Avian influenza in mink: should we care?

The ongoing H5N1 avian influenza outbreak is an unprecedented event in its size, scope and duration (but it’s not getting much press these days). As infections continue to occur is birds in large numbers over a vast geographic range, we worry about spillover events into other species.

There have been various reports of infection of species like foxes. Sporadic transmission into wild mammals that live fairly solitary lives and probably aren’t (currently) great hosts for the virus raises concern, but the broader risks are probably limited because of the low odds that rare infections would result in a relevant change in the virus or recombination with another flu virus. However, more infections create more risk, and infections of species with more human contact amplify that risk.

That’s why the recent report of H5N1 avian flu in mink raises some alarm bells. Infection of a farm with tens of thousands of mink is a whole lot different than infection of the odd free-roaming fox or raccoon.

The good news is that even though the report just came out, the outbreak occurred in October and there’s no evidence I’ve seen that this resulted in a broader issue. The bad news is that it shows what can potentially happen (and surveillance is far from good enough to say that this hasn’t caused an issue).

The report involves a mink farm in Spain that housed >50,000 mink. Concerns were raised when the mink mortality rate increased in October, suggesting something was going on. This seemed like a pretty classic infectious disease problem as it started in one barn and was characterized “multiple ‘hot  spots’  within  the  affected  barns  consisting  of  2–4  pens where all the animals died within a period of 1–2 days.” Mortality rates then increased in neighbouring barns, then eventually the whole farm. SARS-CoV-2 infection was probably the main concern and tests for that were negative. H5N1 flu was eventually confirmed, and sequencing of the virus showed it was the same clade (2.3.4.4b) that’s been circulating in birds in Europe.

The decision was made to cull the mink, and over the course of about a month, all of the mink were killed.

Farm workers were tested at one point and all 11 were negative. That was good news, but single point-in-time testing of people exposed to an infected mink farm over the course of weeks doesn’t rule out transmission. One of the workers later developed flu-like signs but tested negative. Because of the concerns, a ‘semi-quarantine’ of the people was performed to limit contact with other people for 10 days after their last contact with the farm.

The source of the outbreak isn’t known. It’s possible that it was introduced by poultry products fed to the mink, but there’s no evidence that any supplying farms were infected. So, it’s much more likely that wild birds were the source. This highlights concerns about mink farms as a wildlife/domestic animal/human interface. It’s hard to keep wildlife away from a mink farm, creating risk for transmission both from and to wildlife. If wild birds can infect mink, it’s equally likely that wild birds (and other wildlife) could be infected from mink, through contact with the mink farm environment. So, something that spread to mink can then spread to human farm workers, domestic animals on the farm or wildlife. That’s not a comforting scenario.

The report concludes by stating that the mink sector is important economically and “it is necessary to strengthen the culture of biosafety and biosecurity  in  this  farming  system  and  promote  the  implementation  of  ad  hoc  surveillance  programs  for  influenza  A  viruses  and  other  zoonotic  pathogens  at  a  global  level”. I’d agree with the second part of the sentence but we need to think and risk vs reward. Is the benefit from mink farming work the risk that raising large numbers of animals that are susceptible to various human viruses in close proximity and with ongoing contact with people and wildlife worth the risk? The broader societal benefits of mink farming are (to me) negligible and the risks may be low, but they are non-zero and not adequately understood.

Image from https://www.bbc.com/news/business-37679652