Recent reports from federal and state agencies in the US of H5N1 avian flu in cattle in Texas, Kansas and now other states have gotten a lot of attention, and for good reason; but, as is typical, some of the media coverage has gone a bit over the top. This is certainly a noteworthy event, and some aspects of this are concerning, while many aspects remain unclear. At this point, it’s not a game changer in terms of what this H5N1 avian flu virus is doing, but it’s yet another example of why we need to pay attention to animals and influenza viruses.

Remember: For the last 3 years or so, we’ve been seeing an unprecedented, sustained global transmission of highly pathogenic H5N1 avian influenza virus (HPAI) in myriad bird populations internationally. The “highly pathogenic” classification is based on how the virus affects domestic poultry specifically; highly pathogenic strains (as the name suggests) can cause devastating mortality in poultry flocks. That doesn’t necessarily mean it’s highly pathogenic (or transmissible) in people or other mammals, but that depends on the specific virus.

While over 99.99% of infections with this pandemic H5N1 avian flu strain have been in birds, there’s been spillover into a really wide range of mammals, including a lot of wildlife but also some domestic species such as dogs, cats, goats, and now cattle. Often these spillover infections have been fatal to the affected animals (cattle being a notable exception), but whether that’s because infections are always severe when they occur, or we just find the most severe infections more easily isn’t clear.

Spillover into mammals raise a few concerns:

  1. Animal health: animal illness and death are obviously concerning.
  2. Adaptation of the virus to new mammalian species, creating a new flu virus that is transmissible within the species: spillover infections into atypical host species are usually dead end infections, because the virus can infect the host but doesn’t replicate well enough to be able to spread to other individuals. However, flu viruses can change and adapt to other species, with the potential to become an endemic flu virus for that species. The H3N2 canine flu strain that’s endemic in parts of Asia and the US started off as an avian flu strain; then it became host adapted to dogs, and was able to spread widely in the dog population.
  3. Animals acting as a mixing vessel to create a new and even more problematic flu virus: this is the biggest concern. In species that can be infected with multiple flu strains (including human flu strains, there’s the potential for the viruses to recombine with each other and make a new strain. What we don’t want is co-infection with human flu and this H5N1 avian flu (and/or other flu viruses) that could result in creation of a new flu virus that has all the human flu virus bits it needs to readily infect people, but is different enough from other human flu strains that our population won’t have any existing immunity to it. That’s how we get a new human pandemic flu virus. If a new strain like that happens to also cause severe disease in people, that’s even worse.

Back to the US cattle with H5N1 avian flu:

On March 25, the USDA reported H5N1 avian flu infections in cattle from two dairy herds in Texas and two in Kansas. Disease was most often in older cows but it was described only as “decreased lactation, low appetite, and other symptoms.” The virus was identified in unpasteurized milk and oropharyngeal samples from sick cows. Dead wild birds, which had presumably died from the virus (as has been happening all over the world), had been noted on the properties, and wild birds were therefore considered the likely source of exposure for the cattle. Importantly, genomic evaluation of the virus from cattle did not show any apparent changes that would make it more transmissible to/between mammals.

In the latest update from March 29, additional presumptive positive tests for H5N1 had been found in more cattle in New Mexico, Idaho and Texas, and the virus was also found in a herd in Michigan that had recently obtained cattle from Texas. It’s not stated anywhere I’ve seen whether that means an infected herd in Texas or just Texas. That’s a key factor – did the virus get shipped in with the cattle, or is it just a coincidence that the farm had obtained cattle from the same (very large) state that has some affected farms. The Michigan virus strain is similar to the one found in the Texas and Kansas herds, but that means it’s similar to the strain in wild birds all over North America, so we have to be careful not to over-interpret that.

With multiple dairy herds affected, it raises several key question:

  • Is there more widespread infection of birds resulting in more exposure of cattle?
  • Is there something about this flu strain that makes it more transmissible to cattle?
  • Is it starting to infect more and more cattle? It’s certainly been found more since the first cases were reported because people are now looking for it. Since disease in cattle appears to be fairly mild and non-specific, flu testing would have been rare before now.
  • Has there been spread via cattle between farms?

It’s important to sort out these questions, since the implications and response in each scenario would be vastly different.

At this point, we have a situation where the virus infecting cattle is the flu strain circulating in wild birds, without evidence of genetic markers that it’s better able to infect mammals. That’s good.

However, the more this virus spills over into cattle (and other mammals), the more chances it has to adapt and recombine to form a different strain. Cattle don’t have a common species-specific influenza A strain (they have influenza D, but that’s quite different), and they tend not to be highly susceptible to flu viruses from humans, pigs and other species, so recombination is less of a risk at this point, at least in cattle.

If this strain is actually able to spread between cattle and establishes itself as a bovine influenza A, that would be a concern for cattle health and possibly for people who have contact with infected cattle. But I’d still be more concerned with cattle-to-pig transmission, since pigs are susceptible to lots of flu viruses and are a great mixing vessel for creation of a new flu strains.

And the other obvious question:

  • if cattle are infected with flu, are there any foodborne disease concerns? No, not really (but as usual we can’t say absolutely zero).

Fortunately, flu isn’t a very tough virus. It doesn’t survive well outside the host. “Foodborne” transmission has been shown in some animals, but those are situations like cats or wildlife catching and eating or scavenging infected birds, which is very high level exposure to obviously uncooked tissue. There are pretty strict processes for keeping meat and milk from sick animals out of the human food supply. As long as the virus causes some illness when a cow is infected, that can be flagged. If cattle can be infected but not get sick, it’s more of an issue.

For retail meat, the time lag between when it’s purchased and prepared helps. Proper cooking eliminates any concern. Pasteurization of milk would also eliminates any possible concern.

Raw milk is a wildcard for me, and I think we need to know more but should assume there could be some risk since they were able to find the virus in unpasteurized milk samples (but there are a lot more (and more concerning) infectious disease risks from raw milk… that’s another story).

Raw cheese? I’d guess that flu wouldn’t survive the cheese-making process, but that’s not my area of expertise.

The USDA report indicates “Because of the limited information available about the transmission of HPAI in raw milk, the FDA recommends that industry does not manufacture or sell raw milk or raw/unpasteurized milk cheese products made with milk from cows showing symptoms of illness, including those infected with avian influenza or exposed to those infected with avian influenza.” That’s a pretty reasonable statement.

Time will tell, but efforts to control the spread of H5N1 flu to and between cattle are important – sitting back and just “hoping” that it won’t be a problem is a dangerous proposition. As always with emerging diseases, we need to balance proactivity and practicality, by liberally appling the precautionary principle while not going overboard and panicking.

  • Cattle farmers: Pay attention but don’t panic. Use good biosecurity practices when bringing in cattle from other farms. Report strange disease occurrences to your veterinarian.
  • Veterinarians: Use good routine infection control practices when visiting farms, and keep an eye out for unusual diseases or disease patterns.
  • Consumers: Relax. Handle and cook food properly (for reasons beyond flu).