Less than a month after reporting its first human case of bird flu, California has confirmed that 16 people have now been infected with the disease, and a Newsweek map shows how the numbers compare with the rest of the U.S.
Nationwide, the total number of people infected with the avian influenza H5N1 in 2024 now stands at 36.
“To date, [the Centers for Disease Control and Prevention (CDC)] has confirmed infection of 16 farmworkers with H5N1 bird flu in California,” the CDC said in an update on Tuesday. “Genetic sequencing of nine of these cases confirms that all are H5N1 viruses from clade 2.3.4.4b and that all are closely related genetically to the virus causing infections in domestic dairy cattle.”
All of the cases in California have arisen in dairy farmworkers, the CDC said, as is the case with the majority of infections across the country. Elsewhere, 15 cases—nine in Colorado and six in Washington—have been associated with poultry farms.
The source of infection for one patient in Missouri has still not been identified.
California has been by far the hardest hit state since the virus, which had been infecting wild bird and poultry populations around the world, was detected in cattle earlier this year.
According to the latest CDC data, 186 livestock herds have been affected in California by the outbreak, 142 of which have been hit in the last 30 days.
Numbers of deaths among cows in the state have climbed so rapidly that authorities have been unable to remove the carcasses at times, leading to some farmers simply dumping them at the side of the road while they await collection.
“The current bird flu situation in the U.S. is quite disturbing and odd,” Jeremy Rossman, senior lecturer in virology at the University of Kent, U.K, previously told Newsweek when California reported its first cases among farmworkers. “I do not think they are doing a good job at containing the outbreak, and put simply, they are not containing the outbreak.”
So far, all cases are thought to have arisen via animal spillover, but as more and more humans get infected, the risk that the virus could jump between people increases.
“The more transmission that occurs in cows—and the more human spillover that occurs—the more chances that the virus will mutate and start to spread from person to person,” Rossman told Newsweek.
If that were to happen, especially as flu season approaches in the U.S., the implications could be serious. “The biggest concern is that we just don’t know what it would be like if, in fact, we did get this human transmissible bird flu coming from cattle,” Rossman said.
In its update, the CDC confirmed that “all available data so far suggest sporadic instances of animal-to-human spread” and that “the farmworkers in California and Washington state all described mild symptoms, many with eye redness or discharge (conjunctivitis).”
However, Rossman said that the nature of mutation means that if the virus begins spreading in human populations, its characteristics could change too. Rossman added, “The concern is that we would get person-to-person spread as efficiently as you get with seasonal flu, but that it has a very high case fatality rate.”
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