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Study: Rats Pose Plague Risk in Croplands

Conversion of forests to agricultural use create starkly different habitats for small mammals that carry zoonotic disease such as plague, resulting in increased risk of plague transmission. (Photo Credit: Douglas McCauley)
Conversion of forests to agricultural use create starkly different habitats for small mammals that carry zoonotic disease such as plague, resulting in increased risk of plague transmission. (Photo Credit: Douglas McCauley)

Efforts to boost food production in East Africa may increase the risk of plague. A new study says the conversion of natural lands into croplands in Tanzania has been accompanied by a large increase in the rodent population. The rodents are often infested with plague-carrying fleas.

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Dr. Kristofer Helgen, curator in charge of mammology at the Museum of Natural History in Washington – part of the Smithsonian Institution, says plague is an ancient disease caused by a bacterium called Yersinia pestis.

“It captures the imagination when people hear about plague or the bubonic plague. I think we are instantly are transported to sometime in our imagination, distant in the past, where we think about the Black Death in European history," he said. "It maybe surprises people to learn that plague remains a public health concern in various parts of the world, especially in Africa south of the Sahara.”

Helgen said in recent decades croplands have expanded by 70 percent in northern Tanzania. In areas where maize was planted, the rodent population doubled compared to neighboring land that had not been cultivated.

“Plague circulates in small mammals and rodents. And when the conditions are right it’s a disease that can spread to people and it can still be a deadly disease," Helgen said. "Now, plague is a disease that is treatable with antibiotics. And in that regard, it’s considerably maybe less scary, less alarming than other emerging diseases like Ebola. But we should remember that this is a disease that is still on the ground in Africa and remains a public health concern.”

Helgen said that animal populations vary according to land use.

“As natural conserved lands in parts of East Africa are converted to human-use habitats, specifically agriculture -- we’re especially studying crop fields of corn – not surprisingly the animals that live of those landscapes are changing, too. And one of the most profound changes is the rodents that live in those landscapes,” he said.

Natural land has certain kinds of rodents, while farmland attracts others.

“When we study a conserved landscape in the area we see a particular community of rodents -- quite a few different species doing different things for a living, Helgen said. "When we study crop fields right next to that space what we see if that we have a very different community of rodents. And, in fact, in comes to be dominated by perhaps one species in particular that is very good at living in these human modified landscapes. That animal is called the African rat, or sometimes we call it the multimamate [it can feed 12 baby rats at one time] rate.”

“As agriculture expands it’s a perfect habitat for this animal. Population numbers can breed-up very rapidly along the front of moving and expanding croplands," he continued. "And this is an animal that we often will call a commensal rodent species. And literally that word commensal means ‘eating from the same table.’ In other words, the notion that this is a rat that does go into people’s houses and villages and interacts with people in their living spaces.”

There are thousands of species and they are very successful, including the African rat.

“This species that we’re talking about today and others have huge, huge impacts on people’s lives as crop pests – consuming people’s food and their livelihood – and as animals that spread disease. Plague is just one of many different diseases that are rodent-borne in Africa and some are even, you know, more scary and more deadly,” he said.

One of them is Lassa fever, which can be mistaken for Ebola.

Also, the rats living in agricultural areas carry a larger number of plague-carrying fleas than rats in the forest. They’re also infested with types of fleas not found on forest rats.

The World Health Organization and the U.N. Food and Agriculture Organization are concerned about such rat populations and their possible effect on future food security.

The study was published online in the American Journal of Tropical Medicine and Hygiene.