The FDA and CDC are reporting that an outbreak of infections from Listeria monocytogenes in leafy greens has ended.

The Centers for Disease Control and Prevention first reported on the outbreak on Feb. 15 this year, but the agency determined that the first patient in the outbreak became ill in July of 2018. Laboratory tests showed the victims were infected by the same strain of Listeria.

“When this investigation began, there was not enough information to identify the source of the outbreak,” according to the CDC. “State and local health officials collected as much information as they could by asking people or their caregivers about foods they ate before getting sick. Epidemiologic data showed that leafy greens were a likely source of this outbreak, but there was not enough other data to identify a specific type or producer of leafy greens.”

By the time the outbreak was declared over on June 13 this year, 19 patients had been confirmed with 18 of them requiring hospitalization. The patients were spread across 16 states. No deaths were reported. 

Thirteen of 14 people interviewed reported eating leafy greens before becoming sick. No recalls were initiated in relation to the outbreak. 

Sick people ranged in age from less than 1 to 96 years, with a median age of 72.

The CDC reported that state and local public health officials interviewed people about the foods they ate in the month before they got sick. Of 14 people who answered questions about leafy greens, 13 ate iceberg lettuce and 10 ate romaine lettuce. Twelve people ate packaged salads.

“Three people ate leafy greens at the long-term care facilities they lived in, and one person ate leafy greens at a hospital they worked in. People bought leafy greens and different brands of packaged salads from several stores,” according to the CDC’s outbreak report.

“The true number of sick people in this outbreak is likely higher than the number reported, and the outbreak may not be limited to the states with known illnesses. This is because some people recover without medical care and are not tested for Listeria.”

It can take up to 70 days after consumption of foods contaminated with Listeria monocytogenes for symptoms to appear and they often mimic other illnesses, thus requiring specific testing to confirm Listeria infection.

The outbreak investigation was particularly challenging because it stretched over such a long time period.

“Due to the lack of additional detail in the epidemiological data and the absence of supporting evidence collected from traceback and sample collection, investigators were unable to determine a specific type or producer of leafy greens as the source of the outbreak,” according to the FDA’s final updat on the outbreak.