#17,612
Over the past 3 weeks we've seen two human infections with swine-variant (H3v & H1N2v) viruses reported from Michigan, both associated with agricultural exhibits at county fairs, from two different counties (Oakland & Tuscola).
Although we see sporadic cases every summer (and many more likely go undetected), spillovers of swine viruses are followed closely by health authorities because of their (currently low) potential to spark an epidemic or pandemic.
The CDC's Investigation found:
Presumptive positive influenza A(H3)v virus infection
The Michigan Department of Health and Human Services first reported a presumptive positive influenza A(H3) variant virus infection on July 26 in a person who attended a fair that took place July 7-16. A respiratory specimen from that patient was sent to CDC and received on July 27. Diagnostic testing conducted at CDC was inconclusive, likely due to the respiratory specimen not containing enough virus. However, it is likely the patient was infected with an influenza A(H3)v virus given the presumptive positive test results from Michigan, as well as the epidemiologic findings of the local investigation.
An investigation into this first infection found that:
- The patient was exposed to pigs within 10 days prior to illness onset at an agricultural fair where swine influenza A virus was detected among pigs.
- The patient was treated with flu antivirals.
- The patient was not hospitalized and has recovered from their illness.
- Additional investigation did not identify respiratory illness in any of the patient’s close contacts or household contacts.
- To date, no person-to-person spread of this virus has been identified.
Confirmed influenza A(H1N2)v virus infection
The Michigan Department of Health and Human Services notified CDC of a specimen that tested positive for influenza A but lacked reactivity with diagnostic tests for contemporary human influenza viruses representing either H1pdm09 or H3 subtypes on July 31. A respiratory specimen from that patient was sent to CDC and received on August 2. This was confirmed at CDC to be an influenza A(H1N2)v virus infection. This occurred in a person who attended a different fair, which took place July 23-29.
An investigation into the second infection found that:
- The patient was exposed to pigs within 10 days prior to illness onset at another agricultural fair.
- The patient was treated with flu antivirals.
- The patient was not hospitalized and is recovering from their illness.
- Additional investigation did not identify respiratory illness in any of the patient’s close contacts or household contacts.
- To date, no person-to-person spread of this virus has been identified.
Over the past dozen years we've seen nearly 500 swine variant cases (H1N1v, H1N2v, H3N2v) reported by the CDC, and two months ago the CDC held a webinar for clinicians on recognizing, treating, and reporting zoonotic influenza cases in the community (see COCA Call : What Providers Need to Know about Zoonotic Influenza).
Prior to 2017, H1N2v was the least commonly reported swine-variant virus (of the top 3), but over the past 6 years it has outpaced H1N1v & H3N2v by a wide margin. Internationally, we are seeing an uptick as well (see here, here, and here).
Overnight the World Health Organization published their own update on the Michigan H1N2v case. I've posted some excerpts below, but you'll want to follow the link to read it in its entirety. I'll have a brief postscript after the break.
Influenza A(H1N2) - United States of America
11 August 2023
Situation at a glance:
On 4 August 2023, the United States International Health Regulations (IHR) National Focal Point informed PAHO/WHO of a human infection with a novel influenza A(H1N2) variant virus identified in the State of Michigan. This was confirmed by Reverse Transcription Polymerase Chain Reaction at the United States (US) Centers for Disease Control and Prevention (CDC) on August 2.
The case was not hospitalized and is recovering from the illness. Investigations by local public health officials identified swine exposure by the patient at an agricultural fair.
(SNIP)
The case was not hospitalized. Investigation by local public health officials identified swine exposure by the patient at an agricultural fair, that took place between 23 and 29 July, within 10 days prior to illness onset. Additional investigation did not identify respiratory illness in any of the patient’s close contacts or household contacts. No additional cases were identified related to this agriculture fair. No person-to-person transmission of influenza A(H1N2)v virus associated with this case has been identified. No additional cases of human infection with A(H1N2)v virus have been identified as of 10 August 2023.
This is the first influenza A(H1N2)v virus infection identified in the United States this year. Since 2005, there have been 512 influenza A variant virus infections (all subtypes), including 37 (human infections with influenza A (H1N2)v viruses reported in the United States.
Epidemiology of the disease:
Swine influenza A(H1N2) viruses circulate in swine populations in many regions of the world. Influenza A viruses that normally circulate in swine are called “variant” influenza viruses when isolated from humans.1 Human infections are usually due to direct or indirect exposure to pigs or contaminated environments. There may be important antigenic and genetic differences between seasonal influenza viruses that circulate worldwide in the human population and influenza viruses that normally circulate in swine. Close monitoring is needed to early identify changes in the virus and/or its transmission to humans.2
Non-seasonal or zoonotic influenza viruses infecting humans may cause disease ranging from mild conjunctivitis to severe pneumonia and even death; usually, these human infections of zoonotic influenza are acquired through direct contact with infected animals or contaminated environments. Since 2005, 37 human infections with influenza A(H1N2)v virus, including this one, have been reported in the United States.
Public health response
The public health response efforts led by local officials included enhanced surveillance for cases. The county health department conducted active case finding by reaching out to fair exhibitors and their families to ascertain additional illnesses. In addition, local providers were notified to watch for respiratory illness in persons who had attended the fair or had recent contact with swine.
WHO risk assessment
Swine influenza A viruses circulate in swine populations in many regions of the world. Depending on geographic location, the genetic characteristics of these viruses differ. When an influenza A virus that normally circulates ihttps://www.who.int/emergencies/disease-outbreak-news/item/2023-DON482n swine (but not people) is detected in a person, it is called a “variant influenza virus”. Most human cases are the result of exposure to swine influenza A viruses through contact with infected swine or contaminated environments. Because these viruses continue to be detected in swine populations around the world, further human cases can be expected.
According to the IHR, a human infection caused by a novel influenza A virus subtype is an event that has the potential for high public health impact. A novel influenza A virus is considered to have the potential to cause a pandemic if (1) the virus has demonstrated the capacity to infect and transmit efficiently among humans, and (2) differs from currently circulating seasonal human influenza viruses (i.e., A/H1 or A/H3), circulating widely in the human population such that the hemagglutinin (HA) gene (or protein) is not a mutated form of those; and (3) the population has little to no immunity against it.
To date, only sporadic human infections caused by influenza A(H3N2), A(H1N1) and A(H1N2) variant viruses have been reported in the United States, and there has been no evidence of sustained human-to-human transmission. Human infections with influenza variant viruses tend to result in mild clinical illness, although some cases have been hospitalized with more severe disease. Given the potential impact on public health, human infections with these viruses need to be monitored closely. In this event, there was no reported evidence of sustained human-to-human transmission, the illness was mild and no further influenza variant virus transmission in the community has been identified.
There has been some limited, non-sustained human-to-human transmission of variant influenza A viruses, although ongoing community transmission has not been identified. Current evidence suggests that these viruses have not acquired the ability of sustained transmission among humans. The risk assessment will be reviewed if needed should further epidemiological or virological information become available.
Influenza A viruses that infect pigs are different from human influenza A viruses. Currently, there is no vaccine for Influenza A(H1N2)v infection licensed for use in humans. Thus, influenza vaccines against human influenza viruses are generally not expected to protect people from influenza viruses that normally circulate in pigs.
Although swine influenza surveillance is quite limited around the world, the CDC's IRAT (Influenza Risk Assessment Tool) lists 3 North American swine viruses as having at least some pandemic potential (2 added in 2019).
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.
The CDC routinely 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.
