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WEST NILE VIRUS. West Nile Virus: Background. First isolated 1937 Flavivirus Africa, West Asia, Europe and the Middle East 1999 US isolate similar to a goose isolate from Israel. Mosquito vector. Incidental infections. Incidental infections. Bird reservoir hosts.

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West nile virus background
West Nile Virus: Background

  • First isolated 1937

  • Flavivirus

  • Africa, West Asia, Europe and the Middle East

  • 1999 US isolate similar to a goose isolate from Israel


Mosquito vector

Incidental infections

Incidental infections

Bird

reservoir hosts

Incubation period -- 3 to 15 days


Vectors and Hosts

  • Isolated from over 43 mosquito species

    • Mostly Culex species:

      • Cx. univittatus, Cx. perixiguus, Cx. pipiens, Cx. modestus, Cx. quinquefasciatus, Cx. tritaeniorhynchus, and Cx. vishnui

    • Other mosquito species:

      • Aedes, Aedeomyia, Anopheles,

      • Coquillettidia, Mansonia, Mimomyia


Culex tarsalis
Culex tarsalis

Majorvector potential WNV vector in Western US


Epidemiology
Epidemiology

  • Mosquitoes feed on infected birds

  • Pass the virus to warm-blooded animals.

  • Horses cannot spread WNV to other horses, people or pets.

    Dead-end Host


Mammals
Mammals

  • Infected with WN virus

    • Mortality: bats, rabbit, chipmunk, gray squirrel, horse, cat, alpaca, dog

    • Seropositive: dog, bears, sheep, goats, wolf


Clinical signs
Clinical Signs

  • Ataxia 86%

  • Depression 51%

  • Hindlimb weakness 49%

  • Difficulty or inability to rise 46%

  • Muscle tremors 41%

  • Fever only 24%

  • Death in 25-33% of affected horses


Clinical signs colorado
Clinical Signs: Colorado

  • Clinical signs:

    • Ataxia – 73%

    • Weakness – 69%

    • Lethargy, depression – 61%

    • Muscle fasciculations – 60%

    • Unable to rise on their own – 38%***

    • Fever – 35%

    • Altered mentation – 31%

    • Hypersesthesia – 30%

    • Cranial nerve deficits – 27%


Subclinical infection
Subclinical Infection

  • Symptomatic:asymptomatic ratio is approximately 1:9


Epidemiology colorado
Epidemiology: Colorado

  • 9 years (3 mos – 35 yrs)

  • No breed, gender predilection

  • 71.4% (344/482) survived

  • Horses that died were older

  • Unvax: 36.6% mortality

  • At least one vax: 20.3% mortality


Recumbent horses
Recumbent Horses

  • Average duration: 2.3 days

  • Range: 1-7 days

  • Onset to recumbency: 2.1 days (range = 0-14 days)

  • Recumbency as first sign: 4 horses

  • Nonvax 2.4 times more likely to be unable to rise


Diagnosis
Diagnosis

  • Clinical signs

  • CSF: mononuclear pleocytosis, normal to increased protein

  • IgM antigen-capture ELISA

  • Plaque reduction neutralization assay

  • PCR, IHC


Diagnosis1
Diagnosis

  • Serum and CSF results similar

  • Horses do not develop an IgM response to vaccine


Treatment
Treatment

  • Symptomatic

  • IV fluids

  • Anti-inflammatory medication

    • Flunixin meglumine

    • DMSO

    • Corticosteroids


Treatment of wnv
Treatment of WNV

  • Alpha interferon

  • Hyperimmune plasma


Prognosis colorado
Prognosis: Colorado

  • 82% considered fully recovered

  • Some residual signs in 21% (decreased stamina, weight loss)

  • Estimated 2 unreported cases for every 3 laboratory confirmed cases


Increased likelihood of survival if
Increased likelihood of survival if:

  • Steroid treatment (p=0.10)

  • Hospitalization (p<0.0001)

  • Vaccination (p=0.0005)

  • Not recumbent

  • Affected late in epidemic


Aged horses 18 yrs
Aged Horses (>18 yrs)

  • Survivors avg. age = 8.9 yrs

  • Nonsurvivors avg. age = 10.8 yrs

  • Aged were 2.6 times as likely to die

  • Older horses not more likely to be recumbent

  • Older horses not less likely to be vaccinated


Treatment of wnv1
Treatment of WNV

  • Anti-inflammatory medication

    • Flunixin meglumine

    • DMSO

    • Corticosteroids?


Residual effects
Residual Effects

  • Florida: 15%

  • Minnesota: 40%

    • Average followup time 6 months

    • 59.2% had no neurologic or behavioral abnormalities

    • Average duration of signs: 35 days (1-180)


Residual effects1
Residual Effects

  • Behavioral change

  • Loss of muscle mass

  • More frequent stumbling

  • Diminished energy

  • Weak hind limbs

  • “Abnormal” gait

  • Vision loss

  • Dysphagia

  • Relapses (2 wk to 5 mo)


Prevention of wnv
Prevention of WNV

  • Eliminate mosquito breeding sites

  • Control mosquito larvae

  • Reduce exposure to adult mosquitoes

  • Vaccinate


Mosquito Breeding Sites

1. Storm drains

2. Clogged roof gutters

3. Window wells

4. Uncovered containers

5. Leaky faucets and standing water

6. Garden ponds

7. Swimming pools

8. Tires and wheelbarrows

9. Birdbaths and children’s toys


Control mosquito larvae
Control Mosquito Larvae

  • Stock water tanks with fish that consume mosquito larvae

  • Use mosquito “dunks” in water tanks

  • Clean water tanks weekly


Reduce exposure to adult mosquitoes
Reduce Exposure to Adult Mosquitoes

  • Stable horses during active mosquito feeding times

  • Screen the stables

  • Fans, barrier cloth, flysheets

  • Repellants applied frequently


Wnv vaccine whole virus
WNV Vaccine-whole virus

  • 1cc dose, booster 3-6 weeks

  • Induces antibody response

  • Conditional license


Does the vaccine work

Infect Horse Experimentally

Vaccinate Horse

See if horse gets disease

Does the Vaccine Work?

The BEST way to test a vaccine: Experimental Challenge Model

Most horses do not get sick after experimental infection!


Vaccine efficacy
Vaccine Efficacy

Experimental Challenge

  • One year after vaccination

  • Controls

    • 9 of 11 infected--82%

  • Vaccinates

    • 1 of 19 infected--5% (95% Protective)


Infection mortality vs vaccine status colorado
Infection & Mortality vs. Vaccine Status:Colorado

372 Total

230 = No Vax

97 = 1 Vax

45 = 2 Vax



Post approval reaction rates fort dodge
Post-Approval Reaction Rates:Fort Dodge

  • <0.025% adverse effects reported

  • lumps and bumps

  • fever

  • lack of efficacy ?



Merial vaccine
Merial Vaccine

  • Canarypox vector

  • Mosquito challenge model

  • Similar results for safety, efficacy


Web site resources
Web site resources

For more information about humans and WNV: www.cdc.gov

For more information about WNV, USDA Website: www.aphis.usda.gov/oa/wnv

For more information about WNV in horses:

http://prevmed.vet.ohio-state.edu/


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