#18,156
While H5N1 in cattle is a relatively new concern, since 2010 we've seen over 500 swine-variant influenza (H1N1v, H1N2v & H3N2v) infections in humans reported to the CDC, often linked to direct or indirect contact with pigs.
Prior to 2016, the H1N2v subtype was only rarely reported (n=7), but since then it has been appearing with increasing frequency (n=40). Two weeks ago, in Nature Comms: Potential Pandemic Risk of Circulating Swine H1N2 Influenza Viruses, we looked at growing concerns over this subtype.
Swine variant influenza infections are indistinguishable from seasonal flu without highly specialized testing, which few countries tend to do. Even here in the United States it is assumed we only detect a small percentage of cases (see CID Journal: Estimates Of Human Infection From H3N2v (Jul 2011-Apr 2012)).
Last March, in CDC FluView Wk #12: 1 Novel H1N2v Infection In Pennsylvania, we looked at the first reported swine variant case of 2024. That patient (< 18 y.o) had contact with pigs, was hospitalized last March, but has since recovered.
Today the CDC's FluView #25 reports two more cases of the same subtype (H1N2v) reported from the same state. In this case, both patients were > 18, had recently attended a livestock auction, and one was hospitalized but has since recovered.
Novel Influenza A Virus:
Two human infections with a novel influenza A virus were reported by the Pennsylvania Department of Health. The patients, who are close contacts, were both infected with influenza A(H1N2) variant (A(H1N2)v) viruses. Both patients are ≥18 years of age and sought healthcare during the week ending June 22, 2024 (Week 25). One of the patients was hospitalized, and one was not. An investigation by state public health officials found that the patients had attended a livestock auction where swine were present prior to their illness onset. Investigation did not identify illness among additional close contacts of either patient. The investigation is ongoing.
Including these two reports there have been a total of three human infections with variant influenza A viruses reported in the United States in 2024.
When an influenza virus that normally circulates in swine (but not people) is detected in a person, it is called a “variant” influenza virus. Most human infections with variant influenza viruses occur following exposure to swine, but human-to-human transmission can occur. It is important to note that in most cases, variant influenza viruses have not shown the ability to spread easily and sustainably from person to person.
Early identification and investigation of human infections with novel influenza A viruses are critical so that the risk of infection can be understood, and appropriate public health measures can be taken.
Additional information on influenza in swine, variant influenza virus infection in humans, and guidance to interact safely with swine can be found at www.cdc.gov/flu/swineflu/index.htm.
Additional information regarding human infections with novel influenza A viruses:
H1N2 variant [A/California/62/2018] Jul 2019 5.8 5.7 Moderate
H3N2 variant [A/Ohio/13/2017] Jul 2019 6.6 5.8 Moderate
H3N2 variant [A/Indiana/08/2011] Dec 2012 6.0 4.5 Moderate
The CDC currently ranks a Chinese Swine-variant EA H1N1 `G4' as having the highest pandemic potential of any flu virus on their list. But, as with avian flu, they all have barriers they must first overcome.
These often occur at agricultural exhibits in county and state fairs, where large numbers of people have contact with livestock (see EID Journal: Shortening Duration of Swine Exhibitions to Reduce Risk for Zoonotic Transmission of Influenza A Virus).
While most fairs have requirements the screening and barring of `sick pigs' from exhibition, over the years we've learned that many pigs may be infected asymptomatically (see EID Journal: Flu In Healthy-Looking Pigs and Transmission Of Swine H3N2 To Humans At Agricultural Exhibits - Michigan & Ohio 2016).
Although the public health risks are considered low, the CDC advises those who are at higher risk of serious flu complications (including children under 5, adults over 65, pregnant women, and those with certain chronic medical conditions), to avoid pigs and the swine barn altogether.
