwide. Dead bird submissions also preceded human cases 72% of the time. In each year since 2001, the identification of a positive WNV sample through avian morbidity/mortality surveillance has always preceded the first human infection in Iowa. On average, WNV was isolated from dead bird submissions 3.8 weeks earlier than from humans (Figure 2). WNV positive birds appear to begin in week 25 of the year (late June), while humans and sentinel chickens both appear several weeks later in week 29. Interestingly, positive mosquito samples did not appear until week 32, almost 2 months after the first positive dead bird. Although the total number of samples of mosquitoes and of sentinel chickens far surpassed the number of total dead birds samples (data not shown), nearly ten times the number of counties submitted dead bird samples than either sentinel chicken samples or mosquito samples (Figure 3) providing a wider—albeit less detailed—area of surveillance in Iowa. Detection of WNV by dead bird submissions might produce a positive result earlier than other surveillance methods because of the widespread sampling area or because of differences in sensitivity in surveillance methods, but in either case, this method seems to be the most reliable in predicting when WNV will first appear in humans. This data also reinforces the recommendation by the CDC that dead bird submissions are a necessity in WNV surveillance, regardless of the downfalls in the collection methods.
West Nile virus (WNV) first appeared in Iowa in September 2001 when it was isolated from a dead crow. The virus has since spread throughout the state with consistent epidemic activity during the summer and fall. Several statewide surveillance studies provide detailed information about the activity of WNV and other arboviruses. Samples from dead bird submissions, mosquito trapping, and sentinel chickens all serve to monitor WNV activity; however, little data exist on the usefulness of these surveillance methods in predicting the yearly reemergence of the human WNV epidemic in Iowa. Human and sentinel chicken sera samples collected from 2002-2005 were assayed for WNV activity by an enzyme-linked immunosorbent assay. Oral swabs from dead bird submissions and mosquitoes collected by trapping were tested by a polymerase chain reaction. Human cases of WNV begin in week 29 of the year (early July), several weeks after the first positive dead bird submission (week 25). In each year, WNV was isolated from dead bird submissions earlier than any other surveillance method as positive samples appear from mosquito trapping in week 32 and from sentinel chickens in week 29. WNV surveillance from dead bird submissions was also more widespread as 66 out of 99 counties in Iowa submitted dead bird samples while only 6 counties and 5 counties submitted sentinel chicken and mosquito samples, respectively. Although surveillance from sentinel chickens and mosquitoes provides a more active approach to studying WNV infection and allows for studies of other arboviral diseases, samples from dead bird submissions provide the earliest information on forecasting yearly WNV activity and provide sampling data from a larger area of the state than the other methods of surveillance. In general, the appearance of WNV-positive dead bird submissions precedes human cases by 3.8 weeks, suggesting that future outbreaks of WNV in humans can be predicted. These data could help establish dead bird submissions as a primary means of surveillance in Iowa and help public health officials predict future WNV outbreaks.
I) Which surveillance method provides the earliest detection of WNV?
Figure 1-Week of first positive West Nile virus sample from 2002-2005 for several surveillance methods.
Figure 2-Average week of first West Nile virus positive event per surveillance method from 2002-2005.
II) Which surveillance method provides
the best statewide participation?
After first appearing in 2001, WNV activity has remained consistent during the traditional summer and fall transmission season in Iowa. In each year since since its debut, dead bird submissions have provided evidence of pending human WNV activity in the state earlier than any other surveillance methods. Although collection methods from dead birds are cumbersome and rely heavily on public cooperation, the information provided by dead bird submissions makes this method extremely valuable in forecasting human WNV activity, especially early on in the WNV season. Sentinel chickens and mosquitoes continue to provide valuable information on WNV, additional arbovirus activity and provide a good year to year comparison of host, vector and pathogen trends. Unfortunately, the public is becoming increasingly disinterested in participating in dead bird submissions, therefore it is necessary to encourage public health officials and public sanitarians to not only continue dead bird testing, but also actively promote public awareness about the benefits of dead bird testing.
West Nile virus (WNV) is an arthropod borne virus in the family flaviridae. Infection with WNV is asymptomatic in nearly 80% of cases; however, clinical symptoms range from fever, rash, and malaise to neuroinvasive disease and death. After infecting and killing a crow in 2001, this zoonotic pathogen first infected a human in Iowa in 2002. Currently, local public health officials focus on at least four sources of information in order to monitor WNV activity in the state: dead bird surveillance, sentinel chicken surveillance, diagnostic testing of human sera, mosquito surveillance. Although the number of sentinel chicken, mosquito and human samples have remained fairly consistent over the past 4 years, the number of dead bird submissions has decreased because of decreased public interest in WNV and because of lack of desire in handling and mailing a dead carcass. The usefulness of these surveillance methods in predicting or forecasting the yearly reemergence of WNV in Iowa has not been previously addressed.
Figure 4-Percent of counties submitting West Nile virus surveillance data from 2002-2005. *Denotes statistical significance by an odds ratio analysis.
III) Can any surveillance method predict the
yearly emergence of WNV in humans?
Sentinel chicken samples were collected weekly during the summer and fall from 15 surveillance sites from 2002-2005. Sentinel chicken samples and human serum samples collected for diagnostic testing were both tested for antibodies to WNV with a WNV-specific immunoglobulin M (IgM) antibody-capture (MAC) enzyme-linked immunosorbent assay (ELISA). Dead birds were collected by local sanitarians and submitted to UHL. Mosquitoes were collected using New Jersey Light traps or Gravid traps and speciated at Iowa State University before being tested at the University of Iowa Hygienic Laboratory (UHL). Dead bird oral swabs and pooled mosquito samples were tested directly for West Nile virus using a real-time polymerase chain reaction (RT-PCR).
The findings and conclusions on this report are those of the authors and do not necessarily represent the views of the funding agency. This work was funded by the Epidemiology Laboratory Capacity (ELC) grant from the Centers for Disease Control and Prevention (CDC) to the Iowa Department of Public Health and The University of Iowa Hygienic Laboratory. This research was supported in part by an appointment to the Emerging Infectious Diseases (EID) Fellowship Program administered by the Association of Public Health Laboratories (APHL) and funded by the Centers for Disease Control and Prevention (CDC). Thanks to Ana Capuano for help with statistical analysis. The authors report no conflicts of interest.
Figure 5-Evidence of the first WNV event in humans compared to first WNV event in three surveillance methods in Iowa, 2002-2005.