H5N1 contaminated meat caused the deaths of 30 Polish cats from bird flu

In early Summer 2023, there were reports of mysterious cat deaths in Poland. People reported pets with severe respiratory disease and neurologic symptoms, with death coming fast. Tests confirmed the cats were infected with avian flu, H5N1 clade 2.3.4.4b. Many of the sick cats were indoor cats with no outside exposure. The viruses from the cats carried a rare set of mammalian mutations. H5N1 was later found in a sample of frozen chicken meat from one of the cat owners, and that sample contained the same rare set of mutations as the cats. Many questions remain as to how dozens of cats became infected and died, and how the virus ended up in frozen chicken meat sold and purchased.

On June 21, 2023, many of us were savoring the summer sun. The longest day of the year can be bittersweet, for as we enjoy the last glints of sunlight, we know the days will begin to grow shorter. Last year, well after the last rays of sunshine had met the horizon, reports began to surface of mysterious and unusual cat deaths in Poland.

Cats in Poland had been dying quickly from an unknown disease. On June 18, a veterinarian in western Poland reported the death of a cat suffering from neurological and respiratory signs. All over the country cat owners began reporting shortness of breath and neurological symptoms as well as high fever in their pets. Whatever this disease was it progressed rapidly, with death usually occurring within 24 hours.

A database was created by the cat owners so that anyone could share information or if their cats were suffering similar symptoms. More and more people added their pet to the database, with the entries reaching almost 90. But no clear patterns emerged. The source of the deaths remained a mystery.

Avian flu was one of several suspects. As a precaution, cat owners were advised not to feed their pets raw meat and to avoid contact with birds. It was well known that the virus could be spread to other animals by contact with wild birds or poultry. But it was a puzzling prospect in this case, as many of the cats were indoor cats. With no outside exposure, it would be unlikely that wild birds or poultry could have infected the cats. Poland’s Chief Veterinary Officer initially downplayed concerns about avian flu, stating that: “there is no evidence, in particular no laboratory results, that could support the claim that the symptoms described in the media, observed in cats, result from infection with the avian influenza virus[.]” 

Studies were underway and, several days later, avian flu was confirmed as the cause of deaths for at least 9 of the cats. The strain was identified as H5N1 clade 2.3.4.4b.  

This set off alarm bells and veterinarians issued more warnings to cat owners. People were told to keep their cats inside their homes and away from wild birds. They were told to limit exposure to raw meat and thoroughly sanitize their hands and shoes before going inside. The public was assured that there had never been any confirmed “cats to humans” transmission of avian flu, with the caveat that the virus “can change dynamically” as shown by the recent spread to numerous species of mammals. The European CDC indicated in a risk assessment that too many uncertainties existed “to properly assess the risk to the general public.” 

But how did pet cats get infected with bird flu?

The first time a domestic cat had been infected with this strain of H5N1 was in France in late December of 2022. A family living next to a duck breeding farm, which tested positive for the virus, reported that their cat fell ill several days later. The cat developed severe neurologic and respiratory symptoms and was euthanized. H5N1 clade 2.3.4.4b was confirmed. The family had other pets but they were not infected, nor were there any human cases reported. 

There have also been cases in the United States. In January 2023, two cats from the same household in Nebraska tested positive for H5N1 and were euthanized after suffering severe neurological symptoms. In April 2023, another cat in Nebraska tested positive and died, and within two weeks four more cats from the same household died, although they were not tested for the virus. So it was already known that H5N1 could infect domestic cats, but thus far those have been isolated instances associated with direct exposure to wild birds.

How did so many pet cats get infected with, and die of, the virus in such a short time?

For this question, there was no clear answer. The cats were from 13 different regions across Poland. Some were indoor cats, and some were outdoor cats. Some cats had contact with birds and others did not. As the scientific community hunted for an answer, one clue came from a raw chicken sample submitted by one cat owner during the investigation. The chicken meat was positive for H5N1. This suggested that contaminated raw meat was a possible source of the infection.

In July, there were reports of some Polish scientists voicing concerns that the meat industry and the agriculture department were downplaying this potential source of infection. The scientists urged for testing of all meat for H5N1, emphasizing the potential threat not only to cats but also to humans. Poland’s poultry industry accounts for 20% of the poultry meat supplied in Europe, and therefore any risk of contamination would carry with it a wider concern for public health. This indictment of the meat industry was met with some backlash. Poland’s agricultural department reportedly disclaimed the idea that the country’s poultry products were contaminated with avian flu. The department drew focus to the heavy role Poland plays in the European poultry meat supply, and the risk that unfounded rumors of contaminated meat could case to the supply chain.

