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“On Watch” for West Nile Virus in Oklahoma. Kristy K. Bradley, DVM, MPH, Dipl. ACVPM State Public Health Veterinarian/ Assistant State Epidemiologist Oklahoma State Department of Health (405) 271-4060 kristyb@health.state.ok.us. West Nile Virus: Background. Family: Flaviviridae

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“On Watch” for West Nile Virus in Oklahoma

Kristy K. Bradley, DVM, MPH, Dipl. ACVPM

State Public Health Veterinarian/

Assistant State Epidemiologist

Oklahoma State Department of Health

(405) 271-4060


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West Nile Virus: Background

  • Family: Flaviviridae

    • Genus: Flavivirus

    • Japanese Encephalitis Antigenic Complex

    • Complex includes: Alfuy, Japanese encephalitis, Kokobera, Koutango, Kunjin, Murray Valley encephalitis, St. Louis encephalitis, Stratford, Usutu, and West Nile viruses.

  • All are transmissible by mosquitoes, many can cause febrile, sometimes fatal, illnesses in humans.

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West Nile Virus Transmission Cycle

Mosquito vector

Incidental infections

West Nile


West Nile


Incidental infections


reservoir hosts

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West Nile Virus: Background

  • First isolated in 1937 from the blood of a woman in Uganda

  • First recorded epidemic in Israel in 1950’s.

  • Soon recognized as one of the most widespread Flaviviruses.

  • Distributed through Africa, West Asia, Europe and the Middle East.

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West Nile Virus in the US -Some Possible Pathways of Introduction

  • Infected human host

  • Human-transported vertebrate host

    • Legal importation of birds

    • Illegal smuggling of infected birds

  • Human-transported vector(s)

  • Storm-transported vertebrate host (bird)

  • Intentional introduction (terrorist event)

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West Nile Virus in North AmericaHuman Case Summary 1999 - 2001

  • 1999 thru 2000

    • 83 confirmed human cases

    • All from greater NYC metro area

    • 9 deaths (11%)

    • 85% of confirmed cases > 50 yrs old

  • 2001

    • 66 confirmed human cases from 10 states

    • 9 fatalities (14%); median age = 69 yrs.

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1st U.S. crow

Last crow

Number of WNV-Infected Crows Reported from the North Central and South Central Regions,by Week, 2001

*As of 2/1/02

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WNV Vertebrate Health Impacts 1999-2001

Birds-101 species 11,640 99.9%

Horse 673

Cat 1

Dog 0 ----

Dom. Rabbit 1

Raccoon 1

Gray Squirrel 1

Eastern Chipmunk 1

Striped Skunk 1

Big Brown Bat 1

Keen’s Bat 1

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WNV Impact on Pets

  • Horses and pet birds of most concern

    • vaccine for horses

    • avoid mosquito exposure for pet birds

  • Dogs and Cats resistant to disease

    • no special preventative measures needed

  • Pocket pets may be vulnerable

    • more research needed

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    2002 National WNV Surveillance (Final ArboNet Report):

    • 16,739 avian infections reported from 44 states (439 in Oklahoma)

    • 14,571 equine infections confirmed in 40 states ( 965 in Oklahoma)

    • 6,604 WNV-positive mosquito pools from 36 states ( 18 in Oklahoma)

    • 4,156 human cases from 40 states (21 in OK)

    • 284 human deaths(2 in OK)

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    WNV Surveillance in Oklahoma


      • Supplemental Epidemiologic and Laboratory Capacity Grant from CDC with activities beginning November 2000

      • Formation of InterAgency Zoonotic Disease Working Group:

        • OK Dept. of Agriculture, OK Dept. of Wildlife Conservation, OK Animal Disease Diagnostic Laboratory, and USDA Veterinary Services and Wildlife Services

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    WNV Surveillance in Oklahoma


      1. Early detection of WNV epizootic

      activity so intervention is possible to

      prevent human illness.

