Details have now been released about the information that was briefly posted then removed from the US CDC website a couple weeks ago regarding suspected household transmission of H5N1 flu to and from cats. It’s great to have more details, but (as is common with emerging diseases) the full story probably raises more questions than answers. The gist of the story is similar to what we inferred from the leaked information, but the full report is now available online (Naraharisetti et al. MMWR 2025), and includes some useful details. Here’s the more complete rundown:

Household 1

  • This household contained 2 adults, 2 kids and 3 indoor cats. One of the owners worked on a dairy farm that was not known to have infected cattle, but there were infected farms in the area.
  • Cat 1: One of the adult indoor cats developed loss of appetite, lethargy and disorientation, then worsening neurological signs.  After a few days, she was euthanized because of rapid and severe disease progression. H5N1 influenza virus (genotype B3.13) was detected in the cat’s nasal passages and brain tissue; this is the predominant strain currently circulating in US dairy cattle. 
  • Cat 2: One of the other cats then developed signs of respiratory disease four days after the first cat got sick – that timing is generally consistent with what we’d expect for transmission of a flu virus. The owner was asked to collect a nasal swab from the cat because it was too sick to be taken to a veterinary clinic, but this wasn’t done. Fortunately, the cat recovered after 11 days.
  • Cat 3: The last cat just watched everything unfold and appeared healthy the whole time.

The household investigation:

The owner who worked on the dairy farm didn’t have direct contact with cattle. He reported that he took off his farm clothes and boots when he returned home and that they were stored in an area that the cats could not access. The cats were not fed a raw diet or raw milk.

One of the kids had mild upper respiratory tract disease (cough, sore throat, headache and muscle pain) six days after the cat got sick. The other kid had a dry cough around the same time that was attributed to allergies. 

Eleven days after Cat 1 got sick, samples were collected from three of the people in the household. That’s a bit later than ideal, but still reasonable (logistics often dictate when samples can be collected). All the people tested were negative for flu. The sicker kid had a positive test result for rhinovirus/enterovirus, which could explain the illness, but it doesn’t rule out flu completely. 

Unfortunately, the person who worked on the dairy farm declined to submit a sample. He reported a day of vomiting and diarrhea before Cat 1 got sick… Was that potentially related to flu infection? Maybe, since flu can cause gastrointestinal disease, but it’s far from classic, and most that episode was unrelated. The unanswered question is whether the person had a mild or asymptomatic H5N1 flu infection and was responsible for infecting the cat(s).

The people who were tested for flu also got oseltamivir prophylaxis (antiviral medication) because of their close contact with the infected cat (and potentially the initial source of the virus in the household…); the farm worker declined prophylaxis.

Putting Household 1’s story together:

  • The most likely source of H5N1 flu exposure of Cat 1 was the farm worker. Whether he unknowingly tracked the virus home from the farm or whether he had a mild infection that he transmitted to the cat can’t be discerned, but that’s a really important thing to figure out in future cases.
  • It would also be really useful to know if the farm where the person worked had confirmed H5N1 flu in the cattle. Hopefully follow up testing of the cattle was done. If flu wasn’t present, that raises more concerns about how the farm worker might have gotten infected, or if the farm worker was not the initial source in the household, then how did an indoor cat with no other apparent risk factors get infected? 
  • It’s unclear whether there was cat-to-cat transmission of flu. Spontaneous, transient upper respiratory tract disease is uncommon in adult indoor cats. Maybe stress caused a recrudescence of feline herpesvirus infection in Cat 2, but it sounds like the cat was a lot sicker than we’d expect for that. We therefore have to consider Cat 2 to be a strong flu suspect; based on the timing of illness relative to Cat 1’s illness, if it had flu, it presumably got it from Cat 1.

Ideally, serological testing would be performed to detect anti-H5N1 antibodies in blood from individuals (cats and people) in the household, to determine which individuals ultimately were infected, even though serology can’t tell us the when or how.

  • If the farm worker was seropositive, it still wouldn’t tell us if they infected the cat directly or if the cat was exposed to virus on clothing or other fomites (though the former seems more plausible, and would be concerning). If the farm worker was seronegative, that suggests that clothing or fomites was the source (which is a good reminder of why infection control measures like wearing and changing personal protective equipment are so important).
  • If Cat 2 was seropositive, that doesn’t confirm cat-to-cat transmission, though that would still be most likely.
  • If the kids or other adult were seropositive, then there was almost certainly either cat-to-human or human-to-human transmission, neither of which would be good news.

Household 2

This household consisted of a dairy worker and two indoor cats. It’s not clear whether the person had contact with cattle, but he collected raw milk from farms and reported being frequently splashed with milk. He didn’t remove his work clothing before returning home, and one of his cats liked to roll on his work clothes (that were presumably contaminated with milk).

  • Cat 1: The worker’s six-month-old indoor cat was taken to a referral veterinary hospital with a one-day history of progressive neurological disease. It was seriously affected and died in hospital within 24h. H5N1 influenza virus (genotype B3.13, the dairy-associated type) was confirmed in this cat. Whether it was a coincidence or not, this was the cat that liked to roll in the work clothes.
  • Cat 2: The other cat remained healthy, and swabs collected from this cat the day after Cat 1 got sick were negative for flu. 
  • The worker reported signs of eye irritation that began 2 days before Cat 1 got sick. The person was not tested and declined antiviral treatment. So we don’t know if the person had H5N1 flu, but eye inflammation is consistent with the conjunctivitis seen in other human cases of H5N1 infection in the US in the last year, and would be a likely presentation in someone getting splashed in the eyes with influenza-contaminated milk.

There was little further follow up because the owner shut down contact with public health officials, stating a fear of losing his job. 

As with Household 1, timing of sample collection is always a challenge. While we’d ideally get serial samples over time, it’s simply not possible in many cases. Testing Cat 2 at the start was logical and useful, but if there was cat-to-cat transmission, that was probably too early to test. We’d need at least one more sample a few days later to account for the incubation period if Cat 1 infected Cat 2. The initial sample only tells us that Cat 2 likely wasn’t exposed at the same time as Cat 1.

It’s also hard to say if the owner had H5N1 flu or if his clothing was the source of virus for Cat 1. Ideally they could have done serological testing after the fact to confirm whether or not the person was infected, but it doesn’t sound like the person would have been willing to come back to provide a blood sample.

What about the veterinary staff involved in these cases?

Twenty-four (24) veterinary clinic personnel were potentially exposed to the two confirmed H5N1 flu-infected cats from the two households, 18 of which were contacted and monitored for signs of flu. They were all deemed to have had limited exposure. Hopefully that’s a win for routine infection control practices and quick identification of higher-risk situations warranting use of enhanced personal protective equipment (PPE). Because they were all deemed low risk, they were not offered antiviral prophylaxis. Seven individuals reported signs or symptoms of illness after exposure, of which five were tested for flu, and all were negative.  

To sum up:

  • Was there direct human-to-cat transmission of H5N1 influenza? Maybe.
  • Was there direct cat-to-cat transmission of H5N1 infleunza? Maybe.
  • Was there transmission of H5N1 influenza via contaminated clothing? Maybe.

Unfortunately, we need to learn more about transmission by investigating disease events. It’s almost certain there are undiagnosed infections, and maximizing our recognition of H5N1 flu spillovers between species (including humans) is critical, as it helps inform proper care and management of people and animals (including antivirals, when indicated) and helps us figure out transmission patterns and risk factors to prevent more infections. The more we understand this virus, the better we can control it, all the while trying to walk that ever-changing fine line between protection and practicality.