Global strategies to control mammal-to-mammal avian flu transmission aren't working and pose an ongoing threat to people, especially as the viruses become entrenched in Europe and the Americas, according a team of virologists led by the United Kingdom's Pirbright Institute.
Their bird's-eye view of the current H5N1 avian influenza panzootic in mammals appeared yesterday in Nature.
Assessing fur farms, marine mammals, dairy cows
The scientists examined outbreaks in European fur farms, South American marine mammals, and US dairy cattle, noting the diversity of the species, along with changes in the ecology and molecular evolution of H5N1 in wild and domestic birds that increase the chances for spillover to a variety of mammals.
Also, the team laid out different evolutionary pathways that could turn the influenza panzootic into a human pandemic virus.
Their analysis comes as H5N1 outbreaks continue in US dairy cattle and as health officials in Missouri try to pin down the source of a recent human infection with no known animal or raw milk exposure and discern whether three other contacts with symptoms may have been infected with the same virus.
Tom Peacock, PhD, first author of the paper and a zoonotic flu specialist at Pirbright, said in a news release that influenza A viruses have triggered more pandemics than any other pathogen, and pigs have historically been considered the intermediary hosts. "However, the altered ecology of H5N1 has opened the door to new evolutionary pathways," he said.
Shortcomings of control measures in cattle, wildlife
The experts underscored several potential gaps in controlling the virus, including reluctance to pursue modern vaccine and surveillance technologies and sparse data regarding transmission of H5N1 between cows and humans on dairy farms in the United States.
In the past, US cattle producers eradicated foot-and-mouth disease by rapidly sharing data. The authors note, however, that months of missing data on the extent of the spread on US dairy farms leaves researchers, veterinarians, and policymakers in the dark.
Very few states are doing bulk-tank testing to proactively look for the virus. "H5N1 is a reportable disease in poultry, but not mammals, in the US," Peacock said. "The US Department of Agriculture requires H5N1 testing only in lactating cattle prior to interstate movement."
What keeps scientists up at night is the possibility of unseen chains of transmission silently spreading.
The group also pointed to a gap in wildlife testing, which currently involves just carcass testing. The authors said testing the animals while they are alive could help identify any undetected circulation that might be occurring.
"What keeps scientists up at night is the possibility of unseen chains of transmission silently spreading through farm worker barracks, swine barns, or developing countries, evolving under the radar because testing criteria are narrow, government authorities are feared, or resources are thin," they wrote.
Severity in humans unclear
If the current H5N1 virus starts spreading readily in people, disease severity is unclear, given that recent infections have a greatly lower case-fatality rate—with the illness mainly consisting of eye symptoms—compared to the earlier H5N1 virus that circulated in Asia.
Older people might have partial immunity the H5N1 due to childhood exposure to similar viruses, and people born since the 1968 H3N2 flu pandemic might be more susceptible, the authors said.
