Cases of the bird flu in human beings are going “undetected” in the United States. Centers for Disease Control and Prevention (CDC) researchers found that 7 percent of those tested for avian influenza between June and August 2024 had antibodies from the highly virulent form of H5N1 in their blood.

The CDC further notes that half of those who had antibodies in their blood did not ever report being sick and none had any severe symptoms. The current risk to public health is still low, but some researchers are concerned that allowing the virus to spread without control provides it with a greater opportunity to mutate into more threatening strains, according to a report by Science Alert. 

“None of the workers with HPAI A(H5) virus antibodies reported using the personal protective equipment (PPE) recommended for working with HPAI A(H5)–infected animals, and use of recommended PPE was low among all workers,” CDC health scientist Alexandra Mellis and colleagues write in their report.

A change in the disease’s virulence has occurred in Cambodia, where at least three patients have died from their infection. A hybrid between 2.3.2.1c and 2.3.4.4b, that strain of bird flu was contracted from poultry; a preprint study suggests this hybrid has mutations that assist airborne transmission and infection of mammals.

H5N1 Bird Flu Infects 2 Children In Cambodia

No human-to-human transmission of the bird flu has been detected in Cambodia, even though there have been mutations suggesting that it could eventually.

Canada has also just reported its first known case of bird flu in humans, a teenager who is currently in a critical condition in hospital with a mutated version of the strain identified in the United States dairy cow outbreak. Early genetic sequencing shows a change in a gene known to increase a virus’s ability to infect humans.

Canada’s Bird Flu Case Is A Different Strain Than Is Found In Dairy Cattle

To date, there’s only one known case of bird flu with no known animal origin, which is still sparking fears that human-to-human spread may have begun.

The CDC says it is monitoring the bird flu outbreak closely, even though the risk to humans is “low.”