The H5N1 strain of bird flu has been in the news a lot in 2024. It has caused the culling of millions of domestic birds and has infected farmworkers in various states, most recently in Colorado. To learn more about this complicated issue, The Conversation U.S. spoke with Jenna Guthmiller, a skilled immunologist and microbiologist from the University of Colorado Anschutz Medical Campus. She explains the history of H5N1, how it spreads, and important ways to reduce contact with this tough virus.
H5N1 is a subtype of the influenza A virus, which is part of a family of viruses known for causing seasonal outbreaks in humans, like the well-known H1N1 and H3N2 strains. But unlike these other strains, which mainly affect people, H5N1 mostly infects wild birds, especially waterfowl like ducks and geese. These birds are its natural carriers. This serious bird flu has a scary record, causing high death rates among bird populations, including domestic poultry. H5N1 was first discovered in 1959 during an outbreak in chickens in Scotland, starting a long struggle between humans and the virus.
In 1996, scientists figured out that waterfowl are the main hosts for H5N1, leading to a series of concerning outbreaks. The virus has caused four major waves of infection: in 1997, 2003-2005, 2015, and the ongoing crisis that began in late 2021. The outbreaks in 1997 and 2003-2005 were especially important because they resulted in a lot of human infections. Since 2003, nearly 900 human cases have been reported, with over half of them being fatal. The current outbreak, which started in 2021, has spread to six continents, carried by migratory birds and causing the culling of millions of domestic birds.
This outbreak has affected more than just birds. Researchers have found the current H5N1 strain in various mammals, particularly aquatic animals like seals and scavengers. There have also been cases where the virus jumped to domestic animals like minks, goats, and alpacas. A significant outbreak was reported in March 2024 when the U.S. Department of Agriculture confirmed H5N1 in dairy cows. By late August, 192 herds across 13 states tested positive for the virus, with the strain even spreading back to wild birds.
From May to July 2024, there were 13 confirmed human cases of H5N1 connected to dairy farms and poultry culling. Why H5N1 is spreading so much is still a mystery. Like other flu viruses, H5N1 can change quickly to infect new hosts, and it has special features that might help it adapt better. One important part is hemagglutinin, a protein that helps the virus invade new hosts. My research team discovered a mutation in the hemagglutinin of H5N1 related to the dairy cow outbreak that makes it better at binding to receptors in host animals. H5N1 is also changing genes related to replication and avoiding the immune system, which might lead to more infections in mammals.
In cows, H5N1 mainly infects the mammary glands instead of the respiratory system, which is where most flu viruses enter. Studies show that the mammary glands have specific receptors that make them vulnerable to H5N1. Researchers think H5N1 spreads to cows mainly through dirty milking equipment. The movement of infected cattle and shared tools between dairy farms make the situation worse.
To tackle this urgent problem, the USDA started testing cows that are transported across state lines in April 2024. Colorado, where many positive herds are found, now requires weekly farm testing to catch infections early. H5N1 is not a big threat to public health since it doesn’t seem to spread between humans. But people working closely with infected animals are at a higher risk. Those in these high-risk jobs should watch out for symptoms of H5N1, which are similar to cold symptoms like congestion, sore throat, fatigue, and conjunctivitis. The Centers for Disease Control and Prevention provides helpful information about avian influenza in humans.
Animals living near dairy farms, like barn cats that drink raw milk from infected cows, are also at risk of infection. Sadly, some barn cats have died from the virus. To keep humans safe, people working with poultry or dairy cows should use protective gear like goggles and gloves and follow strict safety rules, such as not wearing the same clothes on different farms.
The good news is that there’s no danger to humans from drinking pasteurized milk. The pasteurization process effectively kills any potential H5N1 virus in the milk, making it safe for everyone. However, the ongoing fight against H5N1 reminds us how important it is to maintain a healthy balance between humans and nature, encouraging everyone to stay alert and take action to protect both animals and people in our interconnected world.