Blood tests from 115 dairy workers in Michigan and Colorado have revealed that cases of highly pathogenic bird flu have been occurring undetected in humans.

Researchers from the US Centers for Disease Control and Prevention (CDC) found that 7% of workers tested between June and August 2024 had antibodies for the highly virulent H5N1 strain of bird flu in their blood.

The positive takeaway is that most individuals who contracted the virus experienced mild symptoms. Half of those with antibodies did not report any illness, suggesting they may have had only minor symptoms. However, while the public health risk remains low, there is concern that allowing the virus to spread unchecked could lead to mutations that make it more dangerous.

In Cambodia, there has been a shift in the virus’s virulence, with at least three deaths from a hybrid strain of bird flu that likely transmitted from poultry. A study indicates this hybrid strain could have mutations enabling airborne transmission and mammal infections.

Fortunately, the outbreak in Cambodia appears to be limited, with no confirmed cases of human-to-human transmission. Canada has also reported its first known case of bird flu in a human, a teenager in critical condition with a mutated version of the strain found in the US dairy worker outbreak. Genetic analysis has shown a change in a gene that enhances the virus’s ability to infect humans.

So far, the Canadian case seems to be isolated, but these developments highlight the potential for rapid changes in the virus’s risk, underscoring the importance of continuous monitoring and management of outbreaks.

There has also been one known case of bird flu in a human with no identifiable animal source, raising concerns about possible human-to-human transmission.

The dairy workers with signs of previous infection had been involved in milking or cleaning the milking parlor. Symptoms they reported included red, itchy, or draining eyes, fever, sore throat, congestion, sneezing, diarrhea, and headaches. Notably, none of the workers with H5N1 antibodies used the recommended personal protective equipment (PPE), and overall PPE use was low among all workers, according to CDC health scientist Alexandra Mellis and colleagues.

The CDC is closely monitoring the situation. Alongside poultry outbreaks, over 400 dairy herds in 48 states have been affected by the bird flu. Experts are emphasizing the need for continued monitoring and better management of animal outbreaks to prevent further spread.

The findings also highlight the need for improved outreach to employers and workers, especially in communities where Spanish is spoken, regarding the risks of infection when working with infected dairy cattle and the importance of using PPE.

Global health authorities are advising the public to maintain good hygiene when handling animals and to report sightings of sick or dead birds. Infected animals may show signs such as drowsiness, uncoordinated movements, head tilting, or difficulty standing or flying.

Since 2022, the highly virulent bird flu has led to the culling of over 90 million domestic birds in the US. Vaccination efforts are underway to protect California condors from the disease, but other species may also require intervention. Meanwhile, research is ongoing to develop a human vaccine.