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In the worst US outbreak of bird flu since 2015, more than 24 million domesticated birds have died of the disease or been killed as of yesterday (April 7) as a highly infectious strain has been making its way through poultry farms and yards, the Associated Press reports. The majority of that tally were culled to try and prevent the spread of the disease, driving up the cost of poultry meat and eggs across the US. NPR reports that so far, the deadly bird flu has spread to at least 24 US states less than two months after the first domestic outbreak was reported in a commercial turkey flock on February 9. 

See “Avian Flu Case Numbers Soar in Europe

According to the US Centers for Disease Control and Prevention, bird flu poses little risk to the general public and poultry and eggs remain safe to eat.

When bird flu cases are found in poultry, officials order the entire flock to be culled. Iowa, the worst-hit state, has culled more than 13 million birds, according to NPR. Five million died on March 31 at an egg-laying facility in Osceola, according to the US Department of Agriculture (USDA).  

Ben Slinger, an Iowa farmer whose family raises turkeys for a meat processor, tells Iowa Public Radio that after he and his family had to cull tens of thousands of birds in 2015, he’s been taking extra precautions. In addition to wearing separate pairs of boots for each barn, workers also walk through disinfectant before entering. “We know what the aftermath of that is like, and it is pretty disheartening,” Slinger says.

The AP reports that the state legislature in Minnesota, where more than 1 million birds have been affected, recently approved $1 million of emergency funding to bolster its flu-fighting efforts. Last week, the number of Minnesota birds affected by the highly infectious avian influenza doubled. Minnesota is the top turkey-producing state in the country.

Rep. John Burkel, a former turkey farmer from Badger, Minnesota, tells the AP that his farm in Roseau County was affected in 2015. That outbreak required euthanizing 9 million birds statewide. “The virus is different this time, and the need and the urgency is greater,” Burkel says. 

According to The New York Timesthe range of the current outbreak, which extends from the Midwest and Plains to New England, is broader than the outbreak in 2015. 

The US is the world’s second-largest poultry meat exporter and a major egg producer, and since the bird flu outbreak, trade has slowed. Reuters reports that the USDA is looking into vaccinating birds against the deadly flu. Although shots would come too late to stop the current outbreak, supporters say it could help keep poultry alive. Previously, the US has shied away from such vaccines, worried that importers in other countries, who test animals for bird flu before receiving a shipment, wouldn’t be able to tell the difference between vaccinated and influenza-infected birds. The USDA’s Agricultural Research Service is investigating the potential for developing a vaccine that could be distinguished in testing from the wild type of virus spread to poultry. 

The virus seems to have spread from wild birds. On January 13 of this year, the USDA announced that a highly pathogenic avian influenza virus was found in wild birds—the first time a strain of avian influenza had been detected in the wild since 2016. According to NPR, the first cases were largely detected in North and South Carolina in birds killed by hunters. Many of the subsequent infections were reported in Iowa and neighboring states—at the intersection of the central and Mississippi flyways used by wild birds during migration. 

Birds can transmit the virus through their droppings and nasal discharge. In the 2015 outbreak, objects such as boots, clothing of poultry industry employees, and vehicles used to spread feed were also thought to be major vectors of disease transmission, reports NPR

The Associated Press also reports that birds in zoos are being isolated indoors to prevent them from contracting bird flu. Penguins may be the only birds visitors can see right now, as they are already kept behind glass in their exhibits. So far, no outbreaks have been reported at zoos.

In 2015, the outbreak ended in June, six months after the first cases were reported. Effects on the supply chain lingered, however, and it wasn’t until several months later that some poultry prices normalized, according to NPR.