West Nile Virus was first isolated in Uganda in 1937. It is a single-stranded RNA virus transmitted primarily by Culex mosquitoes that infects humans, birds, horses and other mammals. While most human infections are asymptomatic, it can cause West Nile fever or severe West Nile disease with symptoms like headache, fever and neck stiffness. There is no vaccine for humans but supportive care is provided. Mosquito control and personal protection are the main prevention strategies.

1. West Nile Virus (WNV)
BP221506
Thulasinathan J
1st M.Sc.Applied Microbiology

2. The Organism
• Flaviviridae
– Flavivirus
• Single stranded RNA virus
• 2 genetic lineages
– Linage 1
• 3 clades (1a, 1b, 1c)
• Infects humans, birds, mosquitoes, horses,
and other mammals

3. History
• 1937: West Nile District, Uganda
– First isolated
• 1950: Egypt
– Ecology studied
• Additional outbreaks
– 1951-54, 1957, Israel
– 1962, 2000: France
– 1973-74: South Africa
– 1996: Romania, 1998: Italy

4. United States - 1999
• New York City
– 62 cases; 7 deaths
• Zoo birds, crows,
horses also infected
• Caused by lineage 1a
– NY99

5. Structure of West Nile Virus
(WNV)
• single-stranded, positive-sense RNA genome.
• Size-40-50 nm in diameter.
• Shape-spherical in shape.
• The glycoproteins on the envelope protrude as
spikes that help in the viral attachment to the
host cells.
• The virus is related to other viruses causing
yellow fever and dengue fever and is more
closely related to viruses causing encephalitis.

6. Structure of West Nile Virus
(WNV)

7. Human WNV Activity
Year Cases Deaths
1999 62 7
2000 21 2
2001 66 9
2002 4156 284
2003 9862 264
2004 2539 100
2005 3000 119
2006 4269 177
2007 3630 124
2008 1356 44
2009 720 32
2010 1021 57

8. Center for Food Security and Public Health, Iowa State University, 2011
Incidental hosts
Vectors
Humans, horses,
and other animals
Amplifying hosts
Birds
Culex spp., Aedes spp.,
Ochlerotatus spp.

9. Transmission
• Primary mosquito
vector
– Culex spp.
• Tick vectors
– Asia, Russia

10. Transmission
• Overwintering of
mosquitoes
• Transovarial
transmission
• Birds
– Contact
– Migratory
transport

11. Human Transmission
• Direct contact
– Infected birds, tissues
• Laboratory acquired
• Blood transfusions
– Screening implemented in 2003
• Organ transplants
• Transplacental transmission
• Breast feeding

12. Replication

13. pathogenesis
• WNV directly infects neurons, particularly in
deep nuclei and gray matter of the brain,
brainstem, and spinal cord.

14. Clinical Manifestations
• About 80% of the infected people remain asymptomatic and
20% of the infected people can develop West Nile fever or
severe West Nile disease.
• The incubation period is usually 3 to 14 days.
• However, an extended incubation period of about 21 days has
been observed among immunocompromised patients.

15. Clinical Manifestations
• The common symptoms in the febrile illness are:
• Headache
• Body aches
• Fever
• Joint pain
• Vomiting
• Diarrhea
• Occasional skin rash (on the trunk of the body)
• Swollen lymph glands
• Neck stiffness

16. Diagnosis in Humans
• Serology
– Serum or CSF
– IgM capture ELISA
• Cross reactions possible
– Plaque neutralization test
• Detection of virus, antigen, or
nucleic acids
– RT-PCR
– Immunohistochemistry

17. Treatment in Humans
• No specific therapy
• Supportive care
• Therapies in clinical trials
– Interferon
– Antisense nucleotides
– IV immunoglobulin
– Antiviral drugs

18. Disease in Animals

19. Species Affected
Horses Black Bear Bats
Goats Wolf Llama
Sheep Alpaca Cattle
Dog Mountain Goat Seal
Rabbit Alligator Cat
Chipmunk Gray Squirrels Deer
Skunk
Crocodile

20. Prevention and Control

21. Vaccination
• Several commercial vaccines available
for horses
– Consult label for instructions
– Usually 2 doses, 3 to 6 weeks
apart
– Annual revaccination
• Vaccines sometimes used
off-label to protect birds

22. Mosquito Management
• Surveillance
• Source reduction
• Personal protection
• Biological control
• Larvicide-type of insecticide
• Adulticide