Avian influenza (H5N1) has led to the deaths of over 52 million birds in the United States this year, surpassing a previous record for the disease set in 2015, new data from the U.S. Department of Agriculture (USDA) shows.
Millions of birds were culled to contain the disease, which has nearly a 100 percent fatality rate in poultry, per CBS News’ Olivia Young. The flu spreads quickly in flocks of domestic birds, through droppings, saliva or contaminated feed or water.
"Because this virus is so contagious, drastic measures have to be put into place to stop the spread and that ultimately results in the death of millions of birds," John Brownstein, an epidemiologist and chief innovation officer at Boston Children's Hospital, tells ABC News’ Mary Kekatos.
A recent outbreak on a Nebraska farm led to the slaughter of 1.8 million chickens.
H5N1 was first reported in domestic waterfowl in Southern China in 1996. In the following years, outbreaks popped up sporadically in Asia. In 2005, the disease spread to poultry in Africa, the Middle East and Europe.
The current U.S. outbreak began last year in Eastern Canada and has led to the first case of avian flu in humans in the United States.
"We have to remind people the risk to humans is low, but at the same time, unprotected contact with birds that look sick can pose a risk," Brownstein tells ABC News. "An additional layer is when you have this much virus spread, there's opportunities for mutation and this is where there's an opportunity for a version of this virus that could actually have deeper impact in the human population as well."
Farmers are battling a subtype of the H5N1 strain, known as the goose/Guangdong lineage in the United States, per Reuters’ Tom Polansek. Though higher temperatures usually reduce the disease, this strain survived over the past summer, Rosemary Sifford, chief veterinary officer for the USDA, tells the publication.
This year’s outbreak is widespread, affecting flocks in 46 states, and unlike previous years, an unusually high number of wild birds across the world have contracted the disease. In recent weeks, more than 5,000 pelicans have died from avian flu in Peru. Over the summer, the disease killed thousands of birds at some of the U.K.’s most important seabird colonies.
Scientists don't know why so many wild birds have been affected. The virus may be better adapted to these species or a cold, wet spring gave it a better opportunity to infect them, per the New York Times’ Emily Anthes.
The United States is monitoring wild birds along four migration paths for the flu, and will do the same next year, per Reuters.
"We really don't know what's next in this outbreak," Maggie Baldwin, state veterinarian with Colorado's Department of Agriculture, tells the publication. "This virus is carried by wild birds, primarily waterfowl and shorebird species. So when we see increased migration, we expect that we're going to see more virus.”