As H5N1 avian flu continues to infect dairy cattle in the US, and because we still don’t really understand some important aspects of how it’s being spread on and between dairy farms, questions keep coming up about the risk of transmission of this strain of the virus from milk.


  • Relax. There’s really nothing to be concerned about.

For a more detailed answer, let’s start with a quick refresher on pasteurization:

Pasteurization was a revolutionary public health intervention that had huge impacts on human health. It was first developed in the 1800s, and ultimately became used as a way to practically and effectively reduce the risks of infectious diseases transmitted through milk, and extend the shelf life of milk (and other food products). Typically pasteurization of milk involves heating it to 72 Celsius (162F) for at least 16 seconds before cooling, or heating it to 63C (145F) for at least 30 minutes before cooling.

Does pasteurization kill H5N1 influenza virus?

We don’t have specific data on the effectiveness of pasteurization to inactivate the current circulating strain of H5N1 in milk. The US Centers for Disease Control has stated (repeatedly) that “… pasteurization has continually proven to inactivate bacteria and viruses, like influenza, in milk – which is an accurate statement. It doesn’t specifically say we know it kills H5N1 influenza, but we are confident in it because we know what pasteurization does to other organisms, including those that should be harder to kill than a flu virus

Why can’t we get a defiinitive answer to this question?

There wasn’t a lot of interest in testing the efficacy of pasteurization to inactivate flu in milk until now. We haven’t recognized cattle as a relevant source of flu virus in the past, so it’s not something anyone would study (or, probably more importantly, get funding to study).

Is testing now being done to confirm the efficacy of pasteurization against H5N1 flu?

Hopefully. We’ve had some discussions about it here, but it’s complicated because of the need to handle this virus using containment level/biosafety level 3 conditions. Most laboratories that work with milk pathogens (or most infectious diseases in general) use a lower level containment (Level 2); level 3 labs are much less common (because they’re also much more expensive to run). We’ve been arguing for years to get one at the university here… despite a series of new threats over the years when we’ve said “this is why we needed Level 3 lab!” no one’s ponied up the money; so we still can’t do this kind of testing here.

There are Level 3 labs elsewhere, and while they tend to be heavily used due to their limited number, presumably (hopefully) someone is working on this now. With the right level of containment, it’s not too complicated to do.

Does pasteurization work against other viruses?

That’s not been studied very intensively, since the main public health risks from milk are bacterial. Antibacterial effects of pasteurization are well described, and pasteurization methods are designed to kill those important bacteria. The good news is that influenza viruses tend to be much less stable than many of the bacteria that pasteurization is designed to kill.

What do we know about pasteurization and flu at this point?

We know that flu viruses aren’t very heat tolerant. That’s a good start. Based on that, and knowing how pasteurization is performed, we can come up with some reasonable interpretations of the available information.

There is a study that examined thermal tolerance of an older H5N1 strain in chicken meat (Thomas & Swayne, 2007) – thanks to @SafeFoodCanuck for sending this. It’s not the same as milk or pasteurization, but still gives us useful heat tolerance information. It showed that at 61C (lower than milk pasteurization temperature), the D value (the time required to eliminate 90% of viable virus) was 33 seconds. At 70C, the D value was 0.28 seconds. When we consider that milk pasteurized at 72C is held for at least 16 seconds and milk pasteurized at 63C is held for at least 30 minutes, that provides pretty good confidence that the process will inactivate flu virus.

What about that study that showed pasteurization wasn’t good for killing viruses?

There’s been a lot of social media buzz about a study that showed incomplete inactivation of foot and mouth disease virus (FMDV) in milk by pasteurization (Tomasula et al. 2007). However, FMDV is a very different and much tougher virus: it’s a non-enveloped virus that’s tolerant to lots of stressors, including heat.

Even so, there was still pretty good reduction of the viral load in milk; it’s not like it didn’t do anything. They found “residual infectivity was still detectable for selected pasteurized milk samples,” so pateurization didn’t consistently and completely eliminate this really tough virus, but still had a pretty big effect. They concluded “Although HTST pasteurization did not completely inactivate viral infectivity in whole and 2% milk, possibly because a fraction of the virus was protected by the milk fat and the casein proteins, it greatly reduced the risk of natural transmission of FMDV by milk.

As a result, we can’t use that study as an indication that flu virus won’t be inactivated by pateurization. Actually, I find it a bit re-assuring that there was such an impact on a much tougher virus like FMDV.

Beyond pasteurization, there are other layers of protection when it comes to the risk of transmission of H5N1 through milk:

1) Diversion of milk from sick cows

Dairy cattle and their milk are very closely monitored. Milk from sick cattle should not make it into the food chain. It’s usually discarded. That’s not a complete guarantee that all potentially contaminated milk will be discarded, since we don’t know whether cattle infected with H5N1 can shed the virus before they show signs of disease or if there can have subclinical infections (infections that don’t result in apparent disease). However, such cases would presumably be uncommon and contribute a proportionately very small amount of milk on any farm.

2) Dilution

A typical glass of milk doesn’t come from a single cow. All the milk from dairy cows on a farm that’s acceptable for human consumption first goes into a big bulk tank on the farm. That milk is then picked up and loaded into an even bigger tank on a truck. Then, it gets mixed with more milk from other farms, so there’s a massive dilutional effect. If a small amount of flu virus got into the bulk tank on farm, it would end up at a really, really low (and probably irrelevant) concentration in milk, even before it got to the pasteurization step.

Can we guarantee that there’s no ability of H5N1 to evade pasteurization?

No, but there’s not really any basis for concern. Personally, if/when H5N1 hits dairy cattle here in Canada, I’ll still confidently give my kids (pasteurized) milk.