Sick? ‘Tis The Season.
You’ve heard of flu season, of course (consider this your friendly reminder to get a flu shot!). But a surprising number of other illnesses also have a seasonal component, peaking at certain times of the year. Chickenpox outbreaks peak each spring, for instance, while polio historically tended to surge in the summer.
Micaela Martinez, an environmental health researcher at Columbia University, believes that all infectious diseases may have some seasonal aspect to them. She collected information on almost 70 different human diseases from African sleeping sickness to Zika and looked at factors that could connect each to the calendar. In some cases, the seasonality of the disease is due to weather, while in other cases more complex interactions of host, vector, and human behavior come into play. She joins Flora Lichtman to discuss her work, reported this week in the journal PLOS Pathogens.
Read the original study in PLOS Pathogens.
Micaela Martinez is an assistant professor in the Department of Environmental Health Sciences at Columbia University in New York, New York.
FLORA LICHTMAN: This is Science Friday. I’m Flora Lichtman. Ira Flatow is away. Later in the hour, we’ll talk about the pioneering early days of heart surgery and the first surgery that put not one but two patients at risk. But first, unless you’ve been living on the moon in recent weeks– which would be really cool, so if that’s you please tweet us– someone has probably told you to get your flu shot because flu season is coming. But do other illnesses have seasons too?
My next guest says she thinks that most, maybe all, infectious diseases have some sort of seasonal component. Micaela Martinez is an assistant professor in the Department of Environmental Health Sciences at Columbia University here in New York. She wrote about this idea this week in the journal PLOS Pathogens. And she joins me here in our New York studios. Welcome to Science Friday.
MICAELA MARTINEZ: Thank you for having me.
FLORA LICHTMAN: So it’s not just the flu?
MICAELA MARTINEZ: No, it’s not just the flu. There’s evidence that all infectious diseases are seasonal.
FLORA LICHTMAN: That’s amazing. I want to start with chicken pox– near and dear to my childhood. What is chicken pox season?
MICAELA MARTINEZ: So chicken pox season– so chicken pox, I should say, is a classic childhood infectious disease. And so chicken pox transmission tends to ramp up when kids go back to school in the autumn. And that’s when we see cases start to climb and climb and climb until the epidemics hit their peak in around March. And then they turn around, and we see their cases fall away. And then this repeats like a clock every single year. It happens over and over in the countries that don’t vaccinate against varicella.
FLORA LICHTMAN: I was going to ask, I mean, this must have changed here when people started vaccinating.
MICAELA MARTINEZ: Yes, we’ve been vaccinating since the 90s in the United States, but there are only five countries right now that use the varicella vaccine. So it’s still epidemic in most of the world.
FLORA LICHTMAN: What about sexually transmitted diseases?
MICAELA MARTINEZ: Yes, so I was actually quite surprised to see that there’s documented seasonality for sexually transmitted infections. There weren’t too many studies that had actually looked into this. But the studies that I found were done in the United States and found the seasonal peaks for some of the STDs like gonorrhea being in the summertime.
FLORA LICHTMAN: Do we know why? Is it for the reason we might suspect?
MICAELA MARTINEZ: So seasonal differences in sexual activity could definitely be a driver of that transmission. We also know that there are seasonal changes in the numbers of babies born so that could also indicate that that’s one of the mechanisms.
FLORA LICHTMAN: What about– I know you looked at polio as well. Is there a seasonal cycle to polio?
MICAELA MARTINEZ: Yes, there is. So polio is kind of this anomaly in terms of childhood infections in that polio epidemics– when they were rampant– occurred in the summertime. And for the other childhood diseases are usually associated with school terms. And for polio it’s still not known why. There have been studies that looked at shutting down swimming pools and theaters and all looking at summertime activities to see if that was driving it. And the causative mechanisms still hasn’t been found.
FLORA LICHTMAN: Do we have an– are there any hypotheses out there?
