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The bird flu outbreak in US cattle has put governments on high alert as they grapple with its potential to contaminate meat and dairy supplies, infect other mammals and transmit to humans.
The surge in cases is testing improvements in disease management since the Covid-19 pandemic, notably in dealing with the threat of zoonotic conditions that can pass from animals to humans.
While scientists say the current evidence does not suggest a high pandemic risk, they are calling for urgent investigations into the virus, its spread into cows and its possible mutations.
“Most likely and hopefully, the outbreak in the US dairy herd can be contained and controlled and this will be a limited one-off event without international consequence,” said Paul Digard, a virology professor at the UK’s Edinburgh university. “It’s too early to say, though.”
The virus has spread to 36 cattle herds across nine states in the US, according to the US Centers of Disease Control and Prevention. Fragments of bird flu have also been discovered in retail milk samples.
However, the CDC is struggling to gain access to dairy farms to monitor the spread. “Discussions are under way with farms in multiple jurisdictions to participate in CDC-led epidemiological studies,” the agency told the Financial Times.
Several strains of bird flu have caused concern in recent years, triggering culls of poultry stocks and infecting mammals including dogs and sea lions. Avian influenza has yet to spread widely in humans but it is often fatal when contracted, causing more than 1,000 reported deaths since 2003.
It is likely the H5N1 strain of the virus had been circulating in US cattle for more than four months before it was first confirmed in late March, according to a scientific paper published on Thursday.
But the CDC still classifies the public health risk from the bird flu cattle outbreak as low. Only one animal-to-human crossover case has been reported during this outbreak — in a Texas dairy worker whose only observable symptom was conjunctivitis, a mild eye infection.
Governments are nonetheless planning for the possible spread to people.
The US said the first batches of a pair of two-dose bird flu vaccines could be available within weeks if widescale human-to-human transmission were to occur, and tens of millions of doses could be available within months. Studies suggested these jabs offered “good cross-protection” against avian flu circulating in cattle, Demetre Daskalakis, director of the National Center for Immunization and Respiratory Diseases, said this week.
A vaccine against avian influenza had already been approved in Europe and others could be updated as needed, the EU’s European Centre for Disease Prevention and Control said. The World Health Organization regularly updates a list of known candidate vaccine viruses (CVVs) to respond to bird flu and other potential pandemic risks.
Tests carried out by the US Department of Agriculture said there was no trace of bird flu virus particles in ground beef samples that were tested for the pathogen.
Cattle futures traded on the Chicago Mercantile Exchange opened sharply higher on Friday, after dropping more than 6 per cent on Wednesday, when the department banned infected cattle from crossing state borders.
Early PCR test results from 297 retail milk samples across 38 states detected the H5N1 virus in one in every five samples. But pasteurisation kills the pathogen and therefore outbreak is unlikely to impede US dairy supplies, according to the National Milk Producers Federation.
“Cows are getting a little sick, which means they have to come out of production for a few weeks, but then after that they come back online,” said Alan Bjerga, NMPF executive vice-president of communications.
Experts pointed to possible obstacles to tracking the virus, with potential friction between public health investigators and the agricultural sector.
“There’s always a tension between the human health people and animal health people,” said Scott Gottlieb, a former US Food and Drug Administration commissioner. “The human health people would swab anything; animal health worry more about economic implications.”
Investigations needed to happen quickly because the danger could increase as the pathogen evolved, said Michael Osterholm, professor of environmental health sciences at Minnesota university. One route would be via a reassortment: an exchange of genetic material between viruses, particularly if the virus spreads in swine populations.
“One of the problems with influenza is that the risk can be very, very low for days and all it takes is a few mutations to occur — and you suddenly have a pandemic virus,” Osterholm said.
The virus appeared already to be the product of a reassortment between types of H5N1 pathogen previously observed in Europe and the Americas, said Prof James Wood, an infectious diseases expert at the UK’s University of Cambridge.
“Because this effectively is a new virus, there is a lot of uncertainty around what its characteristics are.”