Researchers identified traces of the H10N5 bird flu virus in environmental samples linked to the world's first human case, which was reported in China in January, but no human-to-human transmission occurred, according to a study released last week.

The study said the infection most likely stemmed from long-distance transmission through wild birds, and called for enhanced surveillance of bird flu among poultry.

The National Disease Control and Prevention Administration reported on Jan 30 that East China's Zhejiang province had confirmed a case of combined infection of H10N5 bird flu and the H3N2 seasonal flu strain in a 63-year-old woman from neighboring Anhui province. It was the world's first human infection with H10N5, but H3N2 cases are relatively common.

The woman had high blood pressure, diabetes, heart failure and other chronic diseases. She began exhibiting coughing, with a sore throat and fever, in Anhui on Nov 30 and died in Zhejiang on Dec 16.

According to the study, released by disease control center workers and public health researchers in Anhui province, all of the 30 close contacts with the case, including medical staff, her neighbors and family members, tested negative for H10N5.

Research shows that the patient slaughtered a live duck at home on Nov 26. A total of 189 samples from locations and objects linked to the patient, including her home, supermarkets, mobile vendors and the poultry market, were collected, and some returned positive results, including frozen duck meat from her home refrigerator, a mobile vendor and a wholesale factory.

However, there was no evidence of H10N5 in the two farms that were the sources of the live ducks, "suggesting that the virus might have been introduced or transmitted at later stages", the study said.