Highly pathogenic avian influenza (HPAI) A(H5N1) emerged in 1997.1 Since then, it has spread globally by migratory birds, resulting in infections in animals on every continent. HPAI A(H5N1) clade 2.3.4.4b emerged in 2021 and resulted in fatal infections in poultry as well as terrestrial and marine mammals.1 In early 2024, influenza A infection was first recognized in dairy cows with mastitis in Texas. Infection in dairy cows is now widespread in the United States, affecting more than 875 herds in 16 states. Most cow infections are genotype B3.13, whereas most outbreaks in wild birds and poultry are genotype D1.1.
Against this background, more humans have come into contact with HPAI A(H5N1). Investigators now report in the Journal a series of human cases from the United States and Canada.2,3 The former series involves 46 case patients with generally mild, self-limited infection with A(H5N1): 20 with exposure to poultry, 25 with exposure to dairy cows, and 1 with undefined exposure.2 Among case patients with occupational exposure, the use of personal protective equipment (PPE) was not universal. Most case patients presented with conjunctivitis, almost half with fever, and a minority with mild respiratory symptoms, and all recovered. The only hospitalization occurred in the case patient with undefined exposure, although hospitalization was not for respiratory illness. Of cases with sequenceable virus, most were B3.13; four cases in patients with poultry exposure were D1.1.