The number of U.S. states tied to a deadly outbreak of cyclospora has reportedly grown from nine to 15, Reuters and The Associated Press said this week, citing three individuals familiar with a federal investigation into the outbreak.

Missouri, Nebraska and North Carolina health authorities have confirmed the presence of the disease in their respective states, while officials in Arkansas said it had been detected in a statement to the Guardian; Iowa and New Hampshire were first reported to have observed cases by the Washington Post.

Those states were added to a list that previously included Illinois, Indiana, Kansas, Kentucky, Michigan, Ohio, Oklahoma, Pennsylvania and West Virginia, both agencies reported.

The Centers for Disease Control and Prevention (CDC) surveillance trails state reporting by several weeks.

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The outbreak, which is among the largest caused by foodborne illness in recent U.S. history, has produced more than 10,468 laboratory-confirmed cases and 12,255 suspected cases nationwide, as well as 517 hospitalizations and two deaths, the CDC website says.

Investigators initially suspected lettuce sold at Taco Bell to be the source of the outbreak, but federal officials subsequently said they were focused on Taylor Farms, which supplied the green to Taco Bell restaurants, as the source. The company has recalled iceberg lettuce grown in central Mexico. Reports of a false positive from a test conducted on Taylor Farms lettuce from Mexico circulated in late July.

Authorities are still looking for other potential sources.

It can be challenging to narrow down the source of a cyclospora outbreak because symptoms often start to appear several weeks after exposure.

In addition to lagging symptoms, Dr. Juan Luis Marquez, medical director of the Washtenaw County and Livingston County health departments in Michigan, told the AP that the parasite is hard to analyze in a lab and that “it’s a lot of work to interview everyone who is sick and clearly identify common exposures.”

“There have been staffing cuts at all levels of public health — federal, local, state — so there are fewer resources and capacity generally available,” Marquez, who is also an adjunct public health professor at the University of Michigan, also noted.

Two cyclosporiasis-related deaths in Michigan were reported by the state’s health department on Monday, marking the first fatalities in the nationwide outbreak.

State health officials reported 11,234 outbreak-linked cases the same day, up 461 from Friday’s update. The outbreak has resulted in 193 hospitalizations, according to the health department’s latest data.

The U.S. Department of Health and Human Services said it is aware of the deaths in Michigan.

“This development highlights the importance of identifying the source of the outbreak and preventing additional illnesses,” said Wade Syers, a food safety specialist at Michigan State University Extension.

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, it said.

The parasite thrives in heat, infects the bowels and spreads through feces. In the past, people have been infected by consuming fruits or vegetables irrigated with feces-contaminated water.

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 — including two deaths. Adding in the other states that have reported cases, the national total stands at more than 25,000.

— With files from the Associated Press