Cat-to-human or human-to-cat transmission of H5N1 influenza would be concerning but not too surprising.
Suppression of information by the US government would be concerning but not surprising given what’s going on at the moment.
But, did those occur?
The sharp eyes at the New York Times have raised concerns, because they spotted a CDC report that mentioned potential cat-to-human and human-to-cat transmission of H5N1 flu that was available online briefly on February 5, but then removed.
Why was the CDC report removed?
It could be because:
- they found that the information was inaccurate
- the story was incomplete and they wanted to get more details
- the info is coming soon but needed additional sign-off by someone internally or an external collaborator
- they were told not to release it
Each of those potential reasons causes a different level of concern, but the net result is that there might be some really important information that’s not being made available.
- In one household there was potential transmission from a cat to another cat and a child.
- In another household, a dairy worker got sick, then their cat got sick a couple of days later and died, raising the question of whether the cat was infected by the person.
I’ve assumed cats would be able to transmit H5N1 flu based on the amount of virus that has been found in respiratory samples from some cats using PCR. Cat-to-cat transmission would be quite likely given how closely some cats interact (e.g. playing / fighting, mutual grooming, sleeping together). Cat-to-human transmission seems possible for similar reasons. If an infected cat is shedding a reasonable amounts of virus in its respiratory secretions, I’d assume there’s some risk to any people who are in close contact, especially those who may have close contact with the cat’s face (e.g. owner of an infected house cat, veterinary personnel). In the case of potential human-to-cat transmission, we don’t really know if cats may be more susceptible to current H5N1 flu strains than people, or whether cats are just more likely to get serious disease when they are infected.
The nature of the evidence and investigation is important to know.
Sometimes, it’s pretty clear how things were transmitted, based on things like a lack of other potential sources of infection and timing. Other times, it can be very messy. For example, in the second scenario, if the cat was an indoor cat (that didn’t sneak outside), didn’t live on a farm and was not fed raw meat or milk, human-to-cat transmission is by far the most logical source. Transmission by fomites (e.g. clothing worn on the farm) would have to be investigated too.
As I said at the start, cat-to-human and human-to-cat transmission would be concerning but unsurprising. “Concerning” may be a bit of an understatement, as it’s a potentially big deal. If a person can infect a cat, then it’s reasonable to assume that person was shedding a reasonable amount of virus (presumably in respiratory secretions) and therefore could have also infected another person. Human-to-human transmission is a very big concern, because if the H5N1 virus evolves to spread effectively person-to-person, and the general population has no immunity to this virus from previous exposure or vaccination, it could result in rapid widespread transmission (similar to SARS-CoV-2). There are still many gradations in transmission risk, and this one (disappearing) report by no means indicates we’re on the brink of a new flu pandemic, but it would be one more step along the way.
If H5N1 flu ever gets good at human-to-human transmission, cats will ultimately be only a minor risk to people in the grand scheme (even though we know they’re quite susceptible), because we’d be at greater risk of transmission from other people. However, if human-to-human transmission does not occur or remains rare, cats can be a bridge to humans from sources like wildlife, poultry and dairy cows, by bringing the virus into the household. A lot more people have close contact with indoor-outdoor cats than with other major sources of H5N1 flu.
From a personal standpoint, as someone who goes out and samples animals during emerging infectious disease events and who provides guidance to veterinarians, shelters and other groups about management of H5N1 flu suspects, I want more information about this risk ASAP, and it needs to be good, reliable information. I realize that there are inevitably necessary channels and approvals for sensitive information, but these should (hopefully) only cause minor delays in getting the information to those who need it most. The information doesn’t need to be polished, but it should be as close to real time as possible.
Emerging infectious diseases must be managed with active and transparent approaches. Too often, that’s not the case (and the initial US response to H5N1 in dairy cattle showed issues that were present even before the current administration).
Hopefully we’ll get more information about this soon. Hopefully these are rare events that don’t indicate an increased risk of mammal-to-mammal transmission. But, as I’ve said before, hope is not an effective infection control strategy.