(WASHINGTON) -- Federal health officials have been investigating five different clusters of cyclosporiasis cases since the start of parasite season in May, the Food and Drug Administration (FDA) said during a press conference on Monday.

Three of those clusters are considered over, acting FDA Commissioner Kyle Diamantas said. Meanwhile, the largest cluster, linked to shredded iceberg lettuce served at Taco Bell locations in five states, has seen cases as recently as last week.

The majority of cases have been reported in Michigan, where there are now 6,148 cases, according to an update Monday morning from the Michigan Department of Health and Human Services (MDHHS). This is an increase of more than 1,000 cases since Friday.

"I want to stress that our investigation continues, including into other potential sources and commodities for other clusters of illnesses," Diamantas said during Monday's press conference. 

So far, the CDC has confirmed 1,644 infections linked to the outbreak in Indiana, Kentucky, Michigan, Ohio and West Virginia, and has advised the public not to eat shredded iceberg lettuce served at Taco Bell locations in those five states.

The agency said the true number of cases in the outbreak is "likely much higher than the number reported" because many people recover from the illness without medical care and are not tested for the Cyclospora parasite.

Diamantas also addressed a false positive test result over the weekend from a sample of iceberg lettuce from Taylor Farms de Mexico. 

Last week, Taylor Farms de Mexico issued a voluntary recall of all iceberg lettuce sourced from central Mexico, with the recall affecting 27 states. The recalled products were distributed from June 29 through July 16 and have a best-by dates up to Aug. 3.

Taco Bell said Friday that it is no longer using lettuce from Taylor Farms de Mexico at any of its restaurants.

Over the weekend, the FDA said a sample of shredded iceberg lettuce supplied by Taylor Farms, which was not part of the recall, tested positive.

However, the agency later clarified that during a re-review of the sample results, it determined the result was a false positive. Taylor Farms issued a statement on Sunday, saying "we were informed that FDA made a mistake."

"To be clear, at this moment, FDA has not identified a single positive product test result for Cyclospora," Taylor Farms wrote in its statement Sunday.

"Based on initial information provided by health officials, in an abundance of caution, we completed a voluntary recall of iceberg lettuce from central Mexico. Recalled product was limited to iceberg lettuce grown and processed in central Mexico," the Taylor Farms statement continued. "All other Taylor Farms products, including all Taylor Farms brand products available for purchase, are not involved in the recall."

Diamantas said Monday that the false positive did not change the FDA's findings that linked cases to shredded lettuce from Taylor Farms. 

"I also want to be clear that our weakened communications around a false positive test result do not change the basis for FDA's ongoing outbreak investigation or the epidemiological data supporting the current voluntary recall by Taylor Farms,” Diamantas said. 

The FDA similarly shared a post on X Monday afternoon, saying that the epidemiological data supporting the Taylor Farms lettuce recall was "overwhelming."

"FDA's traceback investigation and outbreak data continue to converge on shredded iceberg lettuce from Taylor Farms locations in central Mexico," the FDA's post read, in part. "FDA will continue to work with federal and state partners to investigate this multistate outbreak and ensure products implicated in this outbreak have been removed from the market." 

The CDC said it's also investigating other cyclosporiasis outbreaks and illnesses unrelated to the larger outbreak. At least 29 other states have confirmed domestically acquired cases since May 1, the CDC reports.

Additionally, 440 cases in 35 states have been confirmed among people who ate or drank food or water that made them sick while traveling outside the U.S. in the days before they came ill, according to the CDC.

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