Everyone knows flu strikes most often during wintertime, but new research indicates a number of other infectious diseases are seasonal, too. Chicken pox often peaks in spring. Sexually transmitted diseases tend to strike most often in the summer, at least in the U.S. And bacterial pneumonia is most common in midwinter, according to a study of 69 infectious diseases, published Thursday in PLOS Pathogens.
Lead author Micaela Elvira Martinez says she had expected to find acute infections, ranging from malaria to polio, had seasonal peaks. What she had not anticipated was a similar pattern in chronic ones—including HIV. “I was very surprised,” says Martinez, an assistant professor at Columbia University’s Mailman School of Public Health.
Following the calendar makes perfect sense for mosquito-borne diseases such as West Nile virus, says David Fisman, a professor of epidemiology at the University of Toronto’s Dalla Lana School of Public Health. Toronto, where he lives, is too cold for mosquitoes most of the year, and the disease cannot spread if the insect is not around. But Fisman, who was not involved in the new research, says it is not as obvious why respiratory and gastrointestinal bugs would have a seasonality. “Characterizing them as seasonal is the first step,” he says. “The hard part is: Why on earth are they seasonal?”
The reasons generally vary by disease. Middle East respiratory syndrome (MERS), a dangerous respiratory virus linked to camels, is more often reported during camel calving season, according to the new study. Triggers can also be pegged to socioeconomics and geography. In developed countries measles outbreaks tend to spike during the school year, when children are in close contact; in developing areas outbreaks may be tied more to agricultural cycles, the study says.
Less well-understood is why cases of polio seemed to peak in the late summer in the U.S.—so much so that scientists once investigated whether the illness was caused by ice cream. Similar viruses including enterovirus D68 also seem more common in late summer and early fall, which might explain the recent spike in cases of acute flaccid myelitis—a rare, poliolike condition that can follow infection by EV-D68.
Other researchers have talked about the seasonality of certain diseases, but Martinez may be the first to survey such a wide range of conditions and find this common thread. Previous studies tended to focus on the seasonality of one or two conditions, and it was not possible until recently, Martinez says, to amass the kind of data that allowed her analysis. Her research involved reviewing previous studies of 69 communicable diseases of public health interest as presented on the Web sites of the U.S. Centers for Disease Control and Prevention (CDC), the World Health Organization (WHO) and the European Center for Disease Prevention and Control. She then used Google scholar to systematically search for information about disease seasonality.
Recognizing such disease patterns can boost the fight against them, says Elena Naumova, chair of the Division of Nutrition Data at Tufts University. Naumova, who was not involved in the new study, says this kind of disease calendar could be useful for issuing travel advisories or handing out bed nets to protect against mosquitoes.
The findings about the cyclical nature of many illnesses, Martinez says, also suggest people with chronic diseases might benefit from more frequent checkups or a higher dose of their medication at certain times of year to prevent flare-ups. Understanding this periodicity could also help scientists improve other strategies for fighting illnesses. Before switching to human disease Martinez researched infections in marine mammals; scientists researching such animals worry species are most vulnerable when their population levels are at an annual low point, she notes. Turning that around, perhaps a pathogen could be most effectively attacked at a time of year when its population is relatively small, in order to head off a later, larger outbreak. “I see it as biology that’s right in front of our faces, that we overlook very often,” she says.
Researchers next need to examine in more detail how this calendar varies around the world and whether it is affected by climate change, Naumova says. She is concerned climate-induced changes might drive diseases with different schedules closer together, making them more dangerous—just as HIV becomes more serious if the patient also contracts malaria or tuberculosis.
The new study’s findings also suggest humans are more tied to our natural surroundings than we usually acknowledge, Naumova and Martinez say. Animals that transmit infectious diseases to us often have seasonal peaks and troughs of transmission, a fact that likely interacts with variations in our own immunity.
According to Martinez, fields such as psychology are far ahead of infectious disease experts in understanding the human body changes cyclically over time. The type of depression known as seasonal affective disorder, for example, sets in when light levels dim in fall and then lifts when the world brightens in spring. It makes sense that such mood fluctuations could also trigger biological changes that may affect the immune system, she says. Perhaps that is one reason even chronic diseases can impact us like clockwork.