Scientists are watching for changes in H5N1, potentially signaling its adaptation for human transmission, as it has caused severe infections and is on the list of pandemic threats. Any spread to new mammal species is worrying, according to Reuters.
An outbreak of bird flu among US cattle resulted in the first documented transmission from cow to human, when a Texas farm worker contracted the virus. Previously, human infections primarily stemmed from contact with wild birds or infected poultry.
The spread of bird flu to an increasing number of species and its widening geographic reach have raised the risks of humans being infected by the virus, according to the World Organisation for Animal Health (WOAH).
The cattle infections stem from the same bird flu subtype that has been affecting wild birds and poultry worldwide, also causing fatalities in several mammal species likely due to consumption of infected birds.
In February, a lethal strain of bird flu was detected on Antarctica’s mainland for the first time, posing a potential threat to the region’s vast penguin colonies.
Since H5N1 arrived in South America in 2022, bird flu had already killed dolphins, some 50,000 seals and sea lions along the coasts, and at least half a million birds, in Chile and Peru.
While bird flu infections in humans are rare, they can be deadly when they happen. According to the World Health Organization (WHO), there have been 889 H5N1 infections in humans from 2003 to 2024. Of those, 463 or 52% resulted in death.
U.S. officials have intensified measures to contain the first documented outbreak of H5N1 in dairy cows, now affecting herds in nine states and contaminating the country’s milk supply.
Evidence shows transmission from wild birds to cows, between cows, from cows to poultry, and one instance of transmission from cow to human. However, there is no evidence of human-to-human transmission.
Due to the high viral load in milk and mammary glands, scientists believe H5N1 can spread among cattle during milking, possibly through contact with contaminated equipment or aerosolized virus during cleaning.
In a nationwide survey, one in five commercial milk samples tested positive for H5N1 particles, according to the FDA. However, the agency reassured that the virus in milk doesn’t pose a risk to human health, as pasteurization effectively eliminates it.
“I’m not worried about the milk itself,” said Samuel Alcaine, associate professor, of food science at Cornell University. “It does indicate that the virus is more widespread among dairies than we had previously thought.”
A WHO official warned of the potential spread of H5N1 bird flu to cows in other countries via migratory birds.
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