Climate change is already affecting our planet and our lives, with the past few months marking the hottest global summer ever recorded. As the Earth warms, scientists and physicians are growing increasingly concerned about potential health risks to its human inhabitants, threats that extend far beyond beating the summer heat.
As an infectious disease specialist, farmer, and associate professor of internal medicine (infectious diseases) at Yale School of Medicine, James Shepherd, MD, PhD, has experienced firsthand how climate change is reshaping infectious diseases, both locally and globally. He has formed his observations through decades of study, including nine years of travel in Sub-Saharan Africa, where he researched and treated infectious diseases such as tuberculosis and HIV/AIDS.
“There are lots of changes that we as humans have wrought on the planet, which are operating over very short time frames and currently influencing the sort of diseases that we see,” he says.
To mark Climate Week NYC 2024 (Sept. 22-29), Shepherd discusses below climate-driven changes, such as increasing urbanization, that are rapidly impacting the rise and spread of infectious diseases. This interview has been edited for clarity and length.
How have you seen climate change impacting infectious diseases locally?
It’s very interesting and somewhat alarming. One can see the changes in the environment very clearly here in Connecticut, where the number of vectors for infectious diseases—mainly ticks—are moving northward with warming temperatures, mainly warming winter temperatures.
Even just 20 years ago, there was really only one tick vector in Connecticut, the deer tick, that transmits Lyme disease. Today, there are established populations of four different tick vectors that transmit a whole broader group of infectious diseases, ones that we’re just not used to seeing here. We are entering a time where the number of different infections that someone could contract living in New Haven is probably going up pretty significantly.
It's an extraordinary change in biodiversity, in the environment, and in the dispersal and the prevalence of infectious diseases. And one has to assume that it’s going on all over the place.
Based on your overseas research, what are some of the most pressing global health threats?
In Africa, Asia, and South America, tuberculosis (TB) remains one of the world’s major public health problems. It’s been particularly difficult to control, and it has really thwarted all the efforts to reduce its transmission.
TB has definitely been assisted in its transmission globally by continued inequity, but also urbanization—movement from rural to urban centers. Because TB is an airborne disease, it’s a disease of overcrowding and insanitary conditions, which tend to occur when people live very close together. With human migration from the countryside to the cities, it may be increasing, at least in terms of density.
What is driving these increases in urbanization?
With increased temperatures, the basis for rural life—subsistence agriculture, living in the countryside, and supporting yourself through growing food and raising animals—is becoming more unpredictable and difficult to sustain. The weather gets more intense, rainfall gets more concentrated in deluges and then dry periods, and it becomes harder to farm and harder to survive in the [rural] environment. That’s fueled the movement of people from the countryside into the cities.
How is urbanization contributing to global health crises?
The African and South and East Asian cities in the tropics are growing at a faster rate than any other urban environment anywhere. Largely, people are not moving into well-managed urban environments with good infrastructure; they’re congregating in extremely dense shanty towns and unincorporated communities outside of cities, where there are few services, if any, and no water or closed sewers. As an environment, these become very welcoming places for transmission of infectious diseases.
One vector I find particularly alarming is an emerging mosquito vector for malaria that originated in South Asia, a mosquito called Anopheles stephensi, which is largely an urban mosquito, so it specializes in living in human environments. Rates of malaria are going up in some places, according to the World Health Organization, and places that had good control of malaria are now seeing far higher case numbers as a result of a vector that appears to have adapted to the changes in its host population that have been driven by changes in the environment, in the warming planet, and things such as globalization and inequity of resources.
As a farmer yourself here in Connecticut, growing hops, have you personally experienced these challenges that are driving urbanization globally?
We’ve noticed over the last couple of years a real change in the unpredictability of weather, and I think that’s been reflected in the long-term data in Connecticut and much farther afield.
The increasing temperature and the increase in energy being injected into the atmosphere is driving real changes in growing-season reliability. And if it’s happening to us here, it’s happening all over the world, and it’s only going to make things more unpredictable.
What do you think needs to happen to combat these issues?
We need to begin with an evaluation of our place on the planet, because at the moment—and I’m speaking as a farmer, a doctor, an environmentalist, a nature lover, and an animal keeper—we have assumed the rights to our domain to the exclusion and disrespect of all the other species and their places in environment around us.
My fear is that as we urbanize more and more, we become less and less connected and interested in the environment around us and the other species that we live with because we don’t see them. This sort of self-destructive focus on our own existence is going to escalate rather than diminish. My sense is that the trajectory that we’re on, certainly from an infectious disease viewpoint, will be pandemics that are much more significant than the COVID-19 pandemic.