Like clockwork, the influenza virus rears its ugly head every winter and makes our lives miserable. But the flu isn’t the only germ that likes a particular time of year, according to a new paper published this week in PLOS Pathogens. Seemingly every infectious disease, including polio, gonorrhoea, and even HIV, is seasonal, though not always for the same reasons.
In recent years, author Micaela Martinez, a infectious disease ecologist at Columbia University’s Department of Environmental Health Sciences, had become especially interested in understanding when certain diseases strike. It was a fascination that began with her work on childhood ailments that tend to show up as seasonal epidemics, like measles, chickenpox, and polio.
“What I was finding is that there’s no particular time of the year where all diseases are prevalent, and a time of the year when they’re all gone. For each infectious disease, there’s a time of the year for that infection, and every infection has its own season,” Martinez told Gizmodo. “And for some diseases, where the outbreaks are really explosive, you can almost set a calendar by them.”
The world’s most horrifying calendar. (Illustration: Micaela Martinez/PLOS Pathogens)
Indeed, in her latest paper, Martinez found a seasonal pattern for every infectious disease she was able to dig up any relevant research on. The list of diseases in her calendar—69 in total—includes those caused by parasites, like dracunculiasis (otherwise known as Guinea worm disease); those caused by bacteria, like Lyme; and viral diseases, like chickenpox. Martinez also looked at acute infections that usually last for a short while, like the flu, and chronic infections that remain hidden for years without causing symptoms, like hepatitis B and C.
The reasons for a disease’s timing are more obvious in some cases than in others. Lyme, for instance, is spread by a female tick’s bite, particularly from ticks that are in the immature, or nymph, stage of their lifecycle. And in the US, teen ticks begin to show up en masse around the summertime, which is also peak Lyme season. For flu, the winter weather usually keeps the air dry and cold, which helps the virus stay alive longer and might even lower our natural defences against it.
But for other diseases, there’s still lots we don’t know about why they’re seasonal.
Martinez originally intended to only look at acute infections, expecting that seasonality wouldn’t be seen in chronic infections. Someone with genital herpes, for instance, isn’t sick all the time; instead, their symptoms seem to come and go at random, depending on how active the germ is inside their body. But when Martinez expanded her search, she was surprised that even these infections all had their own seasonal clock (with herpes, symptoms seem to be more common in the spring)
One chronic infection highlighted by Martinez is shingles, caused by the same virus responsible for chickenpox. The virus behind shingles hides out in our nerve cells after we get better from chickenpox, and it’s only decades later that it reemerges, causing the painful disease. These new cases seem to cluster around late spring and summer. And past research by Martinez and others has suggested that the climate itself, particularly the amount of sunshine, might help resurrect the virus within nerve cells.
“It could be that UV exposure essentially suppresses the localised immunity that would generally keep the virus in check inside these neurons,” she said. On the other hand, it might be something else entirely about the spring and summer that affects our body’s defences against the virus.
Martinez hopes her paper will spark more interest in answering these and other questions, as well as in exploring seasonality as a key aspect of infectious disease in general. For many germs, there’s surprisingly little to nothing we know about when they peak and why. And most of the time, the research we do have only focuses on a specific part of the world, which can skew the picture. In the case of flu, for instance, it hits the North Hemisphere in the autumn and winter, but the cooler areas of the Southern Hemisphere in the spring and summer.
That knowledge, aside from helping someone plan out the world’s worst beefcake calendar, could have major public health implications. With herpes, for example, antivirals can shorten the length of symptoms and reduce the risk of transmission during someone’s active outbreak, so it might be worthwhile for doctors to have a larger stock of medicine during a peak season. Polio, on the other hand, is close to being eradicated globally, thanks to mass vaccination programmes. But getting rid of these preventable diseases would be easier if we knew exactly when to vaccinate a particular spot of the world (polio is generally a summertime disease, for reasons that are still unclear).
“It all boils down to having the information to get smarter about how to treat infections, especially for the chronic ones, and how to prevent epidemics of the acute infections,” Martinez said. [PLOS Pathogens]