Fears are growing that the H5N1 outbreak among livestock in the United States could be caused by contaminated livestock feed.
Unlike Britain and Europe, American farmers are still allowed to feed livestock and other farm animals ground-up waste from other animals, including birds.
The US Food and Drug Administration (FDA) is now scrutinizing the safety of that policy, a spokesperson said on Tuesday.
Dairy cows in six US states – and at least one farm worker – have been infected with the highly pathogenic virus, which has killed millions of animals around the world since 2021.
The farm worker, who is believed to have been exposed through infected livestock in Texas, is only the second recorded human H5N1 case in the US – although 8,000 possible exposures are currently under investigation, said Dr Joshua Mott, WHO senior adviser on flu.
The development is worrying because it gives the virus, which has killed millions of birds and wild mammals around the world, more opportunities to mutate.
Experts fear that H5N1, first discovered in cows just a few weeks ago, may have been transmitted through a type of livestock feed called “poultry litter” – a mix of poultry feces, spilled feed, feathers and other waste that comes from the manure is scraped. floors of industrial chicken and turkey production plants.
In Britain and the EU, feeding cow protein from other animals has been tightly regulated since the outbreak of BSE – or ‘mad cow disease’ – three decades ago.
Experts are unsure, but fear it may be the poultry litter used in the US that has transmitted the virus to livestock.
“In the US, feeding poultry litter to beef cows is a known factor in the cause of botulism in cattle, and a risk in the case of H5N1,” said Dr Steve Van Winden, Associate Professor of Population Medicine at the Royal Veterinary College.
Dr. Tom Peacock, a virologist and fellow at the Pirbright Institute, agreed: “This latest case would not be the first time there have been concerns that H5N1 could spread through various mammals via contaminated feed,” referring to the outbreak of bird flu in cats in Poland last year, which experts suspected could have been transmitted through mink by-products used in raw cat food.
The US livestock industry is worth more than $100 billion, and animal standards regulations there have long been controversial in Europe – especially over the use of hormones in livestock farming for meat.
While the presence of H5N1 in U.S. livestock herds increases the risk of the virus spreading to humans from farm workers, the spread of the virus to pig farms poses the greatest threat.
This is because pigs have receptors on some cells that are similar to those of humans, making it much more likely that the virus could mutate and jump to humans if pig farms become infected.
So far, however, the virus shows no signs of worrisome mutation.
“Infection of H5N1 in pigs is of particular interest – they are highly susceptible to strains of human influenza virus and thus could act as mixing vessels for avian and human viruses to mix and generate viruses that can infect humans more efficiently,” said Dr. Tom Peacock.
Poultry litter is not only cheaper than other food sources such as soy and grains, but it is also higher in calories, meaning farmers can increase their flocks much more quickly.
According to previous FDA statements, this practice is safe: “With regard to pathogenic microorganisms, drug residues, and contaminants in poultry litter, the FDA is not aware of any data indicating that the use of poultry litter in livestock feed poses a human or animal hazard. health risks that justify restrictions on its use,” the agency previously noted.
However, this decision is now “under review”, the agency told The Telegraph on Tuesday.
There are several other theories about how H5N1-infected cattle – identified so far in Texas, Idaho, Kansas, Ohio, New Mexico and Michigan – contracted the virus.
Many experts say the most likely route of infection is through wild birds – which have been found dead on some farms.
“The spread of this around the world has to do with feral and feral bird populations and where they land and where their droppings go,” WHO’s Dr Johsua Mott points out.
“At some point, contact with wild birds in the area produced a virus to which the cows were exposed, but how that exposure occurred is something many people are trying to figure out,” he added.
It’s also unclear whether the virus spreads from animal to animal, Dr. Mott said.
Multiple animals on each farm are infected, but this could be because they are feeding on a common source of infection – feed or wild birds – rather than passing it on to another.
The U.S. Department of Agriculture’s director of ruminant health, Mark Lyons, suggested at a meeting last week that the virus could possibly be transmitted through contamination of workers’ clothing or through the suction cups attached to cows’ udders during milking are confirmed.
However, others argue that poultry manure cannot be ruled out as a potential source of contamination.
“The flu can be spread via fecal-oral routes, so it is not an impossible scenario that chickens infected with H5N1 shed live virus through their faces, which the cattle then eat. So it is a potential transmission mechanism, although there are other explanations,” said Dr Brian Ferguson, professor of infectious diseases at the University of Cambridge.
“The BSE scandal has shown us the reality of what happens when biosecurity is not a priority, and has shown us that it really needs to be prioritized – which is not always the case due to the economics involved” , he added.
Despite widespread culling of poultry flocks during outbreaks to limit the spread, it appears that a similar approach will not be taken for cattle.
The CDC has advised farmers with affected herds to throw away milk from infected cattle, although the pasteurization process is also thought to destroy the virus – meaning the risk to people consuming animal products remains low.
Currently, the WHO has said the risk to humans is considered low, but surveillance efforts should continue.
“There were twelve H5N1 cases worldwide in 2023, and a similar rate so far in 2024. Since the disease emerged in 1996, there have been more than 800 cases worldwide.
“So you get the sense that there’s nothing unprecedented about the number of human cases we’re seeing – but we need to keep an eye on the virus. We need to keep an eye on the epidemiology to see if it changes in some way,” said Dr. Mott.
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