The head of the World Health Organization is warning about an “alarming” plague outbreak in Madagascar that could worsen, particularly as fleas that transmit the disease to humans have developed immunity to insecticide.

In her address to the WHO’s executive board meeting Monday, executive director Margaret Chan reviewed the ongoing threat from “emerging and epidemic-prone diseases,” including the Ebola outbreak in West Africa, avian influenza, and the growing threat of human resistance to antibiotics.

But she also warned of a plague outbreak in Madagascar that began last November, but is receiving little public attention despite its “disturbing dimensions.” Of particular concern is the fact that fleas that transmit the disease from rodents to humans have developed resistance to the main insecticide used to control their population.

Plague is “endemic in Madagascar,” where seasonal outbreaks can be exacerbated by poverty and an increasing number of people living in close urban settings, Chan said in her address. Plague responds well to treatment when detected early, which is possible with a “cheap and reliable” diagnostic test that provides results within 15 minutes.

However, the current outbreak has managed to establish a foothold in the capital Antananarivo, and could spread easily through the city’s densely populated slums, Chan warned.

“This is alarming, as around eight per cent of cases progress to the lethal pneumonic form, which transmits directly from person to person,” she said.

A recent tropical storm has exacerbated the threat of the disease spreading from “untold numbers of rats” that have been displaced by flooding.

Earlier this month, the International Society for Infectious Diseases (ISID) issued a statement to say that the plague epidemic had reached Madagascar’s capital with 10 suspected cases.

“The progress of the plague epidemic becomes increasingly worrying,” the ISID said. By the end of December, there were 224 cases of plague reported on the Big Island, with 58 deaths.

While bubonic plague had largely disappeared from Madagascar between 1930 and 1990, approximately 200 cases have been reported each year since then and “takes on epidemic form especially in the port of Mahajanga each year,” the ISID says.

According to the agency, at the start of each rainy season, rats flee Madagascar’s sewers “in massive numbers and take refuge in people’s cottages.”

A flea-killing compound must be part of any rodent eradication program, the agency says, or the fleas will merely seek alternate hosts. “That is, humans.”

Despite growing concerns about the outbreak, the WHO has not issued any travel or trade restrictions with Madagascar.