For the second week in a row the CDC is reporting on a novel swine variant flu infection (see last week’s CDC FluView: 1 Novel H1N1v Case Reported From Iowa), this time involving an H3N2v (variant) virus in a patient from Michigan. The details, scant though they may be, are provided in today’s FluView report.
One human infection with a novel influenza A virus was reported by the state of Michigan. The person was infected with an influenza A (H3N2) variant (H3N2v) virus and was hospitalized in June 2015 as a result of their illness, but has fully recovered. 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 rarely reported – partly due to limited surveillance and testing – swine variant influenza infections are probably more common than we know. Illness due to these strains is usually mild to moderate, and the symptoms look like any other flu.
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.
This year we’ve received word of 5 cases (3 - H1N1v & 2 - H3N2v), with three of those reported in the past six weeks. Not an alarming number, but worth noting, as we are heading into the heart of the State and Local agricultural fair season. 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.
The CDC explains their concerns over these types of infections in their FAQ.
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. Just last week we looked at a study that examine two recently discovered swine variant strains (see J. Virol: Novel Reassortant Human-like H3N2 & H3N1 Influenza A Viruses In Pigs).
They described both of these novel subtypes as “. . . virulent and can sustain onward transmission in pigs, and the naturally occurring mutations in the HA were associated with antigenic divergence from H3 IAV from human and swine’” and goes on to warn that ``. . . the potential risk of these emerging swine IAV to humans should be considered”.
Although avian flu gets most of the headlines – mostly due to their high fatality rate – swine flu viruses are considered more likely to jump to humans, simply because they fall in the same H1, H2, and H3 subtypes as `humanized’ flu strains of the past 130 years.
The 2009 H1N1 pandemic virus evolved in pigs, and so easily could the next pandemic virus.
For some more recent blogs on swine and swine variant influenza, you may wish to revisit: