We have had a bad run with avian influenza for years. Since 2016 we have had major outbreaks on the old continent, not only among wild waterfowl, in reservoirs and spreaders of the virus, but also on poultry farms, with the consequent crisis for the sector. Certain genetic combinations have made these avian influenza viruses capable of infecting more efficiently and more different species of birds. But perhaps even more worrying from a public health point of view are the H5 avian influenza infections that have been detected in multiple mammal species such as foxes, seals, porpoises, badgers, and ferrets, among others. Even so, the outbreak of H5N1 avian influenza declared in October 2022 in an American mink (Neovison vison) farm in Galicia is the most relevant event in mammals to date. The journal Eurosurveillance has recently published a detailed report on the outbreak.
The outbreak and the virus: a unique case
Not only is this the first H5N1 infection on a mink farm in Europe, but the level of transmission from animal to animal points to mink-to-mink transmission with spread through contact and probably aerosols. How can this ease of animal-to-animal transmission be explained? Although the avian influenza virus detected on the farm belongs to a genotype that was circulating in seabirds in northern Europe during the summer, this virus differs from that of birds because it contains a rare mutation that favours viral replication in mammals, and therefore may have public health implications. In fact, the same mutation is present in the 2009 pandemic influenza A(H1N1) virus.
How did this virus spread from birds to minks? Research suggests that the virus was introduced into the farm through contact with infected wild birds, because the minks had access to the outdoors. This is also the most frequent route of a first introduction of the virus on poultry farms in a region.
«The outbreak of H5N1 avian influenza declared in October 2022 in an American mink (Neovison vison) farm in Galicia is the most relevant event in mammals to date»
It should be noted that the official veterinary services took swift action immediately after the outbreak was declared: culling, cleaning and disinfection of the premises, surveillance of the poultry and mink farms around the affected farm, etc. Thanks to these actions, which are very similar to those already carried out in the event of an outbreak of avian influenza in poultry farms and which unfortunately have had to be implemented repeatedly in recent years, there has been no further detection of avian influenza in the area. In addition, the virus was not detected in any of the farm workers, possibly due to the personal protective equipment (masks, gloves, etc.) they were wearing for prevention of SARS-CoV-2, because the mink flu virus must not be well adapted to infecting humans, or for both reasons.
Risk of transmission to humans
The H5 avian influenza viruses circulating in Europe are clearly highly adapted to birds and poorly adapted to mammals. However, the continued detection of the virus in wild and domestic mammals, the rapid acquisition of viral mutations associated with mammalian adaptation, the observed transmission among mink on the farm, and sporadic human infections (all asymptomatic and due to close contact with infected birds) highlight the need for intensified surveillance in mammals and humans. The outbreak at the mink farm, and in particular the transmission of the virus between mink, represents a further step towards the emergence of an avian influenza virus increasingly adapted to mammals, which could not only infect people but also spread among people, a risk researchers have warned about if appropriate measures are not taken. Thus, the mink in Galicia would be the «canary in the mine», an expression used to warn of an imminent danger that we cannot yet perceive (English miners used to lower a canary into the mine to detect potential toxic gases).
What measures are we talking about? From an epidemiological and biosecurity point of view, mink and poultry farms have many aspects in common, so it makes sense to apply similar measures, as mentioned above. However, it comes as no surprise that following an outbreak of avian influenza in mink (mammals like us), specific measures adapted to this type of production have to be put in place, such as including avian influenza in the differential diagnosis when neurological or respiratory signs are observed in these animals, or performing (clinical, biological, and serological) monitoring of people exposed to infected mink.
«The mink in Galicia would be the “canary in the mine”, an expression used to warn of an imminent danger that we cannot yet perceive»
On the preventive side, it is key to systematically implement the use of personal protective equipment for workers, to implement surveillance programmes and to monitor viruses detected on farms in order to immediately identify any alarming mutations. Allowing a virus that is potentially infectious to humans to circulate in farm animals is a ticking time bomb that, due to the high mutation rate of influenza viruses, will sooner or later end up leaping species.
«Allowing a virus that is potentially infectious to humans to circulate in farm animals is a ticking time bomb»
Despite recent criticism of mink fur farms following cases of SARS-CoV-2 in mink and between mink and humans, this type of production is still common and has an important economic weight worldwide. In other words, the disappearance of this type of farming is not expected in the short term. Therefore, it is a good time to reflect and realise that it is in our interest to strengthen biosecurity and control (both of zoonotic pathogens and animal welfare) in this production sector on a global scale.
References Agüero, M., Monne, I., Sánchez, A., Zecchin, B., Fusaro, A., Ruano, M. J., del Valle Arrojo, M., Fernández-Antonio, R., Souto, A. M., Tordable, P., Cañás, J., Bonfante, F., Giussani, E., Terregino, C., & Orejas J. J. (2023). Highly pathogenic avian influenza A(H5N1) virus infection in farmed minks, Spain, October 2022. Eurosurveillance, 28(3), 2300001. https://doi.org/10.2807/1560-7917.ES.2023.28.3.2300001
EFSA (European Food Safety Authority), ECDC (European Centre for Disease Prevention and Control), EURL (European Reference Laboratory for Avian Influenza), Adlhoch, C., Fusaro, A., Gonzales, J. L., Kuiken, T., Marangon, S., Niqueux, É., Staubach, C., Terregino, C., Aznar, I., Muñoz Guajardo, I., & Baldinelli, F. (2023). Scientific report: Avian influenza overview September–December 2022. EFSA Journal, 21(1), 7786. https://doi.org/10.2903/j.efsa.2023.7786
Xunta de Galicia (18 October 2022). La Xunta detecta un foco de influenza aviar en una explotación de visones en la provincia de A Coruña [press release]. https://www.xunta.gal/notas-de-prensa/-/nova/73686/xunta-detecta-foco-influenza-aviar-una-explotacion-visones-provincia-coruna