While the investigation is under way, Taylor Farms, which supplies the slivered onions to affected locations, has issued a voluntary recall of its yellow onions.

McDonald’s restaurants in Colorado, Kansas, Utah, Wyoming, Idaho, Iowa, Missouri, Montana, Nebraska, Nevada, New Mexico and Oklahoma have temporarily pulled Quarter Pounders from their menus, though other items, including other beef burgers, remain available.

Shares of the fast-food giant dropped more than 2% in early afternoon trading in the US.

Meanwhile, the law firms Ron Simon & Associates and Meyers & Flowers have filed lawsuits on behalf of two separate consumers from Colorado and Nebraska who fell ill after consuming the burgers.

Each suit seeks a minimum of US$50,000 ($83,600) in damages, and attorney Ron Simon told AFP he plans to represent a total of 25 victims.

“When a consumer goes to McDonald’s to buy a meal, they’re placing an enormous amount of trust that McDonald’s has done everything it can to make your food safe,” said Simon.

“And here, whether it was faulty testing, faulty oversight, faulty handling, somehow poison got in that food, and trust is broken.”

The CDC advised those who consumed a Quarter Pounder and developed symptoms of E. coli poisoning – such as diarrhoea, bloody diarrhoea, a fever over 38.9C, and vomiting – to seek medical attention.

Symptoms typically begin three to four days after exposure, and most individuals recover within five to seven days without treatment. However, some cases can become severe and require hospitalisation.

McDonald’s said in a statement Tuesday it had taken “swift and decisive action” and that food safety was its “top priority”.