For the 4th time this year, the CDC is reporting the detection of a novel swine variant virus in a human. Human infections with these swine-origin viruses are only rarely reported, but presumably happen more often than we know because testing for novel viruses is very limited.
The CDC describes Swine Variant viruses in their Key Facts FAQ.
What is a variant influenza virus?
When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a “variant influenza virus.” For example, if a swine origin influenza A H3N2 virus is detected in a person, that virus will be called an “H3N2 variant” virus or “H3N2v” virus.
Up until about six years ago the CDC only received 1 or 2 swine variant infection reports each year. In 2010, that number jumped to 8, and in 2011 to 12. In 2012 we saw more than 300 cases – mostly mild - and nearly all associated with exposure to pigs at state and local agricultural fairs.
Of the 376 swine variant infections reported in the United States, that vast majority (350+) have been of the H3N2v variety. Far behind, in second place, is H1N1v. A bit surprisingly, of the 4 cases discovered this year, 3 have been of the less common H1N1v variety.
This update from today’s FluView report.
One human infection with a novel influenza A virus was reported by the state of Iowa. The person was infected with an influenza A (H1N1) variant (H1N1v) virus and was hospitalized as a result of their illness. No human-to-human transmission has been identified and the case reported close contact with swine in the week prior to illness onset.
Early identification and investigation of human infections with novel influenza A viruses are critical so that risk of infection can be more fully appreciated and appropriate public health measures can be taken. Additional information on influenza in swine, variant influenza infection in humans, and strategies to interact safely with swine can be found at http://www.cdc.gov/flu/swineflu/index.htm.
While occasional cases are not particularly alarming, we keep an eye on these viruses because they belong to the same subtypes as do human flus (H1, H2 & H3), and presumably would need less of an evolutionary leap to adapt to humans than avian flu strains.
The CDC’s FAQ states:
Why are human infections with variant viruses of concern?
Influenza viruses that infect pigs may be different from human influenza viruses. Thus, influenza vaccines made against human influenza viruses are generally not expected to protect people from influenza viruses that normally circulate in pigs. In addition, because pigs are susceptible to avian, human and swine influenza viruses, they potentially may be infected with influenza viruses from different species (e.g., ducks and humans) at the same time. If this happens, it is possible for the genes of these viruses to mix and create a new virus that could spread easily from person-to-person. This type of major change in the influenza A viruses is known as antigenic shift. Antigenic shift results when a new influenza A virus to which most people have little or no immune protection infects humans. If this new virus causes illness in people and can be transmitted easily from person-to-person, an influenza pandemic can occur. This is what happened in 2009 when an influenza A H1N1 virus with swine, avian and human genes emerged in the spring of 2009 and caused the first pandemic in more than 40 years.
With the fall county and state fair season getting full swing, we’ll be watching to see if more cases are detected this fall.
While infections have been generally mild (with a few exceptions), fairgoers should consider taking a little extra care washing their hands, particularly around the animal exhibits. The CDC’s webpage Take Action to Prevent the Spread of Flu Between People and Pigs at Fairs provides additional advice.
For more on swine variant influenza, you may wish to revisit: