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Perspectives Seasonal Patterns of Infectious Diseases Mercedes Pascual*, Andy Dobson eningococcal meningitis from year to year so important? Isn’t In other words, there are only in western Africa shows it more important for us to instead weak correlations between seasonal Mrecurrent seasonal patterns understand the effects of long-term variations and climate variables at small every year. Epidemics typically start climate change on human health? scales because of the multiple other at the beginning of February and last At fi rst sight, understanding seasonal factors that play a local role and act as until May. We can try to explain the patterns seems disconnected from noise. observed patterns on the basis of some understanding the impact of long-term But we should be cautious about seasonally varying environmental factor climate change. However, seasonal the suggestion that appropriate larger that favors disease transmission. Air patterns are one major pathway for scales will always resolve the problem dryness produced by strong dust winds the subtle but potentially drastic of local variability and present strong is the most likely candidate. effects of climate change on disease linear associations between climate and But while there are qualitative dynamics. Long-term climate change disease. Public health measures might “stories” of this kind in the literature affects seasonal patterns through the require predictions not only at national for many seasonal phenomena, lengthening of the transmission season and regional scales, but also at a variety convincing quantitative evidence and the crossing of environmental of smaller scales. to support them remains largely and demographic thresholds that Moreover, one important source of elusive. Instead, we tend to see weak underlie seasonal outbreaks [2]. variation in how infectious diseases associations between environmental Thus, identifying the specifi c and transmission variables when environmental factors underlying Citation: Pascual M, Dobson A (2004) Seasonal measured by simple, linear correlations. seasonal transmission is a critical step patterns of infectious diseases. PLoS Med 2(1): e63. The study of meningococcal meningitis towards predicting and understanding in Mali by Sultan and colleagues in this how long-term environmental trends in Copyright: © 2005 Pascual and Dobson. This is an open-access article distributed under the terms issue of PLoS Medicine is a remarkable mean climate and their variability will of the Creative Commons Attribution License, exception [1]. The study reports a impact human health. which permits unrestricted use, distribution, and reproduction in any medium, provided the original strong association between the yearly work is properly cited. onset of epidemics and a large-scale The Problem of Scale regional index for atmospheric One important diffi culty in uncovering Mercedes Pascual is an associate professor in the Department of Ecology and Evolutionary Biology, circulation related to the Harmattan seasonal drivers of infectious diseases University of Michigan, Ann Arbor, Michigan, United winds in Sahelo-Sudanian Africa. is to identify the appropriate scale of States of America. Andy Dobson is a professor in the analysis. The relationship between Department of Ecology and Evolutionary Biology, The Importance of Seasonality Princeton University, Princeton, New Jersey, United disease and climate described by States of America. Why is a focus on the seasonality of Sultan and colleagues only becomes Competing Interests: The authors declare that no infectious diseases and its variation apparent at large spatial scales. The competing interests exist. authors argue that these large scales are The Perspectives section is for experts to discuss the necessary to eliminate “idiosyncratic” *To whom correspondence should be addressed. E-mail: [email protected] clinical practice or public health implications of a variability in the relationship between published study that is freely available online. cases and climate at the local level. DOI: 10.1371/journal.pmed.0020005 PLoS Medicine | www.plosmedicine.org 0018 January 2005 | Volume 2 | Issue 1 | e2 | e5 understand the interaction between only simple seasonal forcing functions susceptibility levels and climate (mathematical functions that are variation [3,4,5]. periodic in time and therefore describe The problem of scale also arises in a generic way the seasonal variation when we need to identify the in the transmission rate or some other appropriate timing (the temporal seasonal parameter—a sine wave is an window) to detect strong associations example). There are some important between disease outbreaks and exceptions to this—some models environmental covariates. This is do incorporate more complicated particularly important when strong seasonal forcing functions that describe couplings between environment and the actual processes underlying the transmission occur only transiently. seasonal drivers of transmission. This seems to be the case for cholera Examples are models of childhood in Bangladesh, where couplings diseases that describe the regular are strong during El Niños, but stopping and starting of school terms considerably weaker the rest of the [9,10,11], and recent malaria models time [6]. Intermittent couplings that include the seasonal dynamics provide insight into how the system of mosquito births and pathogen might behave if pushed into specifi c incubation as functions of temperature dynamic regions by a change in and rainfall [12]. DOI: 10.1371/journal.pmed.0020005.g001 climate. Intermittent couplings also The explicit way in which models Figure 1. The Role of Rainfall in Driving the suggest the existence of thresholds treat seasonal environmental drivers Seasonal Nature of Cholera Is Unclear in the response to climate, an area may be critical in addressing the links This photograph was taken during a of research that remains in need of between within-year seasonal cycles cholera and nutrition survey during fl ooding in Bangladesh in 1974. In quantitative approaches. and those of longer period that are Bangladesh, monsoon rains appear observed in many infectious diseases. to have a seasonal “dilution” effect on Seasonal Drivers May Be Elusive For meningococcal meningitis, we transmission, producing a decrease in Besides scale, specifi c seasonal still need to examine the connection cholera cases during that season. We don’t know whether extreme rains also drivers are often elusive because of between the seasonal association produce a lagged increase in cases later the simpler reason that in nature described by Sultan and colleagues on in the cycle. In other parts of the seasonality is ubiquitous. Multiple