West Nile virus
AssessmentsWest Nile virus
West Nile virus
USMLE® Step 1 style questions USMLE
A 24-year-old man comes to the emergency room for evaluation of headache and photophobia. He returned from a trip to central Africa last week, where he hiked and camped in the Jungle. Physical examination reveals increased neck stiffness. His temperature is 38.5 C° (101.3 F°). Cerebrospinal fluid analysis reveals pleocytosis and polymerase chain reaction is positive for West Nile virus. West Nile virus belongs to which viral family?
Content Reviewers:Rishi Desai, MD, MPH
Contributors:Kaia Chessen, MScBMC, Tanner Marshall, MS
West Nile virus is an arthropod-borne virus, or arbovirus, which are viruses that get transmitted through insects called vectors.
The vector for west nile virus is the mosquito, and gets transmitted in highest frequency through the female Culex species, which feeds on birds.
The virus was first discovered in Uganda - west of the Nile - but has since been reported throughout the world.
The virus causes a disease called West Nile fever, which normally causes mild symptoms, but can progress to full blown encephalitis or meningitis.
Normally, west nile virus is found in birds and mosquitoes.
Birds act as a reservoir for the virus, meaning the virus can replicate at high enough levels to cause significant viremia, or elevated viral blood counts, which allows for transmission to other uninfected mosquitoes.
The virus will then replicate inside the mosquito and ultimately move into its salivary glands.
So when the mosquito bites another animal, it injects its infected saliva into the host, since mosquitoes normally use their saliva as an anticoagulant.
When the vector mosquito bites a larger animal, like a horse or a human, the virus can't spread from these larger animals because their blood doesn’t reach high enough levels of the virus to be passed on to any mosquitoes that happen to bite them.
And this is called a dead end host.
West Nile virus is composed of positive single-stranded RNA.
This means that their RNA is actually mRNA, and the host cell ribosomes use this mRNA to make a long polyprotein chain, which is then broken into smaller pieces by viral proteases.
This all happens in the cytoplasm of the host cell, since that’s where ribosomes are found, and results in the production of several viral proteins.
West Nile virus is surrounded by an icosahedral capsid, which is a spherical protein shell made up of 20 equilateral triangular faces.
West Nile virus is also an “enveloped virus” because the capsid is covered by a lipid membrane.
Now, west nile virus enters host cells using a lipid membrane protein called E2 in a process called clathrin-mediated endocytosis.
Clathrin-mediated endocytosis is a cellular process that creates a vesicle to internalize a certain substance, in the case here, the virus, with the help of the proteins called clathrins.
The virus’ membrane will then fuse with the host’s cell membrane, releasing its RNA genome into the cytoplasm of the host cell.
The replication of the viruses then will take place before exiting the host cell by outward budding of the host’s plasma membrane.
After people are infected with west nile virus it takes two to fourteen days to develop symptoms.
Most people infected with the virus, about eighty percent, have few to no symptoms at all.
West Nile virus is an arbovirus transmitted by mosquitoes, with birds acting as a reservoir. The virus causes West Nile fever, which can progress to encephalitis or meningitis. Symptoms include fever, headache, vomiting, and rash. Diagnosis is made by detecting elevated IgM levels in the serum or CSF using a specific immunoassay test. Treatment involves supportive care, and prevention is the best therapy, including the use of mosquito repellent and wearing protective clothing.