Kerala sounds warning on West Nile Virus; neighbouring States on guard

Kerala’s annual battle with vector-borne diseases has begun early this year, even before the South West monsoon, with West Nile Fever (WNF) being reported from several districts.

The rst o©cial alert was sounded by the Health department on May 7 and according to the IDSP report till date, the State has reported 20 suspected cases (only 10 cases conrmed so far) and two deaths are also suspected. Given that Kerala has been endemic for the WN virus for at least two decades and that 80% of cases are asymptomatic, for every o©cially reported case, there could be several unreported and asymptomatic cases in the community.

In symptomatic cases, patients usually have fever, headache, fatigue, myalgia, nausea and vomiting, and sometimes, swollen lymph glands. As clinical symptoms of most mosquitoborne viral diseases are similar, the possibility of WN or JE is considered only when the patient exhibits symptoms of neuroinvasive diseases like encephalitis or meningitis, collectively known as Acute Encephalitis Syndrome (AES). Thus, because of the diagnostic di©culties in identifying WNF in its acute phase, only a few cases get recorded in the State’s o©cial surveillance mechanisms.

Diagnostic di culties

Only 1 in 150 cases aªected by the WN virus gets a severe disease, and even fewer get encephalitis. WHO states that while serious illness can occur in people of any age, people over the age of 50 and some immunocompromised persons, like transplant patients, are at the highest risk of falling severely ill when infected with WNV.

Kerala’s Director of Health Services, K. J. Reena, said that WNF has been in the o©cial surveillance records of Kerala every year since 2011 and that cases are being reported from almost all districts now. Districts that have reported WN cases this year include the northern districts of Thrissur, Malappuram, Kozhikode, and Palakkad.

The principal vectors of the disease are mosquitoes of the genus Culex, generally found breeding in stagnant, large water bodies like paddy elds. WNV circulation is maintained in mosquito populations through vertical transmission (adults to eggs).

In a study, published in the Indian Journal of Medical Research in 2017, B. Anukumar, et al. say that the rst reported acute encephalitis syndrome (AES) outbreak in Kerala occurred in the Kuttanad region between January and February 1996, causing 105 cases and 31 deaths.

Exceptional features

Although the Japanese Encephalitis virus was reported to be an aetiological agent associated with the outbreak, there were some exceptional features noticed during the outbreak. The seasonality of the outbreak was diªerent from the one known for JE in Kerala, and most patients were from adult age groups, whereas JE occurs mainly in children.

Another outbreak occurred in 1997, causing 121 cases and 19 deaths. The role of WNV in AES cases was not ruled out, Dr. Anukumar says.

In 2006, during the Chikungunya epidemic in Alappuzha, abundant in paddy elds and prone to waterlogging, the possibility of the co-circulation of the WN virus was mooted because of the high mortality rate during the epidemic. However, NIV (National Institute of Virology, Pune) provided conclusive evidence about the major presence of the WN virus in the region in 2011 during an AES outbreak when 208 cases were reported.

Public health experts say that the fact that the northern districts too have begun reporting the presence of the WN virus frequently could be either due to improved diagnostic facilities or the fact that the WN virus itself has been spreading. Most cases of AES are still reported by the health system as AES/JE or JE-WN complex because only the plaque reduction neutralisation test (usually done only in NIV) can isolate the WN virus antibodies.The WN virus is maintained in nature in a mosquito-birdmosquito transmission cycle. More than 250 species of birds are reservoir hosts of WNV. “In Kerala, the presence of large stagnant water bodies and migratory birds provides an ideal eco system for the WN virus to thrive. In the era of climate change, when mosquitoborne diseases are emerging as major public health problems, it is important that the State establishes efcient surveillance systems on the One Health platform to identify new pockets where WN and similar arboviruses may emerge,” points out T.S. Anish, a public health expert and Associate Professor of Community Medicine, Government Medical College, Manjeri.

Avian reservoir

“Members of the crow family are said to be particularly susceptible. All our information on the WN virus is based on Western literature. Have there been any studies to identify if our common crow could be spreading any viruses?” Dr. Anish wonders.

Kerala’s health department, while issuing a public alert on WN fever and asking people to take protective measures to prevent

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mosquito bites, perceives WN virus to be a lesser villain because of the low mortality prole of the disease, when compared to JE or dengue. WNV rarely turns fatal.

The health department also contends that unlike dengue fever, which is spread rapidly and e©ciently by Aedes mosquitoes, the WN virus does not cause huge outbreaks. Viremia due to WN virus is transient in humans, and hence Culex mosquitoes cannot transmit the virus e©ciently to more people.

Neurological sequelae left behind by the virus is not something that can be discounted however. Neurological sequelae reported by doctors post WN infection include cognitive dysfunction, memory loss, seizure episodes and motor decits. science@thehindu.co.in