      2. Obtain good baseline epidemiologic

      data on incidence of other

      arboviral infections (SLE, WEE,


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    Distribution of Important Arboviruses in the U. S.

    Eastern Equine Encephalitis

    California serogroup viruses

    Western Equine Encephalitis

    St. Louis Encephalitis

    Probably Highlands J

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    Diagnosis of WNV


      1. IgM-capture ELISA (serum or CSF)

      Diagnostic if CSF-positive; IgM antibodies

      usually present in serum by 8th day of illness

      2. ELISA, IFA, HI

      Need four-fold rise in paired titers for case

      confirmation; postive tests need confirmation by


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    Diagnosis of WNV

    • PCR (polymerase chain reaction)

      • CSF, brain

      • Not sensitive enough to test serum or blood

  • Viral Isolation (requires BSL-3 lab)

    • blood, brain, CSF

  • Immunohistochemistry

    • brain tissue (autopsy)

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    WNV Surveillance in Oklahoma

    • Dead Wild Bird Surveillance and


    • Equine Encephalitides Monitoring

      and Testing

    • Enhanced Passive Surveillance for

      Human Meningoencephalitis

    • Mosquito Trapping & Testing

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    Dead crows per square mile vs. human cases, Staten Island 2000

    Ref: Eidson. Emerg. Inf. Dis. 7(4):662-4; 2001

    Conclusion: Weekly dead crow densities> 1/sq. mi. provided an early warning for a human outbreak

    First Positive Bird


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    WNV Surveillance in Oklahoma: Avian 2000

    • Established dead bird reporting hotline --

      • 1-800-990-CROW

    • Dept of Ag triages phone reports; OSHD and local county health departments used as drop-off sites

    • Developed testing capabilities at Oklahoma Animal Disease Diagnostic Lab

      • Necropsy, RT-PCR test for WNV,toxicology screen

    • Success depends on many community partners

      • Animal control, zoos, bird watchers, parks workers

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    Instructions on Submission of Freshly Dead Birds: 2000

    • Wear gloves or cover hand with plastic bag when collecting bird.

    • Double bag in small garbage bag or other plastic bag.

    • Keep at refrigeration temperature - not frozen-(place in cool location, refrigerator, or bucket with ice) untiltransported to laboratory or pick-up point.

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    Avian WNV Positivity Rates by County; Oklahoma: 2000

    Range: 2% (Canadian) - 100% (Grant)

    • Wagoner 53.3% (32/60)

    • Ottawa 39.5% (32/81) 

    • Muskogee 38.2% (63/165) 

    • Haskell 37.5% (3/8)

    • McCurtain 35.7% (10/28)

    • LeFlore 30% (6/20)

    • Tulsa 28.4% (151/531)

    • Beckham 25% (3/12) ….

    • Oklahoma 15.5% (51/329) 

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    WNV Surveillance in Oklahoma: Avian 2000

    • 3,481 dead wild birds submitted for testing

      • 438 positive (28 different species)

    • Ceased bird testing October 15 due to depletion of resources

    • 38 counties origin of WNV+ birds

      • 10/13 counties with human cases had positive bird as first indicator

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    Symptomatology of West Nile Disease in Horses 2000

    • Spectrum of illness

    • Ataxia, limb weakness, and muscle fasiculations commonly reported

    • Sometimes progression to “down in rear” or complete recumbency very rapid (hours)

    • Recovery not always complete.

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    West Nile Virus Surveillance in Oklahoma:Veterinary 2000

    • Oklahoma ranked #2 in no. of horses/capita

    • Equine owners and veterinarians urged to report suspect equine encephalitis cases to OSDH, ODA, or USDA.

    • Blood test or testing of brain tissue performed at OADDL.

    • All dead or euthanized horses first examined for rabies at State Public Health Laboratory.

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    West Nile Virus Diagnosis by IgM Capture ELISA 2000

    • Can provide diagnosis on single specimen (serum or CSF)

    • IgM antibodies detectable by 8-10 days post-infection.

