Unexpected Consequences: Climate Change and Malaria

My name is Paul Roepe, I am a professor of chemistry in Georgetown College and a professor of biochemistry & cellular & molecular biology at the Georgetown University Medical Center.  I recently participated in a “Three Professors” podcast that had to do with unexpected consequences of climate change. Some additional thoughts that originated in the podcast are summarized in this blog post.

Over the past decades that I have studied the disease, malaria has been called an “emerging” infectious disease, a “re – emerging” disease, a “neglected” disease and other names.  Regardless the language that is used to describe the horrendous global burden of malaria, these days it is also a disease increasingly characterized by issues related to climate change.  It wasn’t when I started many years ago, but it is now.  Like most Americans I have followed news related to climate change with increasing interest and some alarm.  I am ashamed to admit that I did not much earlier dig a bit deeper into the scientific literature surrounding the topic.  In my defense it was not my specific field of scientific study, many other things were on my plate, and it was not clear to me how climate change science might overlap with the science I was trying to do.  Until recently that is.  In surfing this literature these past years, I became amazed at how much there is yet to learn about the consequences of climate change that we are now facing, as well as the new and incredibly diverse scientific challenges that these consequences reveal.  When I would think specifically about malaria and a few other infectious diseases during my reading of the climate change literature, I was surprised to learn how much can be done to change outcomes; often in simple ways, but dramatically and for the better. This blog is all about facing facts, learning more, and how that can empower any of us, regardless the complexity of what it is we are trying to do.

It has always been difficult, if not impossible, to perform productive infectious disease research without viewing the disease in it’s entirety. Young medical students and M.D.s in training are often taught to think about an infectious disease in terms of symptoms and best use of available treatments.  Young natural scientist Ph.D.s are trained to think about the molecular causes of the disease, or perhaps the science needed to create more effective  treatments, and young social scientist Ph.D.s might study why current infectious disease control policies work well in one region but not another.  But, particularly for a global disease like malaria that is actually multiple diseases that infect hundreds of millions annually in more than half the countries of the world, it is difficult to understand the disease using any one perspective.  So, within my research group, we try to tackle pressing issues related to malaria control using whatever concepts are necessary at the time.

For us, along with much of the rest of the infectious disease research community, it is now necessary to integrate new observations related to climate change into what we have always tried to do.

Most everyone knows that malaria is an infectious disease transmitted by mosquitos.  More precisely, it is a parasitic infectious disease caused by Plasmodium parasites that are members of the “Apicomplexa” phylum. There are > 160 species of Plasmodia that infect most all vertebrates on the planet, causing a diverse set of malarial diseases in birds, apes, rodents, humans, etc. Humans can be infected with at least 5 (possibly 6) of these species.  Children vs adults vs pregnant women acquire malaria disease of different severity, with different symptoms.  The point being that malaria is not one disease, it’s actually a spectrum of diseases.  When a female mosquito looking for a blood meal bites someone infected with malaria, a few parasites in the patient’s blood can get sucked into the mosquito’s belly.  Male and female parasites find each other and mate in the mosquito’s belly.  Parasite offspring migrate away from the mosquito’s belly and eventually wind up in the salivary glands of the mosquito.  When the infected mosquito then bites another (uninfected) person, the new parasites are transmitted via mosquito saliva.  We used to think that the only mosquitos that effectively transmitted malaria to humans were female Anopheles gambiae, but we now recognize that > 100 Anopheles species are capable of doing so, with 30 – 40 of them particularly good at it.  No one knows for sure if this has always been the case or whether it is more recent, perhaps another consequence of climate change.  Regardless, we know surprisingly little about most of these mosquito “vectors” that are capable of transmitting one of the most common and lethal infectious diseases on the planet.  We know even less about how climate change is affecting their behavior and their biology (and the biology of parasite development within their bellies). For a long time, it was thought (for good reasons) that malaria was only transmitted at dusk, because that was the time during the day when parasite infected mosquitos preferred to feed.  But that’s not necessarily true anymore. Climate change alters the way mosquitos develop from larvae into the adults that bite humans, where they sleep, what they eat, when they eat, and so on.  We don’t understand all of this as well as is needed.  We also don’t fully understand how parasites that mate in different mosquito bellies exchange genetic information so that drug resistance genes may then spread across a population of parasites.  But there are plenty of new data that tell us that all of this is changing, and quickly, driven largely by changes in temperature and rainfall in various regions of the globe.  Just 10 years ago acquiring certain types of malaria above certain elevations was unheard of because mosquitos could not live at those elevations, presumably because larvae could not successfully “winter over” at temperatures found at those elevations.  That has changed too. And so on.

But my laboratory doesn’t study mosquitos, we study the biochemical details that explain how malarial parasites become resistant to drugs and try to use that information to develop more effective drugs in the face of drug resistance.   My laboratory is full of chemists, biochemists, and molecular biologists.  Why would climate change issues that are affecting the habits of mosquitos be something that would preoccupy molecularly oriented researchers in my lab? There are many answers to that question that would take several more blogs to properly answer.  In brief, the larger picture is how do we control malaria in the field if we don’t fully understand the mosquitos the parasites are hiding in? More specifically related to our current research, the spread of parasites resistant to one drug but not to another appears to be evolving differently as mosquito biology changes due to climate change. Drug resistant malarial parasites evolve via a very complicated lifecycle that includes parasites living in the mosquito, then human blood, then the liver, then human blood again.  Then the mosquito again.   Do some drug resistant parasites spread faster via certain mosquito species? How will the parasite life cycle change as climate continues to force mosquitos to change even more? Ultimately, my lab wants to answer the question: what are the best control strategies to use for a given malaria outbreak in a given region at a given time? Can we answer this question comprehensively if we can’t answer the preceding two? Nope.

Thinking even more broadly, if we can’t yet answer such a question for malaria, what about other “re emerging” or “emerging” or “neglected” diseases transmitted by mosquitos? There are lots of those.  What about other infectious diseases transmitted by different insects whose biology we understand even less well than the biology of mosquitos?  Lyme disease is one example.  Lyme is caused by bacteria, not parasites, and transmitted by Ixodes ticks, not Anopheles mosquitos, but many of the same concepts are relevant.  Is climate change altering the geographical distribution of Ixodes ticks? Their behavior? Do we find Lyme disease in places that we didn’t a decade or two ago? Are these things changing how we view control of the disease? Yup Yup Yup Yup. And so on.

What I think I have learned is that climate change is not an incomprehensible problem, it is a set of specific and very interesting challenges.  Neglecting occasional evidence to the contrary, humans are not stupid, we’re actually quite clever.  We can solve any problem that we set our minds to solving, regardless the complexity, but we first need to define the most important aspects of the problem if we are to make expeditious progress.  To an infectious disease researcher, more and more, every day, climate change is changing the way we think about problems.  Not making the problems unsolvable, just changing the way we think about them.  Research simply needs to adjust.   Like it always does in the face of new challenges.