Bloomberg For influenza viruses, imperfection is a strength. They constantly mutate, producing new strains that challenge immune systems primed to fight earlier varieties. That’s what makes flu a life-long threat to humans and the animal species — mainly birds — that are vulnerable to it.
Since 2020, a strain of highly pathogenic avian influenza called H5N1 has been decimating both wild and domestic birds. Now it’s spreading among US dairy cows, and has infected US farmworkers exposed to sick cattle and poultry. A man in Mexico died with a strain of bird flu, H5N2, that hasn’t been seen in humans before.
The US Centers for Disease Control and Prevention and the World Heath Organization say the overall risk to the general public remains low, but public-health officials remain on alert for any indication of the most feared outcome: human-to-human transmission that could trigger a pandemic.
1. When did H5N1 begin spreading?
The H5N1 strain was first discovered in 1996 in geese bred in southern China. It’s proved to be alarmingly adept at jumping continents and species and has rampaged through captive, commercial birds across the globe; governments ordered millions of birds slaughtered to limit the contagion. A variant that emerged in 2020, clade 2.3.4.4b, led to infections in mammals like farmed mink in Spain and Peruvian sea lions. The same strain is ripping through wild birds and poultry in the US and has gained a foothold among dairy cows, with cases popping up among people who work with sick animals.