Announcement of one of worst outbreaks in years raises fears that disease could spread to towns and cities

Once feared as the Black Death – the rodent-borne disease that wiped out a third of the world's population in the Middle Ages – bubonic plague has killed 20 villagers in Madagascar in one of the worst outbreaks globally in recent years, health experts have confirmed.

The confirmation that bubonic plague was responsible for the deaths last week near the north-western town of Mandritsara follows a warning in October from the International Committee of the Red Cross (ICRC) that the island nation was at risk of a plague epidemic.

The Pasteur Institute of Madagascar revealed on Tuesday that tests taken from bodies in the village last week showed that they had died of bubonic plague. The institute added it was concerned the disease could spread to towns and cities where living standards have declined since a coup in 2009.

The deaths are doubly concerning because the outbreak occurred both outside the island's normal plague season, which runs from July to October, and apparently at a far lower elevation than usual – suggesting it might be spreading.

Bubonic plague, which has disappeared from Europe and large parts of the globe, is spread by bites from plague-carrying rat fleas – Xenopsylla cheopis – whose main host is the black rat. In Europe the threat of the Black Death pandemic, which appeared with black rats brought by merchant ships from Asia, eventually died out as black rats were displaced by brown rats and health and hygiene improved.

Victims often develop painful swelling in the lymph nodes called buboes, flu-like symptoms and gangrene. Although the disease is treatable with antibiotics, without treatment the mortality rate is almost two-thirds of those infected, according to the US Centres for Disease Control.

Last year about 60 people died of plague in Madagascar – the highest number globally. The disease is prevalent in the island's central highlands, where between 200 and 400 confirmed cases are reported each year to the World Health Organisation – between a third and a fifth of globally reported cases.

The disease first appeared in Madagascar in 1898 and was responsible for successive outbreaks until the 1920s, largely confined to the island's ports. While it disappeared from coastal areas it spread to inland areas above an altitude of 800 metres. However, there was no serious outbreak for 60 years until 1991, when it appeared again around the coastal town of Mahajanga.

After a series of outbreaks that lasted until 1998, health experts warned almost 10 years ago that plague appeared to be spreading again to lower altitudes with men and children most susceptible.

The risk of the disease has increased, say experts, amid increasing poverty and insanitary conditions on the island, not least in its overcrowded prisons. Prisoners are usually most affected by outbreaks.

Following the warning from the ICRC in October, the organisation and Madagascan prison authorities launched a campaign against rodents in Antanimora prison in the capital, Antananarivo, where 3,000 people are behind bars, to reduce the risk of the disease spreading.

Christoph Vogt, head of the ICRC delegation in Madagascar, told the Guardian then: "The chronic overcrowding and the unhygienic conditions in prisons can bring on new cases of the disease. That's dangerous not only for the inmates but also for the population in general.

"Rat control is essential for preventing the plague, because rodents spread the bacillus to fleas that can then infect humans. So the relatives of a detainee can pick up the disease on a visit to the prison, and a released detainee returning to his community without having been treated can also spread the disease."