Although cows have been infected with avian influenza subtype H5N1 for the first time and viral remnants have been found in milk, the World Health Organization (WHO) and the US Centers for Disease Control and Prevention (CDC) characterise its current risk to humans as “low”.

The risk for people exposed to infected birds and other animals is low to moderate, they add.

So far, there have been no cases of avian flu being transmitted from person to person in the current outbreak – only from infected birds and animals to humans.

The last human-to-human transmission of avian flu was detected in 2017 and involved infections between a small group of health workers.

Only one person has been infected in the current outbreak in US dairy herds, a man working on a Texas dairy farm who developed conjunctivitis. Swabs of the man’s throat and eye tested positive for H5N1, but he had mild symptoms and did not infect anyone in his household.

Meanwhile, some 220 workers who work at the 36 US dairies affected by the H5N1 outbreak have been screened, but none has been infected with the virus, according to the US CDC’s Dr Todd Davis, speaking at a WHO Information Network for Epidemics (EPI-WIN) briefing on Monday.

“After sequencing several hundred viruses from cattle, we don’t see any molecular changes that would indicate increased possibilities of infection or transmission from person to person,” said Davis. 

“So we still consider this public health risks to be quite low. I think some of the exceptions may be prolonged unprotected exposure to infected dairy cattle, so there are some likely risk associated with occupational exposure.”

Milk and meat risks

About 20% of milk samples collected by the US Food and Drug Administration (FDA) tested positive for H5N1 viral RNA, said Dr Richard Webby, director of the WHO’s Collaborating Centre for studies on the ecology of influenza in animals and birds.

Meanwhile, a smaller sample set targeted at the states where outbreaks had occurred found  40% of the milk products contained viral remnants,  added Webby, who is based at St Jude Children’s Research Hospital in Memphis in the US.

“There has been a relatively large number of samples tested, but so far from a safety perspective, it does look like the pasteurisation process is removing viable virus from those samples,” added Webby.

Dr Moez Sanaa, head of the WHO’s Standard and Scientific Advice on Food Nutrition, confirmed that while viral RNA has been found in pasteurised milk, none of this was live virus “suggesting that the pasteurisation process effectively inactivates H5N1,” said 

“Preliminary results [of ongoing studies] indicate that virus is inactivated by heat treatment similar to pasteurisation,” said Sanaa, but added that more studies of milk with higher viral loads was still needed. He warned people to avoid raw milk.

Meanwhile, last week the USDA’s Food Safety and Inspection Service (FSIS) announced that all 30 samples of ground beef from retail outlets in the states with infected dairy cattle herds tested negative for H5N1. These results reaffirm that the meat supply is safe.

Webby’s group has also tested eggs and found them to be free of H5N1.

Cow transmission: mechanical?

From the genomic analysis, it appears that the outbreak in the dairy farms stemmed from “a single introduction” but that “the moving of dairy cattle has spread that to multiple farms and different locations”, according to Dr David Swayne, a US influenza veterinarian.

Swayne added that as transmission seemed to occur “in the unique environment of a dairy parlour”, there were two leading hypotheses about how the rapid transmission was taking place.

One was that there was “mechanical transmission” with infections being spread via milking machines, for example.

The other was that transmission occurred during the “continual cleaning” in dairies that enabled viral spread through “large droplets produced from that washing down process”. 

Meanwhile, Dr Aspen Hammond from WHO’s Global Immunization Programme (GIP) said that H5N1 had been found in other animals near the affected dairy cattle herd, including cats, raccoons and wild and domestic birds nearby.

‘One Health in action’

Dr Maria van Kerkhove, the WHO’s acting head of Epidemic and Pandemic Prevention and Preparedness (EPP), described the outbreak as “one Health in action”.

“You cannot look at human health risk without looking at the risk in animals,” said Van Kerkhove, stressing that partnerships with bodies in the animal health field were essential.

“Right now, there’s a lot of focus on the US but we are seeing a global epizootic of avian influenza, and we’ve seen H5N1 infection in wild birds and poultry and marine mammals and land mammals,” she said.

“But what is concerning is that we are seeing new species that are being infected… We need much stronger surveillance in animals globally, not just in the US, looking at the species that we know can be infected with H5N1, but also in humans at the animal-human interface. “

She urged those doing surveillance to continue to sequence and share those sequences to enable regular assessments of the viruses as well as “what any changes in these viruses mean, in terms of transmissibility in terms of severity.”

Van Kerkhove also stressed that occupationally exposed people needed to be protected from infection, including by using personal protective equipment and washing hands frequently, “because prevention is key”. 

She also said that, while it was not yet necessary, the current H5N1 flu was covered by the candidate vaccines in the influenza prevention pipeline.

Image Credits: pxfuel, Charyse Reinfelder.