The number of hospitalizations has risen to 22, though no additional deaths have been reported beyond that of an elderly patient in Colorado, according to the Food and Drug Administration (FDA) and the Centers for Disease Control and Prevention (CDC).

One child and one adult developed hemolytic uremic syndrome, a serious condition that damages blood vessels in the kidneys. Investigators have yet to confirm a specific ingredient as the contamination source, though they are assessing whether slivered onions or beef patties could be the origin.

McDonald’s restaurants in the 13 impacted states have temporarily pulled Quarter Pounders from their menus, though other items, including other beef burgers, remain available.

The CDC advised those who consumed a Quarter Pounder and developed symptoms of E. coli poisoning – such as diarrhea, bloody diarrhea, a fever over 102 degrees Fahrenheit (38.9°C), 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 hospitalization.

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