West Nile fever cases detected in Kerala: What is the disease, how can it be prevented?

The minister said that there was no need to be concerned about the virus, but requested anyone showing signs of fever or other symptoms of the West Nile infection to seek treatment immediately, a PTI report said.

According to the health department, West Nile virus can cause a fatal neurological disease in humans. However, approximately 80 percent of those infected will not show any symptoms. Here’s what you need to know about the disease, how it spreads, its symptoms, treatment and prevention.

What is the West Nile Virus?

The West Nile Virus (WNV) is a mosquito-borne, single-stranded RNA virus. It is a flavivirus and is related to the viruses that cause Japanese encephalitis and yellow fever.

How does the West Nile Virus spread?

Culex species of mosquitoes act as the principal vectors for transmission. Infected mosquitoes spread the disease among humans and animals, including birds, which are the reservoir host of the virus.

How the spread of West Nile Virus takes place. (Via Centers for Disease Control and Prevention)

“Mosquitoes become infected when they feed on infected birds, which circulate the virus in their blood for a few days. The virus eventually gets into the mosquito’s salivary glands. During later blood meals (when mosquitoes bite), the virus may be injected into humans and animals, where it can multiply and possibly cause illness,” says the World Health Organisation (WHO).

WNV can also spread through blood transfusion, from an infected mother to her child, or through exposure to the virus in laboratories. It is not known to spread by contact with infected humans or animals.

According to the US Centers for Disease Control and Prevention (CDC), it does not spread “through eating infected animals, including birds.” The CDC notes that the incubation period for WNV disease is typically 2 to 6 days, but can range from 2 to 14 days, and can be several weeks in immunocompromised people.

To date, no human-to-human transmission of WNV through casual contact has been documented, says the WHO.

What are the symptoms of West Nile fever?

The disease is asymptomatic in 80% of the infected people. The rest develop what is called the West Nile fever or severe West Nile disease. For them, the symptoms include fever, headache, fatigue, body aches, nausea, rash, and swollen glands.

“It is estimated that approximately 1 in 150 persons infected with the West Nile Virus will develop a more severe form of disease… Recovery from severe illness might take several weeks or months. Some effects to the central nervous system might be permanent,” says the CDC.

It usually turns fatal in persons with comorbidities and immuno-compromised persons (such as transplant patients). But the death rate is relatively low compared to Japanese encephalitis, which shows similar symptoms.

What is the treatment for West Nile Virus disease?

No WNV-specific prophylaxis, treatment or vaccine is available. Only supportive treatments are given to neuroinvasive WNV patients. Health authorities globally advise for personal protective measures to reduce the risk of mosquito bites, such as using mosquito repellents.

Public health departments are to also ensure larval source reduction, especially at breeding and resting sites for the mosquito vectors.

What are some preventive measures against West Nile Virus?

In most countries, the peak for WNV infections generally coincides with the period when mosquito vectors are most active and the ambient temperature is sufficiently high for virus multiplication. Since WNV outbreaks in animals precede human cases, the establishment of an active animal health surveillance system to detect new cases in birds and horses is considered essential.

Health Minister George also suggested wearing clothing that completely covers the body, use of mosquito nets and repellants, and keeping one’s home and surroundings clean.

Why is it called West Nile Virus?

The virus was first isolated in a woman in the West Nile district of Uganda in 1937. It was identified in birds (crows and columbiformes like doves and pigeons) in the Nile delta region in 1953. Before 1997, WNV was not considered pathogenic for birds, but then, a more virulent strain caused deaths of different bird species in Israel, presenting signs of encephalitis and paralysis.

In 1999, a WMV strain, believed to be one circulating in Israel and Tunisia, reached New York and produced a large outbreak that spread across the United States and eventually across the Americas, from Canada to Venezuela.

WNV outbreak sites are found along major bird migratory routes. Today, the virus is found commonly in Africa, Europe, the Middle East, North America, and West Asia.

West Nile Virus in India

In India, antibodies against WNV were first detected in humans in Mumbai in 1952 and virus activity has since been reported in southern, central, and western India. WNV has been isolated in India from Culex vishnui mosquitoes in Andhra Pradesh and Tamil Nadu, from Culex quinquefasciatus mosquitoes in Maharashtra, and from humans in Karnataka.

In 2013, the complete genome sequence of WNV was isolated from human samples during an acute encephalitis outbreak in Kerala between May and June 2011. In 2022, a 47-year-old man died of the fever in Thrissur district.

This is an updated version of an explainer first published in 2022.