Certain strains of bird flu spread ‘efficiently’ among ferrets, suggest potential for human transmission: new research

New Canadian research has found that certain strains of bird flu, responsible for the deaths of millions of birds worldwide, are capable of spreading quickly and “efficiently” between ferrets in a laboratory setting, raising alarm bells that it may be able to jump species to humans as well.

Avian influenza — known more commonly as bird flu — has been spreading across Canada in farmed birds since 2021, with more than 7.5 million birds impacted as of last week, according to the Canadian Food Inspection Agency (CFIA).

Human infections of bird flu are rare, and mostly occur after close contact with infected birds through poultry farms or live bird markets.

The real concern would be if the virus were able to make the full jump to humans, and then be spread from human to human — a possibility that scientists have been keeping an eye on.

In this new study, released in a preprint this week, researchers with the Public Health Agency of Canada, CFIA, SickKids Research Institute and the University of Manitoba, among others, are reporting that a specific strain of bird flu was able to spread easily among mammals in a lab setting.

Specifically, they found that samples of H5N1 highly pathogenic avian influenza (HPAI) taken from a red-tailed hawk were able to spread “efficiently” in direct contact between ferrets, the mammal used most often to judge the risk of a virus achieving human-to-human transmission.

“Highly pathogenic” refers to the virus’ ability to create disease, underlining how dangerous H5N1 is. 

When research is presented in a preprint, that means it hasn’t passed peer review yet. The research is currently under review for publication in the peer-reviewed journal Nature Portfolio.

Avian influenza has been devastating wild and farmed bird populations across the globe for years. Last November, HPAI was announced to have led to the deaths of over 52 million birds in the U.S. in 2022, either through contracting the disease, or due to flock culling to prevent the spread of the virus.

The virus has also been reported in mammals, such as mountain lions, red foxes, skunks and black bears, although this study is the first time mammal-to-mammal transmission of HPAI has been observed.

Previous studies have demonstrated that H5 subtypes generally do not transmit well among mammals, and researchers noted that recent studies of currently circulating H5N1 strains have only produced mild infection in ferrets, a far cry from the lethal outcomes researchers observed in this latest study.

“Our research has determined that certain, as yet uncharacterized, genetic signatures may be important determinants of mammalian adaptation and pathogenicity of these viruses,” researchers wrote in the study.

In order to study the virus, researchers isolated five distinct H5N1 HPAI strains from infected wild animals in Canada, including three birds and two red foxes who had died of the virus.

They found that the virulence of the virus itself varied widely depending on the specific isolate, with the sample from the red-tailed hawk causing the most severe and lethal disease in the ferrets. Ferrets introduced to viral samples taken from red foxes also experienced severe and lethal disease. Researchers found that specifically there were high levels of viral replication in the upper and lower airway of the mammals.

Direct transmission between the ferrets was clearly indicated in the case of the virus taken from the red-tailed hawk.

Researchers also created a cell culture consisting of primary nasal, tracheal and human airway epithelial cells obtained from healthy human subjects, in order to test how the virus would interact with these isolated cells. The viral sample from the red-tailed hawk replicated the most rapidly within these cell types compared to others.

It’s noted in the study that researchers were “struck by the high virulence and efficient transmission” of the viral samples from the red-tailed hawk among the ferrets, as that specific strain was of avian origin, unlike the virus drawn from red foxes.

They theorized that “passage through multiple animal species” could’ve have contributed to that strain’s “enhanced transmissibility.”

“This is a scenario that is likely to be observed with increasing frequency as the outbreak in wild animal species continues to transpire,” the study stated.

Currently, Canada is experiencing numerous outbreaks of bird flu, largely within commercial poultry farms, although it appears to be circulating at lower levels than in 2022.

Outbreaks of bird flu have been reported in Ontario, Quebec, Nova Scotia, New Brunswick, British Columbia and Alberta in 2023, with Ontario, Quebec and Alberta all reporting at least one infected premises in April.

Earlier this month, CFIA reported that there had been a rare case of a domestic dog contracting bird flu in Oshawa, Ont. The dog reportedly was infected with H5N1 after chewing on a wild goose, and subsequently died.

“The number of documented cases of avian influenza H5N1 in non-avian species, such as cats and dogs is low, despite the fact that this virus has caused large avian outbreaks globally over the last few years,” a release from CFIA stated.

“Based on the current evidence in Canada, the risk to the general public remains low and current scientific evidence suggests that the risk of a human contracting avian influenza from a domestic pet is minor.”

It’s still unclear why certain strains of avian influenza may be more virulent or transmissible than others, but researchers say this new evidence supports the idea that we need to be keeping an eye out now.

“Ongoing surveillance of circulating HPAI A(H5N1) viruses across species, including humans, should be a top priority so as to promptly identify viruses that may have pandemic or outbreak potential in mammals,” the preprint states.

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