A wild marmot was the likely source of an outbreak of pneumonic plague in China that killed three people, according to the World Health Organization.

"According to the epidemiological investigation, the source of this outbreak was a wild marmot, which had contact with the dog of the index case," the United Nations agency said in a statement Tuesday.

China reported the outbreak Aug. 1. A 32-year-old herdsman was the first person to die from the disease. His 64-year-old father-in-law and a 37-year-old male neighbour have also died.

The herdsman became infected after he was bitten by fleas while burying the dog, the Xinhua news agency quoted professor Wang Hu, director of the Qinghai disease control bureau, as saying.

Pneumonic plague occurs when the lungs become infected by the bacterium Yersinia pestis — the same microbe that causes bubonic plague, an infection of the lymphatic system. Pneumonic plague can spread from person to person through the air, according to the U.S. Centers for Disease Control and Prevention.

It is curable if treated early with antibiotics. The three deaths have been attributed largely to delayed treatment, WHO said.

Nine other, non-fatal cases that tested positive for plague were mainly relatives who attended the funeral of the first victim. One remains in critical condition, one had acute symptoms of fever and cough, and seven are in stable condition.

As of Aug. 6, the local health authority in the town of Ziketan, in northwestern China's Qinghai province, had isolated 332 close contacts for medical observation, the WHO said. The town of 10,000 is about 2,000 kilometres west of Beijing.