SHANGHAI, CHINA—With thousands of dead pigs found floating in local rivers and a government with a history of covering up outbreaks, rumors are swirling in China over the deaths of two people here from avian influenza that the government announced on Sunday. But scientists say that it's still too early to draw substantive conclusions about the virulence or source of the virus, H7N9, found in the patients; nor is it clear that there is a link to the more than 16,000 pig carcasses found in the Huangpu River and its tributaries in March.

H7N9 killed two men, ages 87 and 27, and infected a woman in nearby Anhui province in late February and early March, health officials said Sunday; according to an AFP report today, the health bureau in the eastern province of Jiangsu has reported four new cases, which would bring the total to seven.

The virus, which had never been found in humans before, appears to be "more pathogenic than other known H7viruses to humans," says Chen Hualan, a virologist and director of China's National Avian Influenza Reference Laboratory in Harbin. The three infected people reported about on Sunday suffered from severe pneumonia, she says, while "other H7 viruses mainly cause conjunctivitis in humans." But researchers have not yet tested how virulent the patients' viruses are in animal models, she adds. The other big unknowns are whether the strain is circulating in pigs, which could make it a significant threat to humans, and whether people can transmit it to each other.

The China Center for Disease Control and Prevention (China CDC) in Beijing confirmed the cases on 29 March, after ruling out avian influenza strains H3N2, H1N1, H5N1, and the novel coronavirus that has begun to spread recently, according to the World Health Organization's (WHO's) Global Alert and Response Web site. China CDC has sequenced two different H7N9 strains, says George F. Gao, the agency's deputy director-general. It is still unclear where the outbreak originated. The patients may have been infected through contact with wild birds or poultry, Gao says; H7 viruses have been detected in both.

Health authorities have examined 88 people who came into close contact with the first three patients and have not yet found additional infections, according to reports. But two sons of the deceased 87-year-old man both had pneumonia around the time he fell ill, raising fears of human-to-human transmission of the H7 strain. One died at age 55.

While there is no evidence of airborne transmission, Gao says, the possibility had not yet been ruled out as of Monday, and China CDC researchers are now investigating the matter. "This question should be answered very soon," he adds. "At this point, these three are isolated cases with no evidence of human-to-human transmission," WHO China representative Michael O'Leary told reporters at a briefing in Beijing on Monday. But O'Leary noted that the causes of illness in the elderly man's two sons were unclear and acknowledged the speculation they raised. "Naturally, if three people in one family acquire severe pneumonia in a short period of time, it raises a lot of concern."

Previous outbreaks of other H7 viruses elsewhere in the world have led to only one confirmed death, a veterinarian in the Netherlands who contracted the virus at a poultry farm during a massive H7N7 outbreak in 2003. "The unusual thing is that this H7N9 virus killed more humans compared to past outbreaks of other H7 viruses elsewhere in the world," Chen says. But Yi Guan, a virologist at the University of Hong Kong's School of Public Health, says that there may have well been human deaths that went unreported during outbreaks of H7 viruses in birds in Mexico and Pakistan.

Researchers are trying to identify the source of the Chinese outbreak by collecting samples from poultry and wild birds, Chen says. "If we can get the virus from poultry, we will suggest to [cull infected] poultry" in the area, she adds.

That could turn out to be a mammoth undertaking. Public HeaThe fact that the three patients lived at some distance from each other "would imply that the virus itself is also widespread in the animal population," says Malik Peiris, a virologist at the University of Hong Kong's School of lth. The challenge now will be to quickly contain any animal outbreaks, he adds, to avoid "a situation like H5N1 where the virus becomes entrenched."

As for Shanghai's thousands of dead pigs, Peiris says that a link is unlikely. Online speculation homed in on the fact that one of the dead patients was identified by the Chinese press as a "pork trader." But, Peiris says, "It is not expected that any form of influenza would lead to such a huge die-off in pigs." While other H7 viruses have infected pigs, they have not caused large numbers of deaths, he says. Still, the possibility is important to rule out as soon as possible, he adds. "If these pigs actually did have H7N9 infection, it changes the whole risk assessment of the situation very dramatically." Pigs share receptors used by the influenza virus with humans, making it easier for the virus to hop between the two species.

On Monday, the Shanghai Animal Disease Prevention and Control Center tested 34 samples taken from pig carcasses and found no avian flu viruses, according to the Chinese state news agency Xinhua.

Scientists, meanwhile, focused on the virus itself. One of the strains seems to be Tamiflu-resistant, while the other responds to the antiviral drug, Gao says. Both strains appear to be sensitive to Relenza, another antiviral. More details should emerge soon, he adds. "We haven't carefully analyzed the virus sequences yet," Guan says. "So far we have very, very limited information."