Population Migration Through Railroads And Spatial Diffusion Of Polio In India: A Cross-Sectional Pr

Background The recent occurrence of polio outbreaks in countries previously considered polio-free such as Tajikistan, Russia and China, and the economic and public health burden of managing such outbreaks underscored the importance of importation of infectious diseases across geographical regions, transcending political boundaries. It also questioned the appropriateness of classifying geographic regions, countries, states and provinces, as endemic and non-endemic and the implications such classifications had on the quality of surveillance and immunization strategies. Although molecular epidemiology and field-surveys have been useful in demonstrating the possible source of reintroduction of virus transmission in disease-free areas, the mode of importation often remains unexplored. The goal of this study is to explore the role played by popular modes of transport, such as the railways, in the importation of polio transmission from endemic to non-endemic states in India. Methods To explore the association between cases in non-endemic states and the geographical distribution of the railroads, we evaluate the spatial pattern of spread of polio from endemic to non-endemic states in India during the last four years (as of April 2013) of virus circulation in the country, 2008 2011, in a cross-sectional design. We geo-coded addresses of serotype 1 (P-1) cases followed by mapping and proximity analysis using geo-spatial tools. Fisher’s exact 2-tailed test was used to assess the statistical significance of the proximity analysis. Results Our analysis established that cases in non-endemic states were more likely (64.7% in non-endemic states compared to 38.9% in endemic states) to be geographically closer to the railroads compared to cases in endemic states. The results were statistically significant (Fisher’s exact 2-tailed P: 0.01) and the null hypothesis of no association between location of P-1 cases in non-endemic areas and proximity to the railroads was rejected. All P-1 wild type cases (n = 173) from all states and Union Territories in India were included for the most recent 4-year period, and knowing the predominant sub-clinical nature of polio infection, the results were based on a robust set of data from a large cohort of infected population.