    • Sera collected prior to 8 days with equivocal results should be retested a few days later.

    • Cross reaction with other flaviviruses; PRNT confirmatory.

    • IgM antibodies generally persist 2-3 months; IgG > 2 years.

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    WNV Equine Vaccine 2000

    • Conditional license approved August 1, 2001to Fort Dodge Laboratories, Inc.

    • Two dose series given 3 -6 weeks apart

    • Blood test (IgM capture ELISA)will distinguish between acute disease and vaccination antibody response

    • Full vaccine licensure approved February, 2003

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    WNV Equine Vaccine 2000

    • Ft. Dodge Labs estimates 35% of nation’s horse population (~6.9 million) have been vaccinated.

    • In highly enzootic areas, more than one annual booster may be necessary to provide protective immunity.

    • Very good safety and efficacy record.

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    West Nile Virus Surveillance in Oklahoma:Preliminary Equine Findings

    • 964 (+1 zebra) laboratory-confirmed cases

    • Majority of cases had onsets after Sept. 30

    • 13.4% mortality rate (grossly underreported)

    • Twenty-eight horses (2.9%) had received 2 dose primary series within 9 months of disease onset.

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    THE TOP FIVE Findings

    Texas 1,598

    Illinois 1,120

    Nebraska 1,092

    Minnesota 992

    Oklahoma 964

    Neighboring States

    Kansas 792

    Missouri 662

    Colorado 376

    Arkansas 73

    Number of Equine West Nile Cases by State, 2002

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    Summary of Equine Surveillance 2002

    • Complement to avian surveillance

    • First indicator of transmission risk in western OK

    • Funding depleted October 15, 2002

    • Payment for equine testing in 2003 will be responsibility of animal owner.

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    Culex Mosquitoes as Primary WNV Vectors 2002

    • Over 36 different types of mosquitoes have tested positive for WNV, but Culex species of mosquitoes are most involved with transmission.

    • Culex characteristics:

      • prefer to feed on birds

      • most prevalent July- Oct

      • weak fliers

      • breed in small areas of standing water

      • adults live ~ 3 weeks

      • can overwinter and harbor


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    Culex tarsalis 2002

    • Most abundant in western agroecosystems and wetlands

      • types of habitat vary from peridomestic sources to pasture and other flood irrigated crops to wetlands.

    • Feeds most frequently on upland birds that roost and nest at elevated vegetative ecotones

      • house finches, house sparrows, doves, quail

    • Competent vector of SLE and WEE and WNV

      • suitable bridge vector to horses or humans

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    Other Animals Affected by 2002West Nile Virus??

    • Dogs and cats rarely develop illness after infection with the virus

    • Sheep and goats may be more susceptible

    • Documented outbreaks in squirrels in several states

      • 25% squirrels tested in Oklahoma were positive

  • More research needed in this area.

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    Epidemic WNME, United States, 2002 2002

    • Western hemisphere’s largest arboviral ME epidemic

      2,946 WNME cases

      1,300 WNF / unspecified

      284 deaths

      39 states & D.C.

    • Largest WNME epidemic EVER

    • Spread to Pacific coast

    • New clinical syndromes

      Acute flaccid paralysis (“West Nile polio”)

    • Five new transmission modes

    4,156 illnesses

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    New Transmission Modes 2002

    • Transplantation

    • Transfusion

    • Breast Milk

    • Congenital

    • Occupational

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    West Nile Fever: Human Disease 2002

    • Incubation period: 3 - 15 days

    • About 20% of those infected develop a mild illness (West Nile Fever)

    • Acute aseptic meningitis or encephalitis develops in ~ 1 out of every 150 infections.

