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Since last Friday's FluView report of 32 Presumptive & Confirmed H3N2v Cases stemming from exposures to swine at 3 separate Maryland County Fairs, we've seen status reports of additional cases picked up by surveillance, but no details.
Although a far cry from the 307 cases reported during the summer of 2012, 2017 has been a particularly active year for swine variant flu cases. In addition to Maryland we've seen cases reported from Ohio, North Dakota and Texas - all linked to direct contact with infected pigs.Since the influenza subtypes that commonly circulate in swine (H1, H2 & H3) are similar to those that have caused all of the human pandemics going back 130 years (see Are Influenza Pandemic Viruses Members Of An Exclusive Club?), we pay close attention whenever a swine flu virus manages to jump - even in a limited way - to humans.
This from today's FluView Report.
Novel Influenza A Virus:
An outbreak of influenza A(H3N2) variant (H3N2v) virus infections in Maryland was first reported to the state health department on September 18, 2017. Although the number of reported human cases in Maryland is declining, an additional 8 human infections associated with this ongoing investigation were reported to CDC.
In addition, Michigan reported a single H3N2v case, unrelated to the ongoing investigation in Maryland (Maryland [8] and Michigan [1]). Six of these viruses have been confirmed as H3N2v viruses; the remaining three viruses have tested presumptive positive for H3N2v at the Maryland public health laboratory; further testing is being performed by CDC to characterize these viruses.
All nine patients reported exposure to swine at one of two agricultural fairs during the week preceding illness onset. Swine influenza A(H3N2) viruses were identified from respiratory samples collected from pigs at one of the fairs. Eight of the nine patients were children younger than 18 years and one patient was an adult aged >50 years. One of the nine patients was hospitalized and all patients are recovering or have fully recovered from their illness. No human-to-human transmission of these viruses has been identified.
With the addition of the nine cases reported for this week, a total of 61 variant virus infections have been reported to CDC in the United States during 2017. Fifty-six of these were H3N2v viruses (Delaware [1], Maryland [36], Michigan [1], North Dakota [1], Ohio [15], Pennsylvania [1], and Texas [1]) and two were influenza A (H1N2) variant (H1N2v) viruses (Ohio [2]). An additional three viruses have tested presumptive positive for H3N2v and further analysis is being conducted at CDC (Maryland [3]).
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.
Individual infections with H3N2v tend to produce generally mild or moderate illness, and community transmission of these viruses (at least so far) appears to have been limited - albeit probably greater than the numbers we've seen reported.
In 2013, a review (see Outbreak of Variant Influenza A (H3N2v) Virus in the United States) of 2012's major outbreak (300+ cases) conducted by CDC and local Health department researchers, found the vast majority appear to have contracted the virus from direct exposure to infected pigs, while only about 5% appeared to be the result of H-2-H transmission.
In contrast, another study published in 2013 (see CID Journal: Estimates Of Human Infection From H3N2v (Jul 2011-Apr 2012)- estimated that during a time when only 13 cases were reported by the CDC - that the actual number of infections was likely 200 times (or more) higher.
Results. We estimate that the median multiplier for children was 200 (90% range, 115–369) and for adults was 255 (90% range, 152–479) and that 2055 (90% range, 1187–3800) illnesses from H3N2v virus infections may have occurred from August 2011 to April 2012, suggesting that the new virus was more widespread than previously thought.Since relatively few who get flu symptoms are ever tested for variant influenza, the real number infected (and the estimates) are impossible to verify, and so both numbers should be taken with a large grain of salt.
