BEIJING, April 20 (Xinhua) -- Chances of a plague outbreak in the quake zone were slim, but the health sector still needed to be prepared for the worst, a specialist with China CDC told Xinhua Tuesday.

"The human transmission of plague in Yushu requires direct contact with the Himalayan marmot, but the marmots won't wake up from hibernation till the end of April," said Ni Daxin, an epidemiologist with the Chinese Center for Disease Control and Prevention (China CDC) in charge of epidemic risk evaluation of the quake zone.

However, he said the Himalayan marmot, or Tibetan snow pig, might wake earlier because of the quake, so the situation must be closely monitored.

The health ministry warned about outbreak of plague immediately after the 7.1-magnitude earthquake struck Yushu last Wednesday, as the region is the origin of the Himalayan marmot plague.

China CDC started assessing the risk of plague on the day of earthquake, as ordered by the ministry.

Yu Dongzheng, a research fellow specialized in epidemic prevention with China CDC, was quoted by the health-ministry-run newspaper as saying, the past five decades witnessed 20 outbreaks of human-to-human transmission of plague in Yushu, the latest in 2004 claiming six lives.

So far, no marmots have been found. As of Tuesday morning, health institutions in the worst-hit Yushu prefecture reported no cases of plague or any other communicable disease, said Ni.

The health ministry Monday issued a notice requiring the epidemic prevention team on the ground to expand monitoring for plague.