The strain of avian influenza, or bird flu, known as H5N1 has proved to be alarmingly adept at jumping continents and species. First discovered in 1996 in geese bred on a farm in southern China, the virus has rampaged through populations of captive, commercial birds in recent years, prompting governments to order the slaughter of tens of millions of turkeys, chickens and other poultry to limit the opportunity for contagion. One result: higher food prices. Carried by infected wild birds, particularly geese, swans and gulls, the virus has also been gaining a foothold in many types of mammals. That includes a small number of humans, where it has proven to be lethal. Now seemingly able to spread from mammal to mammal — as seen with mink on a Spanish farm and Peruvian sea lions — the ever-evolving virus has public-health officials on alert for any indications of the most feared outcome: human-to-human transmission that could trigger a pandemic.
1. What’s different about this strain of bird flu?
Since 2020, when a new variant of the H5N1 strain emerged in Europe, outbreak numbers that typically ebb and flow have been persistently elevated. Health experts say the new version, clade 2.3.4.4b, is especially efficient at spreading among birds, both domesticated and wild, and the mammals that prey on or scavenge them. It’s led to an unprecedented number of deaths in wild birds and poultry in Europe as well as Asia and Africa, and it spread to the Americas in 2021 and 2022. In the UK, there were 23 infections in wild animals, including in red foxes, Eurasian otters and harbor seals, through July 4, 2023. Dozens of domestic cats and five dogs have been infected in Europe. Twelve fur farms in Finland, housing as many as 50,000 foxes, mink and raccoon dogs, have reported cases. Almost 3,500 sea lions in Peru succumbed to the infection. In the US, H5N1 has been detected in mountain lions, bobcats, bears, dolphins, skunks, raccoons and other animals.
2. What’s the impact on humans so far?
From the start of 2020 through July 14, 2023, there were 17 confirmed cases of H5N1 in humans, according to the World Health Organization. The number includes four infections in people with no symptoms detected through a proactive surveillance program in the UK. All were thought to have contracted the infection via direct contact with sick birds or surfaces contaminated with their saliva or feces. Although case numbers are modest, the death rate among humans isn’t: Of 878 people known to have been infected from 2003 through July 2023, 458 died. With H5N1, as with Covid-19, flu-like symptoms — fever, sore throat, congestion — can advance to pneumonia and respiratory failure.