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Scientists have traced most dairy cattle herds in the United States (US) getting infected with the highly pathogenic avian influenza to a single event when the flu virus (HPAI H5N1) ‘jumped’ from wild birds to cows.
They said the B3.13 genotype detected in the virus poses a pandemic threat to humans as it has shown mammalian adaptation and evolution in cattle.
The virus has already demonstrated severe consequences for animal health and the agricultural economy.
The clade 2.3.4.4b of the H5N1 virus has caused widespread avian flu outbreaks in various parts of the world. It was first detected in 2021 and has since spread widely among poultry, wild birds and mammals. The clade has infected 1,031 dairy cattle herds across 17 US states.
According to the paper published by the scientists, clade 2.3.4.4b viruses have been reported in more than 12,875 wild birds and 200 mammals of at least 20 species across 29 US states and Washington, DC, from 2022 to the present. They have also been reported in more than 1,120 poultry flocks across all US states.
Veterinarians observed dairy cattle displaying decreased feed intake and changes in milk quality and production in late January 2024, according to the analysis, published in the journal Science.
“On 25 March 2024, HPAI H5N1 clade 2.3.4.4b was confirmed in dairy cattle in Texas. Shortly thereafter, the virus was identified in cattle in eight other United States (US) states by members of the National Animal Health Laboratory Network,” it noted.
The paper’s authors analysed genetic sequence data after introduction of HPAI H5N1 in 2021 in order to trace the route of the virus from wild birds to cattle.
They identified clade 2.3.4.4b with genotype B3.13 influenza A virus in milk from 26 samples of dairy cattle sourced from eight states.
“The sequences isolated from cattle clustered within a single group in phylogenetic analyses, supporting a single spillover event in late 2023,” they observed.
A reassortment event in wild bird populations likely preceded this spillover, enabling the virus to jump to cattle, which eventually allowed mass spread of HPAI H5N1 in US dairy cattle herds.
From Texas, it moved to cattle in Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, and South Dakota.
“Continued transmission of HPAI H5N1 within dairy cattle increases the risk for infection and subsequent spread of the virus to humans and other host populations,” the study warned.
The experts said the B3.13 genotype possesses a ‘pandemic threat’ as once infected, a cow may host the virus for two to three weeks, increasing possibility of zoonotic transmission.
The scientists also found some amino acid mutations at sites linked to mammalian adaption that had “already become fixed in the virus population”, indicating a four-month evolution in dairy cattle.
Low frequency sequence variants in the cattle population were also detected, even with limited time exposure post first spillover.
After the virus was introduced in cattle, it persisted and then transmitted from cattle into poultry and peridomestic animal species, according to the study.
Viral transmission happened from cattle to nine poultry birds, a raccoon, two cats and three wild birds post the spillover incident, the scientists.
“The wild bird transmission events were restricted to the common grackle (Quiscalus quiscula), blackbirds, and pigeons. These animals were collected from premises with cattle where genotype B3.13 HPAIV was identified,” the scientists said.
The analysis revealed that the virus had moved to six different host categories including poultry, wild birds, cattle, wild mammals, cats and humans.
The authors warned that the possibility of the genotype B3.13 circulating in unsampled locations and unknown hosts cannot be ruled out as the current analysis indicated missing data owing to incomplete surveillance.