As tensions between the scientific community and the Polish poultry industry were flaring, there were, coincidentally, reports from other countries about recent H5N1 infections in cats.

In July 2023, at least 42 cats died in South Korea from H5N1.  It was first reported that two cats at an animal shelter in South Korea tested positive.  Further reporting revealed that, out of 40 cats at the shelter, 3 cats had died on June 24, 2024.  Within two days, 38 more cats died, all presumably from the bird flu virus.  Several days later, a cat at another animal shelter tested positive.  Ultimately there were four confirmed cases of H5N1 from the second shelter, and all of those cats died.  

As to the South Korean cat outbreaks, an investigation found that raw duck meat fed to cats in the second shelter was positive for the virus, and a recall was ordered for all pet food from the manufacturer.  It was presumed that the cats at the first shelter were fed the same food.  In August, an investigation found that one pet feed manufacturer had not complied with “proper sterilization and disinfection processes” in producing “balanced duck” and “balanced chicken” products sold to consumers from May to August 2023.

 

A few months earlier, in April 2023, a farm in Italy reported that one cat and several dogs had tested positive for H5N1. The animals were asymptomatic and were only tested after poultry on the same farm had tested positive for the virus.  There was no indication that these infections were related to consuming contaminated food.

Meanwhile back in Poland, there still was no clear answer to explain the unusual cat deaths. More and more cat owners had watched their pets suffer severe respiratory symptoms including pneumonia, and neurological symptoms like seizures before dying. The European CDC had reported in June that at least 24 cats tested positive for the virus. Investigations were ongoing to determine the source of the outbreak. During this time, it was confirmed that at least 13 of the infected cats been fed raw poultry meat prior to falling ill, but no tests on the specific food sources had been performed.

Ultimately, it was found that at least 30 cats tested positive for H5N1. Genetic analysis showed that the viruses from the cats were similar to an H5N1 virus found in a white stork and a virus in a backyard chicken a month earlier. H5N1 clade 2.3.4.4b had been present in wild birds and poultry since late 2022, but the mass infection of dozens of domestic cats was unusual and incredibly concerning. 

The analysis of the cat viruses revealed something even more usual: All of the infected cats had H5N1 with the same two mutations: E627K and K526R. These mutations are independently known as markers of viral adaptation to mammals, so finding just one of them would be noteworthy. It is incredibly rare to find them together.

So what does this all mean?

On August 3, two studies were published in Eurosurveillance aiming to provide insight into this mysterious outbreak of H5N1 in cats.

• The first study focused on the epidemiological investigation and analyzed information supplied by individual cat owners. The infected cats varied by age, breed, and sex, and were located all across Poland. They were fed a variety of food, some of which included raw poultry meat. The high similarity of the viruses pointed to a common source of infection, but what source? The distribution of cases all across the country pointed to “an unidentified intermediate food source, e.g. poultry meat contaminated with the virus, that had accidentally entered the cats’ food chain.” Further investigation would be needed to confirm the details of this potential source of infection. The study recommended heightened surveillance on poultry farms and on farmed mammals in close proximity to farms.
• The second study analyzed the frozen chicken meat that had been fed to the cats. Five cat owners had submitted samples of the food their cats ate before becoming sick, and as noted above, one of those samples tested positive for the virus. The authors of the study found “high levels of viral RNA” in the chicken meat. The virus from the chicken meat showed a “high degree of similarity” to the cat viruses. Remarkably, the meat virus had the same rare set of mutations as the cat viruses: E627K and K526R. This study stopped short of concluding that raw poultry meat was the source of the H5N1 infections, noting such was a possibility. Whether any other cat owners had raw meat also positive for the virus is unclear. But the presence of the virus in meat echoed earlier warnings from the Polish authorities, cautioning cat owners against feeding their pets raw meat.

Further, a case study published in September describes the clinical picture of one of the cats who died. This cat lived outdoors and was one of many that was fed raw chicken meat. Upon examination, it was found that the virus caused a “systemic inflammatory response” in the cat and affected many internal organs, including the lungs and brain. Notably, two other cats who lived in the same house died after showing severe respiratory symptoms. All three were cats that went outside and were fed raw chicken meat, so it was concluded that they likely were infected from the “same external source” although the possibility of transmission was not ruled out entirely. The vivid details of the cat’s condition after death need not be outlined here. It is sufficient to say that these animals suffered before they died.

No conclusions as to how all these cats became infected were stated with certainty. Based on all the facts, contaminated raw poultry meat seems the most likely cause here, especially given that the cats were located all over Poland, and some were solely indoor cats. 

This conclusion can be reached even without factoring in the rare set of mutations, and yet the occurrence of these mutations really stands out. . . 

