HONG KONG (CNN) -- Four more people in China have fallen ill from a strain of avian flu not previously detected in humans, the Chinese state-run news agency Xinhua reported, as authorities ramped up efforts to monitor the virus.
The four latest victims, from four different cities in the east coast province of Jiangsu, are in a critical condition and receiving emergency treatment, the provincial health bureau said Tuesday, according to Xinhua.
The disclosure of new infections comes only days after authorities announced the first three known cases of humans infected with the H7N9 bird flu virus on Sunday, two of whom died.
The two men who died lived in Shanghai, which is next to Jiangsu. The other early victim, a woman who is in a critical condition, is from nearby Anhui province.
Chinese authorities are trying to find the source of the human infections. They have so far said there are no signs of transmission of the H7N9 virus between any of the victims or people they have come into close contact with, suggesting the virus isn't highly contagious among humans.
They have also dismissed suggestions linking the infections with the discovery of thousands of pig carcasses from the Huangpu River that runs through Shanghai.
The Shanghai Animal Disease Prevention and Control Center on Monday tested 34 samples of pig carcasses pulled from the river and found no bird flu viruses, Xinhua reported.
The four latest cases are a 45-year-old woman from Nanjing, a 48-year-old woman from Suqian, an 83-year-old man from Suzhou, and a 32-year-old woman from Wuxi, the provincial health bureau said.
The Nanjing woman worked culling poultry, it said.
Malik Peiris, a professor at Hong Kong University's School of Public Health, said Monday that the H7N9 strain of avian flu, already known to exist in wild birds, had probably been transmitted to poultry, from where it infected the humans.
"It's really important to understand where this virus is coming from," he said.
Authorities in Shanghai are gathering daily data on cases of pneumonia resulting from unknown causes and will set up a team of experts to assess the "severity and risk" of H7N9, Xinhua reported Tuesday.
Since the transmission of these types of virus from animals to humans is usually "extremely inefficient," there are often tens of thousands of infected birds for every human case, according to Peiris.
As a result, "it is very likely that there is a quite widespread outbreak happening" among the animals from which it came, he said, underscoring the urgent need to track down the source.
The World Health Organization (WHO) said Monday it was "in contact with the national authorities and is following the event closely."
Because there are so few cases of H7N9 detected so far, little research has been carried out on it, according to Xinhua. There are no known vaccines against this virus, it said.
But Peiris said it was likely that existing anti-flu drugs, such as Tamiflu, are likely to work against the H7N9 strain. He also noted that the WHO has identified the H7 virus family as a potential threat and earmarked possible vaccine candidates.
He said other strains from the H7 family had caused previous outbreaks in poultry in countries including the Netherlands, Britain, Canada, the United States and Mexico. Human infection was documented in all of those cases except the Mexican one.
The outbreak of the H7N7 strain in the Netherlands in 2003 infected 89 people, one of whom died, according to Peiris.
The better known H5N1 avian flu virus has infected more than 600 people since 2003, of which 371 have died, according to the WHO.
In February, China reported two new human cases of H5N1 in the southern province of Guizhou, both of whom were in a critical condition, the WHO said.
A spike in H5N1 deaths, many of them children, has been reported in Cambodia, prompting concern among health authorities.
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