USDA Updates Mammalian Wildlife With HPAI H5 List

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Yesterday the USDA updated their HPAI H5 in wild mammals list, adding 8 more cases.  While many cases go unreported, since January 1st 2025, 25 new cases have been added; 16 domestic cats, 1 each (mountain lion, seal, & Serval) and 6 mice (3 deer mouse, 3 house mouse). 

The USDA only began tracking house cat infections 7 months ago (see May 2024's USDA Map Now Tracking Domestic Cats With H5N1), although a few older cases were added retrospectively.  Mice were first added a week later, in June (see USDA Adds House Mouse To Mammals Affected by H5N1).

The susceptibility of cats (both domestic and wild) to HPAI H5N1 has been long known (see 2015's HPAI H5: Catch As Cats Can), but the role that rodents may play in its ecology is less studied. 

• Last August, in Pathogens: Susceptibility of Synanthropic Rodents to H5N1 Subtype HPAI Viruses, we looked at a study where researchers challenged several rodent species (house mice, brown rat, black rat) with two (older 2010, 2007) HPAI H5N1 viruses, and found they are both susceptible to the virus and could potentially play a role it its evolution and spread.

• Also in August 2024, in Emer. Microbe & Inf.: HPAI Virus H5N1 clade 2.3.4.4b in Wild Rats in Egypt during 2023, we a surprisingly high percentage of wild rats testing positive for H5 antibodies in Egypt, suggesting that some number may survive the infection. 

So far the USDA lists 104 confirmed H5 infections in mice, 80 in house cats, and 63 more assorted wild felines (bobcats, mountain lions, tigers, etc.).  Combined, they make up more that 50% of all of the infected mammalian species on the USDA's list. 

That list is far from exhaustive, however, since many states have reported zero - or only a few - infections.  It would appear that some states may be looking harder that others. 

But even in states that are actively looking, testing and reporting is often limited by animals dying in remote and difficult to access places, or by animals that survive the infection. It is fair to say that what is reported is just the tip of the pyramid. 

In addition to rodents, we've recently seen a number of studies showing that shrews, voles, and other small (often peridomestic) mammals are susceptible to novel flu (see Virology: Susceptibilities & Viral Shedding of Peridomestic Wildlife Infected with Clade 2.3.4.4b HPAI Virus (H5N1)) 

Last summer, in  Nature: Decoding the RNA Viromes in Shrew Lungs Along the Eastern Coast of China, we looked at a study that found a wide range of zoonotic viruses - including HPAI H5N6 - in shrews. Previously, in 2015's Taking HPAI To The Bank (Vole), we looked at that species' susceptibility to both H5N1 and H7N1.

Not so very long ago, HPAI H5 was pretty much just an avian virus, with occasional spillovers to humans and cats unlucky enough to be fed a diet of raw chicken.  But starting in 2021 we began to see reports of numerous spillovers into a much wider range of mammals (see chart below).

Credit ECDC/EFSA

As the HPAI H5 virus continues to find new mammalian hosts it is likely to become more deeply entrenched in our shared ecology, increasing the risks that it will find new evolutionary pathways that were unavailable to it when it was primarily a disease of birds.

Where that takes us is anyone's guess, but it is unlikely that HPAI H5 will be going away anytime soon.