During the past 20 years, highly pathogenic avian influenza A (HPAI) H5N1 viruses have been reported from 23 countries. More recently, HPAI H5N1 clade 2.3.4.4b has spread widely in wild birds, poultry and other animals. During the past few months, the virus has been detected in dairy cows in the U.S. suffering from reduced appetite, lethargy, fever, dehydration, and reduced mild production. The virus has also been detected in unpasteurized milk.
In March 2024, an adult dairy farm worker suffered bilateral conjunctivitis with subconjunctival hemorrhage from what was later identified as HPAI H5N1 clade 2.3.4.4b (Uyeki TM, Milton S, Hamid CA, et al. Highly pathogenic avian influenza A(H5N1) virus infection in a dairy farm worker. N Engl J Med. 2024, May 3 DOI: 10.1056/NEJMc2405371.) The patient, who had no respiratory symptoms and a normal chest X-ray, had direct contact with sick cows. The worker wore gloves but not eye protection.
Although we are now in the midst of a bird flu pandemic among wild birds, poultry and other animals, it is important to note that this virus has not yet developed changes in the hemagglutinin gene that would allow for ready binding to the alpha-2-6-linked sialic acid receptor located in the upper respiratory tract of humans. For this reason, the current strain of HPAI H5N1 is unable replicate easily in the upper respiratory tract, which would allow for the person-to-person spread necessary for a pandemic. Should this strain evolve to bind readily to cells in the upper respiratory tract, a major pandemic could occur. To date, however, pandemics have only been caused by H1, H2, and H3 viruses because the other strains (i.e., H5, H7, and H9 viruses) have not evolved to bind sialic acid residues on human respiratory epithelial cells.
During the past 20 years, highly pathogenic avian influenza A (HPAI) H5N1 viruses have been reported from 23 countries. More recently, HPAI H5N1 clade 2.3.4.4b has spread widely in wild birds, poultry and other animals. During the past few months, the virus has been detected in dairy cows in the U.S. suffering from reduced appetite, lethargy, fever, dehydration, and reduced mild production. The virus has also been detected in unpasteurized milk.
In March 2024, an adult dairy farm worker suffered bilateral conjunctivitis with subconjunctival hemorrhage from what was later identified as HPAI H5N1 clade 2.3.4.4b (Uyeki TM, Milton S, Hamid CA, et al. Highly pathogenic avian influenza A(H5N1) virus infection in a dairy farm worker. N Engl J Med. 2024, May 3 DOI: 10.1056/NEJMc2405371.) The patient, who had no respiratory symptoms and a normal chest X-ray, had direct contact with sick cows. The worker wore gloves but not eye protection.
Although we are now in the midst of a bird flu pandemic among wild birds, poultry and other animals, it is important to note that this virus has not yet developed changes in the hemagglutinin gene that would allow for ready binding to the alpha-2-6-linked sialic acid receptor located in the upper respiratory tract of humans. For this reason, the current strain of HPAI H5N1 is unable replicate easily in the upper respiratory tract, which would allow for the person-to-person spread necessary for a pandemic. Should this strain evolve to bind readily to cells in the upper respiratory tract, a major pandemic could occur. To date, however, pandemics have only been caused by H1, H2, and H3 viruses because the other strains (i.e., H5, H7, and H9 viruses) have not evolved to bind sialic acid residues on human respiratory epithelial cells.