(Photo by Felipe Caparros on Shutterstock)
In a startling development, researchers have documented the first known case of a highly pathogenic avian influenza virus, commonly known as bird flu, being transmitted from a cow to a human. The findings, reported in the New England Journal of Medicine, signal a worrying new chapter in the ever-evolving saga of influenza viruses and their potential to spark global pandemics.
The case in question involves an adult dairy farm worker in Texas who developed redness and discomfort in his right eye in late March 2024. Upon visiting a healthcare provider, the worker was diagnosed with subconjunctival hemorrhage — bleeding under the conjunctiva, the clear membrane that covers the white part of the eye. Thankfully, the worker’s vital signs were normal, with no fever, respiratory symptoms, or vision changes reported.
What sets this case apart, however, is the worker’s exposure history. While he reported no contact with sick or dead wild birds, poultry, or other animals, he did have direct and close contact with dairy cows on the farm where he worked. Some of these cows showed signs of illness, such as decreased milk production, reduced appetite, lethargy, fever, and dehydration. The same symptoms had been observed in cows at other nearby dairy farms with confirmed cases of highly pathogenic avian influenza A(H5N1) virus infection.
Alarmed by the potential connection, healthcare providers swiftly collected conjunctival and nasopharyngeal swab specimens from the worker’s right eye for influenza testing. The results were both striking and concerning: real-time reverse-transcription–polymerase-chain-reaction (RT-PCR) testing came back presumptive for influenza A and A(H5) virus in both specimens.
This prompted immediate action, with the worker being placed under home isolation and prescribed the antiviral drug oseltamivir for treatment. The worker’s household contacts were also given oseltamivir as a precautionary measure. Over the next few days, the worker’s condition improved, with the conjunctivitis resolving without any respiratory symptoms developing. Household contacts remained well, providing a glimmer of hope amidst the troubling implications of the case.
“It’s a huge thing that the virus has jumped from birds to mammals, dairy cows in this case, and then to humans,” says Steve Presley, director of The Institute of Environmental and Human Health and Texas Tech University’s Biological Threat Research Laboratory. “That’s why this paper is very significant. It’s going to lay the foundation, I believe, for a lot of research in the future of how the virus is evolving.”
Subsequent testing by the Centers for Disease Control and Prevention (CDC) confirmed the presence of highly pathogenic avian influenza A(H5N1) virus in the worker’s specimens. Genetic sequencing revealed that the virus belonged to a specific clade, 2.3.4.4b, and was closely related to viruses detected in Texas dairy cattle and wild birds in the area during the same timeframe.
Dangers of contracting bird flu
The discovery of this human case of H5N1 infection is particularly concerning because this virus strain has caused sporadic human infections in 23 countries over the past two decades, with a staggering case fatality rate of more than 50%. The virus’s ability to infect a wide range of hosts, including wild birds, poultry, and now domestic cattle, underscores its adaptability and potential for further spread.
While the viral sequences from the infected worker and cattle maintained primarily avian genetic characteristics, one key mutation was identified in the worker’s virus sample. Known as PB2 E627K, this change has been associated with viral adaptation to mammalian hosts and has been detected in humans and other mammals infected with various avian influenza A virus subtypes in the past.
The report is yet another reminder that influenza viruses are constantly evolving, and even small genetic changes can have significant implications for their ability to infect new hosts and cause disease.
The case also raises important questions about the potential modes of transmission from cattle to humans. While the worker reported wearing gloves when working with cows, he did not use any respiratory or eye protection. This suggests that the virus may have been transmitted through direct contact with the eyes, highlighting the need for comprehensive personal protective equipment and infection control measures when working with potentially infected animals.
How do we stop avian influenza spread?
As the world grapples with the implications of this groundbreaking case, public health authorities and the scientific community are racing to fill in the gaps in our understanding of this emerging threat. Key unanswered questions include the extent of H5N1 virus transmission among cattle, the potential for further cow-to-human transmission, and the risk of human-to-human spread.
To address these concerns, a multi-pronged approach is needed. Enhanced surveillance of both animal and human populations will be critical for detecting new cases and monitoring the evolution of the virus. Research into the mechanisms of cross-species transmission and the genetic factors that facilitate viral adaptation to new hosts will also be essential for developing targeted interventions and countermeasures.
On a positive note, the researchers found that the virus detected in the worker was closely related to two existing H5N1 candidate vaccine viruses. These candidate vaccines, which are available to manufacturers, could be used to produce a vaccine if needed, providing a potential tool for mitigating the impact of a future pandemic.
Though it may sound rare and far from serious to many folks, it is more important than ever that we remain vigilant, proactive, and united in our efforts to understand and combat the ever-present threat of influenza viruses. The case of the Texas dairy farm worker serves as a powerful reminder that the line between animal and human health is often blurred.
By investing in research, strengthening our surveillance and response capabilities, and fostering collaboration across disciplines and borders, we can work towards a future in which the specter of a devastating influenza pandemic is no longer a looming threat, but a challenge that we are well-equipped to face head-on.
