#18,358
Just shy of a year ago (March 21st, 2024) we learned of the first report of HPAI H5N1 in infected goat kids in Minnesota - which was followed 4 days later by the first USDA Statement on HPAI In Dairy Cattle in Texas & Kansas Herds.
Although not entirely unheard of (see A Brief History Of Influenza A In Cattle/Ruminants), this was quite unexpected, and within 60 days infected cattle had been detected in 9 states.
In May of last year the USDA also announced that HPAI H5N1 had been Detected In Alpacas the previous March. This sudden expansion of the host range of H5N1 was largely attributed to the emergence of a B3.13 genotype, and immediately concerns were raised over its potential for being introduced into swine.
An Unusual Report Of H5N1 in Pigs (Indonesia 2016)
Past attempts to infect pigs experimentally have suggested pigs may not be an ideal host for H5N1 (see 2023's EID Journal: Low Susceptibility of Pigs against Experimental Infection with HPAI Virus H5N1 Clade 2.3.4.4b), although the evolutionary trajectory of HPAI H5 the past few years has been towards greater mammalian adaptation.
In May of last year, in Emerg. Inf. & Microbes: Pigs are Highly Susceptible To But Do Not Transmit Mink-Derived HPAI H5N1 Clade 2.3.4.4b, a study reassuringly reported that this mink-derived variant did not transmit between pigs.
While the amount of testing of swine isn't at all clear, last fall H5N1 was detected in two pigs on a farm in Oregon - although not from the bovine B3.13 genotype - but rather from a D1.2 genotype reported in poultry. Both pigs were euthanized, and no further spread was detected.
Until a month ago, only the B3.13 genotype had been detected in dairy cows.
That exclusivity vanished in early February with the announcement that the D1.1 genotype had been detected in Nevada - and later in Arizona - dairy herds (see USDA Summary Of Nevada's H5N1 Genotype D1.1 Spillover Into Cattle).
Today we have a preprint which describes the experimental infection of pigs with the bovine B3.13 genotype of the virus, and once again it finds that while pigs are moderately susceptible to infection, they aren't yet able to transmit the virus efficiently to other pigs.
Pathogenicity and transmissibility of bovine-derived HPAI H5N1 B3.13 virus in pigsTaeyong Kwon, Jessie D Trujillo, Mariano Carossino, Heather Machkovech, Konner Cool, Eulim Lyoo, Gagandeep Singh, Sujan Kafle, Shanmugasundaram Elango, Govindsamy Vediyappan, Wanting Wei, Nicholas R. Minor, Franco Matias Ferreyra, Igor Morozov, Natasha N Gaudreault, Udeni Balasuriya, Lisa Hensley, Diego Diel, Wenjun Ma, Thomas Friedrich, Juergen A Richt
doi: https://doi.org/10.1101/2025.03.04.641414
This article is a preprint and has not been certified by peer review [what does this mean?].
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Abstract
Since the first emergence of highly pathogenic avian influenza (HPAI) H5N1 viruses in dairy cattle, the virus has continued to spread, reaching at least 16 states and at least 950 dairy herds in the United States. Subsequently, spillovers of the virus from dairy cattle to humans have been reported. Pigs are an important reservoir in influenza ecology because they serve as a mixing vessel in which novel reassortant viruses with pandemic potential can be generated.
Here, we show that oro-respiratory infection of pigs resulted in productive replication of a bovine-derived HPAI H5N1 B3.13 virus. Infectious viruses were mainly identified in the lower respiratory tract of principal infected pigs, and sero-conversion was observed in most of the principal pigs at later time points.
In one animal, we detected the emergence of a mutation in hemagglutinin (HA) previously associated with increased affinity for mammalian-type alpha 2,6-linked sialic acid receptors, but this mutation did not reach consensus levels. Sentinel contact pigs remained sero-negative throughout the study, indicating lack of transmission.
The results support that pigs are susceptible to a bovine-derived HPAI H5N1 B3.13 virus, but this virus did not replicate as robustly in pigs as mink-derived HPAI H5N1 and swine-adapted influenza viruses.
(SNIP)
In summary, our data suggests that pigs are moderately susceptible to the bovine-derived HPAI H5N1 B3.13 virus but do not transmit to sentinel pigs. Given the important role of pigs in IAV ecology as a mixing vessel for generating the novel reassortant viruses with pandemic potential, enhanced surveillance of pigs is warranted.
Although the news regarding swine and H5N1 continues to be reassuring, the virus continues to evolve at a furious rate, and what we can say about genotype B3.13 or the mink-derived variant from Spain, may not apply to D1.1, D1.2, or any future variants that may emerge.
Eleven months ago, in ECDC/EFSA Report: Drivers For a Pandemic Due to Avian Influenza, the authors cautioned that we cannot afford to be complacent:
- The available scientific information suggests that the current 2.3.4.4b H5 strains are poorly adapted to pigs. However, field and experimental evidence indicates that infection in this species is possible and generally of a subclinical nature. Active surveillance should be routinely implemented in pigs exposed to or in proximity of either HPAI infected poultry or wild birds or other mammals, given the important role played by this species in the emergence of reassortant zoonotic viruses.
- Considering the plethora of new H5N1 genotypes which have emerged in the last 3 years and the ability of these viruses to acquire mutations that confer an increased adaptation to mammals, a periodic reassessment of the permissiveness of pigs to emerging HPAI H5N1 viruses is needed.
A reminder of how quickly things can change.
