From African Sleeping Sickness to Zika, a new journal article presents the evidence for seasonality in 69 infectious diseases.
Micaela Martine, Columbia University’s Mailman School of Public Health Professor of Environmental Health Sciences, gathered data for the diseases from the U.S. Centers for Disease Control and Prevention (CDC), the World Health Organization and peer-reviewed journal articles. She then mapped the time of year when outbreaks tended to occur, ranging from common infections to rare tropical diseases.
Martine found that in a given year, flu outbreaks occur in the winter, chickenpox in the spring and polio and gonorrhea in the summer.
She found seasonality occurs not just in acute infectious diseases like flu but also chronic infectious diseases like Hepatitis B, which depending on geography, 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 paper 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.
Identifying the drivers of seasonal outbreaks is not always straightforward, but can pay dividends. 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.
“Seasonality is a powerful and universal feature of infectious diseases, although the scientific community has largely ignored it for the majority of infections. Much work is needed to understand the forces driving disease seasonality and understand how we can leverage seasonality to design interventions to prevent outbreaks and treat chronic infections.”
Micaela Martine, Columbia University’s Mailman School of Public Health Professor of Environmental Health Sciences
In the case of polio, public health researchers once thought summer outbreaks were driven by the seasonal mixing of children in swimming pools or theaters or by the climate, but neither of these factors could explain the summertime outbreaks around the world. In 2001, a researcher at the CDC hypothesized that seasonal changes in the hormone melatonin may play a role in modulating the immune system. In an ongoing study funded by a prestigious National Institutes of Health Director’s Grant, Martinez is investigating this possibility by comparing levels of immune molecules in blood drawn from patients at different times of the year.