Deforestation, Zika and Other Deadly Diseases

The emergence of the Zika virus and its rapid spread are adding increased relevance to a number of studies of the impact of deforestation on the spread of animal borne disease to humans.  Several factors are involved, but there is no question that the widespread destruction of forests is contributing to the likelihood and increase in the spread of diseases such as Ebola and Zika, as well as increasing the prevalence of malaria, dengue fever and a variety of others.  From the Smithsonian Magazine:

"The idea was that something fundamental is going on in this era that is driving all these pandemics," says Peter Daszak, who has studied wildlife and human disease for more than two decades, "but no one was bringing the whole thing together."

Now, a series of studies, built upon research over the past two decades, provides increasing evidence that the loss of forest creates the conditions for a wide range of deadly diseases to jump from animals to humans.

Throughout history, diseases have moved from forests into humans through animal carriers. But the increasing proximity of humans to recently deforested areas magnifies the risk.

With deforestation continuing around the world, the implications of studies such as this become even more frightening.

Research in the late 1990s into deforestation and malaria in the Peruvian Amazon by Amy Vittor, now an assistant professor of medicine at the University of Florida, first sounded the alarm.

Clearing forests for agriculture increases sunlight exposure and often disrupts small streams, creating pools of warm water perfect for mosquito breeding.

Eventually, farming becomes unsustainable as the land becomes infertile and people depart, abandoning land to low-lying shrubbery, also conducive to mosquito breeding.   

Research by Vittor and others show that the malaria-carrying species in a deforested area of Peru bit 278 times more frequently than the same species in an untouched forest.

It doesn’t take much to tip the scales. 

Even small decreases in forest cover increase malaria exposure. Cutting four percent of a forest in Brazil, according to a 2010 study, was associated with a nearly 50 percent increase in human malaria cases.

The global spread of the Zika virus is another example of the impact of human interaction both in driving climate change and then becoming victims of the results of climate change.

The Zika virus, the cause of birth defects in Brazil, is another example. It emerged in mosquitoes in the Zika forest of Uganda in the 1940s, but there were few human cases until 2007. Aedes aegypti, the mosquito species that carries Zika and many other diseases, spread first to Asia where it likely mutated, then gained a foothold in the Brazilian Amazon, thanks to global travel. There, the mosquitoes carrying the disease flourished in the heat of places like Recife, a Zika hotspot and a city that had its hottest three months on record late last year.

Deforestation there has contributed to a record drought in Brazil, which leads to more people storing water in open containers. That leads to a rise in the mosquito population. Too, when temperatures go up, mosquitoes require more blood so they feed more often and reproduce faster.

It isn’t just mosquitoes.  Deforestation is creating more opportunities for disease carrying bats, rats, wild dogs, even snails to spread a wide variety of diseases to humans.  Destruction of habitat weakens these populations increasing the number of diseased members or depletes their natural predators.  Reduction of habitat puts these disease vectors in closer proximity to humans.  The result isn’t beneficial.

The snails that carry flatworms that cause schistosomiasis prosper in warm, open areas created by deforestation. A 2015 Lancet Commission study concluded there is "circumstantial" evidence that changes in land use increased the likelihood of Ebola outbreaks. The prevalence of hantavirus, which can have a mortality rate as high as 30 percent, has increased in rodent populations in areas of Panama disturbed by human activity.

The nipah virus, a neurological disease with no known cure, emerged in the late 1990s in Malaysia in the aftermath of slashing and burning to create pig farms. Bats ate fruit in nearby orchards. Pigs ate the mangoes in those orchards and the virus made its way into humans. In the initial outbreak, 257 people were infected, killing 105.

Deforestation continues across the world directly impacting climate change.  We now can add the potential for increased epidemics of once rare diseases to the cost of deforestation.