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

The authors explain some of the limitations. It was only a study of one farm, they didn’t direct the response (samples were sent to them) and they got involved a bit late in the outbreak. Yet, they still have some really interesting and useful findings.

They studied a Texas dairy farm that was one of the earlier affected farms. 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. These are different than earlier messaging that initial concerns were based on drops in milk production and cat deaths. In particular, the early concern based on human illness is worrying, from the standpoint that we might be underestimating human involvement and still not be getting transparent information. No cat deaths were reported on the 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, while milk seems to be the prime source. Since this was one of the earlier farms, the milk story wasn’t yet on the radar, I assume. Timing of sampling wasn’t ideal, as is often the case early in outbreaks. Illnesses were observed in cattle starting Mar 6, 2024, and several people were sick Mar 4-6, However, samples weren’t collected until Mar 21, Mar 28 and April 1. That’s really late for a virus like fu that tends to have a fairly short shedding period. They indicate that the outbreak was 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 I guess this study doesn’t necessarily contradict that since the PCR Ct values suggested the viral load was quite low. However, 7/24 (29%) is still relevant, especially if samples were collected quite late in disease. Peak respiratory flu virus shedding tends to be very earlier in disease. After a few days, I’d expect things to be dropping, both in terms of the percentage that were positive and the amount of virus that was present. So, we have to be careful not to take <50% PCR positivity with a high Ct (low viral load) as an indication of limited risk of viral shedding.

Rectal samples were negative. That’s good. The sample timing disclaimers apply as above but with no positives, it would suggest that we don’t have 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 they 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 this time of the year if there wasn’t yet communication about potential influenza exposure from dairy cattle. This furthers the ongoing concern that we are underestimating the amount of cattle-human transmission through limited testing. It’s absolutely critical to know how often cattle-human transmission occurs and to continue to look at the genome of H5N1 in people (to look for more human adaptations).

What does this report tell me, overall?

  1. We still have a poor understanding of what’s happening.
  2. We are still not getting adequate and transparent information.
  3. As with some other recent reports, there may be more of a cattle health component beyond mastitis than was initially messaged.
  4. Human disease may be underestimated.

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