The H5N1 avian influenza virus transmits between mammals via contaminated cow’s milk and reaches the mammary glands
The H5N1 avian influenza virus can be transmitted between mammals, according to a study published in Nature. The research team isolated the virus from the milk of an infected cow in New Mexico (USA) and confirmed that it spreads in mice and ferrets, reaching the mammary glands of both animals. Furthermore, the virus was also transmitted from infected lactating mice to their offspring. The European Centre for Disease Prevention and Control (ECDC) published a press release this Monday recommending increased vigilance for these viruses.
A scientific team has confirmed the most feared scenario: the highly pathogenic avian influenza virus, which has been spreading across US dairy farms for months, has managed to jump from cow to cow, and from cattle to cats and to a raccoon.
The researchers, from Cornell University, have issued a warning: “efficient and sustained mammal-to-mammal transmission is unprecedented and is concerning because it may allow the virus to adapt, improving its infectivity and transmissibility to other species, including humans.” Their study was published on 24 July in the journal Nature and in it the authors call for strict measures to prevent transmission to cattle and “reduce the risk of a pandemic in humans.”
The Cornell researchers, led by Brazilian virologist Diego Diel, have observed that the H5N1 subtype has a strong capacity to infect udder cells and concentrate in milk, although it also appears in the lungs of cattle. The scientists speculate that the pathogen may be transmitted via the respiratory or oral route, but also suspect it could enter through the teat orifice from contaminated ground or milking machinery. As for the cats and the raccoon, the hypothesis is that they became infected by drinking contaminated milk. Two months ago, a mouse experiment demonstrated that raw milk could contain viruses capable of transmitting the disease.
Meanwhile, virologist Elisa Pérez is concerned about what is happening in the USA, where 168 affected herds have been identified across 13 states since the first case was detected in a cow on 25 March. “It is very worrying, because it implies that cattle constitute a new reservoir for avian influenza, at least for this particular genotype, and that they can act as a source of infection for other species, both avian and mammalian. This is something that had never previously been observed with this virus, as the only natural reservoir for avian influenza was birds,” states the expert from the Animal Health Research Centre in Madrid. Infected cattle may display symptoms such as reduced appetite, digestive disorders, respiratory problems and decreased milk production. To date, outbreaks in livestock have only occurred in the United States.
The nine farms assessed in the Cornell study are located in Texas, New Mexico, Ohio and Kansas. These are open-air farms where wild birds come into contact with dairy cattle, both directly and indirectly, through contaminated feed or water. The first affected farms are situated along a route commonly used by migratory birds crossing North America.
US epidemiologist Maria van Kerkhove, Director of the WHO’s emerging diseases unit, declared two months ago that Covid-19 would not be “the last pandemic we will deal with in our lifetimes.” Diel points to H5N1 from his laboratory at Cornell University. “It is difficult to predict which virus will be next, but given the propensity of influenza viruses to cause pandemics, it is very important to closely monitor the H5N1 situation in dairy cattle,” he explained in statements to the newspaper EL PAÍS.
On 25 January 2024, a scientific team found version B3.13 of the 2.3.4.4b subtype of the H5N1 virus — the root of the unprecedented outbreaks in dairy cattle — in a Canada goose in Wyoming. Whole genome analysis of the virus in cattle has not detected mutations that would favour spillover to humans.
Pérez highlights that cattle on a farm in Ohio became infected following the arrival of asymptomatic livestock from another facility in Texas. “This confirms that apparently healthy cows can transmit the virus. That is, not all cattle develop clinical signs following infection. This has very significant implications for surveillance systems,” the virologist explains. “With this data, it is clear that we must not only conduct passive surveillance (testing only samples from symptomatic cattle), but it is also essential to include active surveillance programmes, such as, for example, testing milk from bulk tanks across all livestock operations, as will be the case in Colorado, one of the states most affected by avian influenza in cattle.”
For further reading:
-. Caserta, L.C., Frye, E.A., Butt, S.L. et al. Spillover of highly pathogenic avian influenza H5N1 virus to dairy cattle. Nature 634, 669–676 (2024). https://doi.org/10.1038/s41586-024-07849-4
-. Avian Influenza on NeXusAvicultura.com