There’s a great new early-release paper in Emerging Infectious Diseases (Oguzie et al. 2024) about the emerging situation with H5N1 influenza in dairy cattle. It provides some important new information, and more details that somewhat challenge the early narrative.

The authors explain some of the limitations of the study to keep in mind…

  • The study was only done on one farm
  • They didn’t direct the response (samples were sent to them)
  • They got involved a bit late in the outbreak

…yet they still have some really interesting and useful findings.

They studied one of the earlier affected dairy farms in Texas. Interestingly, the research team was contacted because of “rumours among cattle veterinarians of possible influenza A virus detection among cattle and conjunctivitis among dairy farm workers.” They also say that cattle had transient respiratory and gastrointestinal signs. That’s different from earlier information that initial concerns were focused on drops in milk production and unusual cat deaths. In particular, the mention of an early concern about human illness is worrying, from the standpoint that we might still be underestimating human infections. No cat deaths were reported on this particular farm.

There’s not much clinical information in the paper, since it’s based on test results, but they report cattle had “decreased appetite, lethargy, increased respiratory secretions, high temperatures (up to 105°F or 40.56°C), abnormal bowel movements, and decreased milk production.” Most of that is unsurprising, but I’m not sure I’ve seen much about respiratory signs before. An increase in respiratory secretions (a runny nose) doesn’t tell us much about the degree of respiratory involvement but suggests there was some. That’s important because it brings in questions about respiratory transmission.

They only got nasal and rectal swabs from cattle for this study, even though milk seems to be the prime source based on what we’ve learned since then. Timing of sampling wasn’t ideal, as is often the case early in outbreaks: illnesses was observed in cattle starting March 6, and several people were sick March 4-6, but samples weren’t collected until March 21, March 28 and April 1. That’s really late to be testing for a virus like influenza that tends to have a fairly short shedding period. They indicated that the outbreak was already waning at the time of sample collection, yet there were 7 PCR positive nasal swabs from 29 sick cattle. No healthy cattle were positive.

It’s been stated from the start that there’s limited respiratory shedding in cattle, and this study doesn’t necessarily contradict that since the PCR Ct values suggested the viral load was quite low in those nasal samples. However, even low shedding in 7/24 (29%) cows is still relevant, especially if samples were collected quite late in the disease process. Peak respiratory virus shedding with flu tends to be very earlier in disease. After a few days, I’d expect shedding to drop, both in terms of the percentage of positive animals and the amount of virus being shed by each one. So, we have to be careful not to take <50% PCR positivity with a high Ct value (i.e. low viral load) as an indication of limited risk of viral shedding.

Rectal samples were all negative for flu. That’s good. The same sample timing disclaimers as above still apply, but with no positives, it suggests that we don’t have much (or as much) to worry about in terms of exposure to feces and manure handling. That simplifies matters.

The human component raises a lot of questions. Details are sparse, but the authors say “Several workers experienced influenza-like symptoms and missed work during March 4–6. A maternity worker visited a local clinic and received treatment for influenza-like symptoms; 2 milkers also experienced influenza-like symptoms and stayed home.”

There’s no mention of anyone being testing and I’d guess that none were. Routine flu testing would be uncommon in mildly ill people at that time of the year if there wasn’t yet any communication about potential influenza exposure from dairy cattle. This furthers the ongoing concern that we are underestimating the amount of cattle-to-human transmission because of limited testing. It’s critical to know how often cattle-to-human transmission occurs and to continue to look at the genome of H5N1 when it infects people (to look for more human/mammalian adaptations).

What this report tells me overall:

  1. We still have a poor understanding of what’s happening with H5N1 flu in dairy cattle.
  2. There is a lack of transparency; we are still not getting all the available information.
  3. Consistent with some other recent reports, there may be more of a cattle health component beyond the drop in milk production than was initially flagged.
  4. Human disease from cattle-to-human transmission may be underestimated.

There’s nothing to panic about here, but it shows that much more work and communication is still needed.