H5 viruses, a subtype of Influenza A or ‘bird flu’, primarily infect wild birds like ducks, geese, and swans.
When they spread to domestic birds such as chickens and turkeys, they can cause severe outbreaks with high mortality rates. These avian viruses have long been thought to pose the greatest risk of another human pandemic.
Strains like H5N1 have shown a worrying ability to infect mammals and humans. It has killed around 400 people since its discovery in the late 1990s, while another clade – H5N6 – caused 18 deaths in China in 2021.
Experts are racing to understand whether the H5N2 case was an isolated incident, or if it somehow links to the ongoing H5N1 bird flu outbreak in the US – which has so far infected 82 herds of dairy cattle in ten states, and at least three farm workers.
“This could be a fully avian H5N2 strain from poultry in Mexico, but we won’t know until we get genomic sequence data, which has not yet been released,” said Dr Rick Bright, a former head of the US Biomedical Advanced Research and Development Authority (BARDA).
“This case raises many more questions that we haven’t come close to answering about the virus on our doorstep,” he added.
“Mexico and the US are very closely linked, if both of these viruses – H5N2 and H5N1 – are in the environment and circulating around us, the risk for a more significant outbreak increases greatly,” Dr Bright said.
‘Could a mouse be a mixing vessel?’
One theory is that the H5N2 virus has somehow mixed with the H5N1 virus circulating in the US.
Just this week, the US Department of Agriculture released data showing that eleven house mice in the state of New Mexico – around 2,000km away from Mexico City – had contracted H5N1.
Although not the most likely scenario, it’s not out of the question that mammals like rodents could have somehow facilitated the transmission or reassortment of these viruses, potentially creating new and more dangerous strains.
“This needs to be ruled out quickly and clearly – if a reassortment has happened, we’ll need to pick up the response. Could a mouse be a mixing vessel?” said Dr Bright.
Another theory is that the victim might have had contact with people in the US working closely with sick cattle. About half of all US dairy workers – 51 per cent – are undocumented migrants, a significant proportion of which are Mexican.
“It’s unclear if any of his contacts or visitors had any travel to the US, or if any of the migrant workers from the US were someone in contact with him,” Dr Bright said.
Again, although H5N2 is a different strain of bird flu from H5N1, scientists have not yet seen genomic data – meaning they are ruling nothing out for the time being.
