An increase in the number of people in China infected with bird flu this year is raising concern among experts. They say an earlier bird flu strain appears to have changed and may be more infectious to people.

The World Health Organization (WHO) says China has reported 21 human infections with the H5N6 strain of bird flu in 2021. That compares with only five reported cases last year.

The numbers are much lower than the hundreds infected with H7N9 in 2017. But the infections are serious, with many severely ill and at least six dead.

"The increase in human cases in China this year is of concern. It's a virus that causes high mortality," said Thijs Kuiken. He teaches at the Erasmus University Medical Center in Rotterdam, The Netherlands.

The WHO said in a statement earlier this month that most of those infected with the H5N6 virus had come into contact with poultry. The WHO noted that there are no confirmed cases of human-to-human virus spread.

The statement added that further investigation was "urgently" required to understand the risk and the increase in human infections.

While human H5N6 cases have been reported, no outbreaks of H5N6 have been reported in poultry in China since February 2020.

China is the world's biggest poultry producer and top producer of ducks. These animals act as carriers for many flu viruses.

The Chinese Center for Disease Control and Prevention (CDC) could not be reached for comment on the rise in H5N6 human cases. But a study published on its website in September said the increasing spread of H5N6 is "a serious threat to the poultry industry and human health."

Bird flu viruses continuously spread in domestic and wild birds, but rarely infect people. However, the increase in changes of the viruses is a major concern. Experts worry that a variant – or new version -- of the virus that spreads easily between people could cause a pandemic.

The largest number of H5N6 infections have been in China's southwestern Sichuan province. Cases have also been reported in neighboring Chongqing and Guangxi, as well as Guangdong, Anhui and Hunan.

At least 10 cases were caused by viruses very similar to the H5N8 virus that struck poultry farms across Europe last winter and killed wild birds in China. That suggests the latest H5N6 infections in China may be a new variant.

Thijs Kuiken said, "It could be that this variant is a little more infectious (to people)...or there could be more of this virus in poultry at the moment and that's why more people are getting infected."

Four of the people who developed the virus in Sichuan raised poultry at home and had been in contact with dead birds, according to a September report by China's CDC. Another had bought a duck from a live poultry market a week before developing signs of illness.

Filip Claes is with the Food and Agriculture Organization. He said China vaccinates poultry against bird flu. But he said the vaccine used last year may only partly protect against new viruses. He added that the vaccine may prevent large outbreaks, but still permit the virus to keep spreading.

China's Ministry of Agriculture and Rural Affairs did not respond to a Reuters request for comment.

Backyard farms in China are common, and many people still buy live chickens at markets.

In September, officials in Guilin city in the Guangxi area said trading of live poultry in 13 city markets had been suspended and that all trade would end within a year.

I'm Jonathan Evans.

Dominique Patton reported on this story for the Associated Press. Jonathan Evans adapted this story for Learning English. Ashley Thompson was the editor.

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strain – n. a group of closely related living things that look similar but possess one or more unique characteristics

poultry – n. birds such as chickens and ducks that are raised on farms for their eggs or meat

outbreak – n. a sudden start or increase of fighting or disease

domestic – adj. living with or under the care of human beings

province – n. any one of the large parts that some countries are divided into

backyard – n. an area in back of a house