With yet another pandemic in the news, I thought it prudent to talk about bird flu. While this new pandemic hasn’t officially started yet (in humans, at least) and there’s no guarantee that it will, scientists and health organizations the world over are taking seriously the possibility of a situation similar (though not identical) to Covid-19. Allow me to explain why…
The Disease
I’ve talked about influenza before in one of my first posts here, and I would encourage you to re-read it. But as a refresher, Influenza A is a family of viruses with a high transmissibility (R0 averages between 0.9 and 2.8) and a lot of variance. You’ve certainly heard terms like H1N1 or H5N1, which are subtypes of Influenza. Influenza viruses have two important proteins on their surface, hemagglutinin (H) and neuraminidase (N), and these are the proteins by which one’s immune system recognizes the threat. Scientists have identified 18 different subtypes of H and 11 different subtypes of N, with each subtype being better or worse at infecting any particular species. These subtypes can be further divided into different strains, slightly different variants of each H or N protein that can have different virulence, deadliness, and resistance to the immune system. In general, an antibody response to a strain of one subtype of a flu protein will be generally effective against every other strain within that subtype; antibodies for one strain of H1 will recognize all other strains of H1. That said, the fast mutability of flu means that new strains are constantly appearing and changing, so each new strain becomes more resistant to antibodies of a related strain with enough time.
Bird flu, or avian influenza, refers to any flu strain that primarily affects and can jump between species of birds, both wild birds and human livestock such as chickens. Since birds fly they can transport the virus a very large distance thus increasing its effective infectiousness, and the enormous global poultry industry means there is plenty of opportunity for humans to come into contact with infected birds. Viruses very rarely jump species,* but having a large number of infected birds increases the chances that some birds will carry a mutant strain that can infect humans and a lot of interaction between humans and said animals increase the chances that a human will be nearby for this strain to make the jump. These species-jumps can make for particularly dangerous viruses because they will often have proteins and antigens that no human immune system has seen before, similar to Covid-19 when it first originated.
While other strains of flu have been called bird flu in the past, the dominant strain of avian-carried influenza for the past two decades has been H5N1. Scientists first noticed this subtype among livestock in Guangdong, China in 1996 and it was quickly found to be able to infect a wider range of bird species than previous strains, primarily poultry and waterfowl, and even some mammal species such as pigs and cats. In 1997, the first human cases were identified in Hong Kong, specifically eighteen cases with six deaths. The first major outbreak was in 2003, with chicken farms throughout southeast and far east Asia experiencing high infection rates and die offs among their animals. At the same time, there were 35 human cases in Thailand and Vietnam resulting in 24 deaths. All of these human cases were chicken farmers or close family of said farmers, so there was very little human-to-human spread of the disease. The following three flu seasons would see the outbreak spread further, causing infected farms and human cases and deaths in other parts of the world, but the overall infection rate among humans has remained low. To date, there have been less than 900 human cases of bird flu globally in the past twenty years.
The Present
While H5N1 has remained active in bird populations and has been causing human cases ever since it first emerged, the disease has been in the news recently due to a significant increase in animal cases. This outbreak has been ongoing since 2020 but has in recent months become globally spread. The primary cause of this major outbreak appears to be a particular new strain of hemagglutinin-5 (H5) called H5-2.3.4.4b, which has been found to be significantly more virulent. Flu subtypes containing this new H5 were detected in birds as early as 2018, but only in flu subtypes that couldn’t infect non-avian species such as H5N6 and H5N8. Different H and N subtypes have different species they are better suited for infecting, with humans only being particularly susceptible to flu subtypes containing H1, H2, H3, N1, or N2, with the most common human flus having two of these such as H1N1. But when two subtypes of flu infect the same organism, they can trade their Hs and Ns to create new subtypes. This is believed to be what happened in 2020 when the first cases of H5N1 containing the new H5-2.3.4.4b strain were identified. N1 is able to infect numerous species of birds and mammals, so this new flu subtype has the potential to become a big problem.
