Influenza is not the only infectious disease with a season. Analyzing published data, Micaela E. Martinez, PhD, assistant professor of environmental health sciences at Columbia University’s Mailman School of Public, created an “epidemic calendar” she said shows the seasonality of 69 infectious diseases, including Ebola, gonorrhea and Zika.

According to Martinez, seasonal drivers affect not only the transmissions of acute infectious diseases like measles but also contribute to flare ups of chronic infections such as hepatitis B. Her findings were published today in PLOS Pathogens.

“Seasonality has been recognized for a number of acute infectious diseases, especially epidemic-prone diseases — things like influenza, cholera, polio, measles,” Martinez told Infectious Disease News. “But aside from those classic epidemic-prone diseases, which have been studied with great depth, infectious disease seasonality had not, to this point, been recognized as a ubiquitous feature of acute or chronic infectious diseases.”

Timing of Epidemics

According to Martinez, the timing of epidemics is influenced by seasonal variation — also known as seasonal forcing — in infectious disease transmission. Martinez said the seasonal forcing of a few diseases important to public health, such as measles, influenza and cholera, has been monitored over the past century but that “disease seasonality has yet to be systemically and/or rigorously characterized for the majority of infections.”

Martinez collected data from the websites of the CDC, WHO and the European Centre for Disease Prevention and Control, compiling a list of 69 communicable diseases of public health interest and categorizing them as acute or chronic. Martinez also used Google Scholar to search for additional information about disease seasonality.

Transmission Dynamics

According to Martinez, four seasonal drivers can affect transmission dynamics: environmental factors, host behavior, host phenology and exogenous biotic factors. Environmental factors can influence transmission through the effect of climate conditions, such as rain or temperature, on hosts and/or parasites, Martinez noted. Host behavior is best exemplified in the elevated transmission of measles during the school year, when children have more contact.

Host phenology, Martinez said, “includes host life history, annual cycles (e.g., migration and hibernation), and endogenous circannual rhythms. Exogenous biotic factors are any “interactions that take place within hosts and interactions within the ecological community of hosts, reservoirs, and vectors.”

Public health agencies

It is well known that winter months include influenza season. According to Martinez, March is the peak of varicella season in the U.S. May through July is gonorrhea season and polio transmission historically occurred each summer. According to Martinez, public health agencies and officials could improve infection control by knowing the mechanisms of seasonality for diseases.

“There needs to be a systematic review of seasonality for each infectious disease,” Martinez said. “Especially for the chronic infections, if we understand when individuals might experience relapse or flare ups, we might be able to anticipate illness and be more proactive about treatment or screening.” – by Marley Ghizzone