Although bird flu (H5N1) has circulated among wild and domestic flocks for 30 years, it has surprised public health experts in the past two years with the longest, largest and deadliest outbreak in history. Either through infection or preventive culling, the avian flu virus has affected a record 90 million domestic birds since January 2022 in the United States alone.
Last month, the addition of dairy cattle to a growing list of mammals infected by the avian strain – from skunks and bears to cougars and sea lions – added more urgency to government surveillance of the virus, which has spread to bird populations across the globe. As of May 2, the virus has been detected in 36 herds in nine states, including on a dairy farm in northeastern Colorado.
“This raises concerns from a pandemic preparedness standpoint, because the more animals that the virus is capable of infecting, the more likely it is that we could be one of those animals that it becomes efficient at replicating in,” said Jenna Guthmiller, PhD, an assistant professor of immunology and microbiology at the University of Colorado School of Medicine.
Only two human cases have been reported with the outbreak so far – a Colorado worker from an infected poultry farm in 2022 and a Texas worker from an infected dairy farm this year. Those cases were mild, and the threat to humans not exposed to infected animals remains low, according to the Centers for Disease Control and Prevention.
So why the concern about humans?
While fewer than 1,000 human cases have been documented in the past 30 years, H5N1 has been highly pathogenic in humans with about a 50% death rate, said Daniel Olson, MD, PhD, an associate professor of pediatric infectious diseases with SOM and of epidemiology with the Colorado School of Public Health and its Center for Global Health. And, as an influenza A virus, H5N1 also has an unusual mutation ability, he said.
All influenza viruses (which include A, B, C and D) can change through a process called antigenic drift, where they develop small mutations over time. That’s why modified flu vaccines are produced each year, said Olson, who studies the intersection of viruses between humans and animals.
But with influenza A, when more than one virus infects a host, entire segments of the viruses (whether human or avian) can get scrambled up and combined, creating a completely new virus through a process called antigenic shift, Olson said. That new virus can be highly infectious or deadly to humans, he said.
“That’s what led to the H1N1 swine flu outbreak,” Olson said. “Most of the big pandemics with influenza happened with antigenic shift, and pigs are the ultimate mixing vessel, so they are able to be infected with a lot of different types of influenza A. If this avian influenza somehow managed to jump into a pig, then that would definitely be concerning.”
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Signs and Symptoms While rare, human cases with avian flu range in severity from mild to fatal. Symptoms can mimic regular flu (fever, cough, sore throat, runny nose, body aches, shortness of breath).
In the most recent Texas case, the dairy farm worker’s symptoms were confined to the eyes and were incorrectly reported as mild conjunctivitis. Pain, subconjunctival hemorrhage and serous drainage were symptoms noted upon exam.
Anyone with symptoms should contact a medical provider immediately. Antivirals given within the first hours of infection can reduce symptoms. |
Some influenza viruses, such as H1N1 (2009 flu pandemic) and H3N2, that cause seasonal influenza infections in humans came from animals, said Guthmiller, whose lab studies viral spillover and human immunity to influenza viruses with an end goal of developing the first universal flu vaccine. “So those viruses caused pandemics and now have persisted within the population,” she said.
What makes the cattle stand out?
H5N1’s jump to cows, however, was highly unexpected, Guthmiller said. “Influenza A has never been recorded like this in cows before. There’s the occasional cow infected, but they are not a natural host for influenza A viruses. And so this is really quite shocking to the field.”
How the virus was transmitted remains a question being studied, but because high levels of the virus have been found in the cows’ udders, and the infection appears restricted to dairy cows, some focus has been on the milk and milking machines. A CDC report noted that on one of the infected dairy farms, half of the barn cats that drank the unpasteurized milk died, with autopsies finding high levels of the virus in the felines.
Also unusual, unlike birds and humans, the infected cows appear to have generally mild or nonexistent symptoms, though some have moderate disease with lethargy and decreased milk production.
“And while that’s good for the cattle, that also makes it harder to prevent transmission,” Olson said. “So, likely, these cattle have been infected for some time, and this virus has been spreading across the country among different cattle populations.” Scientists suspect the virus struck cattle as early as December 2023.
Is it safe to drink milk?
Scientists testing dairy products in stores found DNA evidence of the virus but no sign of active virus, confirming that pasteurization is effective.
“As long as it’s pasteurized, I have no concerns,” said Guthmiller, who grew up on a dairy farm. “Influenza viruses are actually pretty fragile viruses, so anything you throw at them, they will fall apart. They can’t get around being heat killed.”
“Unpasteurized milk is a different story,” Olson said, referring to the farm cats in the CDC study. “While we haven’t heard of human cases from drinking unpasteurized milk, I’m not going to be drinking unpasteurized milk. And that’s the recommendation of the CDC right now.”
Can the outbreak be controlled?
Infected cattle present bigger challenges than poultry when it comes to halting the virus spread. “For the avian influenza, if you have cases detected in a flock of birds, you cull the entire flock,” Olson said. That would not be an acceptable solution with the much more valuable cattle. “You have to weigh the risks and benefits, even economic risks and benefits.”
The government has already restricted transfers of herds across state borders. Expanding wastewater testing, increasing biosecurity measures and implementing influenza surveillance and vaccination of farm workers (the highest risk population) are also preventive options, Olson and Guthmiller said.
But first, officials need to get a better understanding of the extent of the outbreak, both experts said.
“The things that concern me is that we don’t have a strong handle on what the breadth of this outbreak is,” Guthmiller said. “There’s also already evidence that it’s jumping back into birds. So now you have a virus that’s somewhat adapted to mammals jumping back into birds, and those birds can go transmit it to other mammals and cause further outbreaks.”
Right now, it’s important to remember that the human threat is low, and that a lot is happening on the government front in response to the issue, Olson said. Also, if needed, antivirals such as oseltamivir (Tamiflu) and vaccines being stockpiled are so far reportedly effective against circulating strains, he said.
“There will certainly be more information that’s coming in the next few weeks,” Olson said. “And it’s always possible that this one virus just infected cattle, it spread, and there isn’t much more to the story than that. But I think only time will tell whether that is the case.”
