Explainer: Is West Nile Fever that killed a Kerala man this week going undetected in other parts of the country?

Female mosquito sucking human blood

Earlier this week, a 47-year old man in Thrissur district in Kerala died of West Nile Fever, sending the state health department on a high alert.  This is the second reported death from the zoonotic disease in the state over the last three years — the disease had killed a six-year-old boy in 2019.

Many biologists say that the virus has been lurking not only in Kerala but in many other parts of India too, and keeps striking from time to time.

What is West Nile Fever?

Dr Trupti Gilada, an infectious diseases expert from Mumbai, explained that West Nile Fever is a viral illness caused by the West Nile Virus (WNV) transmitted by the Culex mosquito after feeding on infected birds.

Culex mosquitoes are found across the country, both in north and south India, and so is the WNV.

The WNV can cause neurological disease and death in people but nearly 80 percent of the people infected with it may remain totally non-symptomatic, while nearly 20 percent of those infected go on to get fever.

According to the World Health Organisation (WHO), till date no human-to-human transmission of WNV through casual contact has been documented, and no transmission of the virus to health care workers has been reported when standard infection control precautions have been put in place, However, transmission of WNV to laboratory workers has been reported.

Signs and symptoms

Nearly 20 per cent of the people who get infected with the virus will develop fever, and according to Dr Amrita Singh, chief of lab services at Neuberg Diagnostic in Delhi, mild symptoms include fever, headache, bodyache, fatigue, vomiting, diarrhoea, skin rash.

However, some people infected by the virus go on to develop severe symptoms such as high fever, stiffness in the neck, coma, tremors, convulsions, disorientation, muscle weakness, vision loss, numbness and paralysis, said Singh.

The WHO estimates that about 1 in 150 people infected with WNV get severe disease, and while it can occur at any age, those above 50 years and some with compromised immunity may be more vulnerable to getting seriously ill.

“The complications related to this condition may be higher in elderly people,” said Dr Vaishali Lokhande, a senior clinician with the Apollo Hospitals in Navi Mumbai.

Treatment and prevention

Doctors say that treatment is supportive for patients with neuro-invasive WNV, mainly involving hospitalisation, intravenous fluids, respiratory support, and prevention of secondary infections. There are no vaccines currently available against the disease for humans.

According to Singh, preventive measures include proper sanitation, wearing full covered clothes, reducing mosquito breeding sites, covering water storage containers and eliminating puddles and water accumulation.

Outbreaks in India

Antibodies against the WNV, first isolated in West Nile district, Uganda, in 1937, were first detected in Mumbai in 1952, but its presence has since been confirmed in several states, including Rajasthan, Tamil Nadu, Andhra Pradesh, Odisha, Madhya Pradesh, and Assam.

Since the outbreak of the acute encephalitis syndrome in Kerala in 2011, the disease has been regularly reported from the state.

Under the radar outside Kerala?

Many biologists say that while in Kerala, due to high level of awareness amongst physicians, the disease is mostly identified, it is possible that diagnosis may be getting missed in several other states.

“Kerala also has a strong disease surveillance system when compared to other states, and therefore, it is likely that cases, even severe ones, may be going under the radar in other places,” said a scientist attached with an institute under the Indian Council of Medical Research.

Gilada too echoed similar sentiments.

The reason Kerala is often the first state to report cases is not so much to do with just the disease prevalence but with their efficient healthcare system, she said.

“Other states may be having these cases too, but there needs to be both knowledge and suspicion of the illness to trigger the right diagnostic algorithm to identify it,” Gilada said, and added that surveillance, diagnosis and notification are all very robust in Kerala, thus often leading the path for new disease alerts and public health measures for disease control.