GENEVA (Reuters) - An outbreak of the plague has killed 40 people out of 119 confirmed cases in Madagascar since late August and there is a risk of the disease spreading rapidly in the capital, the World Health Organization (WHO) said on Friday.

So far two cases and one death have been recorded in the capital Antananarivo but those figures could climb quickly due to "the city's high population density and the weakness of the healthcare system", the WHO warned.

"The situation is further complicated by the high level of resistance to deltamethrin (an insecticide used to control fleas) that has been observed in the country," it added.

Plague, a bacterial disease, is mainly spread from one rodent to another by fleas. Humans bitten by an infected flea usually develop a bubonic form of plague, which swells the lymph node and can be treated with antibiotics, the WHO said.

If the bacteria reach the lungs, the patient develops pneumonia (pneumonic plague), which is transmissible from person to person through infected droplets spread by coughing. It is "one of the most deadly infectious diseases" and can kill people within 24 hours. Two percent of the cases reported in Madagascar so far have been pneumonic, it added.

The first known case of the plague was a man from Soamahatamana village in the district of Tsiroanomandidy, identified on Aug. 31. He died on Sept. 3 and authorities notified the WHO of the outbreak on Nov. 4, the agency said.

The WHO said it did not recommend any trade or travel restrictions based on the information available about the outbreak.

(Reporting by Stephanie Nebehay; Editing by Janet Lawrence and Hugh Lawson)