Dynamics of Japanese encephalitis virus transmission among pigs in Northwest Bangladesh and the potential impact of pig vaccination

Abstract

Background: Japanese encephalitis (JE) virus infection can cause severe disease in humans, resulting in death or permanent neurologic deficits among survivors. Studies indicate that the incidence of JE is high in northwestern Bangladesh. Pigs are amplifying hosts for JE virus (JEV) and a potentially important source of virus in the environment. The objectives of this study were to describe the transmission dynamics of JEV among pigs in northwestern Bangladesh and estimate the potential impact of vaccination to reduce incidence among pigs.

Methodology/principal findings: We conducted a comprehensive census of pigs in three JE endemic districts and tested a sample of them for evidence of previous JEV infection. We built a compartmental model to describe JEV transmission dynamics in this region and to estimate the potential impact of pig vaccination. We identified 11,364 pigs in the study area. Previous JEV infection was identified in 30% of pigs with no spatial differences in the proportion of pigs that were seropositive across the study area. We estimated that JEV infects 20% of susceptible pigs each year and the basic reproductive number among pigs was 1.2. The model suggest that vaccinating 50% of pigs each year resulted in an estimated 82% reduction in annual incidence in pigs.

Conclusions/significance: The widespread distribution of historic JEV infection in pigs suggests they may play an important role in virus transmission in this area. Future studies are required to understand the contribution of pig infections to JE risk in humans and the potential impact of pig vaccination on human disease.

Figure 1. Map of pig-raising households, nomadic pig routes, pigs with antibodies to Japanese encephalitis virus, and human population density in Rajshahi, Nawabgonj, and Naogaon Districts, Bangladesh, 2009.

Figure 2. Proportion of pigs seropositive to Japanese encephalitis (JE) virus by age.

312 pigs, divided into seven age groups, were tested for presence of IgG antibodies to JE. The points are plotted at the midpoint of the age groups. The blue line represents the fit of the model assuming a constant force of infection.

Figure 3. (A) Model structure indicating pigs with maternal antibodies (M), susceptible (S), exposed (E), infected (I), recovered (R), and vaccinated (V) compartments.

(B) Estimated decrease in incidence of Japanese encephalitis (JE) in pigs under four vaccination coverage scenarios, assuming that 10%, 25%, 50% or 75% of susceptible pigs are vaccinated each year and that 5% of infections originate from outside the study area or from other hosts. (C) Estimated reduction in incidence of JE among pigs by proportion of the susceptible pig population vaccinated each year and proportion of all pig infections that originate from an external source. The colored crosses represent the scenarios in (B).