NEW YORK – A team of US researchers has traced the spread of highly pathogenic avian influenza (HPAI) in cattle back to an initial bird-to-cattle transmission event that took place toward the end of 2023. The virus appears to have spread from there to other dairy herds directly from cattle to cattle.
"Our genomic analysis and epidemiological investigation showed that a reassortment event in wild bird populations preceded a single wild bird-to-cattle transmission episode," Tavis Anderson, Mia Kim Torchetti, and Suelee Robbe-Austerman, researchers at the US Department of Agriculture and the study's co-corresponding authors, and their colleagues wrote in a paper published in Science on Thursday. The study included researchers from the USDA's Agricultural Research Service and the Animal and Plant Health Inspection Services, as well as from other US centers.
"The movement of asymptomatic or presymptomatic cattle has likely played a role in the spread of HPAI within the United States dairy herd," they added.
For their analyses, the investigators focused on the spread of H5N1 HPAI A virus from a hemagglutinin phylogenetic clade related to the goose/Guangdong 2.3.4.4b clade, which has been involved in ongoing outbreaks since reaching North America toward the end of 2021 and is believed to have pandemic potential.
So far, the virus has caused massive wild bird mortality and prompted large culls of infected poultry, the authors wrote. It was identified in dairy cattle in Texas in the spring of 2024 after veterinarians encountered animals with altered milk production and lower-than-usual food consumption and has since been found in several other states in the US.
"It is critical to determine how HPAI clade 2.3.4.4b evolves in wild birds and in nonhuman mammals after spillover to assess the potential for human infection and transmission," the authors explained.
With that in mind, the team performed phylogenetic analyses on genome sequences of HPAI H5N1 isolates in wild birds, poultry, and US dairy cattle — including 227 viral isolates collected from cattle between early March and early April of 2024 and 1,250 viral genomes representing isolates collected between April and late October of that year — and low-pathogenicity avian influenza strains.
They also included epidemiological information, the Global Initiative on Sharing All Influenza Data (GISAID) resource, and the Influenza Research Database Sequence Feature Variant Tool to narrow in on a single avian spillover event in Texas in mid- to late 2023 that led to under-the-radar transmission between cattle until the end of March 2024.
From Texas, their study showed, the virus spread in cattle to Idaho, New Mexico, Ohio, Kansas, and Michigan before spreading further from Michigan to South Dakota and North Carolina.
"The goal of this study was to analyze genetic sequence data collected after the introduction of HPAI H5N1 in late 2021 into the Atlantic flyway of North America and its onward circulation and reassortment with North American wild bird-origin low-pathogenicity viruses," the authors wrote, noting that those data "were combined with whole-genome sequence data and epidemiological information from the HPAI H5N2 outbreak among US dairy cattle."
The reassorted H5N1 influenza A virus behind the spillover event has since spread from cattle back to birds and to other animals in or around affected farms, including cats, the team reported. Along the way, the virus appears to have undergone some subtle, low-frequency genetic changes linked to virulence and host specificity.
"The potential remains for these viruses to acquire other genetic markers that mediate the probability of transmission in mammalian hosts," the authors suggested, adding that "[m]onitoring cattle and other agricultural animals will inform future risk assessments and provide an early warning for whether this interspecies transmission event and dissemination of the viruses represents a future threat to human health."