1 west nile virus. 2 west nile virus: background l first isolated 1937 l flavivirus l africa, west...
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WEST NILE VIRUSWEST NILE VIRUS
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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
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Mosquito vectorIncidental infections
Bird reservoir hosts
Incidental infections
Incubation period -- 3 to 15 days
Incubation period -- 3 to 15 days
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Isolated from over 43 mosquito speciesMostly 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
Vectors and Hosts
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Culex tarsalis
Major vector potential WNV vector in Western US
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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 HostDead-end Host
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Mammals
Infected with WN virus
Mortality:Mortality: bats, rabbit, chipmunk, gray squirrel, horse, cat, alpaca, dog
Seropositive:Seropositive: dog, bears, sheep, goats, wolf
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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
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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%
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Subclinical Infection
Symptomatic:asymptomatic ratio is approximately 1:9
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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
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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
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Diagnosis
Clinical signs
CSF: mononuclear pleocytosis, normal to increased protein
IgM antigen-capture ELISA
Plaque reduction neutralization assay
PCR, IHC
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Diagnosis
Serum and CSF results similar Horses do not develop an IgM
response to vaccine
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Treatment
Symptomatic IV fluids Anti-inflammatory medication
Flunixin meglumineDMSOCorticosteroids
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Treatment of WNV
Alpha interferon Hyperimmune plasma
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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
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Increased likelihood of survival if:
Steroid treatment (p=0.10)
Hospitalization (p<0.0001)
Vaccination (p=0.0005)
Not recumbent
Affected late in epidemic
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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
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Treatment of WNV
Anti-inflammatory medicationFlunixin meglumineDMSOCorticosteroids?
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Residual Effects
Florida: 15% Minnesota: 40%
Average followup time 6 months59.2% had no neurologic or
behavioral abnormalitiesAverage duration of signs: 35 days
(1-180)
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Residual Effects
Behavioral changeLoss of muscle massMore frequent stumblingDiminished energyWeak hind limbs “Abnormal” gaitVision lossDysphagiaRelapses (2 wk to 5 mo)
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Prevention of WNV
Eliminate mosquito breeding sites
Control mosquito larvae
Reduce exposure to adult mosquitoes
Vaccinate
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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
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Control Mosquito Larvae
Stock water tanks with fish that consume mosquito larvae
Use mosquito “dunks” in water tanks
Clean water tanks weekly
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Reduce Exposure to Adult Mosquitoes
Stable horses during active mosquito feeding times
Screen the stables
Fans, barrier cloth, flysheets
Repellants applied frequently
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WNV Vaccine-whole virus
1cc dose, booster 3-6 weeks Induces antibody response Conditional license
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Does the Vaccine Work?
The BEST way to test a vaccine: Experimental Challenge Model
Infect Horse ExperimentallyVaccinate
HorseSee if horse gets
disease
Most horses do not get sick after experimental Most horses do not get sick after experimental infection!infection!
Most horses do not get sick after experimental Most horses do not get sick after experimental infection!infection!
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Vaccine Efficacy
Experimental Challenge
One year after vaccination Controls
9 of 11 infected--82% Vaccinates
1 of 19 infected--5% (95% Protective)
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Infection & Mortality vs. Vaccine Status: Colorado
396
45
7522
97
16268
230
27696
372
0 100 200 300 400
2nd Dose
1st Dose
None
Totals
Vaccination vs. Mortality Status of WNV Horses
Totals
Dead
Alive
(25%)
(30%)
(23%)
(13%)
230 = No Vax
97 = 1 Vax
45 = 2 Vax
372 Total
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California Cases - 2004
Vaccination Status
Cases
(168)
Fatalities
(78)Unvaccinated 110 56
Partially vaccinated
48 16
Fully vaccinated 5 3
Unknown vaccination
5 3
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Post-Approval Reaction Rates:Fort Dodge
<0.025% adverse effects reported lumps and bumps fever lack of efficacy ?
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http://lost-foals-group.4t.com/
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Merial Vaccine
Canarypox vector Mosquito challenge model Similar results for safety, efficacy
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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/