All infectious diseases have a seasonal element, say scientists who have found that, depending on geography, even chronic infections like Hepatitis B and Zika flare up with greater regularity certain times of the year.
Most of us are aware of the seasonal cycle of influenza outbreaks.
Micaela Martinez, a scientist at the Columbia Mailman School of Public Health in the US, collected information from the World Health Organization, the US Centers for Disease Control and Prevention, and peer-reviewed publications to create a calendar of epidemics for 69 infectious diseases, from commonplace infections to rare tropical diseases.
A given year will see outbreaks of flu in the winter, chickenpox in the spring, and gonorrhea and polio in the summer -- to name a few of the best described seasonal outbreaks, according to the study published in the journal PLOS Pathogens.
Martinez found that 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 research 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 mosquitoes.
Host behaviours 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.
"Seasonality is a powerful and universal feature of infectious diseases, although the scientific community has largely ignored it for the majority of infections," said Martinez.
"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," she said.
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 faecal-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.
