West Nile fever is a zoonotic disease (an animal disease affecting humans). Disease is caused by West Nile virus (WNV), which is a flavivirus related to the viruses that cause St. Louis encephalitis, Japanese encephalitis, and yellow fever.
West Nile virus is mainly transmitted to people through the bites of infected mosquitoes. West Nile virus (WNV) is maintained in nature by transmission between birds and mosquitoes as birds are the natural hosts of the virus. Humans, horses and other mammals can be infected.
West Nile fever can cause severe neurological illness and death in people, however about 80% of people who are infected show no symptoms.
Causes of West Nile virus (WNV)
West Nile fever is caused by West Nile virus (WNV), belongs to the Japanese encephalitis virus (JEV) antigenic complex under family flaviviridae.
Transmission of disease – West Nile virus is happened in natural cycle that involves transmission between birds and mosquitoes. Humans, horses and other mammals can be infected.
Birds acts both as carriers and amplifying host for West Nile virus. Some bird species especially the crow family (corvidae) are more susceptible to the virus than others. Migratory birds are important in West Nile virus transmission.
When mosquitoes feed on an infected bird they become infected with the virus. The mosquitoes play their role as carriers by spreading the virus from an infected bird to other birds and to other animals. West Nile virus is maintained in mosquito populations by transferring the infection through adults to eggs.
Infection of other animals (e.g. horses, and also humans) is incidental to the cycle in birds since most mammals do not develop enough viruses in the bloodstream to spread the disease. They are “dead-end” hosts.
The virus can be transmitted through a direct contact with other infected animals, their blood, or other tissues.
A small proportion of human infections may be transmitted from organ transplant, blood transfusions and breast milk of infected person.
Diagnosis of West Nile virus (WNV)
Various diagnostic tests are-
- (ELISA): IgM can be determined from cerebrospinal fluid (CSF) and serum specimens received from West Nile virus infected patients at the time of their clinical presentation. Serum IgM antibody can persist for more than a year.
- IgG antibody sero-conversion in two serial specimen collected at a one week interval by enzyme-linked immunosorbent assay (ELISA)
- Viral detection by reverse transcription polymerase chain reaction (RT-PCR) assay, and
- Virus isolation by cell culture.
Symptoms of West Nile virus (WNV)
The incubation period (the period between exposure to an infection and the appearance of the first symptoms) of West Nile fever is usually 3 to 14 days.
20% of people who become infected with West Nile Virus will develop West Nile fever whereas about 80% of infected people are asymptomatic (show no symptoms).
Some of the symptoms of West Nile fever involve – Fever, headache, tiredness, and body aches, nausea, vomiting, sometimes skin rash and swollen lymph glands.
Severe disease such as West Nile encephalitis or meningitis (inflammation of brain and surrounding tissues) show symptoms of headache, high fever, neck stiffness, stupor, disorientation, tremors, coma, convulsions, muscle weakness and paralysis.
Serious illness can occur in people of any age however people over the age of 50 and some immune-compromised persons (such as person who has received organ transplant) are at greater risk of serious illness.
It takes several weeks or months to recover from serious illness. Some of the neurologic effects may be permanent resulting in long-term sequelae or death.
About 10 % of patients with neurologic complications may die.
Treatment of West Nile virus (WNV)
Specific antiviral treatment for West Nile virus infection is not available. In mild infection pain relievers can be used to reduce fever.
Treatment is supportive involving hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections for the patients who are suffering with neuro-invasive west Nile virus infection. No vaccine is available for humans.