WEST NILE VIRUS

• West Nile Fever (WNF) is a viral infection caused by the West Nile virus (WNV), primarily transmitted to humans through mosquito bites. The virus is named after the West Nile region of Uganda, where it was first identified in 1937.
• The principal vectors of WNV are mosquitoes of the genus Culex, commonly found in areas with stagnant water bodies like paddy fields. The virus circulates in mosquito populations through a transmission cycle involving birds as reservoir hosts.
• Kerala, India, has been endemic for WNV for at least two decades, with sporadic outbreaks reported. The virus can cause outbreaks of acute encephalitis syndrome (AES), particularly during the monsoon season.
• Symptoms of WNF typically include fever, headache, fatigue, myalgia, nausea, vomiting, and swollen lymph glands. Severe cases may progress to neurological complications such as encephalitis or meningitis, collectively known as AES.

3. Diagnostic Challenges of West Nile Fever

• Only 1 in 150 cases affected by the West Nile (WN) virus develops severe disease, with even fewer progressing to encephalitis. According to the World Health Organization (WHO), individuals over 50 years old and certain immunocompromised individuals, such as transplant patients, face the highest risk of severe illness when infected with WNV.
• The West Nile Fever (WNF) has been included in the official surveillance records of Kerala since 2011, with cases reported from almost all districts. Among the districts reporting WN cases this year are Thrissur, Malappuram, Kozhikode, and Palakkad.
• A study published in the Indian Journal of Medical Research in 2017, conducted by B. Anukumar et al., documented the first reported outbreak of acute encephalitis syndrome (AES) in Kerala's Kuttanad region between January and February 1996. This outbreak resulted in 105 cases and 31 deaths, highlighting the historical presence of mosquito-borne diseases in the region.

• During outbreaks associated with the Japanese Encephalitis (JE) virus, which shares antigenic similarity with West Nile (WN) virus, several exceptional features were observed. Unlike the typical seasonality of JE outbreaks in Kerala, these outbreaks occurred at different times of the year. Moreover, the affected demographic skewed towards adult age groups, contrasting with JE's predominant occurrence in children. Notably, an outbreak in 1997 resulted in 121 cases and 19 deaths, suggesting a potential role of the WN virus in these AES cases. 
• In 2006, during a major Chikungunya epidemic in the Alappuzha district, where paddy fields and waterlogging are prevalent, suspicions arose regarding the co-circulation of the WN virus due to the high mortality rate observed. However, conclusive evidence of WN virus presence was provided by the National Institute of Virology (NIV) in 2011 during an AES outbreak, with 208 reported cases.
• Recent reports indicate an increasing presence of the WN virus in northern districts of Kerala. This may be attributed to improved diagnostic capabilities or the actual spread of the virus to new areas. Despite advancements, most AES cases are still categorized as AES/JE or JE-WN complex due to limited diagnostic tests available, such as the plaque reduction neutralization test conducted primarily at NIV.
• The WN virus is sustained in a transmission cycle involving mosquitoes and birds, with over 250 bird species serving as reservoir hosts. Kerala's environment, characterized by large stagnant water bodies and migratory birds, creates an ideal ecosystem for WN virus proliferation. Given the rising prominence of mosquito-borne diseases amid climate change, establishing efficient surveillance systems on the One Health platform is crucial to identify emerging WN and similar arboviruses.

• The common crows, known to be highly susceptible to the West Nile (WN) virus, could be contributing to the spread of the virus. This inquiry highlights the need for localized studies to understand the role of avian species, particularly crows, in the transmission dynamics of the WN virus.
• The Kerala health department's perception of the WN virus differs from other mosquito-borne diseases like Japanese Encephalitis (JE) or dengue. While acknowledging WN fever as a concern, they consider it less severe due to its lower mortality rate compared to JE or dengue. Unlike dengue, WN virus outbreaks are not as widespread, partially attributed to the transient viremia in humans, which limits efficient transmission by Culex mosquitoes.
• Although WN virus-related encephalitis is fatal in only about 1% of cases, the long-term neurological consequences are significant. Patients may experience cognitive dysfunction, memory loss, seizures, and motor deficits, as evidenced by a follow-up study. This study highlights the importance of addressing neurological sequelae post-WN infection, especially among individuals over 60 years with comorbidities, to mitigate long-term health risks.

• Studies suggest that the West Nile Virus (WNV) may persist in the kidneys of some individuals even years after infection, potentially leading to chronic kidney disease. This highlights the need for public awareness campaigns to educate individuals about the risks associated with mosquito bites and the importance of adopting personal protective measures to reduce infection risk.
• The absence of systematic epidemiological and entomological surveillance systems poses future risks, as viruses like WNV can adapt to new hosts or mutate to increase virulence. The scarcity of expertise in entomology at the national level exacerbates this issue, with insufficient training available for healthcare professionals entering the field.
• While vector control efforts primarily focus on Aedes mosquitoes responsible for diseases like dengue, Chikungunya, and Zika, other mosquito-borne diseases like filariasis, malaria, and WN/JE remain prevalent due to the lack of effective vector control measures targeting other mosquito species. This underscores the need for comprehensive and integrated vector control strategies beyond Aedes species.
• The rise in climate-sensitive zoonotic diseases emphasizes the importance of investing in science and adopting a One Health approach to infectious disease management. This approach emphasizes the interconnectedness of human, animal, and environmental health, necessitating collaborative efforts across sectors to address emerging health threats effectively.

7. Way Forward

By implementing the strategies, Kerala can strengthen its preparedness and response to the ongoing threat of West Nile Fever and other mosquito-borne diseases, safeguarding the health and well-being of its population.