The geographic spread of human infections with a new swine influenza virus has widened, the U.S. Centers for Disease Control revealed Friday as it announced a new case, this time in West Virginia.
The agency actually announced two human cases with swine-origin flu viruses on Friday. One was an infection with the H3N2 virus that has been popping up over the past few months, and a second was with a new virus, a swine-origin H1N2. That case was spotted in Minnesota.
Both cases were in children under five years old, and neither child had known contact with swine, Lyn Finelli of the CDC's influenza division said in an interview.
The widening geographic spread of the H3N2 cases has the CDC thinking these swine origin viruses may be transmitting at low levels among people, suggested Finelli, who is chief of surveillance and outbreak response in the influenza division.
"It does make us take it pretty seriously," she said of the evidence of infections in five different states -- Maine, Pennsylvania, Indiana, Iowa and now West Virginia.
The virus is an influenza A virus of the H3N2 subtype. It's a distant cousin of the human H3N2 viruses that circulate every year. But it is sufficiently different, genetically, from the human virus that experts say the H3N2 component of the seasonal flu shot is unlikely to offer any protection against it.
That said, it is believed most people over the age of 20 or so would have been exposed to similar viruses in the past and would probably have some protection against this virus were it to continue to spread in people. All but one of the 11 cases spotted so far have been children under 10; the exception was a 58-year-old.
Most of the cases have experienced only mild infection though three were hospitalized. The three all had other health problems which may have contributed to the severity of their symptoms.
The swine H3N2 virus has picked up a gene from the H1N1 virus that caused the 2009 pandemic. That gene -- the M gene -- has been shown in animal studies to make flu viruses more transmissible.
The virus was first spotted in July and initial cases occurred in people who had contact with pigs or contact with people who had contact with pigs.
The most recent cases -- the West Virginia case and a cluster of three children in Iowa a few weeks back -- seem almost certainly to have been the result of viruses passing from person to person, not from pigs to people.
"We're not exactly sure how many generations these viruses are away from pigs. But it looks at least like those transmissions are person to person," Finelli said.
"(But) we haven't seen any cases in densely populated areas like in big cities in the U.S. And that makes us think there's not that many degrees of separation between pigs and people since these are all rural areas."
The Public Health Agency of Canada said Friday that it has not seen human cases with either of these swine viruses. And Finelli said the CDC has not heard of any other country spotting these viruses either.
The Minnesota case, with the unrelated virus, is still under investigation, Finelli said. The child had no contact with pigs, but may have been in contact with a sick child a day or two before becoming ill.
The rising number of infections with the H3N2 viruses poses a quandary for public health officials. Because of the mercurial nature of influenza, the situation must be watched closely. But authorities know the public largely wrote off the 2009 pandemic as a false alarm and don't want to ring alarm bells if there is no need.
"The important question for humans is whether this particular clade (group of viruses) has potential to become successful, and the answer to that is we don't know," said Dr. Allison McGeer, a flu expert at Toronto's Mount Sinai Hospital.
McGeer said it's likely that more cases of human infection with swine origin viruses are being spotted because surveillance for new flu strains has been enhanced since bird flu -- the H5N1 avian flu virus -- hit the global radar in 2004.
Finelli said improved surveillance may be playing a role. But she noted the flu situation in pigs is very dynamic these days, with the variety of flu viruses infecting pigs having proliferated greatly over the past decade.
"We only used to see one or two reports a year. Now we see many more than that," she said of human infections with swine influenza viruses.
"It's many, many more than we would expect, even given the previous few years with good surveillance in place."
She said the CDC feels rapid investigation of all cases and their contacts is needed to get a better picture of what is going on. The agency is trying to figure out what are the risk factors for infection, the probability of transmission probability and the severity of the virus.
"If we see an acceleration of cases, all of these things will help us figure out the appropriate public health responses," she said.
