A listeriosis outbreak linked to prepackaged caramel apples has sickened 28 people and killed four, the U.S. Centres for Disease Control and Prevention said Friday.
The CDC issued a warning to the public that any caramel apples already purchased should not be eaten. The agency said it has launched an investigation, working with the U.S. Food and Drug Administration and state health departments.
Listeriosis, caused by the bacterium listeria monocytogenes, is most dangerous for people with compromised immune systems, women who are pregnant and the elderly. Symptoms include stomach problems, fever and aching muscles and it can also cause encephalitis and bacterial meningitis.
Dr. William Schaffner of Vanderbilt University Medical School said the incubation period can be as long as 70 days. That means many people may have eaten contaminated caramel apples and have not yet developed symptoms.
"We can anticipate that more illnesses will occur over time," he told ABC News.
The CDC said listeriosis cases have been reported in 10 states across the country. The first were in Minnesota, where four people became sick and two died, all after eating caramel apples.
CDC investigators have interviewed 18 people with the infection, and 15 have reported eating caramel apples, the agency said. A fifth listeriosis death has not been definitively linked to the apples.
Investigators said homemade caramel apples appear to be safe, and the outbreak does not seem to involve fresh fruit.
Nine cases of illness have occurred in pregnant women or newborn babies. In three case, children have developed invasive meningitis, a potentially fatal inflammation of the brain or spine.