Published in Eurosurveillance in January 2023, a report by European researchers details an outbreak of highly pathogenic avian influenza (HPAI) A(H5N1) detected in “intensively farmed minks in Europe”. This took place in the Galicia region in Spain in October 2022. The report explores the investigations of the outbreak and the public health implications it bears.  

Mortality in minks 

The authors explain the in the first week of October an “acute increase” in the mortality rate was identified at an American mink (Neovison vison) farm in Carral, A Coruña. The expected range of 0.2%-0.3% was exceeded as the rate rose to 0.77%.  

As a result, on 4th October 2022, the farm clinical veterinarian collected oropharyngeal swabs from two affected animals. These samples were analysed at the Central Veterinary Laboratory (LCV) of Algete. The samples tested negative for SARS-CoV-2, and positive for HPAI A(H5N1) virus.  

“Post-mortem examination revealed haemorrhagic pneumonia or red hepatisation of the lungs”.  

The mortality rate continued to increase on a weekly basis, reaching a peak during the week 17th-23rd October. The mortality pattern was originally characterised by “hot spots” but increased to affect the “whole premises”. Clinical signs of infection included “loss of appetite, hypersalivation, depression, bloody snout, and neurological manifestations”.  

A census  

On 13th October the animal health services conducted a census to estimate the number of minks on the farm in question. They found 51,986 animals were kept, housed in wire cages in “partially open barns”. They were fed with “raw fish and poultry by-products, cereals and blood meal”. The suppliers of the poultry by-products, also in Galicia, had not reported any outbreaks of H5N1, nor have they done since then.  

Prior to the identification of the mink outbreak, several cases had been reported in wild birds found sick or dead along the coasts. This raised the suspicion of H5N1 virus infection in the minks, allowing the link to be drawn more efficiently.  

A public health response 

The report states that “culling activities started soon after the official order by animal health services on 18th October 2022”. Animals were culled in “batches of 150-200 animals” until all minks were culled, and waste destroyed, by 17th November.  

The mink farm had 12 workers, 11 of whom had been in contact with the animals. Nasopharyngeal swabs were taken, and the workers tested negative for avian influenza virus (AIV). Nonetheless, a “semi-quarantine” was established to isolate the workers for 10 days. Workers and their cohabitants were warned to report any influenza-like symptoms to the public health authorities, and when, in November, a worker developed a “runny nose” they were able to test a nasopharyngeal sample. This produced negative results.  

The authors note that, following the identification of SARS-CoV-2 infection on mink farms in the Netherlands in April 2020, face masks have been compulsory for mink farm workers in Spain. Furthermore, “increased biosafety measures” have been adopted since initial suspicion in October.  

How did this happen? 

The report presents to “the best of [their] knowledge”, the “first report of clade HPAI H5N1 virus infection” of minks farmed for their fur in Europe. The possibility that the mutation “could have arisen de novo in minks” is not excluded. However, the data available are “not sufficient” to rule out the possibility of an “unobserved circulation of avian viruses bearing this substitution in the avian population”.  

“The source of the outbreak remains unknown.”  

Despite this uncertainty, it is “assumed” that wild birds “may have played a major role in the virus introduction into the farm”. Although the authors acknowledge the “criticism” that fur farming has earned after SARS-CoV-2 cases arose in farmed minks, the “production sector is still common worldwide with an important economic impact”. Thus, the report recommends a strengthened culture of biosafety and biosecurity in the system.  

 We previously reported on avian flu restrictions in Britain, which were lifted in 2022, prompting calls for vaccination. To learn more about challenges associated with developing vaccines for this, come to the World Vaccine Congress in Washington 2023.