The H5N1 subtype of avian influenza had been circulating in cows for months before it was detected in late March, according to a new pre-print analysis published by scientists from the US Department of Agriculture on Wednesday.
The H5N1 subvariant of bird flu has been increasingly reported in cattle in recent weeks, with 36 cases reported in nine states since March 25.
According to the analysis, which has not yet been peer-reviewed, veterinarians started noticing unexplained reductions in milk production, decreased feed intake, and changes in milk quality in late January, leading to testing which found the virus in milk and nasal swabs of the cows.
The scientists noted that the same genotype of the virus has been found in cattle herds with no known links to one another, indicating that there are affected herds that still haven't been found.
The scientists also found that considering all the cattle seem to have a type of the virus descended from the same type, it's likely that there was a single spillover event when the virus infected cattle and then it began spreading between cattle. The first infection is estimated to have been around early December, according to an analysis by the scientists.
Additionally, the scientists found that there were five incidents in which the virus was spread from cattle to poultry, one case of transmission from a cow to a raccoon, two cases of cats getting infected from cows, and three cases of wild birds getting infected from cows.
The analysis also found several cases of mutations in the virus associated with increased virulence and transmission, as well as mutations that allow the virus to adapt to infect mammals better, although all of these mutations were found infrequently in the tested cattle. However, the scientists noted that as the virus spreads more, it will give these mutations more chances to spread and develop, stressing that these should be monitored to detect whether or not they increase in frequency.
Outbreaks of avian influenza have been reported among cattle and other animals on dairy farms since March.
Outbreaks may have occurred among farm workers too
Last month, a person who had exposure to infected cows in Texas was found to be infected with the H5N1 subtype, with conjunctivitis being their only symptom. The infected person was treated with a flu antiviral drug and recovered.
While that person remains the only confirmed case of infection among dairy workers, health officials have expressed concerns that there may be more farm workers infected who aren't getting tested.
Dr. Barb Petersen, a dairy veterinarian in Amarillo, Texas, told NBC last week that several dairy workers had fallen ill with "classic flu-like symptoms," including high fever, sweating at night, chills, lower back pain, upset stomach, vomiting, and diarrhea. They also suffered from conjunctivitis. These workers were not tested for H5N1 so it remains unclear if they were infected with the bird flu or another illness.
Petersen told Fortune on Wednesday that she "had people who never missed work, miss work" on affected farms due to illness.
Dr. Keith Poulsen, director of the Wisconsin Veterinary Diagnostic Laboratory, told NBC at last week that he had also heard reports of flu-like illnesses on affected dairy farms.
Gregory Gray, an infectious disease epidemiologist at the University of Texas Medical Branch in Galveston, told NPR on Thursday that he suspects there are at least several cases of farm workers being infected with the virus, as some workers sought medical care for "influenza-like illness and conjunctivitis" at the same time that H5N1 was spreading on the dairy farms.
In total, about two dozen people have been tested since the outbreak began among cattle, Dr. Demetre Daskalakis, a CDC respiratory diseases official, said Wednesday, according to Fortune.
Health officials have called for more widespread testing, noting that there is a stigma around getting tested among many farm workers, partially due to concerns about affecting their farms. However, without widespread testing, it will be difficult for health officials and scientists to track the spread of the virus and monitor any developments in its ability to spread or cause severe illness.
Meanwhile, the USDA announced on Wednesday that testing on retail ground beef had found that all the samples had tested negative, reaffirming that the commercial meat supply is safe for consumers. The USDA is conducting further tests on muscle samples from slaughter facilities and to test if cooking beef containing the virus is effective in killing the virus.
Additionally, the FDA announced on Wednesday that it had found that pasteurization is effective in killing H5N1. No live, infectious virus was found in any samples from retail dairy products.
Outbreaks of bird flu continue among mammals
Outbreaks of avian influenza have been increasingly reported in mammals as the virus has continued to spread around the world in the past few years.
Since 2021, Europe and the Americas have been suffering from a nearly continuous outbreak of H5N1 avian influenza, which has been described as "the largest-ever" on the three continents. The virus has affected tens of millions of birds and thousands of mammals worldwide. Outbreaks of the virus have also become more common in Africa and Asia in the past year and have even spread to Antarctica in recent months.
The World Organization for Animal Health (WOAH) noted in March that, while estimates vary, about 485 bird species and 37 mammal species have been infected with avian influenza since 2021. Only the Pacific Islands, Australia, and New Zealand have been spared from the virus.
“The loss of wildlife at the current scale presents an unprecedented risk of wildlife population collapse, creating an ecological crisis,” noted WOAH.