Bird flu can spread to and be passed on via the mammary glands of mammals, a study has confirmed, explaining why the first outbreak in dairy cows is proving so “alarming”.
Scientists have reiterated their warning that people must not drink “raw” or unpasteurised milk as it carries the risk of being infected with the H5N1 virus. Four cases have been reported in humans linked to an outbreak of what is being dubbed bovine or “cow flu” in the United States.
The US Centers for Disease Control and Prevention (CDC) has described the outbreak in cattle as an “ongoing multi-state outbreak”, prompting fears that the virus may have taken a step closer towards posing a serious problem for humans.
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A new study in the journal Nature has found that mice who drank milk from a cow from New Mexico that was infected with bird flu became infected themselves. They also passed on the virus to their pups via their own milk.
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It shows that bird flu can spread to the mammary glands of mammals and be passed on through their milk. The possibility had not been investigated while the virus was largely contained to birds, which do not have mammary glands.
The study from researchers at the University of Wisconsin-Madison also found, however, that ferrets which became infected by drinking contaminated cow milk did not pass the virus on via their breath.
Professor Ian Brown from the Pirbright Institute, which researches infectious diseases in farm animals, said that a “mouse model … does not always produce data that can be directly correlated with humans” but said the study “emphasises the need for strong monitoring and surveillance”.
Dr Ruth Harvey, deputy director of the Worldwide Influenza Centre at the Francis Crick Institute in London, said the study’s findings are “of concern as it suggests a possible increase in the potential of this virus to infect other mammals, including humans, although more research is needed to confirm how this would happen in practice”.
Scientists have reassured people that there is still no sign of “onward transmission” from human to human.
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Bird flu has killed hundreds of thousands of birds globally, spreading around the world along migration routes. It has briefly crossed into humans, with a few hundred known cases over the past 27 years, half of which have proved fatal. It does not currently appear to spread between humans, but scientists fear that future mutations could allow this to happen.
When the UK Health Security Agency (UKHSA) conducted surveillance of poultry workers last year, it found four cases, but three were asymptomatic and did not belong to the same type of H5N1 that is currently spreading in the US. The UKHSA has not changed its assessment “that bird flu is primarily a disease of birds and the risk to the general public’s health is very low” in the UK at the present time.
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Dr Ed Hutchinson, from the MRC-University of Glasgow Centre for Virus Research, who was not involved in the study, warned: “This new H5N1 influenza virus would be even harder to control and even more dangerous to humans if it gained the ability for effective respiratory spread.”
He added: “The ongoing outbreak of H5N1 influenza in American dairy cows was a shock for virologists. It was surprising because cattle were not a known host for this sort of influenza virus. It was also surprising, and alarming, because very large amounts of H5N1 virus are shed into the milk of infected cattle.”
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He said that the risk for humans “applies only to unpasteurised ‘raw’ milk” as studies have shown that “pasteurisation of milk effectively inactivates influenza viruses”.
He said the new study wanted to investigate “how a virus that entered through the mouth or nose was getting into milk” and said: “Until the virus turned up in an animal we harvested milk from, we hadn’t paid much attention to the mammary glands. Now that we’re looking, it seems like spreading into the mammary glands is something that any of these highly pathogenic H5N1 viruses could do.
“These findings reinforce the need for urgent and determined action to closely monitor this outbreak and to try and bring it under control as soon as possible.”
