There’s not a lot new to report since last week about the finding of H5N1 influenza in dairy herds in multiple US states (see map below). That’s often a good sign.

• I haven’t heard of any new affected farms.
• There have been a few more reports that the virus is not being found (or is present at only low levels) in respiratory secretions from infected cattle.
• No additional human cases of H5N1 flu have been reported.

The investigation is focusing more and more on milk as the key means of virus transmission on affected dairy farms. Viral loads seem to be quite high in milk, which is really surprising, as is the limited to no apparent shedding by the typical respiratory route. These two factors support the hypothesis that transmission has been driven by milk, or more specifically by milking practices that result in cow-to-cow exposure via virus in milk, such as milking equipment that is used on multiple cows. If that is indeed the case, I’d expect the cases to be limited to milking cows, and not younger cows (heifers) or cows not currently lactating (dry cows). I haven’t seen any data on that yet, but it will be informative to learn more about case distribution on affected farms.

With the same strain of H5N1 flu being reported on multiple farms across multiple states, contact tracing will be critical. If there’s a solid link between these farms, then cow-to-cow transmission (probably via milk) would explain things. If there’s no known movement of infected cows between all the affected farms, that would suggest there’s a variant circulating in birds that may be more amenable to infecting cattle, and that wild birds on each of the farms are the most likely (but possibly not the only) route of entry. I can argue for either situation being both good and bad:

Scenario 1: The virus is moving with the cows

If this was a single introduction of a cattle-friendly strain with multistate spread, then it might be containable through good testing and management of exposed and infected cattle. That would be good. If the outbreak is being driven by transmission on farms, and not repeated reintroduction from birds, then containment in cattle might work. Flu virus isn’t usually shed for long in most animals, so I assume that will be true in cattle as well, which makes containment more feasible than for some other viruses that can be shed for a long time. The bad part of this scenario is the unexpected transmissibility of this strain of the virus in a manner and species that really wasn’t on the radar, and the potential that this has spread more widely between dairy farms than we know yet.

Scenario 2: The virus is moving with wild birds

If there’s actually only been limited cow-to-cow spread and the virus wasn’t tracked between farms on cattle, that’s good from the standpoint that it would mean the virus actually does not spread well between cattle. However, it would also mean that there’s a broader reservoir of infected birds across the US (and presumably beyond) with a strain of H5N1 that’s amenable to infecting cattle, such that multiple farms were infected by birds. So, we couldn’t just use a really aggressive containment strategy in cattle and expect that we could eradicate the strain. If we eliminated it from cattle, there’d still be a risk of exposure from bird reservoir.

Both scenarios need good on-farm infection control to limit cow-cow spread via milking and perhaps other contacts. Scenario 1 focuses more on the need for strict measures to prevent farm-to-farm spread. Scenario 2 means the long-term focus needs to be on preventing re-introduction from birds into more cattle herds. That’s really tough since it’s hard to strictly isolate cattle from birds, especially cattle on pasture. So, scenario 1, while it has some concerning aspects, is probably what we’d prefer at this point. Given the bits of epidemiology that I’ve seen and the strain similarity across farms, it’s probably the leading candidate too, which I guess is good.

Regardless of which scenario turns out to be true, what’s needed right now is:

• Continue and increase H5N1 surveillance in cattle (and other animals).
• Be on the lookout for new infections in cattle, where disease is likely going to be subtle.
• Work with farms to increase routine practices that help prevent cow-to-cow spread (e.g. routine hygiene and infection control, review of milking practices and equipment management).
• Work with farms to reduce the risk of inter-farm spread through movement of cattle (e.g. short term quarantine after arrival, health checks before introducing new animal to the main herd). That includes precautions when bringing animals onto dairy farms, as well as when sending animals off dairy farms (such as when excess calves are sold).
• Ensure farmers and veterinarians are using routine, and when necessary enhance, infection control practices to reduce the risk of occupational infection.