Canadian food safety and agriculture officials will test retail milk for harmless traces of the avian flu virus as they step up efforts to determine whether a highly pathogenic version of H5N1 has spread undetected into Canadian cattle from herds in the United States.
The Canadian Food Inspection Agency announced Friday that it would also facilitate the voluntary testing of apparently healthy cows for bird flu. The statement did not provide details on how they would do so.
Earlier this week, in a notice to the Canadian dairy industry, the agency said that lactating cows could no longer be imported from the U.S. without proof of negative flu tests. That change took affect April 29.
H5N1 has not yet been found in cows in Canada, but some doctors and scientists worry cases may have been missed because officials have been waiting for farmers and field veterinarians to report symptoms in cattle, rather than actively hunting for the virus.
The U.S. Department of Agriculture announced on March 25 that it had confirmed for the first time cases of highly pathogenic avian flu in cows in Texas and Kansas. The virus has since been found in 36 herds of dairy cattle in nine states, including in the border state of Michigan.
Finding avian flu in cows set off alarm bells because of the threat the virus could pose to livestock, and because wider transmission in mammals could provide the virus more opportunities to evolve into a pathogen that spreads from human to human.
For now, however, public health officials say the risk to people is very low, unless they work directly with farmed or wild birds. USDA testing of retail samples of milk and milk products, such as sour cream and cottage cheese, found them to be safe because of pasteurization, the process of heating milk to kill pathogens.
However, USDA testing did turn up harmless viral fragments in one in five retail milk samples tested in April – a clue that the bird flu may be more widespread among cows than currently known. Testing retail milk in Canada could serve the same surveillance purpose.
The USDA also said this week that testing of ground beef from states with infected herds found no trace of the virus.
Since 2003, nearly 900 human cases of avian flu have been confirmed in 23 countries. Just more than half were fatal, but the true fatality rate is believed to be lower because only the sickest patients come to the attention of authorities.
“We obviously have been paying very close attention to H5N1 since it was first identified in 1996 and it has not acquired a suite of mutations and mechanisms that would allow it to replicate easily in the human respiratory tract and spread from one person to another,” said Greg Rose, an infectious disease and infection control consultant at the Queensway-Carleton Hospital in Ottawa.
However, Dr. Rose said the risk of that happening is becoming less remote as a highly pathogenic version of the virus known as H5N1 2.3.4.4b traverses the globe, wipes out poultry and wild bird populations, and infects more types of mammals, including raccoons, skunks, red foxes, cats, dogs, sea lions and dolphins.
More than 11 million farmed birds have died or been culled since the virus was first detected in Canada in December of 2021.
So far, H5N1 2.3.4.4b is causing relatively mild illness in cows, marked by loss of appetite, a precipitous drop in milk production, lethargy and fever. No cows are known to have died of the virus.
The lone confirmed human case linked to the outbreaks was found in a Texas farmworker whose only symptom was pink eye.
A new report published in the New England Journal of Medicine Friday suggested the worker may be the first detected example of avian flu transmitting from a mammal to a human. People infected with the virus in the past have had direct contact with sick birds. The Texas worker didn’t, according to the report, written by scientists from the Centers for Disease Control and Prevention and the Texas Department of State Health Services.