Human West Nile Virus Disease Outbreak in Pakistan, 2015-2016

Abstract

Like most of the world, Pakistan has seen an increase in mosquito-transmitted diseases in recent years. The magnitude and distribution of these diseases are poorly understood as Pakistan does not have a nation-wide system for reporting disease. A cross-sectional study to determine which flaviviruses were causing of arboviral disease in Pakistan was instituted. West Nile virus (WNV) is a cause of seasonal fever with neurotropic findings in countries that share borders with Pakistan. Here, we describe the active and persistent circulation of WNV in humans in the southern region of Pakistan. This is the first report of WNV causing neurological disease in human patients in this country. Of 997 enrolled patients presenting with clinical features suggestive of arboviral disease, 105 were positive for WNV IgM antibodies, and 71 of these patients possessed WNV-specific neutralizing antibodies. Cross-reactivity of WNV IgM antibodies with Japanese encephalitis virus (JEV) occurred in 75 of these 105 patients. WNV co-infections with Dengue viruses were not a contributing factor for the severity of disease. Nor did prior exposure to dengue virus contribute to incidence of neurological involvement in WNV-infected patients. Patients with WNV infections were more likely to present with altered mental status, seizures, and reduced Glasgow Coma scores when compared with JEV-infected patients. Human WNV cases and vector numbers exhibited a temporal correlation with climate.

Keywords: Dengue virus; Japanese encephalitis virus; West Nile virus; arboviral disease; encephalitis.

Figure 1

Locations of the patient enrollment sites in the Sindh region of Pakistan (8). Sites included medical colleges, teaching hospitals, and civil hospitals.

Figure 2

Locations of the mosquito traps used for vector collection, May–November 2015. Sites were selected based on their close vicinity to trees/shrubbery, water, and human activity. The area encompassed 0.34 km² and included high-rise buildings, lakes, water pools, and green patches.

Figure 3

Number of patients with both West Nile virus (WNV)-specific IgM and neutralizing antibodies. Data include patient specimens from all five study sites over a 2-year period.

Figure 4

A graphical representation of how WNV infections (A), WNV encephalitis (B), and percentage of cases with neurological symptoms (C) occurred in different age groups.

Figure 4

A graphical representation of how WNV infections (A), WNV encephalitis (B), and percentage of cases with neurological symptoms (C) occurred in different age groups.

Figure 5

Correlation of human West Nile virus cases, vector abundance, and average monthly temperatures in Karachi, Pakistan.