#18,726
The good news is - except for dairy cows - we've not seen evidence of widespread transmission of HPAI H5N1 in mammalian livestock.
Studies - such as Pathogenicity and Transmissibility of Bovine-derived HPAI H5N1 B3.13 Virus in Pigs - suggest the virus isn't well-adapted to these non-bovine mammalian hosts. Infection may be subclinical and transmission may be weak or non-existent.
But of course, surveillance around the globe is quite limited, and over time the virus can always learn new tricks.
Today the Norwegian Veterinary Institute announced the discovery of a (previously) H5N1 infected sheep in Finnmark county using updated antibody tests. This sheep appears to have been exposed to infected wild birds in the summer of 2023 (see photo above).
First the link and text of their announcement, after which I'll return with a postscript.
(translation)
Research project shows earlier bird flu infection in a Norwegian sheep
It is a research project at the Veterinary Institute that has detected antibodies to bird flu in the sheep in question, one of several animals that grazed during the large outbreak of highly pathogenic bird flu in Finnmark in the summer of 2023.
- It is very likely that the infection to sheep occurred in connection with the bird flu outbreak in crutches in the summer of 2023, says bird flu coordinator and senior researcher Ragnhild Tonnessen at the Veterinary Institute.
infection from birds to individual animals has probably occurred in a situation with a very large infection pressure. We do not have an indication that the animals have been sick because of this, nor have we made any findings that indicate infection between the sheep, she emphasizes.
New knowledge
In March 2024, bird flu was detected among dairy cows in the United States, which showed that bird flu can infect ruminants. In June 2024, the Veterinary Institute conducted a pilot study in which blood samples were taken from 220 adult sheep in Vadso. In this way, the researchers were able to investigate whether the sheep could have been infected in connection with the outbreak in Finnmark the previous year.
Several of the sheep sampled had had direct contact with dead and sick birds on the pasture during the outbreak, but no sick sheep were observed. Now the research results indicate that infection with bird flu virus of the subtype H5 probably occurred in at least one sheep in the summer of 2023.
Updated tests
The researchers at the Veterinary Institute analyzed the blood samples from the sheep in June 2024, but the studies at the time showed that none of the 220 samples contained antibodies to influenza A virus, which includes H5 bird influenza virus. Later, the international research community has come up with updated methods to test for antibodies to bird flu in various mammals.
The researchers therefore recently decided to use these updated methods to re-examine 85 of the samples from the most infected sheep herd. As a result, the Veterinary Institute has now detected antibodies against influenza A virus in two sheep, and one of these also had antibodies against H5 bird influenza virus.
What do we know about bird flu in sheep?
In March 2025, H5N1 was first detected in a sheep in England. This was the first detection by ruminants in Europe. Sheep were infected in connection with an outbreak of birds in captivity on the same farm. In addition, a study has detected antibodies to bird flu in sheep in Pakistan in 2023. Apart from these findings, there is generally little knowledge of bird flu in sheep.
the discovery we have now made of antibodies to bird flu in a Norwegian sheep shows that farmers should keep grazing animals away from areas with a lot of sick and dead wild birds. This can prevent infection in future outbreaks, says Ragnhild Tonnessen.
worldwide, several cases of bird flu have been reported in recent years to infect mammals, which is worrying about further adaptation of the virus to humans. This project from the Veterinary Institute contributes with new knowledge and makes us better equipped for handling future bird flu outbreaks, she says.
Mattilsynet wants increased vigilance in pet owners and veterinarians about bird flu infection in mammals. Suspicion of illness is reported to Mattilsynet by calling 22 40 00 00. Here you will find more information.
Read more about the crutch outbreak in Northern Norway in the summer of 2023 here.
Here you can read more about bird flu on the Veterinary Institute website.
Contacts:
Ragnhild Tonnessen
Senior researcher virology, avian influenza coordinator
Given the events of the past 14 months, the detection of H5N1 in a sheep in Norway is hardly an earthshattering revelation - but it is important - because it reminds us that the H5N1 outbreak in U.S. cattle wasn't some rare one-off event; it is the exposed tip of a much larger iceberg.
This sheep infection, and that of sheep and goats in Pakistan, likely occurred months before cattle in Texas began to show the first signs of infection. Likewise, the positive samples from Mongolian horses were collected in 2021 and 2022.
In 2023, we also saw a report from Italy - Seroconversion of a Swine Herd in a Free-Range Rural Multi-Species Farm against HPAI H5N1 2.3.4.4b Clade Virus - and the Netherlands: Zoonoses Experts Council (DB-Z) Risk Assessment & Warning of Swine As `Mixing Vessels' For Avian Flu.
Although much of what happens with the evolution and spread of HPAI H5 viruses occurs outside of our view - from what little we can see - the virus continues to make significant inroads into mammalian species.
Where it goes next is unpredictable, but we underestimate its impact at our our peril.