A probable case of plague in the Seychelles, imported from Madagascar, is believed to have sparked the Indian Ocean country’s first outbreak of the disease, the World Health Organization (WHO) said.

Plague, which is mainly spread by flea-carrying rats, is endemic in Madagascar. A large outbreak has killed 57 people since late August, according to the U.N. agency, the first time the disease has appeared in non-endemic urban areas, including in the capital Antananarivo.

Seychelles health authorities reported a probable case of pneumonic plague on Oct 10 in a 34-year-old man returning from a visit to Madagascar, the WHO said.

“The patient continues to be hospitalized in isolation until completion of the antibiotic treatment. He is currently asymptomatic and in stable condition,” the WHO said.

Nearly 70 percent of cases in Madagascar have been pneumonic plague, a form spread human-to-human that is more dangerous than bubonic plague and can trigger epidemics. The pneumonic form invades the lungs, and is treatable with antibiotics. If not treated, it is always fatal and can kill a person within 24 hours.

An initial diagnostic test on the Seychelles man had been “weakly positive” for pneumonic plague, but definitive laboratory results are expected from the Institute Pasteur in Paris, the WHO said.

Eight of his contacts “developed mild symptoms and have been isolated,” it said, adding that two other suspected cases, without any known link, had been isolated and were on treatment.

In all, 11 close contacts and one foreign national, who was not exposed to the probable case but recently arrived from Madagascar, are hospitalized as a precautionary measure but do not present signs of respiratory distress, the WHO said.

Nearly 1,000 people who were exposed to the man or his close circle were given a prophylactic course of antibiotics to prevent infection and spread, it said. They included children, teachers, passengers, flight crew and health workers.

Seychelles, which has 93,000 people, largely depends on tourism for government revenues.

“Air Seychelles flights to-from Madagascar were stopped from 8 October to reduce likelihood of further importation of cases from Madagascar,” the WHO said. However, the WHO did not recommend restrictions on travel or trade.

The Madagascar health ministry has implemented exit screening for passengers at Antananarivo airport to prevent international spread, it said.

“The risk of further spread in the Seychelles (should the case be confirmed) is considered as low and the overall regional and global risk levels as very low,” the WHO said. REUTERS