As the saying, ‘there is a season for everything’ implies, it now appears that infectious diseases too have a season of their own. Most people are aware that influenza outbreaks appear in seasonal cycles, but scientists at Columbia University in the United States now say that all infectious diseases have a seasonal element of their own. Using information from the World Health Organization (WHO), the US Center for Disease Control and Prevention (CDC), and other peer-reviewed publications, the researchers created a calendar of epidemics for 69 infectious diseases, from common infections to rare tropical diseases.
The extensive list of infections shows that in a given year, flu outbreaks occur in winter, chickenpox in spring, and gonorrhea and polio in summer, to name just a few. The study found seasonality occurs not just in acute infectious diseases such as flu but also in chronic infectious diseases such as Hepatitis B, which depending on geographic location, flares up with greater regularity certain times of the year. Preliminary work has shown that even HIV-AIDS has a seasonal element, thought to be driven by seasonal changes in malnutrition in agricultural settings.
The study describes four main drivers of seasonality in infectious diseases. Environmental factors like temperature and humidity regulate seasonal flu; in vector-borne diseases like Zika too, the environment plays a role in the proliferation of mosquitos. Host behaviors such as children coming into close proximity with each other during the school year are a factor in measles.
Ecological factors such as algae play a role the outbreak of cholera. Seasonal biological rhythms, similar to those that govern migration and hibernation in animals, may also be a factor in diseases like polio, although more research is needed. The scientific community has for long ignored the role of seasonality in the majority of infections, but the new study reveals a powerful and universal role for seasons in the outbreak of infectious diseases. The researchers concede that more work needs to be done to fully understand the forces driving disease seasonality, and to understand how seasonality can be leveraged to design interventions to prevent outbreaks and treat chronic infections. Identifying the drivers of seasonal outbreaks is not always straightforward. For instance, the bacteria that cause cholera, which spread to humans by fecal-oral transmission, can be maintained in water supported by algae.
Public health officials might undertake an intervention to prevent the transmission of cholera from infected individuals, and/or target the bacteria surviving in algal-filled waterbodies; importantly, the key season to undertake each of these interventions would likely differ.
In the case of polio, public health researchers once thought summer outbreaks were driven by the seasonal mixing of children in swimming pools or by the climate, but neither of these factors could explain the summertime outbreaks around the world. It was only in 2001 that a researcher at the CDC hypothesized that seasonal changes in the hormone melatonin may play a role in modulating the immune system.
Researchers are now following up on this hypothesis by comparing the levels of immune molecules in blood drawn from patients at different times of the year.