by Fears are growing that the H5N1 outbreak among cattle in the United States may be the result of contaminated animal feed.
In contrast to Britain and Europe, American farmers are still allowed to feed cattle and other farm animals waste from other animals including birds.
The safety of that policy is now under review by the US Food and Drug Administration (FDA), a spokesman said on Tuesday.
Dairy cows across six US states – and at least one farm worker – have been infected with the highly pathogenic virus, which has already killed millions of animals around the globe since 2021.
The farm worker, who is believed to have been exposed through infected cattle in Texas, is only the second human H5N1 case recorded in the US – although 8,000 potential exposures are currently being investigated, according to Dr Joshua Mott, a senior consultant WHO on influenza.
The development is worrying because it allows more opportunities for the virus, which has killed millions of birds and wild mammals around the world, to mutate.
Experts fear that H5N1, which was not first detected in cows until a few weeks ago, could have been transmitted through a type of cattle feed known as “poultry litter” – a mixture of poultry excrement, spilled feed, feathers, and other waste that scratch off the. floors of industrial chicken and turkey production plants.
In the UK and EU, feeding cows protein from other animals has been strictly controlled since BSE – or ‘mad cow disease’ – broke out 30 years ago.
Experts are not sure but fear that the poultry litter feed used in the US gave the virus to cattle.
“In the US, feeding poultry litter to beef cows is a known factor in causing botulism in cattle, and is a risk factor for H5N1,” said Dr. Steve Van Winden, Associate Professor of Population Medicine at the Royal. Veterinary College.
Dr Tom Peacock, virologist and fellow at the Pirbright Institute, agreed: “This latest case is not the first time there has been concern that H5N1 could be moving through different mammals through contaminated feed,” noting outbreak of bird flu in cats. Poland last year, which experts may suspect was transmitted through mink by-products used in raw cat food.
The US cattle industry is worth over $100 billion and regulations covering animal standards have long been controversial in Europe – particularly regarding the use of hormones in raising cattle for meat.
While the presence of H5N1 in US cattle herds increases the risk of the virus entering humans through farm workers, the greatest threat is the spread of the virus to hog farms.
This is because pigs have receptors on some cells that are similar to 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 has shown no signs of a worrisome mutation.
“H5N1 infection in pigs is of particular concern – they are highly susceptible to human influenza virus strains and could act as mixing vessels for avian and human viruses to more easily infect humans mixing and generating effects,” said Dr Tom Peacock.
Poultry litter is not only cheaper than other food sources such as soya and grain but is also more calorie dense, meaning farmers can grow their flocks much faster.
According to a previous statement from the FDA, the practice is safe: “Regarding pathogenic microorganisms, drug residues and contaminants in poultry litter, the FDA is not aware of any data showing that the use of poultry litter in cattle feed is increasing affects people or animals. health risks that warrant restrictions on its use,” the agency previously noted.
However, this decision is now “under review”, the agency told The Telegraph on Tuesday.
There are many other theories as to how infected H5N1 cattle – identified so far in Texas, Idaho, Kansas, Ohio, New Mexico, and Michigan – brought the virus.
Many experts argue that the most likely route of infection is through wild birds – which have been found dead on some farms.
“This spread around the world comes back to wild and wild bird populations and where they land and where their faeces go,” said WHO’s Dr Johsua Mott.
“At some point, the contact with wild birds in the environment produced a virus that the cows were exposed to at that time, but how that exposure happened is what a lot of people are trying to figure out,” he said. .
It is also unclear whether the virus is spreading from animal to animal, Dr Mott said.
On each farm, many creatures are infected but this may be because they are eating from a common source of infection – fed birds or wild birds – rather than passing it on to another.
The director of ruminant health for the United States Department of Agriculture, Mark Lyons, suggested at a meeting last week that the virus could be transmitted through contamination of workers’ clothing, or the suction cups attached to cows’ udders during milking.
However, others argue that poultry litter cannot be ruled out as a source of contamination.
“Influenza can be spread by faecal-oral routes, so it is not impossible that chickens infected with H5N1 are passing live virus through their faces, which are then eaten by cattle, so it is a possible transmission mechanism , although there are other explanations,” said Dr Brian Ferguson, Professor of Infectious Diseases at the University of Cambridge.
“The BSE scandal showed us what happens when biosecurity is not a priority, and it showed us that it really needs to be a priority – which is not always the case, because of the economics involved,” he said.
Despite large-scale culling of poultry flocks during outbreaks to limit the spread, it seems unlikely that a similar approach will be adopted for cattle.
The CDC has advised farmers with affected herds to dispose of milk produced by infected cattle, although the pasteurization process is also thought to destroy the virus – meaning the risk to people who eat animal products remains low.
Currently, the WHO has stated that the risk to humans is considered low, but that surveillance efforts must be maintained.
“There were 12 cases of H5N1 worldwide in 2023, and a similar pace so far in 2024. Since it emerged in 1996, there have been over 800 cases worldwide.
“So you understand there’s nothing unprecedented about the number of human cases we’re seeing – but we have to look at the virus. We need to look at the epidemiology, to see if it’s changing in some way,” Dr. Mott said.
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