Finding H5N1 avian influenza in mice in the US has caused a lot of angst amongst some – some angst is warranted, but some of it is overblown. That’s not because H5N1 isn’t an issue, or that more species being involved isn’t relevant, but because there are bigger issues to address. Adding yet another species to the susceptible list isn’t a doomsday scenario, even though we’d rather that list didn’t get any longer.

The latest APHIS report involved detection of H5N1 in an additional 36 house mice in Roosevelt County, New Mexico. They’d already found 11 infected mice there earlier in May. Typical of this ongoing outbreak in the US, available details are sparse. I haven’t seen a clear statement about where these additional mice were collected. I assume they were from infected dairy farms, and that’s a pretty basic but critical piece of info. (The first 11 infected mice were reportedly from an infected poultry premise). If the new mice were from farms with infected cattle, it’s not surprising to find the virus in mice at the same location. If they were from other areas, that would be more confusing and more concerning.

How do mice get infected?

I haven’t yet seen any genomic information on the virus found in the mice; it will be helpful to know if they were infected with the dairy cow-associated H5N1 strain, or whether some of the infections might be linked to exposure to wild birds. If we go on the assumption that these mice were from dairy farms, cattle are the most likely source, because we know infected cattle shed lots of virus in their milk, which would make it easy for mice on the farm to be exposed to the virus in the environment. Even though flu virus doesn’t survive long in the environment, if there’s lots of milk loaded with lots of virus (especially in areas where mice are looking for food), mice are likely to encounter some active virus. Fecal shedding of H5N1 in cattle seems to be low, but data are pretty sparse; we need to clarify that risk more since that would be another possible means of exposure for mice (and other animals, and people). It’s obviously highly relevant since cattle produce a lot of feces, and that manure needs to be stored and/or spread somewhere.

If we think about the risks from finding H5N1 in mice, I’d consider four main areas:

1) Risk to people from H5N1 in mice

Yes, there is a risk to humans, to some degree – but we need more information, like the amount of virus the mice were shedding, and how (e.g. fecal shedding vs respiratory shedding). Some infected mammals shed a lot of virus, but others are likely dead end hosts that don’t shed enough virus to spread the infection any further.

Even if mice shed appreciable amounts of H5N1, we don’t tend to have close contact with (wild) mice, so the risk from direct exposure is presumably really low. If someone’s on a dairy farm with infected cattle, mice are very low on the risk scale. Cattle are the biggest risk. Cats are probably #2 on the list.

However, mice do get into peoples’ homes as well, and the risks from that are completely unclear. Flu virus doesn’t survive long outside the host, so it’s not like a virus like hantavirus, where mouse poop in the environment is a significant concern, but we need to know more about virus shedding.

At this point, I suspect the direct risks to people are very low, but not zero.

2) Risk to other species from H5N1 in mice

The biggest risk from this new finding might be mice acting as a bridge from wildlife / livestock to humans, through their potential to infect cats. Cats catch mice, and eating an infected mouse is presumably high risk for H5N1 transmission (just like eating an infected bird). Cats are susceptible to infection, and have close contact with both mice and people (and other domestic species), so anything that increases the risk of cats being infected is a concern.

3) Risk of mice spreading H5N1 farm-to-farm

As we start to (slowly) get more information about H5N1 on dairy farms, we’re seeing more reports of infected farms that did not bring in cattle from other infected farms. That makes us wonder about other sources of introduction, like humans tracking the virus around or spread via wild birds. Fortunately mice, like coconuts, do not migrate (yes, that’s a niche reference – see link below), so they probably pose limited risk for broad geographic spread of the virus because they’re not that mobile (unless they hitch a ride on a human conveyance of some kind…). Mice tend to have very small ranges, so it would probably be tough for them to spread H5N1 even between farms, unless the farms were very close to each other. The bigger risk would be bringing the virus from the barn into the farm house.

4) Risk of virus mutations

The more avian influenza spreads to and within mammals, the more opportunities it has to adapt to mammals. It would take a number of specific genetic steps for an avian flu virus to evolve to effectively infect and spread between mammals (including people), but the more it’s transmitted, the greater the risk that could happen. This is why we don’t want to see avian flu spreading in any mammalian species.

So, while I don’t like seeing more H5N1 infections in more mammalian species, and even though if H5N1 became endemic in mice they could be a long term reservoir, I’m still more worried about birds, cattle and cats at this point.

Back to H5N1 flu in cats

The good news is that infections in cats are still rare. The bad news is that most reported infections in cats have been very severe or fatal. Whether severe disease is the norm in cats, or whether we’ve mainly just tested really sick cats is hard to say. There have been approximately 21 cases of H5N1 influenza identified in domestic cats in the US since the outbreak in cattle was first detected. Other cases have been reported in cats in various parts of the world over the last 2+ years, including in Canada. However, those cases are probably just the tip of the iceberg. We need more surveillance, including testing of healthy and sick cats from locations where H5N1 is present. Cats on dairy farms with infected cattle are at the highest risk, but any cat with outdoor access that might encounter an infected wild bird is at some risk.

What do we need?

More surveillance and more communication. We need broader testing on affected farms, thorough epidemiological investigation of the spread on and between farms and clear (and timely) communications.