MICAELA MARTINEZ: Yes, there is a hypothesis proposed back in 2001 by a researcher named Scott Dowell, who used to be at CDC. Now he’s at Gates Foundation. And he had actually proposed that the summertime seasonality of polio and some of the other infections could be driven by seasonal changes in the human body. So perhaps we have changes in our hormones or immunity that influence disease susceptibility.
FLORA LICHTMAN: How would you go about testing that idea?
MICAELA MARTINEZ: Well, in fact, my lab is currently testing that right now. We want to know whether or not the human body is in fact seasonal. So starting next month, we have people that are going to be coming into specialized labs that we’re using in the United Kingdom and have people come into the lab every single season of the year– so this winter solstice, and then during the spring and autumn equinox, and summer solstice over this next year– come in.
And then we’re going to look at all aspects of their physiology. So their immune system, their hormones, their metabolism, even the microbes in their gut. And we want to see, does a human body change with the seasons? Is some of that because of differences in our activity or things were exposed to? Or is it differences that are just innate to our bodies?
FLORA LICHTMAN: Could it be related to the things people are eating, though, in different seasons? Does it have to be internal?
MICAELA MARTINEZ: It doesn’t necessarily have to be internal, no. Our world is very seasonal. Everything from agriculture, our sleep. Not just the temperature and weather that we’re experiencing. Seasonality is a fundamental property of our planet.
FLORA LICHTMAN: I mean, does that– do you have any hypotheses about when your immune system might be more depleted? I mean, mine would be the winter when people seem to get sick a lot. But that’s as a lay perspective.
MICAELA MARTINEZ: Yeah, well the thing that– so this investigation that I did looking at the seasonality in cross infections really shows that there’s a time of year for each disease. So it’s not that we’re particularly susceptible at one time of year– susceptible to everything at one time of year and then protected at others. It seems to be that there’s a restructuring to what we might be susceptible to as well as restructuring of transmission of various infections throughout the year. So people are getting sick all throughout the year, but the identity of what they’re getting sick from varies.
FLORA LICHTMAN: Like, you might be more susceptible to one illness at one time in the year than you are to another.
MICAELA MARTINEZ: Exactly. And that’s why– so this idea of the calendar of epidemics, you can think of it as that structuring through time. So spring being chicken pox season. Summer being polio season. The common cold season being autumn. And then flu being wintertime.
FLORA LICHTMAN: And it sounds like some of that might have to do with the characteristics of the illness and maybe some of that has to do with us.
MICAELA MARTINEZ: Absolutely. And we still don’t know. So really the identification of seasonality in cross infections is just a kind of enticing bit of evidence that a lot more research needs to be done in this area.
FLORA LICHTMAN: Do you think that understanding the seasonality of diseases can help us eradicate or prevent transmission?
MICAELA MARTINEZ: Absolutely. So because epidemics are structured seasonally, that means that there is a low time in the year for each infection when there are very few cases in the population. And the way I see that is a window of opportunity when we can go in with vaccines or other preventative measures to hit pathogens when they’re at their most vulnerable because they’re few of them.
FLORA LICHTMAN: Hit them while they’re down.
MICAELA MARTINEZ: Exactly.
FLORA LICHTMAN: Other examples of this where people have done interventions based on seasonality?
MICAELA MARTINEZ: No, not to this point. I recently saw a talk by some PhD students that we’re looking in sub-Saharan Africa whether or not it would be possible to administer the measles vaccine seasonally based on this idea. But as of right now I haven’t seen any published papers doing that.
FLORA LICHTMAN: What about something like Lyme disease where you also have an animal reservoir?
MICAELA MARTINEZ: Yes. So Lyme disease is one of these infections that the seasonality is extremely complicated because you have humans, you have animal reservoirs that vary in their competence for transmitting the infection, and then you also have the tick. And so all of these wildlife species, including the ticks, have their own seasonal cycles. And so not only the seasonality but the control and interventions that you would use are quite complicated by the fact that you have all these species interacting to generate these outbreaks.
