H5N1 Update

The virus marches on while we inch towards answers

Jun 20, 2024

It’s been 3 months since H5N1 (bird flu) was reported in American dairy cows. Many of you are asking for an update. So, here we go! A snapshot of where we are today.

Note: If you’re new here and unfamiliar with H5N1, I recommend starting at this previous post to catch up. Glad you’re joining us for the scientific discovery ride!

Is it still spreading?

Yes.

More than 105 dairy herds across 12 states have reported H5N1. Unfortunately, this isn’t the full picture because of the lack of testing. USDA data is also lagged, and we don’t know how many herds have “recovered” (are no longer infected). States continue to report other infected animals, such as alpacas, house rats, and cats.

Three human cases have been linked to this cow outbreak, but there is no onward spread, so the risk to humans is still “low.” Because of limited testing, there are likely more cases we don’t know about.

In Mexico, a fatal human case of H5N2 was reported. Notice the N is different from the virus in American cows. Flu has two proteins on the outside—an H and an N—and each has many different forms.

This death was concerning because:

We don’t know how he got it. He had no contact with birds.
Not great odds. This was the first documented human case of H5N2, which resulted in death.
It might have been linked to H5N1 in the cow outbreak because it has the same H5. After genetic testing, though, it’s clear it’s separate and unrelated to the virus causing cow outbreaks.

Can H5N1 be contained? This is the million-dollar question.

Two possibilities:

It will burn out. The virus will stop finding new cows and die off. This can be accomplished by limiting exposure, or sometimes viruses stop spreading for reasons we don’t know. However, given a new USDA epidemiological report, I’m skeptical. We aren’t doing the work to stop the virus:

0% of farms have enrolled in voluntary milk testing
Only 11% of herds have used federal funds to increase testing or biosecurity
60% of farms said they moved their animals even if they were sick
60% of herds were “closed,” meaning no new cows came in; this means they got sick from another source, perhaps equipment or humans working on other farms.

If it doesn’t burn out, it will become endemic among cows, like we are seeing with wild birds. Given how closely humans work with cows, this would have massive implications, giving the virus more opportunities to jump from cow to human.

What would this look like if it jumped to human spread?

Scientists are trying to figure this out.

Enter ferrets. Ferrets have similar respiratory tracts to humans, so they’re a good proxy. Scientists have found four things so far:

H5N1 can spread even if the ferret has only an eye infection. Protective wear, like goggles, will help.
It can spread through respiratory pathways. This only happened with one ferret with the current strain, but would likely be more if the virus mutates.
H5N1 can result in severe disease. All the ferrets (6/6) died after eye or respiratory exposure. Importantly, the ferrets were immune naive (never exposed to flu before; see more below).
Humans don’t have immunity. Or do we?

The vast majority of us have not been exposed to H5N1, but we are consistently exposed to its relative (H1N1; common flu) every winter. So, we may have some level of immunity. Previous reports have shown the general population (at least in Hong Kong) has significant levels of protective antibodies.

Recently, CDC tested people’s antibodies against H5N1 using blood samples from across the U.S. from the 2021-2023 flu seasons. None had meaningful antibody responses to H5N1, regardless of flu vaccination history.

So does that mean we are screwed? Not necessarily. The type of test used wasn’t included in the press release. The classic test is to see whether red blood cells clump after being exposed to the virus in the presence of antibodies. If there is clumping, antibodies are not blocking flu’s ability to bind its receptors on the red blood cells, suggesting that the antibodies aren’t protective in the amount present. However, this type of test misses a lot of antibodies that could be protective.

We need more clarity on this study—and more studies—to understand its implications.

Why is Michigan a hotspot? It happens to be for good reasons.

Michigan is at the center of the H5N1 outbreak. It has reported the most infected herds and has one human case.

Figure by Helen Branswell; Source Twitter

Why? Michigan may differ a little regarding susceptibility and dairy trade, but the difference likely lies in the response. Because public health is decentralized, states are in the driver’s seat. Infrastructure, financial support, and leadership make a huge difference in public health.

In other words, Michigan is actively looking for H5N1, and it’s reflected in the numbers. For example, they set up a texting program with farm workers asking about their symptoms regularly. This is how they found the third human case in the U.S.

Vaccines: U.S. is getting ready, Finland is vaccinating.

We have over 40 vaccine formulas ready for bird flu, but the formulas would need to be scaled and distributed. The federal government started manufacturing 4.8 million vaccines in case this jumps to humans on a larger scale. They will be ready this summer.

However, two things are unclear:

What is the “oh crap” moment to start vaccinating the front line? It’s unclear in the U.S., but Finland is starting to vaccinate its farm workers.
Will they be effective? While they may match the current virus among cows well, H5N1 must mutate substantially to infect humans efficiently. Vaccine effectiveness will likely be hit. Plus, historically humans have had a harder time responding to H5 than others.

What’s next? This fall brings additional risk.

Flu viruses mutate by either “drifting” with small changes or “shifting” with dramatic changes. Big shifts happen when someone or something (like a pig or possibly a cow) is infected with two viruses at once, and the viruses “swap” genetics and become more effective at spreading among humans.

If H5N1 continues to spread into fall and winter, there is added risk, as regular flu (H1N1, H3N2, and Flu B/Victoria) will also spread, providing an opportunity to swap.

Bottom line

Bird flu continues to spread. As scientific discovery unfolds, it’s important to remember that pandemics are stopped by people. Only time will tell if we have the wherewithal to prevent an H5N1 flu pandemic.

Love, YLE