A second human case of bird flu in America is raising alarm

Editor’s note (May 31st 2024): Since this piece was published a third farm worker has been identified with H5N1

On May 22nd America’s Centres for Disease Control and Prevention confirmed a new human infection with the H5 avian influenza virus, a bug with pandemic potential that has recently spread rapidly among dairy cows in America. This is the second identified human case linked to the cow outbreak. Both are farm workers who probably got infected through contact with infected animals and had only mild symptoms. Although there is no evidence yet of human-to-human transmission, America’s public health authorities are on high alert. Half of the national pandemic stockpile of H5N1 vaccine is being made ready to deploy.

Influenza viruses infect many species and, every now and then, a virus will cross over and infect a species that is not its speciality. When that happens, it could be highly lethal for the new host species. H5N1, an avian influenza virus that has now crossed over into numerous bird and some mammalian species, is one of those with pandemic potential because there is no immunity to it in humans. In the nearly three decades since H5N1 was first detected in geese in China, there have been around 900 known human infections worldwide, usually from contact with infected poultry or other birds. Nearly half of these were fatal. Still, mild or asymptomatic infections would have gone undetected, making the fatality rate lower.

In 2022 H5N1 began to spread more widely among wild bird species, which carried it around the world, infecting numerous species of wild mammals, as well as farmed poultry and mink, as they went. The virus is highly lethal in poultry, so farms in Europe, America and Asia have all had to cull hundreds of millions of birds in an attempt to stop its spread.

This spring, H5N1 turned up in dairy cows on a farm in Texas. It has since been found in dairy herds in at least nine states. In early May, the Food and Drug Administration found that about 20% of milk samples from store shelves across America contained H5N1 (which is killed by pasteurisation), which suggests that the spread in dairy cows was wider than the limited testing of animals had confirmed.

Farmers have balked at reporting outbreaks or testing cows, for fear of losing income. Surveillance for infections in farm workers has also been far weaker than what experts say is needed to keep tabs on how bad the cow iteration of H5N1 may be for humans. This has been exacerbated by the fact that many farm workers, for example, are undocumented immigrants who do not speak English and are paid daily wages (which they will forgo if they have to isolate). Reaching them with information and convincing them to get tested has therefore been a challenge. As of May 22nd only 40 people with suspected H5N1 had been tested.

All of this leaves scientists trying to piece together various bits of imperfect data to assess how H5N1 is evolving. A scientific consortium in America is starting to look for H5N1 in wastewater, but this method can only single out places where there is a lot (or suddenly more) virus circulating. It cannot identify whether the source is cows, humans, discarded milk or infected wild birds shedding it into the sewer systems.

What is clear about H5N1 at the moment is that “if this were spreading human-to-human we’d know”, says Jennifer Nuzzo, an epidemiologist at Brown University. The immediate health concern, she says, is that farm workers are being infected in unknown numbers through exposure to sick animals, because little is being done to stop the spread in cows.

Despite the lack of testing, it is encouraging that only two human cases linked to the cow outbreak have been confirmed so far, given that lots of people on farms are typically in contact with infected cows. Milk from infected cows is loaded with the H5N1 virus, so exposure to it at milking stations would, inevitably, lead to human infections, says Michael Worobey, an evolutionary biologist at the University of Arizona. If serious symptoms from such infections were common, more cases probably would have come to the attention of doctors by now.

Both of the infected farm workers identified so far had only one symptom: conjunctivitis (an eye infection known commonly as “pink eye”). Cells in the human eye have specific receptors that are also present in bird cells—“docking stations” through which a virus enters, says Dr Worobey. That means H5N1 can infect a person when it lands in their eye without having to first develop the specialised mutations it usually needs. It is a similar story with the cells in human lungs, which is why the virus has caused severe lung infections in many people in past outbreaks who have caught it from birds.

But cells in the upper respiratory tract of humans—a part of the body that is important when viruses spread between people through coughs and sneezes—do not have these bird-like receptors. In order to start spreading easily between people, the virus needs to acquire genetic mutations that will allow it to infect those cells in the upper airways. So far, genomic analysis of H5N1 samples from American farms has not found any with these crucial mutations. A nasal swab from one of the infected farm workers did not find any virus; only the eye sample tested positive.

Nonetheless, the situation in America is concerning. The widespread presence of H5N1 in mammals with whom lots of humans are in close contact increases the chances that a version adapted to humans could emerge at some point. The virus mutates as it spreads among cows, so a human-infecting mutation may occur by chance in a viral particle that is picked up by a person. Another possibility is the emergence of a recombinant version of H5N1, which happens when a person is infected with both the cow H5N1 and one of the seasonal flu viruses that circulate every year in humans. Influenza viruses are notorious for swapping bits of their genetic code between them when they infect a cell. The cow H5N1 virus could acquire, in that way, the genes that make the seasonal flu highly contagious among humans.

Though it is impossible to predict what will happen, the way the current outbreak has unfolded in America does not bode well for preventing a future pandemic, whether by H5N1 or another virus. It is worrying that “this was spreading under our noses in a totally new host for months before it was identified”, says Dr Worobey. Surveillance systems that can detect, track and limit the spread of new pathogens in animals that are farmed in large numbers is how future pandemics can be prevented. Without that, the next pandemic could just as easily start in Texas as in Wuhan. ■

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