Converting wilderness areas into farmland in East Africa may be increasing the risk of a bubonic plague or Lassa fever epidemic, scientists have warned.

This follows publication of a study showing rodents crawling with plague-carrying fleas are being drawn to harvested food in the region.

In northern Tanzania, crop lands have expanded by 70 per cent over the last few decades. The number of plague-carrying rodents in these lands has risen dramatically compared with neighbouring wilderness areas, said the study published in the American Journal of Tropical Medicine and Hygiene on Monday.

Scientists linked an increase in maize fields — necessary to feed an expanding population — to a 20-fold rise in the population of African rats in northern Tanzania, which transmit deadly diseases to humans.

"We found that introducing maize production in natural areas appears to create a perfect storm for plague transmission," Hillary Young, a University of California professor and a lead author of the study, said in a statement.

"Local farmers often... store this harvested corn next to or inside their homes — baiting in the hungry field rats and increasing opportunities for human infection."

In Tanzania alone, plague caused about 675 deaths from 1980 to 2011, the study said, and these numbers could rise as new wilderness areas become farmland and rat populations increase.

A rare but serious bacterial infection, bubonic plague (Black Death) wiped out a third or more of Europe’s population in the 14th century. While many people assume it has died out completely, experts say that Africa — especially Madagascar and the Democratic Republic of Congo — accounts for more than 90 per cent of cases worldwide today.

Though less deadly than Ebola or other epidemics, plague — caused by the bacterium Yersinia pestis — is fatal in more than 30 per cent of cases if left untreated, the study said.

As Africa's population soars, and food demand increases, scientists, farmers and politicians will have to balance the need for more farmland with concerns over the spread of disease, scientists said. -- Additional reporting by BDAfrica.com