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Deforestation May Be Helping to Spread the Plague in Africa

As forests are cleared to make way for farmland, the rodents that transmit the deadly bacteria come in more frequent contact with humans.

by Laura Dattaro
Mar 5 2015, 11:30pm

Image via Reuters

The destruction of forests is known to cause the release of massive amounts of greenhouse gases, destroy critical wildlife habitat, and increase soil erosion, which can lead to deadly floods and landslides.

But converting forests to farmland can also increase the spread of the plague, according to researchers at the University of California at Santa Barbara (UCSB). 

"It pops up every other year or so, and the number of cases per year is quite variable and it's also poorly reported," Hillary Young, an ecologist at UCSB, who led the study, told VICE News. "So we don't have a good sense of the number of cases per year in the region."

'You couldn't really come up with a better strategy for encouraging plague transmission.'

About one percent of the forest in Tanzania is cleared every year, mostly to make room for crops. Young and her team trapped rodents in three parts of northern Tanzania from both forested and converted land and tested them for exposure to the bacteria Yersinia pestis, which causes the plague.

After testing more than 100 rodents from eight species, they found that the prevalence of the bacteria was nearly twice as high in rodents trapped from agricultural areas than in forested areas. Farmed land also had a higher number of rodents present than forested land, and minimal presence of larger mammals.

One rodent in particular dominated in the farmed land, the Natal multimammate mouse, also known as the common African rat, which is known to spread the deadly Lassa fever in West Africa. The mice can produce up to 14 pups per litter and reproduce every two to three months, enabling them to proliferate during crop seasons when the food supply is high.

Converted land had nearly 20 times as many of these rodents than forested land.

"People [support] these populations in their fields by growing corn, and then they bring the corn to their house right after harvest and all the rodents go right to the house," Young told VICE News. "You couldn't really come up with a better strategy for encouraging plague transmission."

Fast-reproducing species tend to do better around humans than ones that live longer and reproduce less frequently. Because they spend so much energy reproducing, they may have weaker immune systems and carry more diseases — which would mean the very animals that do best in human environments are the most likely to make us sick.

It's still a controversial theory among epidemiologists, but may at least in part explain why deforestation increases the risk of diseases like the plague.

Related: The bubonic plague is still killing people in Madagascar

"If that's true, you might expect across all habitats, or at least most habitats, that the species that tend to persist are those that carry a lot of diseases," Young told VICE News.

In the United States, the white-footed mouse, a prime carrier of Lyme disease, has thrived in areas disturbed by humans.

There could be a whole host of other mechanisms at work, says Nicole Gottdenker, a pathologist at the University of Georgia. The simple presence of food could increase rodent populations, providing more hosts for fleas and driving up flea populations.

Minute changes in the land and vegetation could also increase flea survival rates and affect the prevalence of antibodies in the host rodents.

"More of this work should be done over a longer term and larger scale," Gottdenker told VICE News. "Studies like this one are critically important."

Converting land for agricultural use has also been linked to the emergence of Nipah virus, which is carried by fruit bats, in Malaysia and the parasitic disease cryptosporidosis in Europe and North America, according to a 2004 review.

Related: Another albino child is murdered and mutilated in Tanzania

Better information about how deforestation spreads disease could help governments decide where to build roads and other development projects, Young said. But on a local scale, providing information about habits that can reduce the risk of infection, like storing crops further away from the house, may be more effective than trying to stop the conversion of land.

"In this part of the world, food provision is a real problem," Young told VICE News. "There's a big effort in east Africa to convert more land to make the country more food independent. So that's a hard message to sell."

Follow Laura Dattaro on Twitter: @ldattaro