My co-author and I wrote a book https://www.press.jhu.edu/books/title/12896/preparing-next-global-outbreak to capture the learnings from the COVID-19 pandemic because we were convinced that it did not serve as the wake-up call that it should have and that memories would be short-lived. In fact, we saw how the global outbreak of MPox shocked the world in 2022 – 2023, even while we were still combating COVID-19, and now we are witnessing a painful reminder that unfortunately, we were right as we watch the U.S. government fail to contain an outbreak of bird flu in cattle and poultry that is spreading to humans.
I have written several previous blog pieces that provide readers with the basics about avian influenza viruses and the developments in this outbreak. This blog post will provide an update on some concerning developments. I’ll state at the onset that I don’t know whether the avian influenza A(H5N1) virus spreading in dairy cattle will develop into a pandemic, but I can state this: Continued failure to contain this epizootic disease (essentially, an epidemic in animals), to take more aggressive steps to understand the mechanisms of transmission of the virus from animals to other animals and from animals to humans, and to increase testing and surveillance of disease transmission to humans is increasing the odds in favor of whatever chance there is for this virus to develop pandemic potential.
A quick review of some of the developments I have previously written about that are of concern:
- Reports from public sources suggest that the virus has changed based upon what we have observed with more recent infections among California dairy herds:
- The virus has appeared to become more virulent in cattle. Previously, what little data has been shared suggested that the case fatality rate among dairy cattle was less than 2 percent. That fatality rate among California herds has been reported to be in the 10 – 15 percent range.We were previously led to believe that the infections in other states resulted in a week-long, or perhaps two, of mild illness in dairy cows. Now California cattle are reported to often be sick for several weeks.
- While we were led to believe that recovered cattle seemed to be completely recovered in other states, a California veterinarian stated that the recovered cattle seemed to only recover about 60 – 70 percent of their milk production.
- It is also possible that the transmissibility of the virus has changed:
- A California veterinarian estimates that as much as 50 – 60 percent of the cattle in the herds with these outbreaks are clinically ill, whereas previously, we were given the impression that isolation of infected cattle had contained the spread of infection to a small percentage of herds, perhaps on the order of 10 percent.
- I also reported on a recent study that demonstrated that the virus obtained from an infected farm worker in Texas was able to transmit through respiratory droplets, although not hugely efficiently, in animal models, and that the virus was capable of infecting human lung cells.
- Another concern of recent is the detection of the virus in pigs. The introduction of H5N1 into pigs increases the risk for genetic reassortments. Reassortments can occur when an animal is infected with two or more different influenza viruses. Pigs are hosts for both avian and human influenza viruses. We are also entering into the human flu season. If wild birds or cattle or poultry transmit H5N1 virus to pigs, and humans transmit H1N1 or H3N2 (our seasonal viruses) to pigs, the coinfection, which pigs seem to tolerate well, allows for the two viruses to swap entire segments of their genetic material with each other resulting in a novel virus that could acquire enhanced ability to transmit to humans, and even more concerningly, perhaps the ability for enhanced transmission from infected humans to non-infected humans.
So, where are things now?
The spread of infection is uncontrolled. In the U.S., for 2024, there are now (as of 11/5 for some of the data, and 11/8 for others):
- 51 jurisdictions with bird flu detected in wild birds;
- Confirmed H5N1 in 10,528 wild birds;
- 48 states with outbreaks among poultry;
- 105,197,601 poultry affected by outbreaks;
- 15 states with outbreaks among dairy cattle;
- 443 diary herds with outbreaks of H5N1 infection;
- 46 confirmed human cases (25 related to cattle exposures; 20 due to poultry exposures (there was one additional case related to poultry culling in Colorado in 2022 that is not included in this number); and 1 case from Missouri for which no potential exposure has been identified. There are an additional 4 suspected, but not confirmed, cases.
Source: https://www.cdc.gov/bird-flu/situation-summary/index.html.
Remember, each infection allows for the risks of mutations and the potential for the virus to develop enhanced transmissibility to mammals. As the human seasonal flu season begins, the risk for reassortments would be expected to increase.
While the human infections to date in the U.S. have been “mild,” it has just recently been reported that a teenager in British Columbia, Canada has been hospitalized with H5N1 infection, and there are unconfirmed reports that the child is requiring intensive care, though we have not been provided with any clinical details of the case. There are many concerns associated with this case:
- Though we have no clinical details, it would appear that this child must be sicker than the illnesses previously reported in the U.S. given the presumed need to hospitalize the child and the unconfirmed report that the child is in intensive care. [The question this raises is whether the strain of the virus acquired by this child had undergone genetic changes that allowed the virus to infect the child’s lungs as opposed to the limited eye infections that have been reported in the U.S. The results of genetic sequencing of the virus have not yet been made available.]
- No source of infection has yet been identified, or at least not yet reported. [The significance of this is whether the infection is a result of zoonotic spread (animal -> human) or whether there is undetected human-to-human spread. Thus far, no clear evidence for the latter has been identified, however, if that were to be the case, the pandemic risk would significantly increase.]
Finally, when the CDC conducted serology (antibody) testing of dairy farm workers, 7 percent were positive confirming that the number of infections are much greater in number than detected, and that many infections go undetected. https://www.cdc.gov/mmwr/volumes/73/wr/mm7344a3.htm. There are a few limitations to this study. First, it was conducted in two states (Michigan and Colorado), and thus, these results may or may not be translatable to what we are seeing in California. Given the data coming out of California, similar testing needs to be conducted in that state given the apparent changes to the virus virulence and potentially to transmissibility. Further, testing was limited to farm workers, so we cannot rule in or out potential forward transmission to family members or other close contacts.
It is notable that only 40 percent of those testing positive for H5N1 antibodies recalled having an unexplained illness. While there is no evidence to date that any of these farm workers were able to transmit the infection to anyone else (though in fairness, we really haven’t looked), recall that asymptomatic spread of disease is something that makes control of an outbreak much more difficult to control, as we saw first-hand with COVID-19.
It is also notable that only one farm worker was known to have worked with an infected cow. The implication is that cattle that were not clinically ill or identified as ill were able to directly or indirectly (e.g., through milking equipment) to transmit the infection to workers. This means that control of the outbreaks will be more difficult and that isolation of infected cattle will not be enough.
Finally, it was notable that none of those infected reported use of PPE, thus the one piece of good news is that we have every reason to believe that PPE will remain effective in controlling human infections among dairy cattle farm workers. On the other hand, the challenge is how to get workers to utilize it.
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