Hey clickbait headlines: pasteurization kills H5N1 flu virus in milk
"Bird flu fragments" are not live virus. Can we stop exacerbating health anxiety and eroding public trust in science?
A few weeks ago, reports of bird flu (officially highly pathogenic avian influenza, HPAI H5N1) starting circulating as a person was infected through a non-bird intermediate. In this instance, it was a cow. While H5N1 influenza viruses can infect other animals aside from birds, including humans and other mammals, this was the first well-documented instance of a person contracting H5N1 from a secondary host.
If you recall, I discussed the virology of influenza viruses previously, here. However, it is time for an update, as more data are emerging to suggest that H5N1 is more widespread, including among cattle, than anticipated.
Highly-pathogenic avian influenza (HPAI) H5N1 is a serious concern for bird species
This outbreak of HPAI H5N1 has actually been ongoing for several years. Normally in wild birds, low pathogenic avian influenza (LPAI) is always present in populations. LPAI rarely causes high mortality in birds.
Unfortunately, HPAI can outcompete LPAI and can near 100% mortality rate in birds, especially domestic poultry. HPAI virus evolves as a result of wild birds infected with LPAI interacting with poultry where the viruses mutate to HPAI.
Unfortunately, because HPAI outcompetes LPAI even among wild birds, HPAI is now circulating more broadly, with wild birds also being killed at high rates by H5N1 (this is a topic for a broader ecological discussion in the future).
What this also means is that there are more opportunities for HPAI to infect less ideal hosts, including non-bird species like humans, cows, and others.
H5N1 has been detected in dairy cattle across 8 states
Currently, 33 herds of dairy cattle have known presence of H5N1. Based on surveillance data, H5N1 is being spread between wild birds, poultry, and cows, all of which are known transmission routes for H5 influenza viruses.
As a result, the USDA has implemented several checkpoints to reduce the risk of ongoing spread, including requiring of testing of lactating dairy cows before transport. This testing involves testing unpasteurized milk directly from these cows for presence of virus.
In order to determine if viral fragments detected are part of intact virus, we must do live viral assays. For flu viruses, we conduct these either by inoculating chicken eggs with the sample to see if we can grow live virus, or by doing the same in cell culture, usually cells called MDCK (Madin-Darby Canine Kidney) cells.
These assays allow us to assess whether the sample contains intact influenza virus that can replicate and infect host cells, or just fragments or inactivated components. However, these assays take longer, because we must incubate them for a period of time in order to see if virus replicates.
The H5N1 reported to be detected in pasteurized milk is NOT live virus.
The FDA and news outlets reported that H5N1 had been detected in commercially available pasteurized milk samples, leading to a frenzy of fear among people who drink dairy products. This detection was based on the previously described PCR-based testing, which cannot differentiate between viral fragments and live virus.
Note: pasteurization is very effective at inactivating potentially harmful microorganisms, including influenza viruses. That’s why we’ve been pasteurizing food products for over 100 years.
The National Institute for Allergy and Infectious Diseases confirmed yesterday that the pasteurized milk samples do not contain active influenza virus.
Pasteurization is an effective method to render pathogens inactive.
Pasteurization is one of the greatest microbiological technologies that improved public health. It was invented by French microbiologist Louis Pasteur in 1862, and it involves heating a food product in order to kill or render inactive potential microorganisms so they cannot replicate and cause food-borne illness.
Today in the US, the primary method used to pasteurize milk is called High Temperature Short Time (HTST) pasteurization.
In this process, the milk is rapidly heated to a specific temperature (usually around 161°F or 71.7°C) for a short period (usually 15 seconds), then rapidly cooled. This kills potentially harmful bacteria such as E. coli, Salmonella, and Listeria, as well as inactivates viruses such as influenza viruses, herpesviruses, parvoviruses, and others that may be found in dairy milk.
This rapid heating and cooling also preserves the taste of milk while limiting protein denaturation, thus also preserving the nutrition of milk.
More than that, influenza viruses are contracted via the respiratory route. Our GI tract does not contact the sialic acid receptors that the virus needs to infect cells, and our gastric acids would denature and inactivate residual virus.
As such, pasteurized milk is safe to drink.
Raw milk products are not safe to consume, now or ever.
Raw milk products are products that have not been pasteurized. While pasteurization was adopted commercially for milk products in 1882, and the first law requiring pasteurization was passed in Chicago in 1908, there are a group of people who, without scientific training, claim that raw milk is superior.
Just to be clear: raw milk offers no benefits and has serious risks associated with it.
Raw milk products have been the cause of over 200 food-borne disease outbreaks in the US in last 20 years. These rates are increasing as certain states relax laws that previously prohibited the sale or distribution of raw milk products.
78% of single state raw milk outbreaks occurred in states where the sale of unpasteurized milk was permitted. The majority of illnesses and hospitalizations as a result of consumption of raw milk are in children.
