Flu season is upon us, but there are actually “seasons” for many other infectious diseases. Chickenpox outbreaks peak each spring and polio transmission historically occurred in the summer. In fact, at least 69 infectious diseases vary seasonally.
Micaela Elvira Martinez, assistant professor in the Department of Environmental Health Sciences at Columbia University, found this out by systematically querying the data of 69 infectious diseases for seasonality. She chose the diseases to study based on those that were of public health interest (including neglected tropical diseases). She searched the literature for a disease name + season and/or disease name + season + human and collected information from the publications found. When needed, she searched for disease name + “seasonality.”
What she found was that seasons matter – but it’s more complicated than cold weather keeping everyone cooped up indoors. Martinez categorizes the impact of seasonality into four factors: environmental, host behavior, life cycle events, and exogenous biotic factors. What’s more, all of these factors can influence disease epidemiology at many levels: from hosts, to reservoirs, and to vectors.
Environmental factors
Environmental factors can range from temperature to rainfall to even water salinity in the oceans. Changes in the environment impact not only host susceptibility, but it can also affect pathogen survival between hosts and the distribution of pathogens among host environments. For example, temperature and humidity both affect transmission efficiency of the flu. And the culprit behind African sleeping sickness? The rainy season affects the geographic distribution of the tsetse fly which changes the likelihood that humans contact African sleeping sickness.
Host behavior
Changes in the behavior of both human and non-human hosts also occur from one season to the next. For example, measles transmission increases during the school year when children are in close contact with others. Some animals may be isolated from one another during most of the year and many animals only have sexual contact during breeding season. Fighting between animals can also result in transmission of disease. Let’s look at the Tasmanian devil for example. Aggressive behavior and fighting among Tasmanian devils result in the spread of infectious facial tumors.
When it comes to humans, tracking behavior easier than ever before: light-at-night satellite imagery and cell phone data can give us insight into mobility patterns across seasons. But monitoring zoonotic diseases is much more difficult, especially in remote areas of the world.
Life cycle events
Seasonal rhythms of plants and animals – patterns of reproduction, metabolism, hibernation, and migration – all have roles in the epidemic calendar. Studying these patterns in humans is a relatively new research area, but these events are well documented in many animals. Martinez suggests that these events could be the most common cause of seasonality in vectors and nonhuman animals, impacting diseases such as Zika and Middle East respiratory syndrome coronavirus (MERS-CoV). Studying the impact of seasonal rhythms on epidemiology becomes more complicated when considering chronic infections, and disease vectors and parasites with their own life cycle events that occur concurrently with human life cycle events.
Community ecology
While hosts, parasites, and vectors are all influenced by environment, behavior, and life cycle, the ecological community they belong to also have seasonal aspects. This includes interactions that take place within hosts (ex: parasite-parasite interactions) and within the greater ecological community. In multi-host systems, such as Lyme disease, community ecology becomes particularly important for disease transmission and each host or vector has its own set of ecological interactions that can affect disease transmission.
“It is, therefore, important to conceptualize the epidemic calendar from the lens of “everything is seasonal,” Martinez wrote in PLoS Pathogens. Take polio for example. Polio cases historically peaked in the summer, along with the sales of ice cream and bathing suits or the length of day. Yet the latter two factors have nothing to do with polio transmission. Seasons clearly matter, but when seasons influence so much, it’s not always clear whether a particular seasonal change is behind the seasonality of a particular disease.
