Some of the large-scale changes humans are causing to the planet—including climate change, biodiversity loss, and the spread of invasive species—are making infectious diseases more dangerous to people, animals, and plants. A new study.
Scientists have previously documented these effects in more targeted studies of specific diseases and ecosystems.For example, they found a Climate warming may lead to the spread of malaria Declining wildlife diversity in Africa could increase Lyme disease cases in North America.
But the new study, a meta-analysis of nearly 1,000 previous studies, shows that these patterns are relatively consistent across the globe and across the tree of life.
“This is a big advance in science,” said Georgetown University biologist Colin Carlson, who was not an author of the new analysis. “I think this paper is one of the strongest pieces of evidence that has been published about how important it is for health systems to start preparing to survive in a world of climate change and biodiversity loss.”
The researchers also found that urbanization reduces the risk of infectious diseases, which may be a more surprising finding.
The new analysis, published Wednesday in the journal Nature, focuses on five “global change drivers” that are transforming ecosystems around the world: changes in biodiversity, climate change, chemical pollution, the introduction of non-native species, and habitat loss or Variety.
The researchers collected data from scientific papers that examined how at least one of these factors affects outcomes, such as severity or prevalence, of various infectious diseases. The final data set includes nearly 3,000 observations of human, animal and plant disease risk on every continent except Antarctica.
The researchers found that overall, four of the five trends they studied — changes in biodiversity, the introduction of new species, climate change and chemical pollution — tended to increase disease risk.
“This means we may be discovering general biological patterns,” said Jason Rohr, an infectious disease ecologist at the University of Notre Dame and senior author of the study. “This suggests that similar mechanisms and processes may exist in plants, animals and humans.”
Researchers found that biodiversity loss plays a particularly important role in increasing disease risk. Many scientists believe that biodiversity protects against disease through a phenomenon called the dilution effect.
The theory is that parasites and pathogens that rely on abundant hosts to survive will evolve to favor common species over rare ones, Dr. Rolle said. As biodiversity declines, rare species tend to be the first to disappear. “That means the remaining species are capable and are the ones that are really good at spreading disease,” he said.
Lyme disease is one of the often cited examples. Dr Rolle said white-footed mice were the main host of the disease and had become more dominant in the landscape as other rare mammals disappeared. This shift may partially explain the rising incidence of Lyme disease in the United States. (The extent to which the dilution effect contributes to Lyme disease risk has been a subject of debate, and other factors, including climate change, may also play a role.)
Other environmental changes may amplify disease risk in a variety of ways. For example, introduced species may introduce new pathogens, while chemical contamination may stress an organism’s immune system.Climate change can alter animal movements and habitats, bringing new species into contact and allowing them to exchange pathogens.
Notably, the fifth type of global environmental change the researchers studied—habitat loss or change—appeared to reduce disease risk. At first glance, the findings appear inconsistent with previous research, which has shown that deforestation increases the risk of diseases such as malaria and Ebola. But the overall trend toward lower risk is driven by one specific type of habitat change: increased urbanization.
The reason may be that urban areas generally have better sanitation and public health infrastructure than rural areas, or simply because urban areas have fewer plants and animals that serve as reservoirs for disease. The lack of plant and animal life “is not a good thing,” Dr. Carlson said. “It also doesn’t mean animals in cities are healthier.”
The new study doesn’t dismiss the idea that forest loss exacerbates disease. Conversely, deforestation increases risk in some cases and decreases it in others, Dr. Rohr said.
In fact, Dr. Carlson noted that while this meta-analysis is valuable for revealing broad patterns, it may obscure some of the nuances and exceptions that are important for managing specific diseases and ecosystems.
Furthermore, most of the studies included in the analysis examined only a single global driver of change. However, in the real world, organisms are dealing with many of these stressors simultaneously. “The next step is to better understand how they are connected,” Dr. Rohr said.
