Species leap: A particularly aggressive strain of the H5N1 bird flu virus has already killed millions of birds, and is now also increasingly affecting mammals. Recently, discoveries in dairy cows and dolphins in the USA caused a stir among experts. This makes it increasingly likely that it will spread to humans. How do health authorities and virologists assess the situation?
Influenza viruses not only affect us humans, but also regularly cause outbreaks of epidemics among birds and other animals such as pigs. Since 1996, the H5N1 bird flu virus, which originated in Asia, has spread worldwide. Since then, new highly pathogenic virus mutants have emerged from the factory farming of poultry.
More and more mammals are in danger
A particularly deadly, highly pathogenic strain of the H5N1 bird flu virus has already led to the deaths of millions of wild birds and farm animals since 2021. This virus strain has recently even spread to remote regions such as Antarctica, probably introduced via migratory birds. Findings in birds of prey such as eagles, owls and vultures suggest that this virus spreads not only within individual populations and species, but also between different bird species through hunting behavior.
Atypical bird species and various mammal species are now also affected by the highly pathogenic influenza strain, both in the wild and in stables. There are reports of such zoonotic species jumps, for example in martens, foxes, skunks, minks and other carnivorous land animals, but also in marine mammals.
Experts therefore fear that the viruses could spread to other mammals or even humans and that the pandemic could continue to spread.
H5N1 also in dairy cows for the first time
A study of dairy cows in the USA recently caused a stir. The US Food and Drug Administration found components of the H5N1 bird flu virus in 20 percent of the supermarket milk tested. However, it is not possible to determine directly from the milk samples how many cows are now infected with bird flu. As the US Department of Agriculture announced, at least 34 herds in nine states in the US are currently infected. The H5N1 viruses from the highly pathogenic clade 2.3.4.4b were first detected in dairy cows in Texas in February, but may have been circulating in livestock barns since December. The affected cows tend to show mild symptoms such as reduced appetite and produce less milk.
The new thing about it: Cattle were not previously considered hosts of the bird flu virus. It is not clear how this was transferred to the cows. “There are a lot of questions and not many answers so far,” virologist Florian Krammer from Mount Sinai Hospital in New York told Nature. However, initial evidence points to a one-time infection of a cow by a sick bird, according to the US Department of Agriculture. The other cows could then have become infected with the virus through their respiratory tract or through their udders during milking, for example through contaminated milking equipment.
If confirmed, this strain of H5N1 could have abilities that allow it to transmit from mammal to mammal. Such direct infection is one of the prerequisites for a pathogen to be able to spread to the new host after overcoming the species barrier.
Dolphins and seals died from bird flu
In addition to the most recent cases in cows, other findings of the H5N1 virus in mammals are raising eyebrows. For example, in 2022, a team led by Allison Murawski from the University of Florida discovered a lost dolphin that was infected with the bird flu pathogen and died shortly afterwards. The brain and lungs of the bottlenose dolphin (Tursiops truncatus) were also infected with subtype 2.3.4.4b of the H5N1 avian influenza virus, according to subsequent necropsy and tissue analysis by various US authorities.
In recent years, other research teams have also reported isolated cases of marine mammal infections with subtype 2.3.4.4b of the H5N1 virus. Various dolphin species in South and North America and Europe were affected. But how did the dolphins become infected with bird flu? The researchers suspect an infection route via local bird and seal populations. H5N1 viruses had previously been discovered in North and South America in deceased sea lions, harbor seals and gray seals.
However, genome analyzes from Florida have not yet been able to confirm this theory: There was no genetic evidence of the origin or infectivity of the virus. “We still don’t know where the dolphin got the virus. More research needs to be done,” says Richard Webby of St. Jude Children’s Research Hospital in Memphis. However, he and his colleagues suspect infection via the blood rather than the respiratory tract.
The tests revealed that the virus from the dolphin contained a mutation that made it more resistant to the common flu drug oseltamivir. This raises concerns that this type of virus would be difficult to treat. However, the team believes “the drug would still be useful.” No further mutations have been found to date that indicate an adaptation of the virus to marine mammals.
H5N1: A danger to humans?
The bird flu viruses of the highly pathogenic H5N1 strain continue to pose a threat, especially to poultry and wild bird populations worldwide. Infected birds die almost 100 percent of the time. “Here we have to look very carefully at Australia with its unique wildlife, the last continent without HPAIV-H5N1 virus infection,” says virologist Martin Beer from the Friedrich Loeffler Institute.
Theoretically, H5N1 viruses can also pose a danger to humans. However, health authorities such as the World Health Organization (WHO) and the United States Centers for Disease Control and Prevention (US CDC) classify the current health risk of H5N1 viruses for humans as still relatively low. “Further spread of the virus in the form of an epidemic or even pandemic [in der menschlichen Bevölkerung] “It’s rather unlikely,” says virologist Martin Schwemmle from the University Hospital of Freiburg.
This bird flu virus can in principle also be transmitted to humans and can lead to mild to severe respiratory illnesses or even death. However, infection of humans from birds or cows has so far been rare. Since 2003, only around 890 cases worldwide have been reported to the WHO. Based on current knowledge, direct transmission of the H5N1 bird flu variant from person to person is not possible.
Close monitoring of wildlife required
Despite the ability of the highly pathogenic H5N1 strain to infect mammals, no mutations have been found in the viruses that indicate a newly acquired adaptation to mammals. So far, H5N1 is “maximally adapted to the bird,” says Beer. “In order to be transmitted to humans, the virus has to overcome a number of hurdles because, for example, we have an effective innate immunity against such influenza viruses.” There is also no danger to humans from contaminated milk from cows because the virus is killed during pasteurization .
In order to keep an eye on the further development and spread of the H5N1 virus and other H5Nx viruses, researchers recommend testing dead birds and stranded marine animals for bird flu as standard. Dairy cows should also be tested regularly. The US Department of Agriculture has already issued a corresponding regulation, and Schwemmle also advises close monitoring in Europe. With every new virus host that comes into closer contact with people, the likelihood of the H5N1 virus being transmitted to humans increases. (Communications Biology, doi: 10.1038/s42003-024-06173-x)
Quellen: University of Florida, Science Media Center, Nature
last updated on April 30, 2024 at 9:45 a.m
30 April 2024 – Claudia Krapp