While the previously known dangers of deforestation are already severe and somewhat grim; less carbon absorption, loss of habitat, biodiversity decreases etc. a new study indicates that it could also be exposing more people to deadly, infectious diseases like the Bubonic plague. While societies of the western world have largely forgotten about the “Black Death”, the plague, transmitted by bites from fleas infected with the bacterium Yersinia pestis, is still endemic to regions of Africa, South America and Asia. The disease is widely rampant amongst wild rodents, but humans can get infected from flea bites. According to the World Health Organization (WHO), worldwide, approximately 1,000 to 2,000 cases of Bubonic plague are reported annually, however they believe incidences occur far more often.
Recently researchers concluded a case study conducted in North Central Tanzania which found that land use change patterns affect the risk of humans contracting the deadly disease. Researchers captured and tested over 100 rodents and their fleas for pathogens. Those caught in agricultural areas were about twice as likely to come up positive for the plague pathogen as those rodents caught in forested conservation areas. Lead author of the study and wildlife epidemiologist from Stanford University states, “What’s interesting, even though it’s a small sample size, is the samples are quite stark.”
The common African rat, Mastomys natalensis, was found to occur twenty times more often on agricultural land than on conservation land and approximately 75% of the species tested positive for the virus. This is the same species responsible for spreading the plague among the humans of Kenya, Mozambique and the Democratic Republic of Congo, according to a WHO physician’s manual. The very competent, X. brasiliensis flea, was also five times more prevalent on agricultural sites studied.
While researchers have a hard time pinpointing one direct cause for the prevalence of plague infected rats on ag-lands, there are a few theories floating around:
The first theory entails the loss of biodiversity on developed land making wildlife populations more susceptible to the “dilution effect”. The dilution effect is defined as the loss of diversity making remaining populations more susceptible to disease. So in an area of high diversity fleas would land on a number of hosts which are not suitable disease vectors, because they quickly succumb to illness, this weeds out the incompetent vectors leaving a population with lower diversity but competent hosts. Most ecologists and even research team members agree that while this can be a contributing factor, it is overly simplistic to be the total solution.
Another theory also relies somewhat on biodiversity loss or reduction. When deforestation occurs, large mammals and predator populations decline greatly, allowing the rodent populations to grow basically unchecked. African rats have litters of about 14 pups each time, combined with the lack of predators and easy access to food in and around croplands population explosions can only be expected. Additionally, at the Tanzanian farms studied, farmers stored their harvests very close to living quarters, further increasing the risk of exposure and infection from flea bites. Co-author of the study and community ecologist at the University of California, Santa Barbara, Hillary Young adds, “The bigger point is that rodents seem to increase in disturbed habitats, the main driver of which probably varies between predators and food.”
Unfortunately Tanzanian rats appear to be a competent reservoir for plague and land use changes are only going to increase as the population climbs in the future. But these changes may have different effects elsewhere. Although interesting, and a little frightening, uncertainty abounds and this study appears to uncover many more questions; more research needs to be conducted on a larger scale. There are already many, many reasons to slow and stop deforestation, but sometimes an angle which directly affects the human population in concrete ways, visible right now can be the most persuasive.
, , , , , , , , , and Original Article: Effects of Land Use on Plague (Yersinia pestis) Activity in Rodents in Tanzania Am J Trop Med Hyg 14-0504; Published online February 23, 2015,doi:10.4269/ajtmh.14-0504
