Raw milk has made headlines recently for widespread samples containing the virus that causes H5N1, or bird flu, with the U.S. Department of Agriculture launching a program in December to track the virus through milk testing. But bird flu is far from the only disease-causing pathogen lurking in raw milk: Unpasteurized milk can contain bacteria such as E. coli and Listeria, for instance.
New Stanford Medicine research has found that another virus affecting both animals and people -- Rift Valley fever virus -- can remain active in raw milk samples for roughly as long as someone might want to drink it. Rift Valley fever is an acute viral illness spread to people through mosquitoes or the bodily fluids of infected animals. These findings mirror recent Stanford research showing that flu virus can be infectious in refrigerated raw milk for up to five days.
While Rift Valley fever virus mostly impacts sub-Saharan Africa, these findings have important implications for understanding human disease risk from drinking raw milk, said Brian Dawes, MD, PhD. He is an infectious disease fellow and postdoctoral researcher in Stanford Medicine's LaBeaud Lab and led the research. Desiree LaBeaud, MD, associate dean of global health, was the senior author. Alina De La Mota-Peynado of the USDA was co-lead author.
"We think Rift Valley fever virus in milk is a big unidentified threat, and our findings show that there's potentially a big problem there that needs to be looked at more closely," Dawes said.
Dawes discussed what the findings tell us about the risk of contracting disease from livestock, as well as the steps we can take to prevent future outbreaks. This interview has been edited for length and clarity.
Why is it important to understand how long Rift Valley fever virus remains infectious in milk?
Decades' worth of LaBeaud's work in Kenya has repeatedly shown that when you look for people who have antibodies to this virus, one of the biggest risk factors in every study is milk exposure. An earlier study from her lab confirmed that the virus can be found in milk. But until now, nobody has studied whether the virus could survive and remain infectious in milk.
We set up a simple lab study to determine whether the virus remains stable in milk or whether milk inactivates it. We found a nearby California farm willing to donate fresh, raw cow's milk. We brought it to the lab and immediately started experiments. We then tested the milk to see if the virus remained viable after we stored it in refrigerated conditions, at room temperature and in a warmer temperature similar to what you find in Kenya.
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What were your most important or surprising findings?
Over five days, there was basically little change in the amount of infectious virus in refrigerated milk -- the virus retained its full infectivity. In warmer conditions, we saw that the levels of virus decreased rapidly over the first 48 hours. In each condition, the virus remained active in the milk for roughly as long as someone would safely drink it. As we all know, unpasteurized milk is a problem and pathogens do grow in it, but to see that level of stability in the virus was surprising.
These findings suggest that people in areas where this virus is common are at risk of exposure to the Rift Valley fever virus if they are drinking raw, unpasteurized milk. People who milk cattle or who are involved in the informal milk transport and processing business are also at risk. But while these experiments and the field studies suggest a large risk of contracting the virus from contaminated raw milk, it's still a big unknown whether people can become infected and develop illness by drinking milk. This requires further study.
People reading this may think of bird flu (H5N1) and recent news of its presence in raw milk samples. How do your findings add to our understanding of how viruses might spread from animals to humans via raw milk?
This is a very similar situation, and it points back to this idea of One Health -- that human and veterinary health are closely linked and should be addressed simultaneously. All these animal products carry a risk of zoonotic infections, especially emerging viruses such as H5N1.
Yet in many parts of the world, there's limited or no surveillance in place to understand how these viruses are circulating in livestock and what risk they pose to people. For instance, widespread surveillance for H5N1 in cow's milk in the U.S. began only about a year after we started detecting the virus in cattle. We need to be more proactive about disease surveillance and research, while also thinking about how diseases that impact animals could affect humans.
Recent changes to federal health department staffing and funding have had tangible impacts on the continuity and reliability of bird flu surveillance. Our ability to detect novel threats requires steadfast and consistent commitment and support from key federal agencies.
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Speaking of surveillance, what does your publication tell us about how milk can be used for this tool?
In our study, we also measured RNA levels in the milk via PCR tests and found they remained constant during all these conditions -- which is great for surveillance. This indicates that we might be able to go out in the field and sample milk containers for the virus. It wouldn't tell us whether the virus is infectious, but it might help us understand how much of the virus exists in a given area, similar to wastewater surveillance.
This could be a simple way to monitor virus levels in a community, and it works in pasteurized milk -- which could make it much easier to obtain samples. However, we need further studies to understand whether this is a doable enterprise.
What are the most important takeaways of this research, and what questions do you hope to ask in future studies?
First, it affirms that pasteurization is super important and that we should continue to pasteurize whenever possible. Our study confirmed that pasteurization effectively kills the virus. Whether it's Rift Valley fever virus or the bird flu, pasteurization is highly effective at killing a host of pathogens that can be found in raw milk. It's a common-sense public health measure that has saved a ton of lives in the past and will continue to do so as long as we use it.
Second, our research underscores the need for a One Health approach to preventing epidemics that recognizes human and animal health as completely connected. As we respond to these challenges, there needs to be close coordination between human and animal providers and agencies.
Image: SimonSkafar via Getty Images
