As telegraphed by yesterday's MMWR, for the 4th week running the CDC today is reporting additional novel flu cases, all three swine variant viral infections linked to attendance of state or county fairs. Over the past 3 weeks we've seen:
statement from the North Dakota Health Department).
Two of the cases have been identified as H3N2v, while final subtyping is awaited on the third case. Preliminary testing indicates it is an H3 variant, making H3N2v the most likely culprit.While rare, it is worth noting we've seen other H3 viruses circulating in North America swine before (see J. Virol: Novel Reassortant Human-like H3N2 & H3N1 Influenza A Viruses In Pigs), so it will be of interest to see what the full lab report finds.
Novel Influenza A Virus:
Three human infections with novel influenza A viruses were reported by three states (North Dakota , Ohio , and Pennsylvania ) during week 32. All three infections were with variant viruses (influenza A viruses that normally circulate in pigs and not people are called variant viruses when detected in people.)
Viruses from two of the infections have been fully characterized and are influenza A (H3N2) variant (H3N2v) viruses; the third infection has been characterized as an influenza A (H3) variant (H3v) virus at this time (further analysis is being performed at CDC to characterize the neuraminidase protein of this virus).
All three patients reported exposure to swine in a fair setting during the week preceding illness onset. Two patients reported attendance at the same agricultural fair. The exposure to swine at the agricultural fair reported by the Pennsylvania resident occurred out of state.
Two of the three patients were children younger than 18 years of age and one patient was an adult aged > 64 years. Two of the three patients were hospitalized but all have fully recovered from their illness. No human-to-human transmission of these viruses has been identified.
To date, a total of 19 variant virus infections have been reported in the United States during 2017. Eighteen of these have been H3 variant viruses (Texas , North Dakota , Pennsylvania , and Ohio ) and one has been with an H1N2 variant virus (Ohio ).
Early identification and investigation of human infections with novel influenza A viruses are critical to ensure timely risk assessment and so that 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 as severe as avian flu in humans, swine influenza viruses nevertheless have some pandemic potential. The CDC's IRAT (Influenza Risk Assessment Tool) Rankings monitors and characterizes 14 different novel flu viruses, and has this assessment on H3N2v.
Swine-origin flu viruses do not normally infect humans. However, sporadic human infections with swine-origin influenza viruses have occurred. When this happens, these viruses are called “variant viruses.” Influenza A H3N2 variant viruses (also known as “H3N2v” viruses) with the matrix (M) gene from the 2009 H1N1 pandemic virus were first detected in people in July 2011. The viruses were first identified in U.S. pigs in 2010. In 2011, 12 cases of H3N2v infection were detected in the United States. In 2012, 309 cases of H3N2v infection across 12 states were detected. The latest risk assessment for this virus was conducted in December 2012 and incorporated data regarding population immunity that was lacking a year earlier.
Summary: The summary average risk score for the virus to achieve sustained human-to-human transmission was in the moderate risk category (less than 6). The summary average risk score for the virus to significantly impact public health if it were to achieve sustained human-to-human transmission was in the low-moderate risk category (less than 5).
For some recent blogs on Swine variant influenza, and why the CDC closely monitors these infrequent human infections, you may wish to revisit: