West Nile Virus Infection
Definitions & Key takeaways
West Nile virus is a type of single-stranded RNA virus belonging to the flavivirus genus that is transmitted by the Culex mosquito. It is spread by mosquitoes from bird reservoirs to other species, such as horses and humans, which are dead-end hosts.
Infected animals, particularly horses, typically display symptoms such as low-grade fever, weakness, muscle twitching, difficulty swallowing, seizures, and sometimes death. Birds may display symptoms such as lying down, lack of coordination, twisted neck, arching of the back, and paralysis of the legs and wings. Survivors of the infection may suffer chronic disabilities.
Human infections are usually mild, but elderly or immunocompromised individuals may develop meningitis or encephalitis. Diagnosis involves viral isolation, PCR, and IgM antibody detection in the blood. Currently only symptomatic treatment is provided for infected people or animals and prevention involves mosquito control and vaccination of susceptible animals. There is currently no vaccine available for humans.
The West Nile virus gets its name from where it was discovered, west of the Nile river in Uganda. It primarily causes infection in birds, especially geese and a range of wild birds, although it can also infect other animals like horses.
The virus affects the nervous system of the animals, causing symptoms that range from weakness and loss of coordination to complete paralysis.
West Nile virus is a zoonotic virus, which means that it can spread from animals to humans. The West Nile virus belongs to the genus flavivirus along with a few other viruses like the zika virus and dengue fever virus.
West Nile virus has a worldwide distribution. Before 1999 it was present mainly in Africa, Asia, the Middle East, and some European countries.
Following an outbreak in New York in 1999, it spread across North America, including all 48 of the contiguous states of the US, and has now also been found in South America and parts of the Caribbean.
West Nile virus is transmitted primarily by mosquitoes, most often the female Culex species, which usually feed on birds.
In people, other routes of infection include blood transfusion, organ transplantation, through breast milk and occasionally across the placenta.
The virus has also been found in some tick species, but it is not known if they play a role in passing the virus to susceptible animals or people.
The virus usually causes meningoencephalitis in animals, which is inflammation of the brain along with its protective membranes — the meninges.
This results in a variety of symptoms including behavioral abnormalities and physical disabilities. In horses, common symptoms include discomfort, lameness, staggering, low-grade fever, weakness, inability to stand, muscle fasciculation, anorexia, and inability to swallow, convulsions, and even death.
Similarly, infected birds also show symptoms like recumbency, uncoordinated movements, and leg and wing paralysis. Some birds may also show torticollis, which is the twisting of neck to one side; and opisthotonos, which is an extreme arching of the neck and the back caused by muscle spasms.
Now, when humans get infected, it's usually asymptomatic or causes mild symptoms like fever, chills, headache, and vomiting.
Only a few people, usually those with underlying medical problems, the elderly or young, develop meningitis or encephalitis.
These conditions can present with seizures, altered mental status, vision changes, severe headaches, neck pain or stiffness, coarse tremor in the upper extremities, and weakness.
In a few cases, weakness becomes severe enough to require mechanical ventilation The occurrence of cases in horses are a good indication of West Nile virus transmission in a locality and should increase the level of suspicion that people with typical symptoms, particularly those with meningoencephalitis, may have West Nile virus infection.
Diagnosis of West Nile virus is usually made by a blood test which shows an increase in IgM-type antibodies against West Nile virus.
In paralytic or dead animals, tissue samples are obtained from the brain, spleen, or kidneys and viral isolation can be done to directly look for the virus, or a reverse transcriptase PCR, or rtPCR, test can be done to look for the viral RNA.
Currently, there is no specific treatment available for West Nile virus infections, and so only supportive care is provided.
Some of the damage caused by the disease may not resolve, so human and animal survivors of severe disease may be left with chronic disabilities.
Preventive measures are necessary to limit the spread of infection. These include controlling mosquito breeding by getting rid of stagnant water and spraying larvicides around farms with animals.
Vaccinating young bird flocks and horses is also an effective solution, but there are no vaccines available for people. ##Summary As a quick recap… West Nile virus is a single-stranded RNA virus that belongs to the genus flavivirus.
It's an arbovirus that's transmitted by the Culex species of mosquitoes that spread the virus from bird reservoirs to other species like horses and humans, which are dead-end hosts.
West Nile virus usually causes meningoencephalitis in infected animals. Common symptoms in horses include low-grade fever, weakness, muscle fasciculation, inability to swallow, convulsions, and even death.
Typical symptoms in birds include recumbency, uncoordinated movements, torticollis, opisthotonos, and leg and wing paralysis.
Animals that do survive are left with chronic disabilities. Human infections are usually mild or asymptomatic.
However, elderly or young people or those with underlying medical problems may develop meningitis or encephalitis. Diagnosis is made by viral isolation, PCR and looking for IgM antibodies in the blood.
Currently only symptomatic treatment is provided for infected people or animals and prevention involves mosquito control and vaccination of susceptible animals.
There are no vaccines for people.
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