‘Most’ Cases of Avian Influenza in USA Cattle Likely Undetected
Milking a cow in Texas. H5N1 Avian influenza is spreading among US cattle herds, most probably during the time of milking.

Most H5N1 infections spreading through US dairy cattle and other animal populations are likely going undetected despite stepped up surveillance by the US Department of Agriculture, Michael Osterholm, director of the Centre for Infectious Disease Research and Policy (CIDRAP), told Health Policy Watch on Wednesday.

Osterholm spoke as a growing number of infectious experts were sounding alarm bells about the expanding spread of the deadly virus to mammalian populations – and especially dairy cattle in the United States, where some 36 herds have so far been infected, according to the latest US Centers for Disease Control and Prevention (CDC) update April 30. Sanitation gaps and lax surveillance in industrial dairy operations are facilitating H5N1 transmission, experts now suggest. 

Total number of H5N1 cases in different animals in the US. Infection spread in dairy cattle, the newest ‘terrain’ for the virus, is causing the most concern.

And “most” infections are likely going undetected due to farmer reluctance to have cattle and employees tested for avian flu, warned Osterholm, an internationally-known expert. 

“We need much more extensive follow up, serology studies and close monitoring of mixing vessel animal species, e.g. pigs,” Osterholm stressed in emailed remarks. 

He noted that pig infections, for example, constitute a “mixing vessel animal species” because they can become co-infected with both animal and human forms of avian influenza, increasing the risks of spread to humans of a virus that has a greater than 50% fatality rate.

On the positive side, Osterholm added that it was “very unlikely” pasteurized milk could transmit the infection if pasteurization “is done properly.”

Policymakers have taken their ‘eye off the ball’ of ‘One Health’ principles

“Policymakers have again taken their eyes off the ball in efforts to protect humanity from new pandemics,” stated Dr Nigel Sizer, an Executive Director of the non-profit Preventing Pandemics at the Source (PPATS), as part of a stiff warning  issued Tuesday by half a dozen international experts on the wave of H5N1 infections.

“In this case, it is hard not to point the finger at lax monitoring and regulation of animal agriculture in the United States and elsewhere,” Sizer said.

Colorized transmission electron micrograph of Avian influenza A H5N1 viruses (seen in gold) grown in MDCK cells (seen in green).

The expanding outbreak takes place as WHO member states are in the throes of finalizing a draft Pandemic Accord agreement, where a commitment to stronger “One Health” measures for preventing spillovers of animal pathogens to humans remains a sticking point for some countries in the Global South. 

But in the case of the USA, a dearth of surveillance is also a hallmark of the rapidly expanding avian flu outbreak in the Global North, other experts also pointed out.  

“The situation with avian influenza across the United States exemplifies the inherent hypocrisy and vested economic interests around Pandemic Prevention, Preparedness, and Response,” said Dr. Christian Walzer, Executive Director of Health at the Wildlife Conservation Society, and a professor at the University of Veterinary Medicine in Vienna, Austria, in the PPATS statement of experts.

“As one hundred ninety-four member states of the World Health Organization (WHO) are negotiating perceived responsibilities and equity around a new Pathogen Access and Benefits-Sharing [PABS] Mechanism – while the Global North is demanding transparent and rapid access to pathogen data from the Global South to develop diagnostics, vaccines, and therapeutics, it seems unwilling to share such information with the world,” Walzer said.

Spread through milking machines

Among dairy cows, H5N1 appears to be transmitting rapidly through their mammary glands via virus contaminated milking machines, Stat News reported on Tuesday. The milking equipment is typically not sanitized between sessions with individual animals, becoming an obvious transmission path to infection by more and more cows from the herd.

Experts such as Jared Taylor, a professor of veterinary pathobiology at Oklahoma State University, noted that the H5N1 infections in cattle herds reported so far have been  limited to lactating dairy cows. And the ones who produce most milk – and therefore spend more time on the milking machines – have the highest disease incidence.

Even more worrisome is the potential presence of the virus in raw milk – which some consumers in the USA as well as in Europe prefer as a more ‘natural’ alternative.

US states where H1N5 has been detected in cattle

There also have been virus remnants found in pasteurised milk. “The role of pasteurization in inactivation of the virus […] is currently being investigated,” states a 23 April assessment by the WHO, issued jointly with the UN Food and Agriculture Organization (FAO) and the World Organisation for Animal Health (WOAH).

Infection with H5N1 from pasteurised milk is, however, “very unlikely, if [the pasteurisation is] done properly,” Osterholm said.

As compared with consumers, farm workers who have everyday contact with cattle are, however, at a far higher risk of infection. And the one case of human infection reported in March, does not reflect the scale of the disease spread to people, experts say because farmers have been reluctant to allow testing of cattle or employees, and such testing remains voluntary. 