    • Most fatal cases >50 years old; majority of surviving patients suffer memory loss and marked muscle weakness

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    West Nile Virus: Mild Infection 2002

    • Lasts 3-6 days

    • Febrile illness of sudden onset often accompanied by

      Tiredness Muscles aches and pains

      Loss of appetite Rash

      Nausea Swollen lymph glands

      Vomiting Headache

      Sensitivity of eyes when looking at light

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    West Nile Virus: Severe Infection 2002

    • Less than 1% of persons bitten by an infected mosquito progress to severe neurologic disease.

    • Encephalitis more commonly reported than meningitis.

    • Hospitalized for supportive care; no specific treatment.

    • Several patients experience severe muscle weakness and flaccid paralysis.

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    Arboviral Surveillance in Oklahoma: Human 2002

    • Free testing available at State Public Health Laboratory

      • WNV and SLE

      • Request must be made through a health care provider

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    Symptom Onset Dates of Human Cases* of West Nile disease in Oklahoma; 2002

    * N = 20 cases of probable or confirmed WNV infection by detection of IgM antibodies in CSF or serum.

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    OK WNV Human Case Summary Oklahoma; 2002

    • Sum of 21 Cases

      • Eighteen patients over 50 years of age

      • 1 exposure in D.C.; 2 transfusion-related

      • Majority of cases had onset of illness during October

      • Counties of residence: Muskogee (2), Tulsa (3), Oklahoma (3), Canadian (3), Washington, Rogers, Ottawa, Okmulgee, Pushmataha, Caddo, Kay, Beaver, and Osage.

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    Mosquito Life Cycle Oklahoma; 2002

    • Most eggs hatch within 48 hours.

    • Larvae must breathe; feed on organic matter.

    • Pupae = “tumblers”

    • Only adult female mosquitoes need a blood meal.

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    Mosquito Prevention Measures Oklahoma; 2002

    • Personal Protection

      • Avoid times and places where mosquitoes active.

      • Cover exposed skin.

      • Apply DEET-based repellant (10-30%) to exposed skin and clothing.

    • Mosquito Source Reduction

      • Dispose of trash piles

      • Mow tall weeds

      • Drain rain barrels, plant containers, child wading pools, etc.

      • Clean roof gutters of leaves and other debris.

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    Mosquito Abatement Measures Oklahoma; 2002

    • Stock Tanks, Ornamental Water Gardens

      • Apply mosquito larvacide (BTI).

      • Stock with fish.

      • Put in aerator pump.

      • DO NOT treat with insecticides.

    • Keep window and door screens in good repair.

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    Larvicides Oklahoma; 2002

    • Interrupt mosquito life cycle

    • Most very environmentally safe

      • microbial (Aquabac, Vectobac)

        Bacillus thuringiensis israelensis

      • chemicals

        methoprene, oils,

        mononuclear films

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    Adulticides Oklahoma; 2002

    • “Last resort”

    • Requirements for successful application

      • spray when target species active

      • low wind

      • dispersal of ultra-low volume droplet size

        Bacillus thuringiensis israelensis, Bacillus sphaericus

      • chemicals

        synthetic pyrethroids (resmethrin, sumethrin), inorganic phosphates (malathion)

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    Oklahoma West Nile Virus Preparedness and Response Oklahoma; 2002

    • Response will focus on public health messages

    • Encourage cities with adulticide applicators to have public health licenses

    • Costly to purchase equipment

      (ULV foggers & chemicals)

    • May need to address

      ordinances dealing with

      public health nuisances

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    Changes in Surveillance Procedures for 2003 Oklahoma; 2002

    • Universal dead bird reporting hotline will not be used this year.

    • Bird testing will only be done in communities with some mosquito control capabilities, e.g. Tulsa, Norman, Muskogee, Wagoner, Ponca City, etc.

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    Changes in Surveillance Procedures for 2003 Oklahoma; 2002

    • State Public Health Lab will expect to test more human specimens with broader clinical presentations.

    • Will try to enhance physician education.

    • Will no longer pay for equine testing.

    • Will attempt to enhance mosquito collections and testing.

      • State Public Health Lab will test mosquito pools collected at 6-7 trapping locations across the state.