Sign of the Apocalypse? Plague Is Back, With a Disturbing TwistIf you thought the film Contagion was frightening, this medical plot twist may scare you even more—because it’s real. Back in November, the island nation of Madagascar confirmed 119 cases of plague, including 40 deaths. But the bad news recently took a disturbing turn: “The fleas that transmit this ancient disease from rats to humans have developed resistance to the first-line insecticide,” Margaret Chan, director-general of the World Health Organization, said in a new report. You probably recognize the infectious disease as the one known as the “Black Death,” which during the 14th century became a devastating epidemic that claimed an estimated 50 million lives throughout Europe, Asia, and Africa. Caused by the bacteria Yersinia pestis, the disease spreads from rodents to humans via infected fleas. Those infected generally develop bubonic plague—exhibiting swollen lymph nodes and flu-like symptoms—or, if it spreads to the lungs, the deadlier advanced form, pneumonic plague. Caught early, antibiotics can effectively treat the disease; left untreated, however, plague kills 30 to 60 percent of those infected.

Steamboats from India carried plague to Madagascar in 1898. What makes the recent outbreak there particularly troubling is that scientists have been warning about insecticide resistance in fleas for years. Plague surveillance in Madagascar was discontinued in 2006 due to a lack of funding, but almost 17 years ago—and just six years after the first-line insecticide was initially used in Madagascar—an article published in the Journal for Emerging Infectious Diseases closed with the admonition, “the increasing resistance of fleas to insecticides have caused much concern.”

A November 2014 study conducted by the health research center Institut Pasteur in Madagascar found conclusive evidence that more than 80 percent of the fleas tested were resistant to Deltamethrin, the insecticide referenced in the WHO report. Out of the 32 flea populations examined, only two demonstrated susceptibility to the insecticide. The report’s authors conclude, “In the…re-emergence of plague…in Madagascar, Deltamethrin is ineffective against fleas. Its use in Madagascar should be stopped and the control program for plague diseases needs to change to another insecticide.”

While the study explains that many factors could contribute to the fleas’ increased resistance to insecticides—including environment, climate, geography, urbanization, and human social and cultural behaviors—the core mechanism at work is natural selection. Each time a population of fleas is treated with insecticide, fleas that by some quirk have a built-in resistance survive and breed to create the next generation of fleas, born genetically resistant to the insecticide that wiped out their parents’ peers. Over time, the insecticide becomes less effective as the flea populations are increasingly comprised of only those with the quirk of DNA that protects against it. To compensate for its lowered levels of efficacy, a higher concentration of the insecticide is often used—which breeds a generation of fleas even more resistant than the last.

For the people of Madagascar, Deltamethrin restistance is a case of déjà vu all over again. Use of the insecticide, a man-made version of a natural insecticide that chrysanthemum flowers produce, began in the 1990s after insects developed resistance to the flea-control chemical being used at the time. Some scientists have hypothesized that fleas’ resistance to Deltamethrin may be a result of the species’ exposure to the old insecticide. More