The health ministry said there had been 138 suspected cases since the beginning of the year, and warned the death toll was likely to rise in coming months.
Two people have been infected in Antananarivo, one of them dying, and health workers have mounted a pest control campaign through slum areas around the city, the World Health Organization (WHO) said.
The health ministry's secretary general, Philemon Tafangy, said "two hundred households have been disinfected" this month.
He said those who had contact with the infected had been given antibiotics in a bid to arrest the spread the disease.
The WHO last week said 40 people had died as a result of plague, which was first identified in August.
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 situation in Madagascar is all the more worrying because of a high level of resistance to insecticides targeting fleas, the UN health agency said.
In Ankasina, a slum outside Antananarivo, the family of the young woman who died from the plague said they have been stigmatised by the community.
Residents of the poor and overcrowded slum speak of squalid conditions, infested with rats, increasing the risk of infection.
"Our neighbourhood is really dirty and has been neglected by the state for a long time," she said.
Plague often breaks out in the vast island nation, and is usually at its worst between October and March.
The last case of plague in the capital was 10 years ago, said Christophe Rogier, of the island's Institut Pasteur.
"It is possible that the plague continued to survive in Antananarivo for 10 years without touching humans," with the virus restricted to its rat population, he said. "Rats are a natural reservoir of the plague, and they also survive the plague."
According to the International Committee of the Red Cross, the country has recorded on average 500 cases of plague every year since 2009.
The Black Death, otherwise known as the bubonic plague, is estimated to have killed some 25 million people across Europe in the Middle Ages.