By Roberta Attanasio, IEAM Blog Editor
The impacts of global climate change on human health and well-being are undeniably alarming. Safe drinking water, sufficient food, and secure shelter are threatened by rising sea levels and severe weather events. Heat waves dramatically increase death rates not only from heat strokes, but also from complications arising from cardiovascular, respiratory, and cerebrovascular diseases. According to the WHO, climate change is expected to cause approximately 250,000 additional deaths per year between 2030 and 2050, mostly from malnutrition, malaria, diarrhea, and heat stress. Unfortunately, these estimates take into account only a subset of the possible health impacts and assume continued economic growth and health progress. The global situation is likely to be much worse.
In addition to malaria and diarrhea, climate change is expected to have a substantial effect on the burden of many other infectious diseases. However, such expectations have generated considerable debate in the past few years—in many instances, factors such as socioeconomic drivers and disease control measures can confound the detection of effects mediated by climate change. Sonia Altizer, Professor of Ecology at the University of Georgia, said: “For a lot of human diseases, responses to climate change depend on the wealth of nations, healthcare infrastructure and the ability to take mitigating measures against disease. The climate signal, in many cases, is hard to tease apart from other factors like vector control and vaccine and drug availability.”
Indeed, while the effects of climate change on some infectious diseases, such as malaria, seem to be established, causal links are harder to assess for others. The report Under the Weather: Climate, Ecosystems, and Infectious Disease points out: “Since the dawn of medical science, people have recognized connections between a change in the weather and the appearance of epidemic disease. Roman aristocrats retreated to hill resorts each summer to avoid malaria.”
Mosquitos, particularly the Anopheles species, which transmits malaria, require temperatures above 16°C to complete their life cycles. Process-based (mathematical) models show that temperature increases of 2-3ºC would increase the number of people at risk of contracting malaria by 3-5%—a major effect, considering that in 2013 an estimated 198 million cases of malaria occurred worldwide, and 500,000 people died. In addition, according to the models, the seasonal duration of malaria would increase in many endemic areas. Thus, rising temperatures could elevate malaria as a great public health concern.
However, a few years ago, Kevin Lafferty, a USGS research ecologist, wrote: “Although the globe is significantly warmer than it was a century ago, there is little evidence that climate change has already favored infectious diseases. While initial projections suggested dramatic future increases in the geographic range of infectious diseases, recent models predict range shifts in disease distributions, with little net increase in area. Many factors can affect infectious disease, and some may overshadow the effects of climate.”
Due to the current uncertainty of the climate change-infectious disease relationship, it is not surprising that a new paper published in the journal Philosophical Transactions of the Royal Society B (“Evolution in action: climate change, biodiversity dynamics and emerging infectious disease”) is drawing extensive attention. Daniel Brooks, one of the two authors, said that the appearance of infectious diseases in new places and new hosts, such as West Nile virus and Ebola, is a predictable result of climate change. He warns that humans can expect more such illnesses to emerge in the future, as climate change shifts habitats and brings wildlife, crops, livestock, and humans into contact with pathogens to which they are susceptible but have never been exposed to before.
Indeed, the paper brings up a novel paradigm. According to Brooks and his co-author Eric Hoberg, scientists have long assumed that infectious pathogens don’t quickly jump from one species to another—as they adapt to a specific host species, they become less effective at infecting other species. Brooks calls this phenomenon the “parasite paradox.” Over time, hosts and pathogens become more tightly adapted to one another.
However, Brooks and Hoberg found that such jumps happen more quickly than anticipated. Even pathogens that are highly adapted to one host are able to shift to new ones under the right circumstances. Climate change represents one of these circumstances. For example, as temperatures increase in some regions, many host species move into new habitats along with their pathogens, thus increasing the risk that their pathogens will encounter and infect native hosts. Brooks said that the new hosts are more susceptible to infection and get sicker from it, because they haven’t yet developed resistance.
“It’s not that there’s going to be one ‘Andromeda Strain’ that will wipe everybody out on the planet,” Brooks said, “There are going to be a lot of localized outbreaks putting pressure on medical and veterinary health systems. It will be the death of a thousand cuts.”
It is not difficult to imagine who will be mostly affected by these localized outbreaks: children, the elderly, the sick, and the poor, especially those in regions with weak health infrastructures and in developing countries. What are possible solutions? Maria Neira, WHO Director, Department of Public Health, Environmental and Social Determinants of Health, recommends general actions and others specific for the control of infectious diseases: “Surveillance systems for climate-sensitive infectious diseases like malaria, dengue, and cholera should be fortified. Countries should make better use of early-warning information to predict the onset, intensity and duration of epidemics. Such predictions allow health officials to pre-position medicines and vaccines, which can reduce the death toll.”
However, forecasting appropriate responses poses a significant challenge. It will require the combined efforts of, among others, evolutionary biologists, biomedical scientists, epidemiologists, geoscientists, and social scientists—such combined efforts could eventually lead to the design of multi-faceted, effective approaches to mitigate the effects of climate change on the spread of infectious diseases.