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Dispatch
Early-Season Avian Deaths
from West Nile Virus as Warnings of Human Infection
Stephen C. Guptill,* Kathleen G. Julian,† Grant L. Campbell,‡ Susan
D. Price,* and Anthony A. Marfin‡
*U.S. Geological Survey, Reston, Virginia USA; †Hershey Medical Center,
Hershey, Pennsylvania, USA; and ‡Centers for Disease Control and Prevention,
Atlanta, Georgia
Suggested citation for this article: Guptill
SC, Julian KG, Campbell GL, Price SD, Marfin AA. Early-season avian
deaths from West Nile virus as warnings of human infection. Emerg Infect
Dis [serial online] 2003 Apr [date cited]. Available from: URL:
http://www.cdc.gov/ncidod/EID/vol9no4/02-0421.htm
An analysis of 2001
and 2002 West Nile virus (WNV) surveillance data shows that counties
that report WNV-infected dead birds early in the transmission season
are more likely to report subsequent WNV disease cases in humans than
are counties that do not report early WNV-infected dead birds.
West Nile virus (formal name: West Nile virus [WNV]) was first
detected in the United States during an encephalitis outbreak in New York
City in September 1999 (1). Since then, WNV activity
has been reported from 42 additional states and the District of Columbia
(2). Avian, equine, and human illnesses are most often
reported. Analysis of surveillance data from 2001 and 2002 chronicles
the spread of infection and may provide a means of locating areas where
human illness is more likely to occur.
The Study
Surveillance data have often been used in the study of arboviral disease
outbreaks (3,4). ArboNET, a cooperative WNV surveillance
program maintained by the Centers for Disease Control and Prevention and
48 states, five cities, and the District of Columbia, collects surveillance
data on a continuous basis. These data include reports of WNV-infected
mosquitoes, sentinel animals, dead birds, and ill humans and horses (5).
In 2001, 328 counties reported a total of 7,333 WNV-infected dead birds
(range per county: 1–350). The first WNV-infected dead bird was found
on April 8, and the last was found on December 26. Sixty-six human cases
of WNV disease were reported from 39 different counties in 10 states,1
including two outpatient West Nile fever cases. No single county reported
more than four human cases. Onset of illness was on July 13 for the first
human case and December 7 for the last reported case. Of particular interest
is the date that the first WNV-infected dead bird was found in a given
county. These dates ranged from the week ending April 14 to the week ending
December 8.
In this retrospective cohort study, all U.S. counties that reported dead
WNV-infected birds were categorized on the basis of whether a WNV-infected
bird was found early in the transmission season (i.e., before August 5)
and whether at least one subsequent human disease case was reported from
the county. A relative risk (RR) statistic was calculated as follows:
The proportion of counties that reported human cases among the counties
that found infected birds before August 5 was divided by the proportion
of counties that reported human cases among the counties that did not
find infected birds before August 5.
Results
Of 93 counties that reported at least one WNV-infected bird before August
5, 28 (30%) subsequently reported a human WNV disease case in 2001 compared
to 11 (4.7%) of 235 counties that did not report an infected bird (RR
6.43, 95% confidence interval [CI] 3.34 to 12.38). In other words, in
2001, counties that reported a WNV-infected dead bird before August 5
were more than six times more likely than other counties to report a human
WNV disease case (Figure 1).
As Figure 1 shows, the 2001 outbreak had two distinct
geographic foci, the Northeast and the Southeast United States. These
areas were analyzed separately to determine if the correlation between
WNV-positive birds and human cases was true in different ecologic regions.
The Northeast region consisted of Maryland, New Jersey, Pennsylvania,
New York, Connecticut, Rhode Island, and Massachusetts. The Southeast
region consisted of Florida, Georgia, Alabama, and Louisiana. The Northeast
region contained 22 of the 39 counties in which human cases occurred.
The Southeast contained the remaining 17 counties. RR statistics were
significant for both regions (Northeast: RR 11.57, 95% CI 3.58 to 47.99;
Southeast: RR 2.38, 95% CI 0.89 to 6.39).
Recently, provisional totals for the 2002 WNV surveillance data have
become available through ArboNET. Given the great increase in the geographic
extent and the 50-fold increase in the number of human cases of WNV, we
repeated this analysis by using the provisional data for 2002 to see if
similar results would be obtained. A great deal of variation in the reporting
of WNV fever cases has occurred between states. For this reason, only
WNV meningitis and encephalitis cases were included in this analysis.
In the provisional figures for 2002, a total of 504 counties reported
human cases of WNV meningoencephalitis, and 1,719 counties reported WNV-infected
birds. Of 632 counties that reported at least one WNV-infected bird before
August 4, a total of 284 (45%) subsequently reported a human WNV disease
in 2002 compared to 220 (19%) of 1,162 counties that did not report an
infected bird (RR 2.37, 95% CI 2.05 to 2.75). Thus in 2002, counties that
reported a WNV-infected dead bird before August 4 were more than two times
more likely than other counties to report a human case of WNV disease
(Figure 2).
The early August date (end of the 31st week of the year) used for classifying
the surveillance data was selected by empirical analysis of the WNV epidemiologic
curves. This date approximates the inflection point where the rapid increase
in case reports occurs. Using an earlier date for classifying the cases
results in an increased value for the RR statistic but a decrease in sensitivity.
This type of analysis could possibly be refined by stratifying surveillance
data by the number of birds and humans tested to compensate for variations
in the intensity of the surveillance effort. Factors such as the size
of the human population also may affect the number of dead birds sighted
and the number of persons exposed to WNV-infected mosquitoes. Other researchers
have attempted to address these issues (6-8). In addition,
we are analyzing the data to determine if the risk for human illness is
greater the earlier the positive bird (or other indication of epizootic
transmission) is detected.
However, the aim of this study was to see if a simple analysis of surveillance
data could provide useful indicators of human disease risk. The results
of our analysis suggest that, in counties where an avian epizootic is
present early in the transmission season, subsequent WNV disease in humans
is more likely. An early epizootic may indicate viral activity that has
sufficient time to escalate to high levels before the end of the transmission
season. WNV-infected dead birds found in spring or early summer thus may
be a warning for increased human risk for WN viral disease.
Dr. Guptill is a
senior research physical scientist at the U.S. Geological Survey. He
is internationally recognized for his contributions dealing with geographic
information systems research and geospatial data policy issues. Currently
he is conducting spatial analysis research with colleagues from the
Centers for Disease Control and Prevention to determine the environmental
influences on emerging zoonotic and vector-borne diseases.
References
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of West Nile-like viral encephalitis—New York, 1999. MMWR Morb Mortal
Wkly Rep 1999;48:845–9.
- Centers for Disease Control and Prevention. Provisional surveillance
summary of the West Nile virus epidemic—United States, January–November,
2002. MMWR Morb Mortal Wkly Rep 2002;51:1129–33.
- Eldridge BF. Strategies
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- Day JF. Predicting
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- Centers for Disease Control and Prevention. Epidemic/epizootic West
Nile virus in the United States: revised guidelines for surveillance,
prevention, and control. Washington (DC): Department of Health and Human
Services; 2001. Available from URL: http://www.cdc.gov/ncidod/dvbid/westnile/resources/wnv-guidelines-apr-2001.pdf
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al. Crow
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1 These figures do not include a human case
in New York that was not located to the county level.
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