SCIENTISTS are racing to understand how someone became infected with H5 bird flu despite claiming to have had no contact with animals.
The patient was treated in a hospital in Missouri, US on August 22 and has since made a full recovery, the US Centers for Disease Control and Prevention (CDC) said.
Experts have speculated the unnamed patient might have come into contact with raw milk infected with H5N1, which is currently circulating in cows in the US.
Another possible scenario is that the virus has mutated, allowing it to spread more effectively between humans, going undetected.
This new case, reported by the US CDC on Friday, is “the first case of H5 without a known occupational exposure to sick or infected animals,” it said in a statement.
Thirteen other human cases were reported this year, all farm workers who worked with cows or birds.
Concerns have grown in recent years as the H5N1 strain has jumped from birds to a wide range of animals, including penguins, polar bears, foxes and even humans.
US health officials discovered a human case of bird flu in March 2024, which was identified after exposure to dairy cows that were potentially infected.
However, the main worry among experts is that an unexplained H5 infection raises the possibility of person-to-person transmission.
The World Health Organisation (WHO) warned that this would be of “enormous concern” due to its potential to trigger a global pandemic.
“This is how pandemics start,” Kruitka Kuppali, from the Infectious Disease Society of America and former WHO medical officer, told The Telegraph.
“We need to scale up preparedness and response efforts.”
The new case was detected through the state’s seasonal flu surveillance system, indicating there could be more flying under the radar, Paul Hunter, professor in medicine at the University of East Anglia, told the Sun.
“It is not possible to say how long the chain of person-to-person transmission was in Missouri and how many undiagnosed people are passing the virus between themselves,” he explained.
“Most people with flu are not tested so we wouldn’t know what type the virus was.”
As it stands, the length of person-to-person transmission of bird flu tends to be very short.
“The big concern is that as the transmission chain gets longer the virus can evolve quite rapidly to get more infectious,” Paul Hunter, professor in medicine at the University of East Anglia, told the Sun.
“And as the disease gets more infectious the probability of it starting to spread in the general population increases.”
He added: “The big uncertainty is whether the virus retains its virulence and lethality as it evolves to become more infectious.
Another theory, suggested by Ed Hutchinson, professor of Molecular and Cellular Virology, is that the person drank some unpasteurised (raw) milk, which has killed several barnyard cats this year.
“If so this could help public health officials to better direct efforts to contain the virus,” he told the Sun.
Lisa Cox, a spokesperson for the Missouri Health Department, said that the patient had not reported consuming raw milk.
Alternatively, “it could be that there was contact with an animal such as a cow that we’re not yet aware of,” Prof Ed added.
The CDC has said Missouri health officials are conducting an on-the-ground investigation to look for the source of the infection, although it is unclear when they will share the results of their findings.
By Isabel Shaw, health reporter
BIRD flu is running rampant in wildlife around the world and is now spreading in cows.
This increase in transmission has given the virus lots of opportunities to mutate – a process where a pathogen changes and can become more dangerous.
Scientists fear it’s only a matter of time before one of these mutations makes it better at spreading among mammals – and potentially humans.
Some experts believe the virus could already be spreading among some animal species.
So far, there is no evidence that H5N1 can spread between humans.
But in the hundreds of cases where humans have been infected through contact with animals over the past 20 years, the mortality rate is high.
From 2003 to 2024, 889 cases and 463 deaths caused by H5N1 have been reported worldwide from 23 countries, according to the World Health Organisation.
This puts the case fatality rate at 52 per cent.
Leading scientists have already warned an influenza is the pathogen most likely to trigger a new pandemic in the near future.
The prospect of a flu pandemic is alarming.
Although scientists have pointed out that vaccines against many strains, including H5N1, have already been developed, others are still in the pipeline.
