The push to boost food production in East Africa that is accelerating the conversion of natural lands into croplands may be significantly increasing the risk of plague, according to a new study published in the American Journal of Tropical Medicine and Hygiene. 

Researchers studied rodents in northern Tanzania, where over the past few decades croplands have expanded by 70 per cent.

They found that in areas where maize production has been introduced, the number of rodents infested with plague-carrying fleas that can cause human infections nearly doubled compared with the numbers in neighbouring wilderness areas.

Scientists also linked the maize fields to a 20-fold increase in the population of the African rat (Mastomys natalensis) that is a major conduit for plague and a number of other diseases, including deadly Lassa fever — an affliction often mistaken for Ebola — that has become a growing concern in West Africa.

“We found that introducing maize production in natural areas appears to create a perfect storm for plague transmission,” said Hillary Young, a community ecologist at the University of California, and one of the lead authors of the study.

“The presence of the crop as a food source caused a surge in the population of a rat species known to carry plague. Local farmers often then store this harvested corn next to or inside their homes — baiting in the hungry field rats and increasing opportunities for human infection.” 

Plague is an ancient disease caused by the bacterium Yersinia pestis that continues to infect kill people even in the 21st century.

Since 2000, most of the outbreaks of plague have occurred in Africa, particularly the Democratic Republic of Congo and Madagascar off the coast of Mozambique. 

Plague has long been a threat in Tanzania and continues to emerge in sporadic outbreaks. Between 1980 and 2011, for example, there were about 8,490 cases and 675 deaths reported in the country. 

According to WHO, plague is treatable with antibiotics and supportive therapy but, left untreated, it is 30 to 60 per cent fatal. 

In June and July of 2011, shortly after the typical annual peak in human plague cases, investigators studied three areas in northern Tanzania. Mr Young said the area of Tanzania where the study was conducted is a historic hotspot for plague, though so far there has not been an uptick in infections documented in the farming communities studied.

But the researchers believe all of the factors that could cause an outbreak are now in place: A population surge in rodents carrying plague and fleas capable of transmitting plague to humans, coupled with crop storage practices and rat behaviours that could increase contact with humans.