West Nile virus infection

Definition

West Nile virus infection in a mosquito-borne viral disease of birds, mammals and reptiles, caused by Flavivirus, characterized by variable clinical signs and death

Etiology

-Agent: West Nile virus

• Genus Flavivirus

• Family Flaviviridae

• RNA virus

• Part of the Japanese encephalitis virus complex or serogroup

• Two genetic lineages

-Does not persist in the environment for long; inactivated by UV and gamma irradiation

Epidemiology

-Host range: birds, mammals, reptiles

-ZOONOTIC

-Clinical cases: birds, horses, human, alligators

-Morbidity: birds <50%, horses <90%

-Mortality: birds <30%, horses <60%, other mammals <100%, humans < 15%

-Worldwide distribution

Transmission

-Excretion: oral and cloacal secretions

-Routes:

• Mosquito bites

• Blood transfusion, transplants

• Cannibalism and feather picking; eating infected animals or mosquitoes

Clinical signs: animals

-Horses:

• Mostly asymptomatic

• Anorexia, depression, neurologic signs

• Recovery after 7 days

-Birds:

• Some species are asymptomatic, other develop clinical signs

• Weight loss, decreased activity, depression, neurological signs

• Affected birds usually found dead

-Alligators:

• Anorexia, lethargy, weakness and neurological signs

• Death in 24-48 hours after the onset of clinical signs

Clinical signs: humans

-Many infections are asymptomatic

-West Nile fever: most common form (20%)

• Flu-like illness: fever, malaise, weakness, head- and body aches

• Most uncomplicated infections resolve in 2-6 days

• Mainly in elderly patients

-West Nile neuroinvasive disease:

• Encephalitis – changes in consciousness, disorientation, focal neurol. signs

• Meningitis – fever, headache, stiff neck and photophobia

• Acute flaccid paralysis – resembles polio, asymmetrical

Postmortem

-Birds:

• Nonspecific gross and microscopic lesions

• Emaciation and dehydration, multiorgan hemorrhages, petechia and congestion

• Histopathologic lesions in CNS, heart, spleen, liver and kidney

-Mammals (horses):

• Gross lesions uncommon

• Small multifocal areas of discoloration and hemorrhage in the spinal cord, brain stem and midbrain

-Reptiles (alligators):

• Moderately sized fat bodies and clear yellow fluid in coelomic cavity

• Liver: mottled red to yellow, enlarged, rounded edges

Diagnosis

-Material: brain, spinal cord, blood, major organs (heart, liver)

-In the lab: virus isolation (BS3!), detection of viral RNA/antigens (immunofluorescence, RT-PCR), serology

Prevention & control

-No treatment, just supportive care

-Vaccination in horses

-Mosquito control

-Quarantine of infected animals

-Carnivores & omnivores: no feeding of contaminated meat

-Surveillance of sentinel birds, dead birds and mosquitoes

-Good biosecurity and hygiene