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In the USA, bird flu was detected in pigs. Why does this matter?
Nov
Last week, the U.S. Department of Agriculture reported that a pig at a backyard farm in Oregon had been infected with bird flu.
As the bird flu situation continues to evolve, we have learned that the A/H5N1 strain of the virus infects a range of animals, including a variety of birds, wildlife and dairy cattle.
Fortunately, we have not seen sustained spread of the virus between people at this stage. However, the detection of the virus in pigs represents a worrying development in the trajectory of this virus.
How did we get here?
The most concerning type of bird flu currently circulating is clade 2.3.4.4b A/H5N1, the influenza A strain.
Since 2020, the A/H5N1 2.3.4.4b virus has spread to a wide range of birds, wildlife and farm animals that have never previously been infected with avian influenza.
While Europe is a hotspot for A/H5N1, attention is currently focused on the US. In 2024, dairy cattle were infected for the first time, with over The problem affects 400 herds in at least 14 US states.
Bird flu has huge consequences for breeding and commercial food production, as infected poultry flocks must be slaughtered and infected cows may result in contamination dairy products. That said, pasteurization should make the milk protected to drink.
Although farmers have suffered massive losses due to H5N1 bird flu, it can also mutate to cause a pandemic in humans.
Birds and humans have different types of receptors in their respiratory tract to which influenza viruses attach, such as a lock (receptors) and a key (virus). The attachment of the virus allows it to enter the cell and body and cause disease. Avian influenza viruses are adapted to birds and spread easily among birds but not in humans.
So far, human cases have mainly occurred in people who stayed at the facility close contact with infected farm animals or birds. In the US, the majority were agricultural workers.
There is concern that the virus will mutate and adapt to humans. One key step for this to happen would be to change the virus’s affinity from bird receptors to receptors found in the human respiratory tract. In other words, if the virus “key” has mutated to better fit the human “lock”.
Recent testing of sample A/H5N1 2.3.4.4bz infected person, disturbing results were obtainedidentifying mutations in the virus that may enhance transmission between human hosts.
Why are pigs a problem?
A pandemic strain of influenza in humans can arise in several ways. One involves close contact between humans and animals infected with their own specific influenza viruses, creating opportunities mixing of genes of avian and human viruses.
Pigs are the perfect vessel for mixing genes to produce a human strain of pandemic flu because they have receptors in their respiratory tract that both avian and human influenza viruses it can get tied up.
This means that pigs can be infected with both avian influenza virus and human influenza virus. These viruses can exchange genetic material, mutate and easily transmit to humans.
Conversation, CC BY-SA
Interestingly, there were pigs in the past less susceptible to A/H5N1 viruses. However, the virus has recently mutated they infect pigs more easily.
In a recent case in Oregon, A/H5N1 was detected in a pig on a non-commercial farm following an outbreak of disease among poultry kept on the same holding. This A/H5N1 strain originated from wild birdsand not the one common in US dairy cows.
Pig infection is a warning. If the virus makes its way into commercial piggeries, it would create a much higher level of pandemic risk, especially as winter approaches in the U.S., when seasonal human flu cases begin to enhance.
How can we minimize the risk?
Surveillance is key to early detection of a possible pandemic. This includes comprehensive testing and reporting of infections in birds and animals financial compensation and support measures for farmers to encourage timely reporting.
Strengthening global surveillance of influenza is crucial because unusual spikes in pneumonia and severe respiratory illnesses could signal a human pandemic. Our EPIWATCH system looks for early warnings of such activity, which may accelerate vaccine development.
If a cluster of human cases occurs and influenza A is detected, further testing (called subtyping) is necessary to determine whether it is a seasonal strain, an avian strain resulting from transmission of the virus, or a modern pandemic strain.
Early identification can prevent a pandemic. Any delay in identifying an emerging pandemic strain allows the virus to spread widely across international borders.
The first human case of A/H5N1 in Australia was in a child who became infected while traveling in India and was hospitalized for the disease in March 2024. Tests then identified influenza A (which could have been seasonal influenza or bird flu), but subtype to identify A/H5N1 was delayed.
This type of delay could be costly if human-borne A/H5N1 emerges and is considered seasonal influenza because the influenza A test is positive. 5% of positive tests for influenza A undergo further subtyping in Australia and most countries.
In lithe of the current situation, there should be a low threshold for subtyping influenza A strains in humans. Rapid tests that can tell the difference between seasonal flu and H5 A flu are emerging and should form part of governments’ pandemic preparedness.
The risk is higher than ever before
The U.S. Centers for Disease Control and Prevention states that the current risk posed by H5N1 to the general public remains low.
However, with the H5N1 virus already able to infect pigs and showing worrying mutations in adapting to humans, the risk level has increased. Given that the virus is so common among animals and birds, the statistical probability of a pandemic is higher than ever before.
The good news is that we are better prepared for a flu pandemic than other pandemics because vaccines can be produced in the same way as seasonal flu vaccines. Once the genome of the pandemic flu virus is known, vaccines can be updated to match it.
Partially matched vaccines and some are already available countries such as Finland vaccinate high-risk farm workers.
