By Amanda Gardner
TUESDAY, April 9 (HealthDay News) -- At this point, there's no reason to believe that the emerging H7N9 strain of bird flu that has sickened at least 24 people and killed seven in China is cause for alarm, health officials in the United States say.
For one thing, no cases of human-to-human transmission of the virus have yet been reported -- a necessary precursor to a full-blown pandemic.
"This is very early in the course of identification of human cases," said Dr. John Midturi, assistant professor of internal medicine at the Texas A&M Health Science Center College of Medicine, in Temple.
"We do see something similar every few years with avian [bird] flu," added Richard Webby, a member of the department of infectious diseases at St. Jude's Children's Research Hospital in Memphis.
But this year's strain does seem a little different.
"What's making everyone a little bit more uneasy is that, looking at the sequence of the virus, it appears to have some mutations we think may indicate that the virus might have increased its ability to replicate in humans," Webby said.
But for now, there's no proof of that ability, he cautioned, and the genetic sequence of the virus would still need to change for it to pass easily from person to person.
The first human cases were not identified until March 31, according to published reports.
It's possible that the H7N9 virus is also found in some type of mammal, such as swine, and public-health officials are working to identify possible hosts.
"We don't think that necessarily just this virus growing in [birds] would cause some of these [genetic] changes," said Webby, who's also director of the World Health Organization Collaborating Center for Studies on the Ecology of Influenza Viruses in Lower Animals and Birds. "We can't say with any sort of gusto that the last host of this virus was an avian host. Avian flu is a very generic sort of term.
Chinese authorities are taking precautions against further spread of the virus, suspending sales of live poultry in Shanghai and slaughtering poultry in markets where the virus has been detected.
On Monday, the World Health Organization announced that it was in talks with the Chinese government about sending experts to help investigate the outbreak.
The good news is that most of the human cases "have been associated with poultry exposure, which is typical with most bird flu outbreaks," Midturi said.
However, the flu strain has also been found in live pigeons being sold as poultry at a market in Shanghai. That has unnerved some experts since any infection among wild pigeons would be tougher to control than among penned-in poultry.
The fatality rate from H7N9 also remains unclear. Although the death rate in China seems high, with six deaths out of 16 confirmed cases, authorities don't know at this point how many people have actually been infected. If hundreds or thousands of people contracted the virus with few or no symptoms, the true fatality rate would be much lower, said Midturi, who is also director of infectious disease at Scott & White Memorial Hospital in Temple.
The current outbreak is reminiscent of the outbreak of H5N1 avian flu in 2003, another bird flu that has killed about 370 people since then, according to the World Health Organization. That outbreak raised fears about a pandemic that never materialized.
There are also differences between the two viruses. Unlike H5N1, H7N9 doesn't seem to cause illness in birds, making it harder to detect and track. But, it's not unusual for flu viruses to not cause symptoms in their bird reservoirs, said Dr. Len Horovitz, a pulmonary specialist with Lenox Hill Hospital in New York City.
But he also added a bit of good news: H7N9 is treatable with currently available antiviral medications.
According to The New York Times, the U.S. Centers for Disease Control and Prevention has started work on what's known as a "seed vaccine" for the virus as a precaution. That vaccine will take at least a month to prepare.
And the Associated Press reported Monday that Chinese health authorities are also at work on a vaccine against H7N9.
"If there was human-to-human transmission, that would be worrisome," said Horovitz. "[But] even then, we don't know how aggressive or fatal it would be."
Although two of the people who were sickened by the virus had had contact with one another, right now it only "smells like the possibility" of human-to-human transmission, Webby added.
"On the whole, it doesn't look like there's any strong evidence that this thing is really running around rampant," he said.
There's more on H7N9 avian flu at the U.S. Centers for Disease Control and Prevention.