It has been one year since the beginning of an outbreak of highly pathogenic avian influenza (HPAI), strain H5N1, in the US. To date, 47 states have been affected. The Animal and Plant Health Inspection Service (APHIS), part of the United States Department of Agriculture (USDA), has reported 6,161 confirmed cases of H5N1 in wild birds since January last year. The 2022–23 outbreak of HPAI may prove to be the worst on record, affecting wild and domestic birds, mammals, and potentially humans.
According to the AHPIS, this outbreak has currently reached 441 backyard flocks and 317 commercial flocks, totalling around 58.39 million birds affected. This includes not only infected birds, but also birds killed to contain the spread of HPAI in a practice known as culling. The death of birds via infection or culling has also impacted the poultry industry and consumers. In December 2022, the Economic Research Service of the USDA reported that 43 million egg-laying hens had been lost in two waves of HPAI, from February to June and from September to December. This contributed to egg prices being 267% higher in December 2022 than in January that year.
The US Centres for Disease Control and Prevention (CDC) note that while H5N1 is an influenza strain that is endemic in birds, transmission to mammals can occur. Animals that prey on infected birds are at elevated risk of contracting the virus. Since April last year, 110 wild mammals in the US have tested positive for H5N1. Foxes account for roughly half of these cases, followed by skunks, seals, and raccoons. All 17 seal cases were recorded during June and July in Maine, and six of the 18 total skunk cases were collected on 28 November in Oregon.
The presence of HPAI H5N1 in mammals is not a guarantee that the virus will adapt to human transmission, but it is an opportunity for mutation and antigenic shift. Antigenic shift is a dramatic rearrangement of two co-infecting influenza viruses to produce a completely new influenza strain. This occurs most often in pigs, as seen in the 2009 H1N1 pandemic strain. It can also occur in humans, but there is little to no record of antigenic shift in species such as foxes or skunks. Antigenic shift is not common and does not always produce a pandemic strain, but mammalian infection with avian influenza can increase this risk.
Human cases of avian influenza are rare. The CDC has reported only a single human case of HPAI H5N1 in the US, which occurred in a poultry worker in April last year. Avian influenza has not resulted in sustained human-to-human transmission. Humans most at risk of contracting avian influenza from birds include poultry workers and those in contact with wild birds. The CDC discourages contact with sick or dead birds and their droppings if possible and advises those in contact with wild birds and poultry to employ personal protective equipment (PPE).
In addition to avian influenza cases in birds and mammals, GlobalData epidemiologists forecast that there will be more than 1.4 million diagnosed incident cases of laboratory-confirmed seasonal influenza in humans in the US by the end of this year.
This may place additional pressure on public health systems, emphasising the urgency of HPAI containment. Transmission prevention measures learned from the COVID–19 pandemic and increased awareness of non-human influenza transmission may serve to reduce this burden. Ideally, a combination of awareness and transmission prevention measures will protect backyard flocks and domestic mammals from HPAI infection.