BEIJING (AP) Scientists taking a first look at the genetics of a bird flu strain that has killed three people in China said Wednesday that the virus could be harder to track than its better-known cousin H5N1 because it might be able to spread among poultry without showing any signs.

The scientists, at several research institutes around the world, urged Chinese veterinary authorities to widely test animals and birds in affected regions to quickly detect and eliminate the H7N9 virus before it becomes widespread.

They said the virus is troubling because it can infect poultry without producing any symptoms, while seriously sickening humans. The virus, previously known to have infected only birds, appears to have mutated, enabling it to more easily infect other animals, including pigs, which could serve as hosts and spread the virus more widely among humans, they said.

The findings are preliminary and need further testing.

China over the weekend reported two deaths in Shanghai in the strain's first known infections of humans. On Wednesday it announced an additional fatality a 38-year-old cook working in Jiangsu province, where other cases also have been reported.

The cook went home to Hangzhou in Zhejiang province for treatment after falling ill in early March, and died March 27.

One other person in Hangzhou, a 67-year-old retiree, was in critical condition, the official Xinhua News Agency reported, bringing the number of seriously ill H7N9 patients in three eastern provinces to six. Those regions stepped up measures this week to guard against the spread of the disease, calling on hospitals to report severe pneumonia cases with unknown causes and schools to monitor for fevers.

In the wake of the outbreak, the Chinese Center for Disease Control and Prevention shared the genetic sequence of the new virus with the global health community. The data allow scientists to make preliminary interpretations of how the virus might behave in different animals and situations. Such hypotheses, while not conclusive, can help provide important early warnings to authorities dealing with the disease.

The scientists said that based on information from the genetic data and Chinese lab testing, the H7N9 virus appears to infect some birds without causing any noticeable symptoms. Without obvious outbreaks of dying chickens or birds to focus efforts on, authorities could face a challenge in trying to trace the source of the infection and stop the spread.