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West Nile Virus Activity --- New York and New Jersey, 2000
In late August 1999, an outbreak of encephalitis caused by West Nile virus
(WNV) was detected in New York City and subsequently identified in neighboring counties
(1). In response, an extensive mosquito-control and risk-reduction campaign was
initiated, including aerial and ground applications of mosquito adulticides throughout
the affected areas. No human WNV infections were found in New York City with an
onset date after the campaign was completed. Cases continued to occur among humans
in surrounding counties that did not undertake mosquito-control efforts until
later, suggesting that the campaign may have reduced human risk. In May 2000, CDC
issued guidelines to direct national surveillance, prevention, and control efforts
(2) and provided funds to support these efforts in 19 state and local health departments
where WNV transmission had occurred or where transmission would probably occur
based on known bird migration patterns. This report presents the findings of
surveillance activities.
From May 6 through July 8, 2000, state and local health departments
confirmed WNV infections in 26 birds from five counties in New York and New Jersey.
Twenty-one infections have been confirmed in American crows in New York and New
Jersey, four in blue jays, and one in a red-tailed hawk. The first infected crow was found
May 22 in Rockland County, and the most recently infected crows were found July 6 and
8 in the same county. Fourteen infected crows identified in New York were found
in Rockland (eight crows), Suffolk (three), Westchester (one), and Richmond
(Staten Island) (two) counties. Seven infected crows were found in Bergen County,
New Jersey. Rockland County also identified four blue jays with WNV infection, and
one infected hawk was found in Westchester County. WNV has been detected
by polymerase chain reaction molecular methods in mosquito pools collected
in Westchester County (Aedes japonicus) and in Suffolk County (mixed
Culex species). No cases of human or equine infection have been reported in the region or
in surrounding states.
Reported by: A Novello, MD, D White, PhD, L Kramer, PhD, C Trimarchi, MS, M Eidson, DVM,
D Morse, MD, P Smith, MD, State Epidemiologist, New York State Dept of Health; W Stone,
PhD, Dept of Environmental Conservation, Albany; Rockland County Health Dept, Pomona;
Suffolk County Health Dept, Hauppague; Westchester County Health Dept, New Rochelle; J Miller, MD,
M Layton, MD, New York City Dept of Health. Bergen County Health Dept, Paramus; W Crans,
PhD, Rutgers Univ, New Brunswick; F Sorhage, DVM, E Bresnitz, MD, State Epidemiologist,
New Jersey Dept of Health and Senior Svcs. National Wildlife Health Center, US Geologic
Survey, Madison, Wisconsin. Arbovirus Diseases Br, Div of Vectorborne Infectious Diseases,
National Center for Infectious Diseases, CDC.
Editorial Note:
WNV is transmitted readily by mosquitoes.
Culex species were the primary vectors of WNV during previous outbreaks and epizootics; however,
WNV also has been isolated from many species of
Aedes and Anopheles (3). In New
York, WNV was isolated primarily from Culex species mosquitoes during the
1999 outbreak; WNV also was detected in overwintering
Culex species in New York City. These findings suggest an important role for these species in the transmission
of
WNV in the United States. Aedes japonicus was detected recently in the
United States, and research is needed to determine the flight range and feeding behavior
of mosquitoes and to better understand the risk for transmission to humans.
The susceptibility of crows to infection and death is a sensitive surveillance
tool that is unique to the United States (4). No data exist from which to infer the
mosquito WNV infection rate associated with a small number of dead crows in an area, or
to infer the risk to humans. Data also are lacking to infer where and how the dead
crows acquired infection. Time of year and reproductive status of the crow population may
be used to indicate whether transmission occurred locally. On the basis of the
known nesting habits of crows, the finding of infected crows in early summer suggest
local transmission in Rockland, Westchester, Suffolk, and Bergen counties. Data from
the U.S. Geologic Survey's National Wildlife Health Center indicate that crows
infected with WNV are likely to have high viremias and also are likely to be
sedentary approximately 4 days before death, suggesting that they can be a source of WNV
for mosquitoes in areas where they are found (National Wildlife Health
Center, unpublished data, 2000).
On the basis of the surveillance indicators described in this report and the
phased response plan (2), CDC recommends the following for those areas where
evidence suggests local transmission of WNV:
- Intensify local and regional
Culex mosquito larval control to prevent
the emergence of adult mosquitoes that feed on birds and may contribute to the
virus amplification/transmission cycle.
- Expand and intensify surveillance activities in and around areas where
WNV-infected birds are found. Additional surveillance data about the species
population densities, virus infection rates in mosquito vectors, seroprevalence in resident
wild birds (e.g., house sparrows), and seroconversion rates in sentinel chickens
will permit a more accurate interpretation of dead bird surveillance data and the
relative risk for human disease.
- Continue active WNV surveillance to determine the presence of new or
expanding WNV transmission foci.
- Reinforce public education and outreach programs to reduce mosquito
breeding sites around the home and use personal protective measures.
- Implement, if necessary, focal adult mosquito control to reduce the number
of virus-infected mosquitoes, thus reducing the immediate risk to humans.
Mosquito species that feed on birds probably are driving enzootic transmission in 2000
and probably are the vector for human cases
(5). Adult mosquitoes should be controlled within approximately a 2-mile radius around the area where a
WNV positive dead bird or infected mosquitoes are found. This radius depends on
the length of time between transmission of the virus and the execution of control;
as the time period increases, larger areas must be treated.
- Consider aerial spraying of adulticides in areas where WNV transmission
is sustained and further amplification is evident despite intensive local
mosquito control efforts.
- Monitor adult and larval mosquito control efforts to ensure that the
control programs are effectively reducing vector mosquito densities and virus
infection rates.
Counties where WNV transmission occurred in 1999, but has not been identified
in
2000, should maintain active surveillance for WNV and continue
larval mosquito-control, such as controlling larval mosquito habitats, particularly
around homes in suburban and urban areas and monitoring
Culex larval habitats regularly for mosquito breeding.
References
- CDC. Update: West Nile virus encephalitis---New York, 1999. MMWR 1999;48:944--6,955.
- CDC. Epidemic/epizootic West Nile virus in the United States: guidelines for
surveillance, prevention, and control: a workshop cosponsored by US Department of Health and
Human Services, CDC, and US Department of Agriculture, Fort Collins, Colorado, November
8--9, 1999. Available at
www.cdc.gov/ncidod/dvbid/westnile/resources/WN_surv_guide_Mar_2000.pdf.
- Hayes CG. West Nile fever. In: Monath TR, ed. The arboviruses: epidemiology and
ecology. Boca Raton, Florida: CRC Press, Inc., 1989.
- Komar N. West Nile viral encephalitis. Rev Sci Tech OIE 2000;19:166--76.
- CDC. Update: West Nilelike viral encephalitis---New York, 1999. MMWR 1999;48:890--2.
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