Questions & Answers on Serosurvey 2000
January 2001
Background:
In September 1999, West Nile virus (WNV) was recognized in the Western Hemisphere for the first time when it caused an outbreak of encephalitis and aseptic meningitis in the New York City metropolitan area, centered in northern Queens. Citywide, 46 cases of WNV were identified including 4 deaths. WNV re-emerged in New York City in 2000. In the second year of WNV in the region, there were 14 cases in New York City, including one death.
The center of West Nile activity in 2000 was Staten Island. To better estimate the public health impact of the outbreaks, household-based seroprevalance surveys were conducted in Queens in 1999 and Staten Island in 2000.
The US Centers for Disease Control (CDC) and the New York City Department of Health
(NYCDOH) announced on January 25, 2001 the results of the door-to-door WNV survey of
households on Staten Island that took place in October 2000. The survey was conducted by
NYCDOH with the assistance of the CDC to help estimate the percentage of people who were
infected with the virus, and what their symptoms were, if any. Similar surveys also took place in the fall of 2000 in Suffolk County, New York, and Fairfield County, Connecticut. A summary of findings from these surveys appears in the January 26, 2001, issue of the CDC's Morbidity and Mortality Weekly Report.
Q. I've never heard of this before the recent WNV outbreak. Is this type of survey unusual?
A. NYCDOH, with the assistance of CDC, conducted a simple community-based voluntary
"seroprevalence survey." This is a standard tool frequently used by public health officials. "Sero" refers to the testing of blood for antibodies to an infectious organism; "prevalence" means the percentage of persons with a particular characteristic at a given point in time. Therefore, "West Nile virus seroprevalence survey" literally means a survey to determine the percentage of persons with antibodies to WNV at a given point in time.
Q. What are the results of the door-to-door serosurvey conducted on Staten Island during the Fall of 2000?
A. CDC reported to NYCDOH that four blood samples out of 871 persons tested positive for an antibody against WNV. This antibody indicates a recent infection that likely occurred in the summer of 2000.
Q. What do these findings mean?
A. Based on these results, it is estimated that approximately 0.5% of the population aged 12 or older on Staten Island (342,152 persons) has been infected with WNV. These results cannot be extrapolated to other areas of the city. However, because Staten Island had the highest rate of West Nile encephalitis cases, researchers believe that infection rates elsewhere in the city were significantly lower. These findings, while not unexpected, demonstrate the impact of mosquito-borne diseases on a community, and the value of vigilance in monitoring and reducing the numbers of urban mosquitoes. As described in the New York City West Nile plan, NYCDOH is emphasizing prevention -- by eliminating areas of standing water where mosquitoes can breed; using larvicides to kill mosquito larvae ("wrigglers", which live in standing water) before they become
adult mosquitoes; surveillance to rapidly detect infection in humans, animals, and mosquitoes; and education and outreach to the public.
Q. Why were the results of the 1999 serosurvey in Queens higher than those of the 2000 serosurvey on Staten Island?
A. A similar West Nile survey was performed in a three square-mile area of northern Queens in the Fall of 1999, it was estimated that approximately 2.6% of persons in the surveyed area were infected with WNV. The higher infection rate in Queens may be explained by the fact that the WNV outbreak was not detected until late August. As a result, mosquito control activities in 1999 did not begin until September (as opposed to April in 2000). Additionally, the survey in Queens took place in a smaller geographic area where nine persons with West Nile virus neurologic illness resided. In Staten Island, because the ten cases were dispersed throughout the island, a sampling method that
included the entire island was used.
Q. In Staten Island, did people who had antibodies to West Nile virus report any illness?
A. Surveyed individuals whose blood tested positive for antibodies to WNV (indicating a previous infection) in Staten Island were more likely to report recent illness characterized by fever and headache. In Queens in 1999, persons were also more likely to report joint pains and muscle aches.
Q. Are people who tested positive still at risk from West Nile virus?
A. No. The infection (and any associated illness) is short-lived. A person with antibodies to West Nile virus is likely to have life-long immunity to a repeat infection of this virus.
Q. How was the serosurvey in NYC conducted?
A. Local officials and community leaders helped inform neighborhood residents about the survey. Residents of selected blocks were pre-notified that they might be visited over the next several days. Mobile health vans were stationed throughout the community and served as alternate sites for interviewing and blood drawing, as well as sources of information for interested residents. Information pamphlets were prepared in several languages and interpreters for most languages were made available during the survey. Of 1,607 households invited to participate, 635 (40%) agreed to participate, and 871individuals (over the age of 12) in those households completed confidential
interviews and provided blood samples.
Q. What tests were conducted on the blood and why?
A. Serum samples were tested for two types of antibodies to WNV: immunoglobulin M (IgM) and immunoglobulin G (IgG). These two tests help determine whether an individual was recently infected with WNV (IgM antibodies) and whether an individual was ever infected with WNV (IgG antibodies).
Q. What were people told about their involvement in this survey?
A. Everyone who participated was fully informed about what would be required if they participated and how blood collected would be used. Everyone who participated was a volunteer.
Q. Why were the results of the 1999 serosurvey in northern Queens anonymous while the 2000 survey on Staten Island was confidential?
A. NYCDOH obtained more detailed information from this year's survey to help determine why certain people get infected and others do not. It may have been necessary to call those individuals who test positive for West Nile virus on Staten Island to ask more detailed questions about their exposure to mosquitoes. Also, additional blood tests may have been needed to determine how recently these individuals were infected since this virus has now been in New York City for more than one year, and a positive test could represent an old infection. Therefore, NYCDOH collected name and addresses so that it could contact participants again, if needed.
Q. What can people do to prevent WNV from returning next Spring?
A. In residential areas, the public is urged to eliminate areas where water can accumulate around their homes. This includes unclogging roof gutters, discarding old tires, buckets, or other containers that may hold water, or covering them, changing water in bird baths at least once a week, and keeping swimming pools clean and chlorinated, or draining and covering them.
Q. Did the people who participated in the serosurvey receive any compensation for their involvement?
A. No. The residents of Staten Island and Queens who took part in these important public health efforts were motivated by a desire to help the community in which they live.
Q. Does WNV usually cause serious illness in humans?
A. No. Viruses like West Nile usually cause serious illness in only a small minority of persons who become infected from a mosquito bite. Elderly people (>50 years of age) are at higher risk of developing an illness from infection with West Nile virus than are younger people.
Q. Do these results suggest that this strain of West Nile virus is more virulent that other known strains?
A. No.
Q. Was a serosurvey conducted anywhere other than Staten Island?
A. In the January 26, 2001, issue of the CDC's Morbidity and Mortality Weekly Report article on the three separate West Nile virus surveys conducted in 2000, estimated rates of infection in the other survey sites ranged from less than 0.5% in Fairfield County, Connecticut, to 0.12 % in Suffolk County. It is noted that many factors could contribute to these different estimates obtained from these surveys, including the intensity of each local outbreak and geographic differences in the surveyed areas.
For additional information call the New York City Department of Health's West Nile Information Line, 24 hours a day, seven days a week, at 1-877-WNV-4NYC.
January 2001