Since 2020, this new strain of H5N1 has been found in numerous species of poultry, waterfowl, seabird, shorebird, passerine, raptor, feline, canine, bear, seal, mustelid, cervid, pig, and cow, and on every continent including Antarctica. This is not to say that all these species and all these locations are experiencing a high transmission rate or death toll, but many of them are. In the United States, the CDC has reported sporadic outbreaks amongst poultry farms in multiple states over the past two years. Currently, there are over a hundred dairy farms in the US experiencing major outbreaks as the virus can be transmitted from cow-to-cow. As of writing, there have only been four cases in the country of humans catching bird flu, all of whom worked directly with infected dairy cows. While viruses that have just jumped species can be more deadly since their host isn’t adapted to fight the virus, these viruses are often far less transmissible since they’re not adapted to work within the new species. It’s to be expected that the virus can’t jump from human-to-human at this time, at least not well enough to cause a pandemic in humans.
For now, the risk posed to humans by bird flu is minimal. But flu viruses evolve incredibly rapidly, as evident by the fact that it’s jumped to so many new species in only a few years. A wider spread virus has more opportunities to interact with humans and thus infect humans. More H5N1 viruses inside humans means a higher chance that one of these viruses can “learn” how to work with the human body well enough to jump from one human to another. Once the virus can do this, it can continue to improve its infectiousness in humans until it’s infectious enough to cause a pandemic. What such a pandemic’s R0 value or its fatality rate would be can’t be predicted ahead of time, though it would likely be within the same range as other flu subtypes. Based on data from the previous outbreaks, roughly half of H5N1 avian influenza patients die from the disease. That said, these were all cases of exposure from an animal which might not be representative of a human pandemic; again, there’s a limit to how well we can predict the hypothetical transmissibility between humans from only the transmissibility between birds. Also, the patients from these outbreaks were disproportionately likely to be from rural regions of developing nations, whose relative lack of access to healthcare might be skewing these results.
There’s no guarantee that bird flu will turn into a pandemic in humans, but such a pandemic is likely enough and dangerous enough that health organizations around the world have already started preparing for the worst. Fortunately, H5N1 avian influenza has existed for twenty years, so there are existing tests and vaccines against it. Also, as I’ve covered before, there is a lot of existing infrastructure for dealing with flu, from vaccine manufacturing to effective antiviral drugs. Unfortunately, health organizations like the CDC have already started running into the same problems they had during the Covid-19 pandemic. The CDC is mass-producing bird flu tests, but they are sending them to the laboratories of public health organizations instead of the clinical and medical labs that the majority of patients would use. Health organizations have also been extremely slow in granting these smaller clinical labs the legal license to use these tests or to grant these labs access to the CDC’s data. This testing is vitally important to determining if a pandemic has started and who needs to be treated once it has. Encouraging the cattle farmers most at-risk of exposure to get tested regularly has also run into problems, mostly because these workers often don’t have the health benefits or paid leave one would need to take a major illness seriously. And while health organizations do keep stockpiles of bird flu vaccine and can manufacture more quickly if needed, there are still concerns about distribution and sufficiency. Some of these problems are just normal issues with emergency logistics or with striking a balance between a fast response and an effective response, but there are undoubtedly some broader administrative and social issues at play here that we really should have learned from the last pandemic.
As it stands, the risk posed by bird flu to the public is still very low. In a year’s time, a human pandemic could start up and we might all be wearing masks again. In a year’s time, the whole thing could fizzle out and be forgotten by the public at large. Everything I’ve talked about here is the preparations health officials are making for a worst-case scenario, which is exactly what we should want them to be doing. The good news is that these agencies have had so much advanced warning and they have been preparing for the worst. And flu is a disease we have much more experience and infrastructure for combating, not to mention the advances in vaccine technology that were developed from Covid. For now, the CDC only advises people to avoid direct contact with wild birds, wear prophylactic protection if you work directly with dairy cattle and chickens, don’t consume unpasteurized milk or improperly cooked chicken (don’t do that regardless), and continue to listen to health officials as things progress. I will certainly continue to write about this topic if it continues to get more interesting.
For More Details
*Specifically, viruses very rarely add a new species or clade of species to their repertoire of infectability. Adapting to a new host species means developing adaptations to a new physiology, new proteins, and new immune threats, all of which take time. In theory, a virus that has jumped species before is more likely to be able to jump to a new species again, but in practice, this can be skewed by how closely related the two host species are and how much regular contact the two host species regularly have.
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