Outbreaks of All Acute and Chronic Infectious Diseases Are Seasonal

Infectious diseases can circulate on some level throughout the year, but in a new study, an investigator from the Columbia University Mailman School of Public Health shared that all outbreaks of infectious diseases have a seasonal element.

According to the World Health Organization (WHO), 3 of the top 10 causes of death worldwide in 2016 were infectious conditions—lower respiratory infections, diarrheal diseases, and tuberculosis. Although the number of deaths caused each year by these conditions has declined since 2000, together they still caused more than 5.5 million deaths globally in 2016, and along with other infectious diseases continue to create a global public health burden.

Researchers have observed that with seasonal patterns of some infectious diseases there are periodic surges in disease incidence which correspond with seasons or other annual conditions, though the mechanisms behind this seasonality of many conditions are not entirely understood. For example, investigators are still working to understand how climate drives outbreaks of influenza each year.

Investigators on a new study, published on November 8, 2018, in the journal PLOS Pathogens, explored documented seasonal cycles of human infections, and posited that all infectious diseases outbreaks have a seasonality caused by varying factors. The findings, according to the author, expand the concept of an epidemic calendar to include seasonal outbreaks of even more infectious diseases.

In the study, Micaela Martinez, PhD, an infectious disease ecologist at Columbia University in the Department of Environmental Health Sciences, noted that we are familiar with outbreaks of influenza in winter and chickenpox in the spring, and pointed to evidence suggesting that each acute infectious disease has its own seasonal window of occurrence, which can vary among geographic locations and differ from other diseases within the region.

The study was based on data of 69 infectious diseases, including neglected tropical diseases, collected from the WHO, the US Centers for Disease Control and Prevention (CDC), and the European Centre for Disease Prevention and Control. In exploring 4 drivers of seasonal outbreaks—environmental factors, host behavior, host phenology, and exogenous biotic factors—Dr. Martinez determined that in addition to acute infectious diseases, outbreaks of chronic infectious diseases such as hepatitis B virus and HIV/AIDS also occur at greater regularity during certain times of the year.

Many infectious diseases can share the same seasonal driver while differing greatly in multiple aspects of their biology. The new findings offer the public health community needed insight into the role of seasonality in disease outbreaks, which the study says is an overlooked driver of infections. In an interview with Contagion®, Dr. Martinez explained why this might be the case.

“Seasonality has been explored in great depth for a few infectious diseases, but for the majority of infections there really isn’t much in the literature about seasonality, particularly for the chronic infections,” Dr. Martinez said. “For the chronic infections, perhaps there’s just not an expectation that they would be seasonal.” She noted that chronic disease reporting is usually limited to prevalence rather than symptoms.

“I really don’t know why [seasonality] isn’t focused on more for acute infections because for those diseases that are notifiable, there are plenty of data that can be used to study their seasonality and nowadays a lot of public health agencies and ministries of health make data available online,” Dr. Martinez continued.

A better understanding of seasonality can help public health officials develop new interventions ahead of disease seasons, such as monitoring individuals for symptoms of chronic infections and more aggressively treating symptoms ahead of anticipated outbreak seasons to prevent downstream infection in the population. For acute infections that have peaks and troughs, the troughs offer windows of opportunity for vaccination campaigns and other preventative measures, and “operationalizing the use of this information is exciting,” Dr. Martinez concluded.