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Of Mice and Cows: Another Wake Up Call for Bird Flu

Gripe aviar en vacas y ratones
Photo: Canva

Bird flu is already a threat to wildlife, and the recent jump to cows and mice increases the risk to humans.


What do cows and mice have in common? They are the two latest species to be affected by the H5N1 avian influenza virus, which is wreaking havoc (it can be called a "panzoosis") on wildlife around the world. Although in the last few years the virus has spread from birds to many other mammalian species, including seals, bears, foxes, ferrets, cats and dogs, this new jump to cows and mice is particularly worrisome. Let's start with cows.

Cow-to-cow transmission

In March 2024, US authorities reported the first outbreak of H5N1 in cattle. In fact, the virus had been circulating under the radar among dairy cows for several months. Genomic analysis suggests that there was a single jump from bird to cow in late 2023, and that the virus has since spread within and between herds via milking machines, although the airborne route has not been ruled out. To date, more than 60 infected herds in 12 states have been reported.

The infection is not fatal in cows, but causes loss of appetite and reduces milk production. In cows, unlike other mammals, the virus replicates mainly in the mammary glands. This is why raw milk from infected cows contains high levels of infectious virus (something suspected after the death of farm cats and demonstrated in laboratory mice). The good news is that the virus is inactivated by pasteurisation.

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Cow-to-human transmission

There have been three documented human infections from dairy cows (one in Texas, two in Michigan). None of the three farmers became seriously ill: two of them had conjunctivitis, most likely after receiving milk splashes in their eyes, and the third also developed respiratory symptoms. However, given the number of herds affected, it is very likely that more people have been infected. The danger is that the more the virus jumps from cows to humans (and other species), the more likely it is to acquire the ability to transmit between humans (fortunately this has not happened, yet). Another concern is that because cows barely get sick, they could act as a reservoir for the virus, promoting recombination with other influenza viruses.

And now mice

The recent discovery of H5N1 in mice on affected farms in New Mexico (USA) is an additional cause for concern. Mice live in close proximity to humans and can transmit a range of pathogens, including Lassa fever virus, through their urine and faeces. The fact that mice have been found infected with H5N1 greatly increases the likelihood of new human cases. In addition, mice are ideal vehicles for carrying the virus from farms to other areas, including residential areas, which would make containment measures extremely difficult.

What is being done?

Surveillance: On the one hand, the US CDC is monitoring symptoms and presence of the virus (and hopefully soon of antibodies) in people who have been exposed to infected animals. It is also testing clinical specimens from the general population for the presence of type A virus, as well as trends in hospitalisations for influenza-like illness (so far, nothing abnormal has been detected). Finally, it is measuring the presence of type A virus in wastewater in all states; as expected, a high signal is observed in states with affected livestock.

Preparedness: Both Europe and the US are taking steps to purchase or produce pre-pandemic (i.e. before a pandemic is declared) vaccines to protect people at risk - those in contact with poultry or dairy cattle, and laboratory, veterinary and healthcare workers. In the US, the company Seqirus has said it can supply 4.8 million doses of its H5N1 vaccine (produced in cells rather than chicken eggs) and can produce up to 150 million doses in the first 6 months if a pandemic is declared. The authorities are also in discussions with mRNA vaccine manufacturers (Pfizer and Moderna) to develop potential pandemic vaccines.

In Europe, the EMA has licensed several vaccines that can be rapidly adapted to any strain if a pandemic is officially declared by the WHO or the EU. Finland, with all its ferret farms, has announced that it will start vaccinating workers at risk of exposure, becoming the first country to do so. Perhaps the most sensible thing to do would be to phase out the intensive farming of these (and other) animals.

Finally, the recent death in Mexico of a person infected (it is not yet known how) with another avian virus (H5N2) is a reminder that there are many influenza viruses out there that can infect humans. And a reminder that animal, human and planetary health go hand in hand. The international pandemic preparedness agreement is more urgent than ever.