https://www.nature.com/articles/s41564-024-01885-6/figures/2
#18,457
While the HPAI H5N1 virus circulating today still lacks the kind of transmissibility that could spark a pandemic, over the past several years we've seen signs of increased transmission among mammals.
- Nearly 700 American dairy herds, across 15 states, have been infected with the virus (which is undoubtedly a significant undercount).
- Hundreds of other species of mammalian wildlife have been infected as well, with cats, foxes, and mice particularly susceptible.
- Tens of thousands of pinnipeds have died around the world, providing worrisome signs of mammal-to-mammal transmission (see Preprint: Massive outbreak of Influenza A H5N1 in elephant seals at Peninsula Valdes, Argentina: increased evidence for mammal-to-mammal transmission)
- In October of 2022, we learned of a mink farm in Spain where H5N1 had spread rapidly, generating rare mutations (see Eurosurveillance: HPAI A(H5N1) Virus Infection in Farmed Minks, Spain, October 2022), which resulted in a new risk analysis from the CDC.
And perhaps most telling of all, during the first 2 years following the arrival of H5N1 to North America, only 1 (suspected) human infection was reported. Over the past 8 months, the CDC has confirmed 55 human cases in the United States, and that number is almost certainly higher.
Something has obviously changed, but exactly what remains elusive.
A lot of research has been directed at looking for changes in the virus that would allow it to bind to - or replicate more efficiently in - mammalian hosts (including humans). And indeed, we've seen some mutations (PB2 E627K, PB2 T271A, PA-K142E, etc.) that may be factors.
But today we've a study, published in Nature, that takes a look at the dynamics of the shedding of the virus by the infected host, and its impact on transmission.
What they found was that ferrets infected with some newer clades of H5N1 shed (via exhalation) more of the virus than with older H5N1 clades. While it was not a huge increase, the authors write:
`. . . . recent A(H5N1) viruses exhibit a low but increased level of infectious virus shedding into the air as compared with older A(H5N1) viruses.'
I've only posted the abstract, and a brief snippet from the discussion, so follow the link to read the report in its entirety. I'll have a brief postscript when you return.
Influenza A(H5N1) shedding in air corresponds to transmissibility in mammalsIlona I. Tosheva, Fabien Filaire, Willemijn F. Rijnink, Dennis de Meulder,Bianca van Kekem, Theo M. Bestebroer, Mathis Funk, Monique I. Spronken, C. Joaquin Cáceres, Daniel R. Perez, Mathilde Richard, Marion P. G. Koopmans, Pieter L. A. Fraaij, Ron A. M. Fouchier &Sander Herfst
Nature Microbiology (2024)
Abstract
An increase in spillover events of highly pathogenic avian influenza A(H5N1) viruses to mammals suggests selection of viruses that transmit well in mammals. Here we use air-sampling devices to continuously sample infectious influenza viruses expelled by experimentally infected ferrets.
The resulting quantitative virus shedding kinetics data resembled ferret-to-ferret transmission studies and indicated that the absence of transmission observed for earlier A(H5N1) viruses was due to a lack of infectious virus shedding in the air, rather than the absence of necessary mammalian adaptation mutations. Whereas infectious human A(H1N1pdm) virus was efficiently shed in the air, infectious 2005 zoonotic and 2024 bovine A(H5N1) viruses were not detected in the air.
By contrast, shedding of infectious virus was observed for 1 out of 4 ferrets infected with a 2022 European polecat A(H5N1) virus and a 2024 A(H5N1) virus isolated from a dairy farm worker.
(SNIP)
Our results indicate that recent A(H5N1) viruses exhibit a low but increased level of infectious virus shedding into the air as compared with older A(H5N1) viruses. Given the ongoing epizootic in cattle and the high risk of exposure for farm and dairy workers and domestic and wild mammals to infected cows and contaminated milk, it is crucial for effective outbreak control and public health safety to understand how this virus spreads among cattle, its potential for mammalian adaptation and its capacity for airborne transmission.
And that means both on the transmitting, and the receiving end of the equation.
While the odds of stumbling upon that `right combination' are nearly astronomical, our passive `Don't test, don't tell' policy provides the virus with nearly unlimited opportunities to try new combinations.
Long odds, or not - we have to stay lucky 100% of the time - while virus only has to get lucky once.
