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| Case Through Jan 2016 - Credit CDC |
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While reports of human infection with novel swine variant influenza (H1N1v, H1N2v & H3N2v) are pretty rare, the H3N2v strain is by far the most common, making up 94% of all reported cases (n=354) in the United States over the past 11 years.
H1N1v comes in 2nd, with 20 cases (4%), but the least common is H1N2v - which up until today - has only numbered 5 cases since 2005 (4 in Minnesota, 1 in Michigan).
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.
In 2015 we've received word of 7 cases (4 - H1N1v & 3 - H3N2v), while last January we saw 1 report of H3N2v from New Jersey.
Granted, these variant virus infections are probably more common than the official numbers would suggest, since most illnesses are mild and testing for novel flu viruses (particularly in mild, uncomplicated cases) is extremely limited.
Today the CDC reports (via FluView) a recently identified swine variant H1N2v case in Minnesota. An investigation is underway to determine how this virus may have been acquired, and to look for additional cases.
Novel Influenza A Virus:
One human infection with a novel influenza A virus was reported by the state of Minnesota. The person was infected with an influenza A (H1N2) variant* (H1N2v) virus. The patient was hospitalized as a result of their illness, but has fully recovered. An investigation is ongoing into the source of the patient’s infection and to determine if there are other epidemiologically-linked cases of H1N2v virus infection.
Early identification and investigation of human infections with novel influenza A viruses are critical so that the 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.
*Influenza viruses that circulate in swine are called swine influenza viruses when isolated from swine, but are called variant influenza viruses when isolated from humans. Seasonal influenza viruses that circulate worldwide in the human population have important antigenic and genetic differences from influenza viruses circulating in swine.
The CDC explains their concerns over these types of infections in their FAQ.
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.
While we’ve not seen sustained or efficient spread of these swine variant viruses in humans - like all flu viruses - swine variant viruses are capable of evolving, reassorting, and adapting to their hosts.
So we watch the progress of these swine variant viruses with interest. Some earlier blogs include:
Eurosurveillance: Seroprevalence Of Cross-Reactive Antibodies To Swine H3N2v – Germany
JID: Evolutionary Dynamics Of Influenza A Viruses In US Exhibition Swine
Live Markets & Novel Flu Risks In The United States
