While it may be well-known that flu season is in the winter, it may come as a surprise that other forms of illness, including sexually transmitted diseases, malaria and even chickenpox have “peak seasons” as well, according to a study published in PLOS Pathogens.
According to the study, there are a couple of main factors that primarily cause a peak season in these diseases.
The first and seemingly most obvious are environmental factors. In acute infections, both the host and the invader can be affected by external factors. For example, during the warmer, wetter seasons, mosquitoes increase their rates of breeding, providing more sources for the disease to be transmitted. In addition, especially with the flu, the colder weather affects the immune system’s response to pathogens.
Host behavior is another cause of these seasonal disease trends. This is because most infectious diseases require some kind of contact, whether its direct or through the air, for the disease to be spread. Seasons can affect our day to day movements and the overall probability that we will spread the disease. For example, the measles peak season is when school is in session because children, susceptible “hosts,” are gathered together in one area.
According to Micaela Elvira Martinez, lead author of the study and assistant professor at Columbia University’s Mailman School of Public Health, finding seasonal trends in acute infections was unsurprising, but finding seasonal trends in chronic diseases such as human immunodeficiency virus (HIV) definitely came as a surprise.
David Fisman, a professor of Epidemiology at the University of Toronto’s Dalla Lana School of Public Health, commented that the seasonality trend in diseases such as West Nile Virus makes perfectly logical sense. But like Martinez, he realizes that there is no clear reasoning as to why many respiratory diseases and gastrointestinal bugs would follow seasonal patterns.
“Characterizing them as seasonal is the first step,” Fisman said according to Scientific American. “The hard part is: Why on earth are they seasonal?”
According to the researchers, the causes are different for most of these chronic diseases, and most has to do with the two previous factors: environment and host behavior. For example, Middle East respiratory syndrome (MERS), a rare and dangerous respiratory illness stemming from Saudi Arabia, is most often reported during calving season, which makes sense because many researchers believe that the virus may have originated in camels or that camels play a role in spreading the illness.
However, some of these chronic diseases are much less understood. Research has yet to uncover why polio is most common in the late summer – research was even conducted to see if the disease was correlated with ice cream consumption.
Elena Naumova, chair of the Division of Nutrition Data at Tufts University, believes that studies like these are especially important because knowledge on the seasonal patterns can help doctors to better prevent and combat these diseases in their patients.
Martinez agrees, stating that information from these studies will give doctors a way of knowing at which times of the year patients with these chronic diseases may benefit from higher doses of medication or more frequent checkups.
“I see it as biology that’s right in front of our faces, that we overlook very often,” Martinez said according to Scientific American.
Furthermore, these findings may also suggest the best time to try to attack a pathogen: at the time of the year when its population is the lowest, which could help prevent large future outbreaks.
Next, Martinez and her team hope to study how these seasonal trends are different around the world and whether some of these diseases are affected by climate change in different countries.
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