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 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 Majorvector potential WNV vector in Western US
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 • Infected with WN virus • Mortality: bats, rabbit, chipmunk, gray squirrel, horse, cat, alpaca, dog • Seropositive: dog, bears, sheep, goats, wolf
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: • 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 • Symptomatic:asymptomatic ratio is approximately 1:9
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 • 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 • Clinical signs • CSF: mononuclear pleocytosis, normal to increased protein • IgM antigen-capture ELISA • Plaque reduction neutralization assay • PCR, IHC
Diagnosis • Serum and CSF results similar • Horses do not develop an IgM response to vaccine
Treatment • Symptomatic • IV fluids • Anti-inflammatory medication • Flunixin meglumine • DMSO • Corticosteroids
Treatment of WNV • Alpha interferon • Hyperimmune plasma
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: • Steroid treatment (p=0.10) • Hospitalization (p<0.0001) • Vaccination (p=0.0005) • Not recumbent • Affected late in epidemic
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 WNV • Anti-inflammatory medication • Flunixin meglumine • DMSO • Corticosteroids?
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 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 • 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 • Stock water tanks with fish that consume mosquito larvae • Use mosquito “dunks” in water tanks • Clean water tanks weekly
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 • 1cc dose, booster 3-6 weeks • Induces antibody response • Conditional license
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 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 372 Total 230 = No Vax 97 = 1 Vax 45 = 2 Vax
California Cases - 2004
Post-Approval Reaction Rates:Fort Dodge • <0.025% adverse effects reported • lumps and bumps • fever • lack of efficacy ?
http://lost-foals-group.4t.com/
Merial Vaccine • Canarypox vector • Mosquito challenge model • Similar results for safety, efficacy
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/
