Polio virus emerged across the globe at the turn of the 20th century and plagued the US with seasonal fears of seemingly random infection that left thousands paralyzed. Poliomyelitis' transition from endemic to epidemic status has long baffled epidemiologists. With the current setbacks in the global eradication initiative we turn to historical accounts of polio in the US to gain insight into the ecological mechanisms of its emergence, transmission, and vaccine-induced retreat. We use state-level monthly counts of polio in the 50 US states and the District of Columbia to understand how long-distance dissemination, host demographics, and environmental conditions influence transmission and polio outbreaks. Wavelet phase analysis was used to determine the periodicity and explore the spatial synchrony of epidemics. In order to elucidate the role of host demographics, we turned to the US Census and National Vital Statistics and obtained geographical population centers, population sizes, birth rates, and infant survivorship for each state. These descriptors of host demographics were then related to disease dissemination, epidemic magnitude, and inter-state synchrony. Lastly, state-level monthly temperature and precipitation was gathered from the National Climate Data Center to investigate environmental-forcing.
Results/Conclusions
Patterns of epidemic emergence differed greatly between states. States with larger populations had rapid establishment of epidemics with pronounced annual periodicity and a multi-annual signatures. Large states harbored cases year-around and were likely to have served as a virus reservoir for the surrounding regions. We find that epidemic synchrony, and possibly interstate transmission, is inversely related to the distance between state population centers. Taking the US as a whole, we found that epidemic magnitude increased with the mean age of infection and the number of children. Furthermore, we suspect to find environmental forcing caused epidemics to begin in the warmer months (July and August) and peak around October after schools opened and children transmitted the virus outside of the home.