H5N1 Bird Flu Adaptation in Cats in South Dakota as a Lethal Neurotrophic Virus Raises Alarms

Medical News: Virus adapted to target the brains of cats aggressively
In recent years, a dangerous form of bird flu known as the highly pathogenic avian influenza H5N1 virus has rapidly spread across various parts of the world, infecting not only birds but also an expanding list of mammals. Initially detected in poultry and wild birds, this strain eventually made its way into animals such as foxes, skunks, sea lions, mink, dolphins, raccoon dogs, and even a small number of humans. Now, a new development has emerged that is causing alarm: the ability of this virus to infect and harm domestic cats in a very aggressive and lethal manner. Researchers from the University of Pittsburgh in the United States have examined evidence, providing vital insights that are covered in this Medical News report.


H5N1 Bird Flu Adaptation in Cats in South Dakota as a Lethal Neurotrophic Virus Raises Alarms

An Unexpected Threat to Our Feline Friends
For decades, cats were not considered a major concern when it came to bird flu. However, this perception has changed. In April 2024, an unusual and distressing event took place in rural South Dakota, where a group of ten outdoor domestic cats mysteriously died. These beloved pets, aged between six months and four years, were part of a rural household and often came into contact with birds, other farm animals, and wildlife. They were commonly fed raw meats, including poultry, and lived in an environment shared with cattle. Researchers from the University of Pittsburgh - USA found that these cats were infected with the H5N1 clade 2.3.4.4.b virus. This particular strain had already been making waves globally due to its ability to infect a wide range of species, and its presence in domestic cats is a major cause for concern.
 
Bird Flu Is Spreading Beyond Birds
H5N1 first emerged in China back in 1996, causing outbreaks in geese and then spreading to other bird species. Over time, it evolved into different groups known as clades. One of these, clade 2.3.4.4.b, is now recognized for its extraordinary ability to cross species boundaries. Since 2020, this clade has caused widespread infections among birds in Europe, Africa, Asia, and the Americas, affecting poultry and wild birds on a massive scale. Alongside birds, more than 21 mammalian species have also been infected, including cattle. This extensive spread in both birds and mammals is unprecedented and raises the potential of the virus adapting even further.
 
The South Dakota Outbreak: A Disturbing Discovery
The cats in the South Dakota outbreak exhibited symptoms including lethargy, anorexia, and potential neurological deficits. Necropsies performed at the North Dakota Veterinary Diagnostic Laboratory revealed that the cats were infected with the H5N1 clade 2.3.4.4.b virus. Genetic sequencing further confirmed that the virus closely matched strains previously identified in local dairy cattle, suggesting a possible spillover from infected cattle or birds.
 
Histopathological analyses provided striking evidence of the virus’s neurotr opic nature, with severe lesions and high viral loads observed in the cats’ brains, particularly in the cerebellum and hippocampus. Immunohistochemistry revealed an extensive presence of viral antigens in these regions, far surpassing levels found in the respiratory tract. The study also noted unique mutations in the viral genome, hinting at possible adaptations to feline hosts.
 
Neurological Impacts of H5N1 in Cats
The study’s findings highlight the virus’s pronounced neurotropism in domestic cats. Key observations include:
 
-Neuronal Necrosis: Significant death of nerve cells in critical brain areas.
 
-Gliosis: Increased proliferation of glial cells in response to brain injury.
 
-Perivascular Inflammation: Immune responses concentrated around blood vessels in the brain.
 
-High Viral Load: Intense viral antigen staining, particularly in the cerebellum and hippocampus.
 
Notably, the study also found widespread co-expression of sialic acid receptors in cat tissues. These receptors are compatible with both avian and human influenza viruses, potentially enabling cross-species transmission and further adaptation of the virus.
 
Digging Deeper into the Virus
Genetic testing showed that the viruses found in the cats closely matched H5N1 strains circulating in local dairy cattle and wild birds. This suggests that the cats may have become infected either by direct contact with infected birds or by consuming contaminated raw meats. It is also possible that exposure to infected cattle or the environment around them played a role.
 
Further genetic analysis revealed some unique mutations in the virus. These included a change known as T143A in the hemagglutinin protein (the part of the virus that allows it to bind to host cells) and two additional changes in another viral protein called PA. Such mutations might help the virus replicate better in cats and increase its ability to cause severe disease.
 
