Over the past few decades, thousands of acres of wildlands in East Africa have slowly been converted into farmlands, largely as an effort to end the threat of food insecurity. While this mostly has had a positive impact, providing more reliable sources of food and income for locals, a new study shows it's also led to an increased risk of plague for the people who work and live there. It brings to light a challenging intersection of public health, food security, and ecology.
In order to assess whether the landscape changes had any effect on plague-carrying rodents, researchers honed in on an area in northern Tanzania, where agricultural lands have expanded by 70 percent in the last few decades.
They discovered that the species of rodent most likely to be infected with plague was almost 20 times more abundant in the agricultural areas as it was in the wilderness, and plague-infected rodents were nearly twice as common on farms, according to their study published today in the American Journal of Tropical Medicine and Hygiene.
"It's not numbers of rodents, per se, but it's more that in the corn fields you have a particular rat that is more common and more likely to have plague," explained Dan Salkeld, a wildlife epidemiologist at Colorado State University and one of the study's authors.
Salkeld said there are a number of factors that might be contributing to the phenomenon.
"Some rodents are not quite as resilient so if you remove the natural vegetation, they disappear with it. This rat might be more resilient," Salkeld said. "We found very, very few of these rats in the undisturbed forests so it does seem that they like something about what people are doing to the fields—which might have something to do with the massive amounts of corn."
The study found the rats carried a higher number of plague-infectious fleas, including a species of flea that wasn't even found on the wild forest rodents. The likelihood of transmission to humans is also increased, the study reads, because not only are there more plague-carrying rodents, but also there are more people present on farms than there are in the conservation areas. However, the study could not determine whether this had led to an increase in plague-infected people.
"We don't have data on the human plague cases. We don't know when, particularly, they're getting plague or how often. These are areas where human health data are not always fantastic," Salkeld explained. "The next step is obviously to tighten in on when people get plague and how and from where."
While the plague has been largely eradicated in developed nations, there are still deadly plague outbreaks in much of East Africa. From 2000-2009, Tanzania had 290 reported cases of plague, Congo had more than 10,000, and a recent outbreak in Madagascar killed at least 40 people.
So how do we weigh the need for food security with the need to prevent deadly outbreaks of infectious diseases?
"That is always the conundrum. You do this research and then realize, 'oh my goodness, look at the size of this issue,'" Salkeld said. "Food is obviously important but in doing so you bring these unintended consequences and you have to mitigate against that whether it's through better education, better food storage, or better land management."
But Salkeld said continued research could hold the key, especially if it's focused on the factors that lead to an outbreak. Too often research doesn't begin until after an outbreak, he said, like the scramble to slow the devastating Ebola outbreak in West Africa. While we now understand the increased risk of plague in these farmlands, it's still unclear how the Ebola outbreak originated.
"When it gets into the human system and it's spreading from person to person then the response has to be 'how do we stop it?' But the question behind that is 'how can we stop it getting into people in the first place?'" Salked said.
"The question about 'what does one do?' is the big one and the one that makes me feel the most uncomfortable," he added. "But this study shows that you should be trying to treat the underlying causes."