Since the end of March, the bird flu virus has spread to many American dairy farms. Could milk, one immediately asked, serve as an intermediary to transmit this pathogen to humans?
It has indeed been official since March 25: several organizations in the United States have confirmed the presence of the virus, H5N1, in dairy cows. What’s more, according to experts interviewed by columnist Zeynep Tufekci of New York Times, the first hatchings probably began in February. Which raises questions about gaps in oversight.
As of April 30, data from the Centers for Disease Control and Prevention (CDC) revealed that 36 herds in nine states were affected by an outbreak. Furthermore, in a situation update on May 1, the US Food and Drug Administration (FDA) reported that one in five milk samples contained fragments of the virus.
These had been detected using PCR tests which did not make it possible to know whether the virus was still infectious. Therefore, additional tests were carried out by inoculating eggs with milk samples. On May 1, the FDA confirmed that no live virus had been detected in the samples tested and that pasteurization appeared sufficient to inactivate the virus. The milk on the shelves therefore remains safe for human consumption. However, the FDA recommends not consuming raw milk.
The situation in Canada
It must be remembered that this virus is carried by migratory birds. That means that cows in other countries could also be infected, explained Wenquing Zhang, head of the World Health Organization’s global influenza program, in an article published on May 1 on the United Nations website. united.
For the moment, there is no indication that Canadian cows have been infected with the H5N1 virus, Nitish Boodhoo of the University of Guelph (Ontario) pointed out on April 29. Furthermore, in an article from The Press, journalist Mathieu Perreault reported on April 27 that Canadian authorities are not currently considering testing pasteurized milk, since no Canadian cow has tested positive for the H5N1 virus. In a press release published on May 3, the Canadian Food Inspection Agency (CFIA), however, changed this position and announced that it would expand screening in cattle and require that lactating cows imported from the United States obtain a result. negative for the avian flu virus test.
Before this date, the CFIA had asked Canadian veterinarians to test cows whose production had recently declined abnormally. This is in fact how the first infected American cows were identified.
Little risk to humans
Still, according to Nitish Boodhoo, many questions remain unanswered. Experts still don’t know how the American cows became infected and whether the virus was transmitted directly from one to another. Several hypotheses are on the table, reported Zeynep Tufekci: for example, the virus could have spread via milking equipment or through food which is sometimes composed of leftover chicken litter.
Tufecki criticizes the American authorities for their slowness in setting up a screening campaign, in the face of a virus apparently capable of spreading very quickly. The big worry being: could cows act as intermediaries between birds and humans? Since 1997, when this H5N1 flu has been closely monitored, cases in humans have been very rare: less than 900 between 2003 and 2023. But the mortality rate has been very high each time. In the case of this outbreak, as of April 30, the CDC has identified a single case of transmission of the virus, following the exposure of a dairy producer to an infected cow. He only had a minor infection.
The CDC considers the risk to public health to be low. This is also the conclusion of the WHO following its assessment of risks to human health.