FLORA LICHTMAN: Could you use an understanding of, say, the seasonality of that animal reservoir to help predict how bad the season’s going to be for, say, Lyme disease?
MICAELA MARTINEZ: You could, but it takes a lot of integration or would take a lot of integration of data from all of the hosts and the vectors to be able to make the intervention on the human side. But there are other infections like mosquito transmitted infections where that might be more straightforward like Zika.
FLORA LICHTMAN: Are we any different from other animals in terms of our seasonality of illnesses?
MICAELA MARTINEZ: Well, I don’t know because there haven’t been these very widespread studies looking across infections for species. To my knowledge, I haven’t seen anything where they’ve done a similar study looking at all the diseases of, I don’t know, bears or some rodent.
FLORA LICHTMAN: I would be interested in that.
MICAELA MARTINEZ: That would be very cool.
FLORA LICHTMAN: You looked at almost 70 diseases in this paper.
MICAELA MARTINEZ: Yes.
FLORA LICHTMAN: What jumped out to you? What surprised you?
MICAELA MARTINEZ: When I set off to do this study, I went in with the hypothesis that the acute infectious diseases– so those that don’t infect for very long, you might be infected for a month or so– that those would be seasonal. But actually the chronic infections that can last your entire life that they would not be seasonal. And I found that everything was seasonal. So I was quite surprised by that. And so–
FLORA LICHTMAN: Such as, what was, like, one of the most surprising?
MICAELA MARTINEZ: HIV. And even though there was only one study that I had found for HIV– well, first of all the researchers that conducted the study are extremely thorough. But what they had found was that the progression to AIDS was influenced by malnutrition. And malnutrition is seasonal in developing countries. And so you can think of that as malnutrition causing stress that makes the body less capable to deal with the progression of the disease. So it happens more rapidly. And so I was very, very surprised by that.
FLORA LICHTMAN: Where do you go next with this research?
MICAELA MARTINEZ: Well, my lab is, like I mentioned before, looking at the seasonality in the human immune system and the human body in general. But we’re also working on studies modeling chicken pox epidemics and if we can do this seasonal tailoring of intervention. So we’re calling them smart interventions where we’re leveraging disease seasonality for eradication and elimination initiatives.
FLORA LICHTMAN: When is that going to roll out?
MICAELA MARTINEZ: In terms of the actual implementation? You know, that’s up to each country. Every country decides on their routine immunization and their mass vaccination campaigns for those countries that use them.
FLORA LICHTMAN: So it’s a suggestion.
MICAELA MARTINEZ: Exactly. And that’s always the best that we can do as academic scientists.
FLORA LICHTMAN: And what about the other study that you mentioned where you’re taking people and trying to understand the seasonality of their bodies? What are you going to be looking for?
MICAELA MARTINEZ: Yeah, so at its basis in terms of the mechanism what we think is happening is like in other mammals and in birds. Well, we know that humans have circadian rhythms. So these are changes in our body around the day-night cycle. And in other mammals and birds, these circadian rhythms are modulated with the seasons as the light cycle changes with the seasons. As we know, it’s a lot darker in winter. And that affects the circadian rhythms. And it can actually generate seasonal biology. And so that’s what we’re going to be looking at for humans.
FLORA LICHTMAN: What do you mean seasonal biology?
MICAELA MARTINEZ: In terms of seasonal changes in our immune system, seasonal changes in our hormone levels, and seasonal changes in our metabolism.
FLORA LICHTMAN: That’s so fascinating. I can’t wait to hear more.
MICAELA MARTINEZ: Thank you.
FLORA LICHTMAN: Thank you so much for joining us. Micaela Martinez is an assistant professor in the Department of Environmental Health Sciences at Columbia University here in New York. Thanks again.
MICAELA MARTINEZ: Thanks.