All the Polish cat viruses carried the same pair of rare mutations.

 

Indeed, the Polish cat viruses were the only viruses known to contain both the E627K and K526R mutations.  That fact, taken with the finding that the raw chicken contained the same rare mutations, provides compelling evidence.  

The E627K mutation is a known mammalian adaptation marker and has been well-studied. The K526R mutation is also known for promoting viral adaptation in mammals. E627K has “frequently been acquired by the virus after transmission to mammals.” (In addition to the Polish cats, it was found in the French cat). Logic suggests that mutations promoting adaptation in mammals, like E627K or K526R, would not be helpful for viral replication in birds, and thus would not be favorable traits in birds, right? So the fact that H5N1 viruses circulating in wild birds could contain these mutations is interesting. It is also indicative of prior mammalian infections. Indeed, E627K was found in a white stork positive for H5N1 in Poland a month before the cats began dying. The stork virus did not carry the second mutation, K526R, and its absence “suggests that the virus that had spilled over to cats was already partially adapted to mammalian species.” Typically, avian flu adapts to mammals after it infects mammals, so this may indicate the virus spilled back into wild birds after initially spilling over into a mammal.

It seems incredibly remarkable, and alarming, that all of the cat viruses and the virus from the raw chicken sample were similar to each other and carried the same rare mutations. Even more alarming was the fact that the H5N1 positive raw chicken meat had reportedly been from the cat owner’s refrigerator and had been “purchased fresh for human consumption” in early June. 

Should we have concerns about the safety of the food chain?

The virus-positive frozen chicken meat still gives me pause. The fact that someone had purchased raw chicken, stored it in their fridge, which was ultimately found to be positive for H5N1, is incredibly concerning. The event invites questions as to how the poultry industry ensures that avian flu does not enter the supply chain, and if any other cat owner in Poland had contaminated poultry meat sitting in their refrigerator. 

These will likely remain questions, as only one cat owner supplied a food sample. But it stands to reason that if one person had chicken meat contaminated with the virus, others probably did as well given the fact that all the cat viruses were similar to each other.  

Indeed, if raw poultry meat was the source, that would mean that the poultry was infected with H5N1 while they were alive, before they became cat food, right? Therefore, that would mean the farm or supplier that the poultry came from had an H5N1 outbreak go undiscovered (or unreported). Or perhaps the poultry became infected during transport to wherever they went to become cat food. Or else the raw poultry meat was somehow contaminated with the virus thereafter. Any of these scenarios invites questions about the quality control measures in place. It is incredibly important that effective measures are taken to ensure that avian flu is kept out of the poultry meat supply, regardless of whether that meat is meant for pets or humans.

Is there a risk to human health?

The loss that many cat owners in Poland suffered is tragic, as any pet owner would know. No one imagines that one trip to the store for cat food (in this instance raw cat food) will lead to your cat’s death. The good news is there were no human cases reported. 

None of the infected cats transmitted the virus to their owners, despite the fact that cat owners likely had some close contact with their pets before or during their illness. The lack of human cases speaks to a lower level of risk to people right now. Indeed, the CDC and the WHO has continually reported that the risk of infection to the public remains low.

But the Polish outbreak does draw focus to a broader concern: that domesticated animals could be an intermediary between birds and humans for the spread of H5N1. 

Pets that spend time outdoors, like dogs and outdoor cats, would presumably be in contact with birds or their droppings at some point. There have already been instances of domestic cats being infected with this clade of H5N1. The absence of human cases in Poland is encouraging, but the fact that the cats did not spread it to their owners does not mean it could never happen. Indeed, the “close interactions and proximity” of cats and humans has been cited by the CDC as a concern as that, along with the “rapid selection of mutations” in a mammalian host, “could result in a virus with potential for interhuman transmission, indicating a considerable public health threat.” Their role as a potential intermediate host may warrant further study as the current clade of H5N1 is showing itself more fit to infect mammals than the virus has before.

Given the unprecedented scale of this H5N1 panzootic, pet owners should exercise more vigilance, even if you have a cat that does not go outside. The Polish cat owners were warned to sanitize their shoes before coming back inside their homes, and the obvious purpose behind this direction was to mitigate against the risk of tracking in bird feces contaminated with the virus. Nothing from the CDC indicates that domestic pets are at a high risk for the virus, but the level of risk is different depending upon the circumstances. For animals that have exposure to wild birds, the risk is clearly higher than for animals that do not. Although in the case of the Polish cats that did not seem to matter, as it was most likely their food source that caused the infections.

Hopefully there won’t be any more instances like this, as there is already enough to worry about with bird flu.

Until next time.