Fifteen milus, or Pere David's deer, have recently been found dead from unknown causes in a nature reserve in north China's Hebei Province.

Fifteen out of the 20 milu deer in the Luanhe Upper River National Nature Reserve have died since last August, when the body of an adult male deer was discovered, said Yuan Minlong, head of the reserve.

"The first was believed to be bitten by a snake, as the chest and mouth of the body was injured and swollen," he said.

The skeleton of a deer calf was found last September. Another deer calf's body was found in January with severe injuries on its waist, which was suspected to be caused by collision with other deer.

"We found another body of an adult female milu in March and found blood and swelling in its internal organs after dissection," Yuan said. "After that, 11 milus died and most of them were adults in their prime and had swollen waist and blood on their mouths and anuses."

Samples of the milus bodies have been sent to animal quarantine authorities in Beijing for research and analysis, he said.

Among all five surviving milu deer, one was born in 2013 and four were born last year, he said.

"There are still 100 sika deer and a dozen red deer, all in good health, but it has been so unfortunate for the milus," Milu keeper Wang Zhen said.

The milu is a species of deer unique to China. It has antlers with a main branched anterior segment, with the points extending backwards. The deer's summer coat is reddish tan in color and becomes woollier and dark grey in the winter.

Overhunting and the loss of its habitat led to the near extinction of the species in the early 20th century in China. The species were reintroduced to China from Britain in the 1980s.