West Nile Virus

Kimberly Signs, DVM

Michigan Department of Community Health

West Nile Virus

Found in Africa, West Asia, Eastern Europe, and the Middle East

Emerging Infectious Disease

Flavivirus Family

Japanese Encephalitis Group

Closely related to St. Louis Encephalitis virus

Electron micrograph of West Nile virus isolated from the brain of a crow in New York

Pathogenicity of Virus

Previous outbreaks in Africa not associated with significant bird or human morbidity or mortality

NYS 1999 strain essentially the same as that associated with stork and goose mortality in Israel

Different lineages of WNV

WNV Surveillance

Types of Surveillance Activity Used by Public Health:

Passive -rely on receipt of reports

Active -efforts made to receive reports or perform testing

Sentinel –groups of susceptible animals placed and sampled to provide early warning of disease

Special –focused studies

Surveillance for WNV

Crows/Corvids

Veterinary (Horses)

Mosquitoes

Humans

All types employ both active and passive surveillance activities to some degree.

WNV Activity in the U.S. - 2002WNV Activity in the U.S. - 2002

Western Hemisphere

North America currently only has experience with related SLE-a sporadic illness hereCentral and South America-other flaviviruses more prevalent-Dengue, Yellow Fever (vaccine)Southern spread may be hindered by immunity to these other flaviviruses

WNV Infection in Birds

most birds are simply found dead

clinical signs may include incoordination, weakness, tremors, abnormal head posture

some are asymptomatic

Susceptible Corvid SpeciesSusceptible Corvid Species

Blue jay

American crow

Common raven

WNV Activity in Birds - 2001WNV Activity in Birds - 2001

WNV activity in Michigan - 2001WNV activity in Michigan - 2001

585 birds submitted

244 birds tested

38 counties

participated

65 positive birds,

in 10 counties

WNV Positive Birds 2002

WNV Activity in Michigan - 2002WNV Activity in Michigan - 2002Bird SurveillanceBird Surveillance

766 birds submitted

588 birds tested

80 counties participated

73 of 83 counties with positive birds

Bird Ecology

Competence: Passerine birds

WNV found in 162 species of birds to date

Reservoirs need to be abundant and exposed to the infection

Sero-prevalence: 30% in epizootic areas

Transmission in Birds

Mosquitoes: (ticks? biting flies?)

Contact: Experimental w/ crows Virus high-oral and cloacal

Oral: Experimental w/ infected bird and mammal carcasses-birds can have high levels of virus in tissues up to 14 days after viremia is cleared

Vector Ecology

Primarily Culex species involved (bird biters), container breeders

Bridge vectors (bite birds and mammals)

Over-wintering in adults (sewer systems)

Vertical Transmission in some species (+ male mosquitoes)

Environmental Determinants

Competent vector and host must occur together with virus

Temperature conditions affect transmission

Appropriate mosquito habitat

Equine Surveillance

~15,000 cases in 2002Fatality rate 30%Dead-end host-viremia low and briefCS: weakness, ataxia, muscle twitchingDX: tissues-VI, PCR

serum-IgM ELISA, PRNTVaccine: Fully licensed

Interferes with PRNT Pregnant mare?

329 Equine Cases

Positive Equine WNV Counties 2002