It’s been detected in birds on every continent except Australia, seals in South America, foxes in Canada, as well as poultry, dairy cows and dozens of farm workers who have had contact with them in the US – and now the highly infectious H5N1 bird flu virus has jumped to a pig in the US for the first time.
This development is, without a doubt, troubling. It affords the virus one of its best opportunities to date to evolve and become dangerous to humans. But it is highly unlikely to become the turning point that sends the US into another pandemic.
There is ample reason to have a healthy fear of H5N1. Since 2003, around have reportedly contracted the virus worldwide, and at least half of those have died from it. The good news is H5N1 is still very much a bird flu virus, meaning it is poorly adapted to infecting humans. It also isn’t known to transmit between people. But that could change.
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Every time H5N1 infects a mammal, it has a chance to develop mutations that make it more transmissible to other mammals – humans included. But this possibility is greater for some mammals than others. Consider cows: while H5N1 has been circulating among US dairy cattle for nearly eight months, it has yet to acquire any mutations that we know of which would make it more dangerous to people.
Pigs are a completely different story. Aside from humans, pigs probably provide H5N1 with the best opportunity to adapt to people. Pigs have both human-like and bird-like influenza receptors in their upper respiratory tracts, so bird flu viruses and human flu viruses can infect them simultaneously. If that happens, the viruses could potentially swap genetic material through a process known as reassortment. This makes pigs a “mixing vessel” for influenza viruses, says at Johns Hopkins University in Maryland.
A prime example of this was the 2009 swine flu pandemic, in which an influenza virus picked up genetic elements from human, avian and swine flu viruses in pigs before spilling over into humans. The risk of viral spillover is further exacerbated by our frequent contact with hogs – there are roughly on US farms.
Given all of this, it is unsurprising that the 30 October from the US Department of Agriculture (USDA) of an H5N1-infected pig in Oregon would stoke anxieties about the virus adapting to humans. However, there are a number of reasons why this is unlikely to have happened in the Oregon outbreak.
For one, the infected pig lived in a small backyard farm with only four other swine. After USDA and Oregon Department of Health officials realised there were H5N1-infected chickens sharing water and shelter with the pigs, they euthanised all of the animals.
“That immediate, sort of heavy-handed response, what it does is reduce any potential forward transmission risk,” says at Emory University in Georgia. While we don’t know if the virus spread to people or other animals on the farm prior to the pigs’ deaths, Lakdawala believes it is unlikely. “We haven’t seen any other cases pop up yet, and I think based on the timing [of the infection] we probably would have seen it,” she says. All of this suggests the small farm was a dead end for the virus.
Two of the pigs also tested negative for H5N1. While we are still awaiting results for the other two, this is good news given how rapidly influenza viruses transmit between swine. The one pig that tested positive showed no symptoms, implying a mild infection. The farm is under quarantine to prevent any further spread of the virus, and officials are monitoring other animals on the premise, including sheep and goats, for signs of illness.
Even though it seems this outbreak was nipped in the bud, it underscores an alarming prospect – that H5N1 can infect pigs. “There are major implications for parts of the country where commercial swine operations and commercial dairy or poultry facilities are in close proximity,” says Davis.
If we don’t contain other H5N1 outbreaks, including those in US dairy cattle, the virus will have plenty of opportunities to spread further through pigs, or even to more humans, potentially developing dangerous mutations along the way.
