EXPLAINED | What Is West Nile Fever? All About The Disease That Claimed A Life In Kerala

West Nile Fever is a disease caused by the West Nile Virus, which is a flavivirus. Mosquito species of the genus Culex are generally considered the principal vectors of West Nile Virus.

Mosquito species of the genus Culex are generally considered the principal vectors of West Nile Virus, particularly Culex pipiens. The virus is maintained in mosquito populations through vertical transmission

Source : ABP Live

New Delhi: A man in Thrissur, Kerala, died from West Nile Fever on Sunday, May 29. Joby, a native of Puthur, Kerala, was shifted to Thrissur Medical College Hospital two days back. He was made to undergo some tests, which confirmed he was suffering from West Nile Fever. 

Two of his bystanders were also found to be suffering from West Nile Fever, media reports said. Hospital authorities have issued warnings and precautionary measures to be followed because West Nile Fever is contagious. No other cases of the disease have been reported so far.

What Is West Nile Fever?

West Nile Fever is a disease caused by the West Nile Virus, which is a flavivirus, and causes disease in humans, horses, and several species of birds. Flavivirus is a group of positive, single-strand RNA viruses which mostly have arthropod vectors, and cause a number of serious human diseases such as yellow fever, dengue, encephalitis, hepatitis C, and West Nile Fever. Flavivirus is the only genus in the family Flaviviridae. West Nile Virus is related to the viruses which cause St Louis encephalitis, Japanese encephalitis, and yellow fever. 

Most of the people who are infected with West Nile Virus show signs of illness. However, only some of them develop severe neurological illness which can be fatal.

West Nile Virus has an extremely broad host range, and replicates in birds, reptiles, amphibians, mammals, mosquitoes, and ticks, according to the World Organisation for Animal Health (OIE). 

How Is West Nile Disease Transmitted?

The reservoir of West Nile Virus is in birds. When mosquitoes bite an infected bird ingesting the virus in the blood, they act as carriers or vectors spreading the virus from the bird to other birds and animals. In this way, the virus circulates from bird to bird through mosquito bites, and is amplified at each cycle.

Mosquito species of the genus Culex are generally considered the principal vectors of West Nile Virus, particularly Culex pipiens. The virus is maintained in mosquito populations through vertical transmission, which is movement from adults to eggs. 

Corvidae, the crow family, is among the bird families more susceptible to the West Nile Virus than others. Therefore, finding dead crows can signal the presence of West Nile Disease. This is why surveillance programmes often target dead crows.

Since most mammals do not develop enough virus in the bloodstream to spread the disease, infection of other animals including horses and humans depends on the cycle. 

West Nile Virus is spread to humans by the bite of an infected mosquito. In very rare situations, the virus is spread through exposure in a laboratory setting, blood transfusion and organ transplant, and from mother to child, during pregnancy, delivery, or breast feeding. 

West Nile Virus is not spread through coughing, sneezing or touching, by touching live animals, from handling live or dead infected birds, avoiding bare-handed contact when handling dead animals. The disease is not spread through eating infected animals, including birds.

How Susceptible Are Humans To West Nile Fever?

West Nile Fever is a zoonotic disease, which is an animal disease affecting humans. Though humans are susceptible to West Nile Fever, most of them have no symptoms. About twenty per cent of of people suffering from West Nile Disease have mild flu-like symptoms such as fever, headaches, and rashes. In rare cases, West Nile Virus can cause encephalitis. Less than one per cent of persons infected with West Nile Virus develop severe symptoms. 

Cases of West Nile Fever occur during mosquito season, which starts in the summer and continues through fall, according to the Centers for Disease Control and Prevention (CDC). Most people infected with West Nile Virus do not feel sick. About one in five people infected with West Nile Virus develop a fever and other symptoms. About one in 150 people infected with West Nile Virus develop a serious, sometimes fatal illness.

What Are The Symptoms Of West Nile Disease?

No symptoms are observed in most people suffering from West Nile Disease. About eight out of 10 people infected with West Nile Virus do not develop any symptoms.

Febrile illness or fever is seen in some people. About one in five people infected with West Nile Virus develop a fever with other symptoms such as headache, body aches, joint pains, vomiting, diarrhoea, or rash, according to the CDC. Though people with febrile illness due to West Nile Virus recover completely, fatigue and weakness can persist for weeks or months.

Serious symptoms are observed only in a few people. About one in 150 people infected with West Nile Virus develop a serious illness which affects the central nervous system. The person may suffer from encephalitis, which is inflammation of the brain, or meningitis, which is inflammation of the membranes surrounding the brain and spinal cord.

The symptoms of severe illness include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. 

According to the CDC, severe illness can occur in people of any age. However, people above 60 years of age are at greater risk for severe illness if they are infected. About one in 50 people aged above 60 years can suffer from severe illness.

Other people who are at greater risk include people with certain medical conditions such as cancer, diabetes, hypertension, kidney disease, and those who have received organ transplants.

People suffering from severe illness may take several weeks or months to recover. About one in 10 people who suffer from severe illness affecting the central nervous system die, according to the CDC.

The incubation period of the disease is usually three to 14 days.

How Can West Nile Disease Be Diagnosed?

West Nile Disease can be diagnosed by a number of tests including IgM antibody capture ELISA, neutralisation assays, and RT-PCR, among others. 

Two serial specimen can be collected at a one week interval, and enzyme-linked immunosorbent assay (ELISA) can be performed to determine IgG antibody seroconversion, which refers to the development of specific antibodies in the blood serum as a result of infection or immunisation.

Antibody-capture ELISA is used for the detection of immunoglobulin M antibodies in sera from patients with Japanese encephalitis, dengue haemorrhagic fever, or West Nile Fever. 

Neutralisation assays detect antibodies which are capable of inhibiting virus replication. 

Reverse transcription polymerase chain reaction (RT-PCR) can be used to detect West Nile virus. 

According to the WHO, IgM can be detected in nearly all cerebrospinal fluid (CSF) and serum specimens received from West Nile Virus-infected patients at the time of their clinical presentation. Serum IgM antibodies may persist for more than a year. 

How Can West Nile Disease Be Treated?

No vaccine or specific medicines are available for West Nile Disease. 

Pain relievers can be used to reduce fever and relieve some symptoms caused by the disease.

Patients often need to be hospitalised in severe cases, in order to receive supportive treatment, such as intravenous fluids, pain medication, respiratory support, and nursing care, and also for the prevention of secondary infections.