Raw milk can contain dangerous bacteria such as Salmonella, E. coli, Listeria, Campylobacter, and others that cause foodborne illness, often called “food poisoning” as well as a variety of viruses that can be found in cows, some of which, like influenza viruses, can be transmitted to humans.
The consumption of raw milk and raw milk products is discouraged by all credible scientific and regulatory agencies. Why would you put your family at risk for absolutely no reason?
Pasteurization inactivates H5N1 viruses. Milk that is not pasteurized can contain active H5N1 virus.
While the likelihood of infection of flu viruses through ingestion is rare, there are potential routes of exposure (like inadvertent inhalation or aspiration) around raw milk products that aren’t worth the risk, in addition to the other risks of raw milk.
H5N1 flu viruses do not efficiently infect humans. This is good news, for now.
During this recent outbreak, there has been 1 reports of human H5N1 in early April, where a man in Texas was infected through exposure to a dairy cow. However, testing among people is currently voluntary, so it is unclear if there are additional cases.
While H5N1 can cause more severe illness and mortality compared to seasonal flu in humans, it does not effectively infect us. There are a few reasons for this:
Receptor Binding Specificity: all influenza viruses bind to sialic acid receptors. H5N1 influenza viruses preferentially bind α2,3-linked sialic acid receptors (SAα2,3), which are predominantly found in the respiratory tracts of birds. This preference for SAα2,3 receptors in avian hosts contributes to the virus's ability to infect and replicate efficiently in birds.
In humans, the predominant sialic acid receptors in the respiratory tract are α2,6-linked sialic acid receptors (SAα2,6). The binding affinity of H5N1 to SAα2,6 receptors is much lower. This means H5N1 is going to less efficiently attach to and infect our cells.
Replication Efficiency in Human Cells: H5N1 virus replication is less efficient in human cells compared to avian cells. The enzymes required for replication of influenza viral RNA, the polymerase complex, have been found to have reduced activity and specificity in human cells compared to avian cells, which limits the speed and ease at which H5N1 could replicate and infect cells in people. In addition, our innate immune system is able to recognize and eliminate H5N1 more effectively than the immune systems of birds, thereby reducing likelihood of infection and viral spread.
Transmissibility: Although H5N1 can infect humans through direct contact with infected birds or their droppings, it doesn't spread easily from person to person. This is crucial for a virus to cause large-scale outbreaks or pandemics.
However, this could change with ongoing spread of H5N1 among birds, cows, and other species.
Right now, H5N1 can infect humans, but doesn’t do so very readily. However, current genomic surveillance does indicate that H5N1 may be evolving increased transmissibility among non-bird species.
This is obviously a serious concern for global ecology and biodiversity, which I plan to discuss in the future. In addition to that though, it means that humans possible points of exposure may increase.
So if the current H5N1 virus mutates through antigenic drift or even the more gradual antigenic shift, it could lead to a new H5N1 (or an entirely different flu virus) that can not only cause high mortality in humans, but also infect us more more readily. That could be the conditions needed to trigger another global outbreak.
On the flip side, mutations could also lead the virus to become less deadly - remember, mutations are random, so anything can happen as viruses replicate and errors during replication occur.
What should you do?
At the moment, nothing too different from what you’re already doing, unless you drink raw milk, then stop doing that.
The US government has a stockpile of H5N1 flu vaccines in case.
We also have a stockpile of chickens that can lay eggs that we produce flu virus in for vaccine manufacturing (fun fact: the neighboring lab at my graduate school was one of the main labs that developed seed virus for flu vaccine, and our walk-in incubator was always filled with chicken eggs).
There are some countries that are working on bird-specific (and even cow-specific) vaccines for H5N1, which could limit and slow the spread amongst poultry populations and also prevent unnecessary death of these animals.
I listed other prevention methods in my previous piece on avian flu, here.
A call to action: help stop the spread of misinformation.
Misinformation travels faster and further than credible facts. We all can play a role in limiting the spread of claims that are created to evoke emotions. We also need to elevate subject matter experts and those who are working to provide real-time updates.
Unfortunately, we are living in an era where science rejection is getting all too common, and if we’ve learned anything from the COVID-19 pandemic and the current measles surges around the world, anti-science disinformation threatens our safety in more ways than one.
If you see misinformation, correct it. There are lots of credible resources out there that we can use to combat falsehoods, even if the social media and media algorithms are not playing in our favor.
And finally, don’t panic, but be aware. And steer clear of fear-mongering and clickbait headlines.
Thanks for joining in the fight for science!
Thank you for supporting evidence-based science communication. With outbreaks of preventable diseases, refusal of evidence-based medical interventions, propagation of pseudoscience by prominent public “personalities”, it’s needed now more than ever.
Your local immunologist,
Andrea