Download A Two-Year Serosurvey of a Rural Population for West Nile Virus

The Frequency of West Nile Virus
Infected Mosquitoes in Fremont
County, WY
Tyler Graham, Russell Goff, Rachel Lamb,
Michael Bostick and Steven McAllister
Department of Science
Central Wyoming College
Background
• West Nile virus (WNV) was
originally discovered in
Uganda in 1937. WNV is
in the family Flaviviridae
belonging to the same
genus as Dengue Fever,
Tick Borne Encephalitis,
and Yellow Fever. WNV is
a (+) RNA arthropodborne virus that enters a
cell, mimics the host cell’s
mRNA, and replicates.
The primary vector of WNV to humans
in Wyoming is the Culex tarsalis
mosquito.
Mosquitoes
carry
infectious
virus
particles in
their salivary
glands.
WNV Life Cycle
Birds, primarily Corvids,
serve as a reservoir for
the virus. Humans,
horses and other animals
can be terminal hosts for
the virus. Infectious-level
viremias are not known
to be sustained in most
mammals.
Epidemiology of WNV
• WNV was first detected in the United States in 1999
• The virus rapidly migrated across the lower 48
contiguous states over the course of 6 years
Pathology of West Nile virus
• The incubation period in humans is usually 3
to 14 days.
• 80% of patients experience symptoms that
include fever, headache, tiredness, and body
aches, nausea, vomiting, occasionally with a
skin rash (on the trunk of the body) and
swollen lymph glands.
• About 20% of people who become infected
with WNV will develop West Nile fever.
Pathology of West Nile virus
It is estimated that approximately 1 in 150 persons infected
with West Nile virus will develop a more severe form of
disease.
• The symptoms of severe disease (also called neuroinvasive
disease)
1. West Nile encephalitis
2. West Nile meningitis
3. West Nile poliomyelitis (an inflammation of the spinal cord).
• People over the age of 50 and some immunocompromised
persons (for example, organ transplant patients) are at the
highest risk for getting severely ill when infected with WNV.
West Nile virus as of January 7, 2014
Nationwide, a total of 5,674 human cases of West Nile virus,
including 286 deaths, have been reported to CDC. Of these,
2,873 (51%) were classified as neuroinvasive disease (such as
meningitis or encephalitis) and 2,801 (49%) were classified as
non-neuroinvasive disease.
Cases per 100,000 people
WNV in Wyoming
 Human cases were first
detected in Wyoming in
2002
 There were 2 human
cases in 2002
 2007 was the last major
outbreak of cases in
Wyoming
 There were 185 human
cases with two fatalities
WNV in
Fremont County, WY
 In Fremont County in 2007 there were 117
human cases of WNV with one fatality in
Fremont County. (63% of all cases in Wyoming
that year)
 Since 2007, the highest number of cases has
been 17 (2013)
Purpose of Research
• Explain the recent drop of cases in Fremont
County since the large outbreak in 2007
• Determine the frequency of West Nile virus
infected mosquitoes (primary vector) in
Fremont County
Mosquito Trapping
• Three different locations
• Kinnear, WY (Longitude
-108.73, Latitude 43.17)
• Riverton, WY (Longitude
-108.43, Latitude 43.02
• Pavillion, WY (Longitude
-108.65, Latitude 43.25)
Trapping Parameters
•
•
•
•
Seven week period trapping once a week
CDC Light Trap
Augmented with CO2
12 hour period (6:00 p.m. to 6:00 a.m.)
RAMP Reader and Results
• Is an immunoassay
based on
fluorescence tags
• Majority of tests
were ≤ 10 units
• 30 or more units, in
optical density,
indicate a positive
Testing Results
• Riverton Golf Course – 0% of tests were
confirmed positive
• Kinnear – 2.2% of tests were confirmed
positive (direct count)
• Pavillion – 0.021% to 0.93% of test were
confirmed positive (statistical count)
Agresti-Coull “Plus Four” Interval:
Pavillion Results
• 𝑃=
2+2
40+4
• P=1–
•
=
20
4
44
1−
Adding 2 pseudo-successes and 2
pseudo-failures
4
44
Taking the complement
= 0.00475
• 𝑃 −+ 1.96
0.00475 1 − 0.00475
880
• Interval: (0.00021, 0.0093)
• CI 95%: (0.021% to 0.93%)
Finding 95% confidence interval
using a z-table or technology
Conclusion
• Results showed that a small proportion of the
mosquitoes were carrying the West Nile virus.
This could potentially explain the drastic drop
in cases since 2007. Future research will
include more trapping locations and the use of
atmospheric and temperature sensors.
Additionally, we will seek collaboration with
Fremont County Weed and Pest Control
(FCWP) to obtain additional data.
Acknowledgments
• This project was supported in part by
grants from the National Center for
Research Resources (P20RR016474) and
the National Institute of General Medical
Sciences (P20GM103432) from the
National Institutes of Health.
• We thank Dr. Jun Ren, Dr. Scott Seville, and
the University of Wyoming INBRE Network
for their support in this research.
Works Cited
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