Cyclospora cases in 15 states have been linked to iceberg lettuce, U.S. health officials said Wednesday, adding six states with cases tied to the ongoing outbreak.

The parasite has been reported in at least 47 states total this year, and investigators have continued to probe how many could be linked to a common contaminated source.

The Centers for Disease Control and Prevention said the number of states tied to the deadly outbreak had grown from nine to 15. Arkansas, Iowa, Missouri, Nebraska, New Hampshire and North Carolina were added to a list that included Illinois, Indiana, Kansas, Kentucky, Michigan, Ohio, Oklahoma, Pennsylvania and West Virginia.

Investigators initially narrowed in on the lettuce served at Taco Bell. Federal officials subsequently said they are focused on Taylor Farms as the source of the lettuce in the outbreak, and the company has recalled iceberg lettuce grown in central Mexico.

Confusion was sparked last month when the Food and Drug Administration announced that a sample of lettuce from Taylor Farms that had tested positive for cyclospora was a false positive. But the FDA clarified the next day that the investigation and evidence "continue to converge on shredded iceberg lettuce from Taylor Farms locations in central Mexico."

"I don't think we can say that this is under control yet," Dr. Ulysses Wu, the chief epidemiologist for Hartford HealthCare, told CBS News last month. "Even if this lettuce was implicated, it doesn't mean that other produce is not implicated as well."

The CDC previously said it can take as long as six weeks for federal and state officials to determine if a sick person is part of an outbreak. There are several reasons cyclospora outbreaks can be hard to investigate and contain, said Dr. Juan Luis Marquez, medical director of the Washtenaw County and Livingston County health departments in Michigan.

It can take weeks between when someone is exposed to the parasite and when they come down with symptoms; it's harder to analyze the parasite in laboratories; and it's a lot of work to interview everyone who is sick and clearly identify common exposures, said Marquez, an adjunct public health professor at the University of Michigan.

Also, "there have been staffing cuts at all levels of public health — federal, local, state — so there are less resources and capacity generally available," Marquez said in a statement.

Cyclospora is a microscopic, spherical parasite that commonly causes watery diarrhea "with frequent and sometimes explosive bowel movements," according to the CDC. Outbreaks tend to occur most often in the late spring and summer.

The heat-loving parasite infects the bowels and spreads through feces. The illness, called cyclosporiasis, is less common than foodborne illnesses caused by other germs, including salmonella and E. coli. Many cyclospora cases are never linked to a specific food or other source.

The worst year in the U.S. for infections was 2019, when about 4,700 illnesses were reported nationally. This year has far surpassed that. Michigan alone has reported more than 12,000 cases — including two deaths. Adding in the other states that have reported cases, the national total stands at over 25,000.

Of the more than 12,000 cases in Michigan, 193 patients had received hospital treatment as of July 30, according to local health officials. The two people who died had "significant underlying health conditions," the Michigan Department of Health and Human Services said.

The FDA says it is tracking five separate ongoing cyclospora outbreaks. The 15-state outbreak is by far the largest. No products have been linked to the other outbreaks, including one that has sickened 115 people.