‘Strong Evidence’ of First mammal-to-human transmission of H5N1 bird flu
Researchers believe that the virus initially crossed over from birds to cows in Texas, and then spread between herds across states, before transmitting to a worker.
Scientists warn that bird flu has likely jumped from mammals to humans for the first time, according to an official report published on Friday.
The US Centres for Disease Control (CDC) said they found “strong evidence” that a dairy worker in Texas who became infected with bird flu in March contracted the virus from a cow.
Although people have been infected with the H5N1 bird flu virus before, previous human cases were all linked to transmission from infected birds, typically domestic poultry.
“This would be the first reported case of mammal-to-human H5N1 virus transmission reported worldwide,” wrote the authors.
The development has increased the risk of spillover to humans, the World Health Organization said, which is of ‘enormous concern’.
The A(H5N1) variant has become “a global zoonotic animal pandemic”, the UN health agency’s chief scientist, Jeremy Farrar, told reporters in Geneva. “The great concern of course is that in ... infecting ducks and chickens and then increasingly mammals, that virus now evolves and develops the ability to infect humans and then critically the ability to go from human to human.”
The report, published by the New England Journal of Medicine, was written by scientists from the Centers for Disease Control and Prevention, the Texas Department of State Health Services, and the Texas Tech University Bioterrorism Response Laboratory in Lubbock.
“Given the infected human was a dairy farm worker with reported exposure to sick, presumably infected cows in Texas and without reported exposure to other mammals or birds, we believe the genetic and epidemiologic data are strong evidence of infection of the human following exposure to presumably HPAI A(H5N1) virus-infected cows,” they wrote in the materials that accompanied their report.
Over the last two decades, 889 people have been infected with the highly pathogenic strain of H5N1 across 23 countries, with all cases caused by infected wild or captive birds. 463 of those cases resulted in fatalities, meaning the 52 percent “mortality rate is extraordinarily high” , said Farrar, due to humans lacking natural immunity against the virus.
Since March 25, the highly pathogenic strain of H5N1 has spread to 36 cattle herds across nine states in the US, as well as grocery store milk, the CDC said. This marks the first time bird flu has been identified in cows, likely due to exposure to wild birds.
According to scientists, the man most likely got sick due to exposure to cows on a dairy farm in Texas presumed to be infected with bird flu, by either handling the milk or breathing in respiratory droplets from the animals. Despite the virus's high mortality rate, his only visible symptom of the virus appeared to be mild conjunctivitis.
Currently, there is no indication that H5N1 is spreading directly between humans. However, when “you come into the mammalian population, then you’re getting closer to humans”, Farrar said, warning that “this virus is just looking for new, novel hosts”.
This was mirrored by Seema Lakdawala, associate professor of microbiology and immunology at Emory University School of Medicine: “The more the virus spills from birds into other hosts like cattle, foxes, seals, etc the more likely the virus is to change and gain properties that could lead to successful human-to-human transmission.”
Farrar called for increased monitoring, saying it was “very important understanding how many human infections are happening ... because that’s where adaptation [of the virus] will happen”.
However, the dairy industry and state officials are reportedly resisting efforts by the CDC to conduct research, with farmers refusing federal health officials access to their property to monitor the health of farmworkers and collect other data.
According to the CDC, although thousands of workers have likely been exposed, only 25 have been tested for the virus and only 100 farm workers are currently being surveyed for signs of infection.
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