Saturday, January 21, 2023

That Touch of Mink Flu (2023 Edition)



The significance of this week's Eurosurveillance journal report HPAI A(H5N1) Virus Infection in Farmed Minks, Spain, October 2022 continues to resonate across the internet as public health agencies and researchers try to gauge the potential risk from mammal-to-mammal transmission of avian H5N1. 

Individual spillover events - such as we've seen with wildlife (Montana FWP: Three Grizzly Bears Test Positive For HPAI H5) and even companion animals (WOAH: France Reports Cat Infected With Avian H5N1) - are obviously a concern, but most appear to be dead-end infections. 

We take reassurance from the fact that while humans have been infected, and sometimes even killed, by avian H5N1 - so far, at least - the virus hasn't demonstrated a strong ability to transmit from human-to-human. A few sporadic cases in households have been documented, but community transmission has not. 

But the outbreak at the mink farm in Spain, where efficient mink-to-mink transmission appears to have occurred - and a rare, and potentially important, mutation ( (T271A), which `enhances the polymerase activity of influenza A viruses in mammalian host cells and mice' was detected -  reminds us that the virus continues to up its game. 

It's no secret that the SARS-CoV-2 virus transmits exceedingly well in mink, and has jumped to humans with new mutations. 

But SARS-CoV-2 was already transmitting efficiently between humans, making that less surprising than when an avian virus like H5N1 shows increased adaptation to mammals. 

While increased concern is justified, long-time readers of this blog are aware that this is far from the first time that farmed mink have been infected with novel (and human) flu viruses.  Starting in October of 2009, in That Touch Of Mink Flu, we looked at several outbreaks, going back more than 20 years. 

In 1984, a rather large outbreak in Swedish mink farms was found to be due to the H10N4 avian virus.  This summary from the Archives of Virology. 

Received: 26 August 1985  Accepted: 26 August 1985 

Summary  During October of 1984 an influenza epidemic occurred on mink farms in the coastal region of South Sweden. Six strains of an influenza A virus were isolated. All six isolates were of the H10 subtype in combination with N4. The H10 subtype in combination with various N subtypes was hitherto only known to occur in avian strains, the prototype being the A/chicken/Germany/N/49 (H10N7) virus.

In 2006, a mink was discovered to have highly pathogenic H5N1 `bird flu’ in Sweden.   This report from CIDRAP news. 

Mink had H5 flu virus

In Europe this week, an H5 virus has been confirmed in a new species, a mink found in Sweden. There have been no confirmed reports of H5N1 avian flu in mink, according to a species list maintained by the US Geological Survey's National Wildlife Health Center. 

The mink had an aggressive H5 virus and was euthanized, the National Veterinary Institute in Sweden said, as reported by Reuters Mar 27. The animal was found in the Blekinge region of southern Sweden, where several infected birds have been found. 

The institute said the mink was thought to have contracted the virus by consuming infected wild birds, the suspected mode of transmission to felines as well.

And in 2007, an H3N2  Swine Influenza was detected in a mink in Canada, as detailed in this 20009 report from  Journal of Clinical Microbiology.

Characterization of a Canadian Mink H3N2 Influenza A Virus Isolate Genetically Related to Triple Reassortant Swine Influenza Virus.

Journal of clinical microbiology. 2009 Mar;47(3):796-9.

In 2007, an H3N2 influenza A virus was isolated from Canadian mink. This virus was found to be phylogenetically related to a triple reassortant influenza virus which emerged in Canadian swine in 2005, but it is antigenically distinct.

In the fall of 2009 - during the midst of the H1N1 swine-origin flu pandemic - we saw another multi-farm outbreak (indicating onward transmission), of a `humanized' H3N2 virus. 

Af: Lars Henrik Aagaard,

16/10/2009 22:30

Mink at least eleven farms in Holstebro area are infected with the new form of human influenza virus. The situation recalls the start of the current H1N1 pandemic in Mexico.

 Danish veterinary and health authorities have issued a warning after the mink at least 11 mink farms in Holstebro area in a globally unique occurrence was infected with a new form of human influenza virus.

"It is a situation that creates some concern because we can see that the virus originates in humans. Against this background, we conclude that there is an increased risk that it also can infect back to people, "says senior scientist Poul Henrik Jorgensen, National Veterinary Institute.

Luckily, this virus never took off, possibly overwhelmed by the surging pandemic H1N1 virus. 

In 2015, we revisited mink flu in That Touch Of Mink Flu (H9N2 Edition), after a study was published in the Virology Journal on a serological survey of antibodies to H9N2 (along with H5 & H7 viruses) in Chinese farmed minks, along with the results of experimental infection of minks with the H9N2 virus.

This 2015 study found that mink inoculated with the H9N2 subtype replicated the virus in their lungs (and to a lesser extent) heart, brain, and kidney. While H9N2 infection was non-fatal for mink, they developed lung lesions, edema, and shed the virus through their respiratory tract. 

Two years later, in That Touch Of Mink Flu (H9N2) - Revisited, we looked at a study in the journal Nature called Intraspecies and interspecies transmission of mink H9N2 influenza virus that demonstrated the ability of mink to transmit H9N2 influenza to other peridomesitic mammals.

It should be noted that mink farming has become big business in China, with more than 60 million raised in 2012. Increasingly fox and raccoon dogs are raised on the same farm, increasing the risks of inter-species transmission of novel viruses,

In China, farmed animals are often fed a diet that includes raw poultry or poultry products (cite), which increases their risk of exposure to avian viruses. This practice inadvertently led to the deaths of hundreds of exotic tigers in Thailand in 2004 (see HPAI H5: Catch As Cats Can) from HPAI H5N1.

We returned to the topic again in 2019, when the journal Nature published an article (see Semiaquatic mammals might be intermediate hosts to spread avian influenza viruses from avian to human) where the authors posited that that some semiaquatic mammals - but most particularly mink - should be considered, along with pigs, as potential sentinels for novel flu. 

Also in November of 2019, just weeks before the world's attention was captured by the events in Wuhan, China, we saw a report from China in Veterinary Microbiology of a reassorted Eurasian Avian-like Swine Influenza H1N1 virus found in farmed mink.

Emergence of an Eurasian Avian-Like Swine Influenza A (H1N1) Virus from Mink in China
Jiahui Liu, Zihe Li, Yanlei Cui, Haiyan Yang, ... Chuanmei Zhang

• We isolated and identified a swine origin triple-reassortant H1N1 influenza virus from the lungs of infected mink.
• Farmed minks were susceptible to H1N1 virus.
• This stain was lethal in mice and had low pathogenicity to mink.
• H1N1 virus could infect minks, replicated in vivo, was eliminated outwards.

There were other studies along the way, including a 2018 report in Transboundary and Emerging Diseases that characterized two H9N2 viruses isolated from mink in 2014 in China and the discovery of H2N2 in another semi-aquatic mammal (muskrat) in western Siberia in 2017.

Although many appear surprised by last October's outbreak of H5N1 in farmed mink in Spain, we have evidence going back nearly 40 years showing that mink are highly susceptible to influenza, and that large, even multi-farm, outbreaks (indicating mink-to-mink transmission) have occurred before. 

While pigs or birds are still the most likely source of the next pandemic influenza virus, an intermediary host - like mink, dogs, cats, or even seals - can't be ruled out. 

Unfortunately, surveillance and reporting from many parts of the world has deteriorated over the past few years (see Flying Blind In The Viral Storm), meaning we may never see the next one coming until it is too late to do anything to stop it.