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West Nile Fever: Background and Epidemiology. First isolated in West Nile district, Uganda, 1937. Commonly found in humans, birds and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East, but has not previously been documented in the Western Hemisphere.

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West Nile Fever: Background and Epidemiology

  • First isolated in West Nile district, Uganda, 1937.
  • Commonly found in humans, birds and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East, but has not previously been documented in the Western Hemisphere.
  • Basic transmission cycle involves mosquitoes feeding on birds infected with the West Nile virus.
  • Infected mosquitoes then transmit West Nile virus to humans and animals when taking a blood meal.

West Nile Virus Transmission Cycle

Mosquito vector

Incidental infections

West Nile


West Nile


Incidental infections


reservoir hosts

west nile virus in domestic animals
West Nile Virus in Domestic Animals
  • Domestic rabbits, (swine, dogs)
  • Others:
    • chipmunks
    • gray squirrels
    • bats
    • eastern striped skunks.
west nile virus in the united states
West Nile Virus in the United States

National Surveillance Plan

  • Monitor geographic and temporal spread
  • Develop more effective strategies for surveillance, prevention and control
  • Define regional distribution and incidence of other arbovirus diseases
  • Provide current national and regional information
partnerships for west nile virus surveillance and other activities in the united states
Partnerships for West Nile Virus Surveillance and other activities in the United States
  • Centers for Disease Control and Prevention
  • U.S. Department of Agriculture
  • U.S. Geological Survey
  • Department of Defense
  • Environmental Protection Agency
  • State and Local Health Departments
  • State and Local Veterinarians
  • State and Local Wildlife Biologists
partners with mda
Partners with MDA:
  • Local Mosquito Districts
  • Local Health Departments
  • Physicians
  • Veterinarians
  • Nature Groups
  • Others
west nile in birds
West Nile in Birds
  • West Nile infects and multiplies in birds
  • Bird serve as reservoirs in nature
  • At least 76 kinds of birds have the potential to carry the virus
  • The disease effects bird in a variety of ways, including death
    • American Crows often die from the infection
west nile in birds9
West Nile in Birds
  • The first bird documented with WNV in the Western Hemisphere was identified in August, 1999
  • Since this first incidence, WNV has spread
    • One reason is sue to migration of birds
birds spread the virus
Birds spread the virus
  • Bird migrating to different areas carry WNV with them in their blood
  • This greatly effects the spread of West Nile Virus across North America
clinical signs in birds
Clinical Signs in Birds
  • Inability to fly
  • Incoordination
  • Abnormal movements
  • Ataxia
  • Weakness
  • Tremors
wnv surveillance modalities for birds
WNV Surveillance Modalities for Birds
  • Dead birds (especially crows)
  • Mosquitoes
  • Live wild bird sentinels
  • Live captive sentinels (e.g., chickens)
  • Veterinary surveillance
  • Human surveillance
  • There has been research into whether or not chickens would contract WNV
    • Inoculation of 21 chickens with needle, oral, and mosquito bite
    • None showed clinical signs
    • All developed neutralizing antibodies
    • Viremia was established in all but one chicken, but the level of virus was insufficient to infect vector mosquitoes
cases in the us
Cases in the US
  • In 1999: 292 avian cases (8% of which were crows)
  • By 2000: 4312 avian cases (still mostly crows)
virus cycle
Virus Cycle
  • The virus circulates from the bird reservoir to mosquitoes
  • Occasionally the virus can infect horses or humans, which are considered incidental hosts because they cannon continue the transmission cycle
cycle of wnv cont
Cycle of WNV cont.
  • “Enzootic” cycle involves transmission from bird to bird
  • Mosquitoes bite, or take a blood meal, from an infected bird
  • The mosquito bites a second bird and transmits the virus
testing of dead birds
Testing of Dead Birds
  • Necropsy crows and blue jays
  • Histopathology of major organs
  • Immunohistochem for WNV
  • PCR for WNV
  • Courtesy of Dr. Jon Patterson
birds collected for testing
Birds Collected for Testing
  • American Crows, Blue Jays, and Fish Crows, members of the Corvid family, which was most affected by the WN virus outbreak in 1999, were a top priority for submission, followed by raptors and house sparrows
  • As the outbreak progressed, birds from counties without documented WN virus were given higher priority, as well as migrating species of birds.
necropsy of birds
Necropsy of Birds
  • Organs collected for laboratory testing included brain, kidney, heart, liver, and spleen.
looking for wnv
Looking for WNV
  • During gross pathologic exam, the pathologist is looking for the following signs of WNV
    • emaciation
    • Splenomegaly
    • Hepatomegaly
    • cardiac or pericardial lesions
    • possible signs of encephalitis
pathological findings
Pathological Findings
  • Some reports indicate that the connection between the bird testing positive for WNV and having suspicious pathological lesions was highest in American Crows (51.8%)
  • The positive predictive value (i.e. the number of birds having suspicious pathological lesions and also testing positive) was relatively low (27.9%)
  • WNV infection was confirmed with at least 2 positive assays
  • Birds tested for WNV in NY state in 2000
wnv surveillance totals 2000 reported to cdc through january 25 2001
WNV Surveillance Totals, 2000Reported to CDC through January 25, 2001
  • Dead birds 4312
  • Mosquito pools 480
  • Horses 63
  • Other mammals 25
  • *Humans 21
  • Sentinel chickens 13

* Confirmed & Probable cases

clinical disease
Clinical Disease
  • Listlessness
  • Stumbling
  • Incoordination
  • Weakness of limbs
  • Ataxia
  • Partial paralysis
  • Possible death
diseases with similar signs
Diseases With Similar Signs
  • Equine Protozoal Myloencephalitis (EPM)
  • Herpes
  • Rabies
  • Eastern Equine Encephalitis (EEE)
  • Western Equine Encephalitis (WEE)
  • Venezuelan Equine Encephalitis (VEE)
need for surveillance
Need for Surveillance
  • Zoonotic Disease
  • Year 2000
    • EEE
    • WEE
    • WNV
2001 vector borne disease surveillance status report
2001 Vector-borne Disease Surveillance Status Report:
  • 32 counties participate in mosquito, bird surveillance.
  • 20 veterinary practices covering 35 counties.
  • Over 25,000 mosquitoes and 1,066 bird sera samples collected July-August.
  • 4 labs: MDA, MDCH, MSU-AHDL & MSU-MEL test for EEE, SLE, and West Nile virus.
diagnosis of wnv
Diagnosis of WNV
  • Testing blood for antibodies
  • Samples are sent to NVSL or MSU AHDL
  • Diagnosing live animals is difficult
treatment of wnv
Treatment of WNV
  • Currently no treatment
  • Supportive therapy and prevention of secondary infection
  • Approximately 65% recover
  • Long term immunity likely
equine wnv vaccine
Equine WNV Vaccine
  • Fort Dodge
  • Provisional USDA License
  • Killed, adjuvanted vaccine
  • Two series vaccine
  • Vaccinated animals will test positive for West Nile Virus
  • Given annually
  • Dr. Jon Patterson, MSU AHDL CVM
  • Dr. Steve Halstead, MDA