How the Virus Attacks the Cat’s Body
In the infected cats, researchers found damage in multiple organs. The lungs showed signs of pneumonia, inflammation, and tissue damage. More surprisingly, the virus was found in large quantities in the brain, where it caused cell death, inflammation, and serious harm to nerve cells. The cerebellum, cerebrum, hippocampus, and brainstem were heavily affected. Viral proteins were detected in these brain regions, confirming that the virus not only reached the brain but actively replicated there. The findings indicate an alarming trend that the virus is evolving to be more neurotrophic and is more intense on targeting the CNS system and brain.
 
Why Are Cats So Vulnerable Now?
One crucial factor is the way the virus attaches to certain receptors on host cells. Birds and humans have different types of sialic acid receptors in their tissues, which influenza viruses use as entry points. Birds typically have receptors known as SA α-2,3, whereas humans have SA α-2,6 receptors. It was once believed that viruses from birds would have difficulty infecting mammals because they prefer SA α-2,3 receptors. However, domestic cats appear to have a wide distribution of both SA α-2,3 and SA α-2,6 receptors in their lungs, intestines, and even their brains. This wide receptor distribution means that cats can serve as a sort of “mixing vessel” where different strains of influenza viruses might exchange genetic material, potentially creating new hybrid viruses that could spread to humans.
 
Cats as a Potential ‘Mixing Vessel’
Because cats come into regular contact with humans and may be exposed to both avian and mammalian strains of influenza, they could allow these viruses to reassort and form more dangerous variants. If such a hybrid virus emerged that could spread easily among humans, the consequences could be severe. Although no human cases have directly resulted from infected domestic cats so far, the possibility is no longer a distant hypothetical situation. As the virus continues to adapt, every infected mammalian host increases the risk that the virus could gain the ability to transmit between humans with greater efficiency.
 
The study’s findings are consistent with earlier reports from Texas and Europe, where H5N1 infections in cats have been linked to neurological symptoms and significant mortality. These cases underscore the virus’s ongoing adaptation to mammalian hosts and highlight the need for enhanced surveillance and preventive measures.
 
Precautions and Public Health Measures
To protect cats and the people around them, it is important to understand the potential sources of infection. Keeping cats indoors, especially if there are known outbreaks of avian influenza in nearby birds or farm animals, can reduce their risk. Avoiding feeding cats raw or undercooked poultry can also help. More broadly, farmers, pet owners, and public health authorities need to remain vigilant, monitor local bird populations, and maintain good hygiene practices. Keeping an eye on unusual behavior in cats, such as unexplained neurological signs or sudden deaths, is vital.
 
A Changing Landscape of Infection
The ongoing spread and evolution of H5N1 clade 2.3.4.4.b is transforming the way scientists and public health officials think about influenza. No longer limited to birds or specific mammals, this virus is demonstrating that it can reach across species barriers. It can thrive in multiple hosts, adapt to new environments, and mutate in ways that increase its pathogenic potential. This is a dynamic and evolving situation, and the role of domestic cats has become a new area of focus. As beloved household pets that bridge indoor and outdoor worlds, cats represent a perfect sentinel species that can warn us of emerging viral dangers.
 
Conclusion
The presence of H5N1 clade 2.3.4.4.b in domestic cats is a stark reminder that influenza viruses are not static entities; they constantly change, find new hosts, and adapt to different environments. In this case, the virus has shown unexpected neurotropism, meaning it prefers to infect brain tissues, causing severe neurological damage in infected cats. Combined with the detection of unique mutations, this highlights the virus’s remarkable flexibility. More importantly, the discovery that domestic cats have both avian and human-type receptors suggests that they could become key intermediaries in the emergence of new influenza strains that are more capable of infecting humans. While we are not yet witnessing widespread cat-to-human transmission, the situation demands attention and action. A coordinated One Health approach, involving veterinarians, farmers, wildlife experts, medical doctors, and public health officials, is needed to monitor outbreaks, protect animal and human populations, and develop preventive strategies. Ultimately, by understanding the evolving relationship between viruses and their hosts, we can remain better prepared for whatever challenges future outbreaks may bring.
 
The study findings were published in the peer reviewed journal: Emerging Microbes & Infections.
https://www.tandfonline.com/doi/full/10.1080/22221751.2024.2440498
 
For the latest H5N1 News, keep on logging to Thailand Medical News
 
Read Also:
https://www.thailandmedical.news/news/h5n1-news-as-h5n1-now-targets-domestic-cats-across-the-united-states-data-shows-that-67-percent-of-infected-felines-do-not-survive
 
https://www.thailandmedical.news/news/warning-sars-cov-2-infections-are-widespread-in-cats-and-could-lead-to-mutations-escaping-human-immunity-upon-spillback
 
https://www.thailandmedical.news/news/china-identifies-new-feline-coronavirus-that-is-extremely-lethal-and-could-pose-a-threat-to-humans