Human Influenza Pandemic? The value of understanding the factors that effect disease dynamics

Infectious diseases are a well-studied public health issue. However, the factors that influence infectious disease dynamics that involve elements of population ecology are just as important to study. Spatial and geographical patterns play a large role in the spread, transmission, and elimination of infectious diseases.  For example, understanding human population density gives insight to the rate of which an infectious disease will spread. Also studying the patterns presented in previous occurrences of infectious diseases are a key element in preventing future outbreaks of these diseases. This understanding of density-dependence, which is population growth dependent on the average number of individuals per unit area, also accounts for the persistence and transmission dynamics of infectious diseases. Studying the patterns presented in previous occurrences of infectious diseases are a key element in preventing future outbreaks of these diseases.

Figure: Infectious diseases that affect human populations can be spread through a multitude of animal hosts

Chowell et al. studied the spatial variations (regions and counties) in transmissibility and mortality of the 1918-1919 H1N1 influenza pandemic in rural and urban areas of England and Wales (2008). The authors collected data from the British Ministry of health about weekly influenza-related deaths. Decennial census data provided the demographic information needed to evaluate the effect of the socio-demographic factors-population size and density, residential crowding and urbanization-that led to differential rates in transmissibility (reproductive number), and mortality rural and urban. This study looked at these factors from the autumn of 1918 compared to the winter of 1919. The rate of influenza transmissibility was the highest and also the most statistically significant during autumn 1918 in both urban and rural areas while transmissibility was low and not significant during the winter of 1919. From the socio-demographic factors analyzed, population size and urbanization were found to be significant predictors of the death rate during both times frames of the influenza pandemic. There was a much higher experienced per capita death rate in urban areas during both autumn and winter periods while rural areas with the smallest population sizes experienced the highest death rate overall. Population size was also a significant predictor of the onset of the disease since early onset of influenza was found in areas with larger population sizes.   

Figure 1: Weekly number of influenza during 1918-1919 pandemic at spatial scales of regions (administrative units) and counties

          This paper addresses important aspects of spatial variations found within a disease epidemic while leaving some questions unanswered. For example, the effects of population density on transmissibility and mortality rates were left unexplained. The Chowell et al. study does not completely compare the factors that differ between rural and urban areas that could have led to these spatial differences except for population size. In a more recent paper, the influenza pandemic of 1918 was studied in England, Wales and the US further exploring the factors that led to the spatial spread of the disease paying attention transmission, population size and distance while specifically analyzing the extent to which density-dependence played a role in the results found without looking at these differences in specifically rural and urban areas (Eggo et al., 2011). I think it would be extremely important to do a qualitative analysis of the factors that influence the transmissibility and mortality of human influenza in rural and urban areas, looking at population density as a major factor. I would conduct this study in Iowa, a state with both rural and urban counties, where I would collect data on reported incidences of influenza through public health records and analyze density through census data. Once this data is collected, I would conduct analyses using statistical tests, spatial tests, and general model testing to  truly quantify my results. The results of this study would add to the knowledge of the more recent human influenza dynamics, while examining the importance of population type that affect the spread of disease, access to preventative measure, etc. 

Works Cited

Chowell, G., L. Bettencourt, N. Johnson, W. Alonso, and C. Viboud. 2008. The 1918-1919 

Influenza Pandemic in England and Wales: Spatial Patterns in Transmissibility and Mortality Impact. Proceedings: Biological Sciences, 275:501-509.

Eggo, R. M., S. Cauchemez, and N. M. Ferguson. 2011 Spatial dynamics of the 1918 influenza pandemic in England, Wales and the United States. Journal of The Royal Society Interface. 8:233-243.