NIH scientists are sounding public health alarms after discovering a highly pathogenic avian influenza (HPAI) H5N1 virus in a Texas farm worker earlier this year.1 The virus, dubbed huTX37-H5N1, was isolated from the worker’s eye and appears to have originated from contact with infected dairy cows. While the worker experienced only mild conjunctivitis, the virus proved lethal in laboratory tests on mice and ferrets. This alarming finding has scientists asking if bird flu can be transmitted in cow’s milk.
This incident raises serious questions about “bird flu milk” and the growing trend of consuming raw, unpasteurized milk. While proponents tout potential health benefits, raw milk can harbor dangerous pathogens like the H5N1 virus, putting consumers at risk of severe illness. The presence of bird flu on dairy farms underscores the importance of pasteurization in ensuring milk safety and protecting public health.
Concerned researchers described, in an October 2024 Nature research paper, how this newly identified strain of bird flu is transmitted through the air more easily than other bird flu strains. The virologists based at University of Wisconsin-Madison watched as airborne respiratory droplets spread the disease between laboratory animals. Healthy ferrets were housed in cages near infected ferrets. They found that 17-33% of healthy ferrets became infected and also developed severe disease and died, similar to the deliberately infected ferrets.
When the scientists analyzed the genome of this conjunctivitis-causing flu strain, they discovered that the huTX37-H5N1 bird flu had a mutation that allowed it to replicate more efficiently in mammals, increasing its threat to humans.
Should we worry about bird flu in cow’s milk?
The huTX37-H5N1 virus is not the first example of a highly pathogenic strain of bird flu being found on dairy farms. It’s not even the first case this year.
Earlier in 2024, there was a large outbreak of highly pathogenic bird flu in US cows, with the virus spreading between herds and even infecting other animals like poultry and cats.2 Epidemiologists have raised concerns about the potential for human cases. Highly pathogenic avian influenza (HPAI) H5N1 viruses usually only infect birds. While they can sometimes infect mammals, bird flu viruses don’t usually spread easily between them, but these recent cases on dairy farms appear to be an exception.
So should we be worried about bird flu in milk?
Given that dairy products are a staple food in North America, public health researchers have begun to look into whether bird flu can be transmitted through cow’s milk. In July this year, the Wisconsin academics showed that HPAI H5N1 infected cows have bird flu viral particles in their milk, and that feeding ferrets infected cow’s milk made them sick. What’s more, infected ferrets and mice went on to accumulate HPAI H5N1 virus in their mammary glands (where milk is produced).
While the virus was easily transmitted via milk, it didn’t seem to spread so fast between the ferrets in different cages. Only one out of four ferrets exposed to the virus showed signs of infection, but no virus was actually detected in their bodies.
While this might seem reassuring, DNA sequencing revealed that this strain of bird flu has mutated to allow it to infect humans more easily. The researchers tested the virus to see how well it could bind proteins on the surface of respiratory cells. Flu viruses use these salicylic acid receptor proteins to enter cells and infect them. The HPAI H5N1 virus was able to bind to receptors in the human upper respiratory tract (nose and throat).
All dairy milk is pasteurized, so there is very little risk of infection from normal milk, however raw milk has not been heat treated to kill off viruses making it a potential source of transmission.
Understanding H5N1
In contrast to the relatively difficult to spread Bovine H5N1 detected in cow herds in the spring, huTX37-H5N1 appears to be airborne with the ability to infect human upper respiratory tract.
Laboratory experiments revealed that huTX37-H5N1, the dairy worker virus, can infect human respiratory tract cells, raising concerns about its potential to cause severe illness in humans. However, the researchers noted that most human cases reported during the current H5N1 outbreak have presented with mild symptoms, primarily conjunctivitis.
They speculate that prior exposure to seasonal influenza viruses might offer some level of protection.
The study also provided reassurance in the form of treatments. The researchers found that huTX37-H5N1 is susceptible to certain antiviral drugs, including favipiravir, baloxavir marboxil (Xofluza), and zanamivir.1.
References
- Gu C, Maemura T, Guan L, et al. A human isolate of bovine H5N1 is transmissible and lethal in animal models. Nature. Published online October 28, 2024. doi:10.1038/s41586-024-08254-7
- HPAI in livestock: Animal and Plant Health Inspection Service (no date) HPAI in Livestock | Animal and Plant Health Inspection Service. Available at: https://www.aphis.usda.gov/livestock-poultry-disease/avian/avian-influenza/hpai-detections/livestock (Accessed: 07 November 2024).
- Eisfeld AJ, Biswas A, Guan L, et al. Pathogenicity and transmissibility of bovine H5N1 influenza virus. Nature. 2024;633(8029):426-432. doi:10.1038/s41586-024-07766-6
