The impact of a physical geographic barrier on the dynamics of measles

Abstract

Spatial-temporal patterns of measles incidence reflect the spatial distribution of human hosts. The heterogeneous spatial distribution of communities has been shown to introduce spatially dependent temporal lags in the timing of measles incidence. Incidence patterns reflect internal dynamics within a community and coupling of communities through the movement of infectious individuals. The central role of human movement in coupling dynamics in separate communities suggests that physical geographic barriers to movement should reduce spatial-temporal correlation. We examine measles dynamics in Maryland and Pennsylvania during the period of 1917-1938. The central feature of interest is the Chesapeake Bay, which separates Maryland into two distinct regions. We find that correlation of measles incidences in communities separated by the bay is reduced compared to communities not separated by the bay, suggesting the bay acted as a barrier to human movement during this time sufficient to decouple measles dynamics in Maryland counties.

Fig. 1

Map indicating the mean phase difference for the multi-annual variance of measles incidence in Pennsylvania. Large circles indicate counties that are on average ahead in phase of other counties in the dataset, meaning changes in incidence on average precede peaks in other counties by several months. The size of a circle indicating a zero average phase difference is shown in the bottom left-hand corner. Changes in incidence in a county with a zero average phase difference precede half of all counties and follow the remaining half. Underlying shading indicates population density with darker grey indicating counties of high population density. Coupled with population density, the map suggests that more densely populated areas are ahead in phase, suggesting epidemics originate in more populous areas.

Fig. 2

K-mean cluster assignments for clustering log normalized measles incidence time-series for two (top map) and three (bottom map) clusters. Members of one cluster appear as the same colour (white, light grey, black). Membership in the same cluster indicates correlation in measles incidence.

Fig. 3

Spatial synchrony of the long-term mode of periodicity of measles incidence in Maryland as a function of distance. Spatial synchrony provides a measure of the correlation of series as a function of spatial distance separating them. The black curve surrounded by black 95% confidence intervals indicates the correlation of counties that are not separated by the Chesapeake Bay. The white curve surrounded by grey 95% confidence intervals indicates the correlation of counties that are separated by the Chesapeake Bay (meaning the line between the centroids of the counties intersects the Chesapeake Bay). The figure indicates that at a distance of ∼120 km the correlation of pairs separated by the bay is significantly less than the correlation of pairs not separated by the bay.