There's already ample evidence of the ways environmental degradation can contribute to the spread of infectious diseases, and now a recent study provides an example of how the disruptions to an ecosystem caused by deforestation and other land-use change can help spread a bacterial pathogen.
There are about 250 known human emerging infectious diseases, which are those that have recently appeared within a human population or those with an incidence rate or geographic range that is rapidly increasing. Rapid urbanization in developing countries and human population encroachment are known to help drive outbreaks of emerging infectious diseases.
Mosquitoes that transmit the Zika virus, for instance, thrive in artificial habitats created by humans, including urban waste such as uncollected garbage piles or old cans, barrels, and tires that collect water and provide a breeding ground for the insects.
As noted by the authors of a recent study in the journal Science Advances, many of the known emerging infectious diseases have been traced to tropical rainforests, especially freshwater aquatic systems. The study shows that the collapse of a freshwater food-web driven by deforestation and land-use change led to the increased prevalence of a bacterial pathogen called Mycobacterium ulcerans, which causes a skin disease in humans known as Buruli ulcer.
According to the World Health Organization, Buruli ulcer has been reported in 33 countries in Africa, the Americas, Asia, and the Western Pacific. Treating the disease involves a complex process that can involve surgery and skin grafting, but even then Buruli ulcer frequently results in long-term disability. And since we have little knowledge of how Buruli ulcer is transmitted, preventive measures are lacking.
But the recent Science Advances study sheds new light on exactly how the bacteria that causes Buruli ulcer spreads and comes into contact with humans.
For the study, a team of researchers led by Aaron Morris of Imperial College London collected more than 3600 invertebrate and fish specimens from 17 freshwater sites in French Guiana, a South American nation where Buruli ulcer is endemic. They discovered that the highest concentrations of M. ulcerans appeared in species lower on the food chain, such as bottom-feeding fish and insects.
Morris and team also examined each collection site to determine how much deforestation or other environmental degradation had occurred. The sites exhibited a range of impacts due to land-use change: some were fully urbanized, while some had been converted to agricultural land and others were still pristine rainforest.
What the researchers discovered was that, up to a point, environmental degradation caused population declines amongst large predators and other species atop the food chain, which in turn meant less predation on lower-level organisms, allowing them to multiply relatively unchecked. And because those organisms are the preferred hosts for M. ulcerans, that allowed the bacteria to become more abundant, as well.
The more deforestation and land-use change that had occurred, however, the more the population density of even these lower-level hosts dropped, suggesting there is a certain range of environmental degradation in which M. ulcerans can be expected to thrive. "There's sort of this sweet spot where you get a lot of the organisms that carry the bacteria, but as you progress to large levels of habitat destruction, then they sort of decline again," Morris told The Washington Post.
Morris and his co-authors write that their findings could be applicable elsewhere around the globe. "As urbanization, agriculture, and deforestation intensify, notably in tropical regions of the world (which also hosts the largest number of developing countries on the planet), similar trends for other [emerging infectious diseases], as observed here for M. ulcerans, may become apparent."
But they caution that, although their study identified a potential mechanism for the outbreak of a specific emerging disease, the "generalizability" of their results still requires further testing in order to be more definitively established.
"Because human populations have greater contact with land-use change generated by global urbanization, it is not unreasonable to predict that emergence and reemergence of infectious diseases will become more common," they write. "We therefore recommend that more empirical evidence must be sought for mechanisms related to disease emergence in areas of anthropogenic disturbance, notably in tropical regions of the world where biodiversity (at both micro- and macroscales) can be high."