Sep 7, 2012 (CIDRAP News) – The swine-origin H1N2 virus found in three Minnesotans last week carries the matrix gene from the 2009 H1N1 pandemic virus, marking the first time such a virus has been found in humans, according to state and federal health officials.
The variant H1N2, or H1N2v, virus is different from the swine-origin H3N2v strain that has cropped up in nearly 300 people within the past year. But the two are alike in that both picked up the pandemic virus's matrix gene.
The number of H3N2v cases has risen to 297, an increase of 7 since a week ago, according to the Centers for Disease Control and Prevention's (CDC's) new count, posted today. Nearly all the cases have been in people who were exposed to pigs, most of them at agricultural fairs this summer. Only one death, that of an Ohio woman who had other medical conditions, has been linked to the virus, but there have been 16 hospitalizations.
No ongoing human-to-human transmission of either H3N2v or H1N2v viruses has been reported, but the CDC has said that the H1N1 matrix gene might increase the transmissibility of H3N2v and that a few instances of likely human-to-human spread of that virus have been found.
In reporting the H1N2v cases in a statement today, the CDC said the population probably has some immunity to it because of past exposure to seasonal H1N1 flu viruses.
"Genetic analysis shows that the hemagglutinin (H) of this virus is similar to human seasonal influenza viruses that circulated in people as recently as 2009, so there would likely be protective immunity against this particular virus in the human population," the agency said.
The three confirmed H1N2v cases reported in Minnesota last week were all in people who were exposed to pigs at the State Fair. The Minnesota Department of Health (MDH) said today that a fourth case has since been reported and is awaiting confirmation by the CDC.
MDH spokesman Buddy Ferguson said the "presumptive positive" case involved a teenage boy from outside the Twin Cities area who exhibited pigs at the State Fair from Aug 23 to 26 and got sick on Aug 28. Ferguson said he didn't know of any connection between the teenager and the other three cases.
The teenager was not hospitalized, Ferguson said, but he had no other information on his illness or recovery.
Although H1N2v is new in humans, the US Department of Agriculture's swine flu surveillance program has detected very similar H1N2 viruses with the 2009 H1N1 matrix gene in pigs in Minnesota and some other states since early 2010, the CDC reported.
In reporting the cases last week, the MDH said two sick pigs at the State Fair tested positive for H1N2. Ferguson reported that the virus in the pigs was also found to contain the 2009 H1N1 matrix gene.
The H1N2v cases demonstrate once again that swine flu viruses can spread to people, and they point up the importance of the CDC recommendation that people at risk for serious flu complications should avoid close contact with pigs and pig barns at fairs this season, the agency said.
Those at risk include children under age 5, people over 65, pregnant women, and people who have long-term health problems such as asthma, other lung diseases, diabetes, heart disease, weakened immunity, and neurologic or neurodevelopmental conditions.
Although the recent H1N2v cases are the first known to contain the 2009 H1N1 matrix gene, a few other H1N2v cases have been reported in recent years. One case was reported in a Minnesota child in December 2011. Another case was reported in 2007 in an 18-month-old Michigan child who had been in a swine barn. Both of those patients recovered.
In 2002 the CDC reported that a total of 11 human H1N2 cases had been detected in Wisconsin, Texas, and Nevada the previous year.
In other developments, one sick pig at the Minnesota State Fair was found infected with an H3N2 virus, according to Tom Hagerty, DVM, the State Fair veterinarian. The case turned up on the last day of the fair, Sep 3, and was the only swine H3N2 case found during the event, he told CIDRAP News.
Three human H3N2v cases have been reported in Minnesota, all of them in people who had visited a live animal market, according to the MDH.
The leading states for H3N2v cases are Indiana, with 138, and Ohio, with 102, according to CDC data.
Sep 7 CDC notice on H1N2v virus in Minnesota
Sep 7 CDC FluView weekly flu update
CDC H3N2v case count
Aug 31 CIDRAP News story "Minnesota reports another swine flu virus in humans"