Geneva (AFP) - The plague has killed 71 people in Madagascar since September, the World Health Organization said Wednesday, adding that the outbreak was slowing but still posed a threat.

Since the outbreak began in September, 263 people have become infected and 71 have died, the UN health agency said, reporting the findings in a probe it conducted with the country's health ministry.

WHO said the outbreak had peaked from November through the end of December and had slowed "for the time being."

"However, the plague season on the island continues until April," it warned, adding that health officials on the island were "keeping a close watch over the situation."

The district of Amparafavarola in the central highlands has been the most heavily affected, with continued reports of cases during the first week of January, WHO said.

Thirteen cases had been reported in the slum areas of the capital city Antananarivo through the end of December, it said, urging "heightened vigilance".

Plague is spread by fleas and mostly affects rats, but humans can also contract the disease if they are bitten by a disease-carrying flea.

The bubonic form prompts swelling of the lymph node, but can be treated with antibiotics. The pneumonic version, affecting the lungs, can be spread from person to person through coughing and can kill within 24 hours.

The ongoing outbreak has a fatality rate of 27 percent, WHO said, adding that the investigation showed early detection and treatment could significantly improve the chances of survival.

Plague is endemic on Madagascar, where outbreaks have resurfaced nearly every year since 1980.

WHO warned that over the past three years the annual number of cases had steadily increased, making the island nation the country in the world most affected by plague.

The situation in Madagascar is all the more worrying because of a high level of resistance to insecticides targeting fleas, the UN health agency said.

There have been three pandemics of the plague since the Middle Ages, inflicting an estimated death toll of 200 million people.

In the 14th century, the disease, also known as the Black Death, wiped out an estimated third of Europe's population.