That’s why “most” cases likely go undetected, Osterholm told Health Policy Watch.

Avian influenza as symptom of world’s failure to apply ‘One Health’ measures

Virus spillovers to more and more species are a direct effect of the poor animal management practices in the  intensive livestock production conditions that are common throughout the developed world today, said Sizer in the statement on April 30.

“Improved animal husbandry conditions, more rigorous inspection standards, as well as better reporting and sharing of animal health information could reduce the risk of these outbreaks as well as improve the welfare of the animals we consume,” he asserted. “We must question for how much longer consumers will have to worry that the price of a cheap sausage or steak is the risk of another global pandemic.”

One Health principles recognize the interlinkages between ecosystems, animal and human health, and call for enhanced collaboration between sectors to prevent zoonotic spillover into human populations. 

“This outbreak highlights the need for One Health approaches for preventing spillovers and interventions to reduce risk of such threats ‘at source’,” argued Dr. Malik Peiris, Professor of Virology at the School of Public Health at The University of Hong Kong, and a leading H5N1 expert.

Pigs are an important possible intermediary host for avian influenza.

“There is still a widespread philosophy of aiming for maximum profit in meat production, when it should be about optimizing food security, food safety, animal welfare and ecological sustainability,” added Dr. Dirk Pfeiffer, Professor of One Health at City University of Hong Kong, in the same statement, adding: 

 “Global investors who see opportunities in making money out of meat production should be aware that ‘growing cattle, pigs or chickens’ is not the same as making parts for mobile telephones.”

WHO still ranks human health risks as low or low-to moderate

In its 23 April assessment, WHO ranked avian influenza risks to humans as low generally insofar as human-to-human transmission has never been documented, and low-to-moderate for those “with exposure to infected birds or animals or contaminated environments.”

Only one human case connected with the US outbreak has been reported, a cattle worker from Texas.

Since 2003, WHO reported only rare instances of human infections all from close contact with animals. Even so, the mortality was “extraordinarily high,” according to WHO Chief Scientist Jeremy Farrar. He was referring to the 52% mortality rate registered amongst the 889 human cases of H5N1 reported to WHO between 2003 and 1 April 2024.

Jeremy Farrar, WHO Chief Scientist, during a press briefing April 18

“Being one of the few people around the world who have actually treated patients with H5N1 […] this remains I think an enormous concern,” Farrar stressed at an April 18 media briefing in Geneva. From 1996 until 2013, Farrar was Director of the Oxford University Clinical Research Unit in Ho Chi Minh City, where he and a Vietnamese colleague identified the re-emergence of the deadly bird flu, or H5N1, in humans in 2004. 

What worries experts the most is the virus’s expanding host range. Naturally found in wild birds, avian influenza has developed a limited ability to infect other species, including mammals. The recent joint assessment by WHO, FAO and WOAH reports mentions the virus as the probable source of infection in sea mammals, multiple fur animal farms and ferret-to-ferret infections, according to some studies.

The list of infected animals is getting longer: cats, dogs, goat kids or polar bears, already known to suffer from the disease were recently joined by a walrus, as The Guardian reported.

“The great concern, of course, is […] that that virus now evolves and develops the ability to infect humans. And then critically, the ability to go from human-to-human transmission,” Farrar told the media briefing.

Reassurance, but… 

Other WHO officials have still sought to issue a more reassuring note.

Speaking at another WHO press briefing on April 24, Dr. Maria Van Kerkhove, a WHO epidemiologist, asserted that appropriate surveillance systems are working well.

“We are concerned about this particular virus because we know influenza has the potential to cause epidemics that have the potential to cause pandemics,” she said. “And that’s why we have a global system in place to monitor, detect, and to rapidly do risk assessments to look at viruses that could potentially be used in vaccines as we go forward. There’s a lot of work in place right now,” she said.

Since the beginning of 2021, 28 detections of H5N1 in humans have been reported to WHO, with all nine cases reported in Europe or North America being assymptomatic or mild cases, notes the WHO/FAO/WOAH joint assessment. Still the three agencies also recommended tougher monitoring – calling on national authorities to  use “active case finding and serologic methods, as well as work with national agencies to understand the exposure and risk from milk and milk products.”

Osterholm, like other experts doubts that US authorities have the situation under control. What’s needed is, among others, a “much more extensive follow up,” said Osterholm. 

He also advises a close monitoring of pigs, which have “influenza virus receptors for both avian and human influenza viruses.

“If they get coinfected with both viruses simultaneously then viral reassortment can occur and a new human virus emerge, like what happened in 2009 with the new H1N1 [strain] that emerged in swine in Mexico.” 

With editing and reporting contributions by Elaine Ruth Fletcher 

Image Credits: Josh Kelahan, US CDC, CDC/ Courtesy of Cynthia Goldsmith, Flickr: Dutchairplaneshooter.

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