The seasonality of many infectious diseases is well recognized. Influenza occurs at the highest rates during winter in temperate climates, and dengue fever occurs during the rainy season in tropical climates. But recent research by Micaela Martinez, PhD, of the Columbia University Mailman School of Public Health suggests that all infectious diseases may have a seasonality component. The article, published in the journal PLOS Pathogens, uses data from the World Health Organization, the U.S. Centers for Disease Control and Prevention, and peer-reviewed articles to create calendars of epidemics for 69 infectious diseases.
A few of the trends that the research identified include a peak of chickenpox outbreaks in the spring, gonorrhea in the summer and fall, and polio and tetanus in the summer. The analysis included not only acute infectious diseases, but also chronic ones like HIV, which may be correlated with seasonal nutrient deficiencies, and hepatitis B, which showed peaks in spring, summer, and/or fall in different regions of the world. While many of these diseases were correlated with the seasons and weather conditions such as temperature and humidity, others were based on the school year, agricultural cycles, or mating and birthing seasons in animals that carry disease.
Awareness of the seasonality of infectious diseases can lead to much more effective prevention measures. According to Martinez, viewing infectious diseases through the lens of seasonality forces us to focus on the causes of that seasonality. This helps us find the true mechanisms of infectious diseases and avoid misinterpreting correlative relationships with seasonal phenomena. For example, it was once thought that polio outbreaks were caused by children gathering in swimming pools or theaters for typical summertime activities, but this did not explain summer outbreaks around the world. It is now hypothesized that seasonal changes in melatonin may have an effect on the immune system that makes children more vulnerable to polio infection during the summer months. Such insights into the true mechanisms of disease, gleaned through awareness of their seasonality, have the potential to generate more effective prevention and control measures.
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