This week we read two disease ecology papers both in Nature: the classic Anderson & May (1979) paper that introduces the compartmental model foundational to most parasite population theory and a review from Keesing et al. (2010) on the multiple impacts biodiversity can have on infectious diseases, a continually debated topic in the study of emerging infectious diseases. Comparing the two papers was difficult. These works are different styles of articles (though both listed as reviews), written about two different scales, with different directionality of effects between hosts and parasites. Anderson & May focus on parasite trait affects on a single host species population through a modeling approach, while Keesing synthesized some case studies on host community traits that affect parasite populations. Inevitably disease ecology studies involve multiple species, which adds complexity to their dynamics. Taking perspectives from both reviews will allow us to apply to address the multiple different axes at play in host-parasite systems.
After working on disease ecology projects and in the midst of ECOL6150 Population Biology of Infectious Diseases, reading through Anderson & May’s work was particularly exciting. Most intriguing was their original use of standard variables, XYZ, (see Figure 3) rather than the now ubiquitous S-I-R compartmental model. I think Anderson & May allow simple adjustments to build to more complicated mathematical models, providing an interpretable framework to approach any population dynamics question. I appreciate that they isolate one variable, the number of hosts, and address this as dynamic in the context of parasite transmission factors. This trend has extended into other variables of focus in my labs, for instance, how the degree of provisioning or animal movement can also be dynamically scaled. Further, Anderson & May provided convincing evidence through their multipronged use of experimental, modeling, and comparative analysis. Lastly, I thought it was particularly intriguing to think about their point of modern vs. non-industrialized societies, in which infectious agents that are more epidemic wouldn’t survive due to low influx of susceptibles. Typically I think of infectious pathogens as more of undeveloped world issue, yet this view may emphasize intermediate levels of developement where there are large populations but poor public health access, as opposed to undeveloped and isolated communities.
Unique to Keesing et al. is the authors’ motivation to influence policy, as evidenced by a current events-based opener rather than a scientific thesis. The authors elaborate on known issues regarding linkages between biodiversity and ecosystem services to include currently unknown consequences on the emergence and transmission of infectious diseases. While we agreed with the authors on many points, much of our discussion focused on the inadequate figures to argue the author’s points. In particular, Figure 1 and 2 suffer from poor data visualization techniques: awkward scaling, using non-interpretable colors and pie graphs. We discussed how the small sliver of green/yellow contrast on Fig. 1 did not hit home the main goal of this case study: that host behavior drives host competence. We thought that this could have been better addressed through bar graphs comparing hosts in a more straight forward way. Similarly, Fig. 2 was also disappointing in that there was too much information. I wish they had further synthesized trends based on continent or GDP of country, as on a map many of the pie charts were lost. I do think this paper set the stage for the classic disease systems used in biodiversity case studies: lyme and hantavirus. We did appreciate that they broke down the underlying mechanisms on how biodiversity loss can increase transmission through either changes in host/vector abundance or behavior. Despite these issues in figures, I think the overarching goal of the article to shed light on the urgency to study biodiversity’s role on infectious disease was effective in motivating science in the 2010s.
References:
Anderson RM, May RM. (1979) Population biology of infectious diseases: part I. Nature 280: 361–367.
Keesing F, Belden LK, Daszak P, Dobson A, Harvell CD, Holt RD, Hudson P, Jolles A, Jones KE, Mitchell CE, Myers SS. (2010) Impacts of biodiversity on the emergence and transmission of infectious diseases. Nature 468(7324): 647.