Avian influenza (bird flu) is a highly contagious viral infection that primarily affects birds. An emerging new lineage 2.3.4.4b of avian influenza has been spreading across the globe since late 2020, carried by migratory birds following specific routes. This panzootic has a significant effect on the avian population with disastrous consequences to the ecology and significant economic loss affecting poultry across the world. In rare instances, the virus can infect mammals from birds causing spillovers, and in recent years, several such instances of spillovers spanning over 200 species have been noted, the most recent being polar bears in Antarctica. Close contact with infected animals could mean the virus could spill over infecting humans and this comes with a significantly large fatality rate.
In late March 2024, a multistate outbreak of H5N1 in dairy cows was detected in the U.S. In what started as a mystery disease affecting dairy herds in Texas, the U.S. Department of Agriculture (USDA) detected the highly pathogenic strain of the virus in herds across the states of Texas and Kansas. The affected animals displayed symptoms including loss of appetite, low-grade fever, and reduced lactation. This marked the first time H5N1 had been detected in cattle, raising concerns about potential transmission routes and the broader impact on the dairy and meat industry. To date, H5N1 has been detected in over 12 herds from six states — Texas, Kansas, Michigan, New Mexico, Idaho and Ohio.
In Texas, other animals in proximity to the affected farms — wild birds and cats — also showed signs of illness and have tested positive for the virus. The exact extent of the spread of H5N1 in cows has not yet been ascertained since cattle are not routinely tested for avian influenza and the symptoms have been relatively mild, leaving possibilities that there may be other undetected infected herds. The initial detections in late March in Texas, Kansas, and Michigan have yet to show a definitive pattern of transmission. Given the pace of the spread of infection across herds, the transmission of the virus within cattle has not yet been ruled out. Investigations are ongoing to determine the exact source of the spread to cows.
Subsequently, in April 2024, a human infection of H5N1 was reported from Texas. The infected individual reportedly had contact with cows presumed to be infected with the virus. Fortunately, the patient had mild illness, with symptoms like eye redness, and has recovered after being treated with antivirals. Following this case, the CDC has reiterated that the risk of infections for humans remains low, although people with prolonged or close exposure to H5N1-infected animals such as farm workers, are at a higher risk. This is the second case of human H5N1 infection reported from the U.S. A previous case was reported in 2022 from Colorado.
Human H5N1 infections are rare but have been sporadically reported in several countries. H5N1 infections in humans can range from mild symptoms of eye infections to severe illness including pneumonia and death. Since 2003, more than 800 sporadic human H5N1 cases have been reported to the World Health Organization from over 20 countries, with a case-fatality ratio of 53%. In January 2024, concerns about H5N1 avian influenza rose in Southeast Asia when Cambodia reported two human cases, with tragically one resulting in death. A fatal case of H5N1 infection was also recently reported from Vietnam.
Genomic insights
Researchers sequenced the H5N1 virus from both the infected Texas cows and the human case and found that both viruses belonged to clade 2.3.4.4b of H5N1, with the human strain having one minor mutation potentially linked to adaptation in mammals. However, this change has not led to increased transmissibility among humans, and the overall public health risk remains low according to the CDC. Since late 2021, H5N1 clade 2.3.4.4b has circulated in wild birds in the U.S., causing outbreaks in poultry and sporadic infections in mammals, including the recent Texas case.
Globally, 2023 also saw potential spillover events with H5N1 linked to the deaths of seals in Russia and infections in marine mammals in Peru. Early in 2023, dead seals in Russia and infected marine mammals in Peru suggested H5N1 might jump from birds to mammals. Additionally, the U.K. reported deaths of other animals including otters and foxes due to H5N1. These incidents underscore the need for further research to understand how H5N1 might evolve and potentially adapt to different species.
In summary, while the current risk of H5N1 transmission to humans is considered low, a combined approach of disease surveillance and monitoring the virus’s genetic makeup (genomic surveillance) will be crucial for managing the outbreak.
(Bani Jolly is a senior scientist at Karkinos Healthcare and Vinod Scaria is a senior consultant at Vishwanath Cancer Care Foundation)
