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Parasite mystery continues; Texas cases not linked to bagged salad

The mystery surrounding a nasty gut parasite that has sickened more than 600 people in the U.S. this summer continued this week as health officials said that cases in Texas aren’t linked to a salad mix from Mexico.

At least 283 people have been infected with cyclospora in the Lone Star State since July, but the Centers for Disease Control and Prevention says those cases don’t appear related to illnesses tied to imported bagged salad served at Olive Garden and Red Lobster restaurants in Iowa and Nebraska. That's according to a preliminary analysis of a cluster of cases where people ate at a Texas restaurant.

“The findings in Texas differ from those from earlier investigations in Iowa and Nebraska,” CDC officials wrote in an update

So far, at least 610 people in 22 states have been sickened by cyclospora, a parasite that typically spreads through contaminated produce or water, the CDC said. Forty-three people have been hospitalized. New cases have been winding down, though illnesses that began in mid-July might not be reported yet.

Taylor Farms of Mexico, the U.S.-based firm that distributed the implicated bagged lettuce, was given the all-clear Sunday by the Food and Drug Administration to start shipping leafy greens and other produce again. The company had halted imports from Mexico on Aug. 12 pending investigations, but chief executive Bruce Taylor told NBC News that the FDA found no evidence of contamination at the site or on five contract farms.

“The firm has committed to a comprehensive cyclospora sampling program for leafy green and other products from their farms and processing facility in Mexico,” FDA officials said in a statement.

But that means no one knows what’s behind the cases in Texas, which have left hundreds with lingering diarrhea and other flu-like symptoms. CDC officials said the quest is complicated because there’s no tool to identify the genetic fingerprint of the parasite, a key to outbreaks of more common bugs that cause food poisoning such as E. coli O157 and salmonella.

“It’s been hard to tell whether we’re watching one game unfold or two games,” Dr. Rob Tauxe, the CDC’s deputy director of the division of foodborne, waterborne and environmental diseases.

Critics in the food safety community have complained that the outbreak investigation has taken too long, and that state officials were slow to identify the parasite early. But Tauxe and other CDC officials said they reviewed investigations done in Iowa and Nebraska and that state officials have done “appropriate work.”

“This is not taking surprisingly long,” Tauxe told NBC News.

Not all cases of cyclospora, even in the same season, are necessarily caused by the same source, the CDC said.

In 1997, for instance, there were three separate and unrelated outbreaks of cyclosporasis caused by different fresh produce from several sources, the CDC noted.  

Health officials say they’ll continue to work with states to track down the source of the outbreak in Texas and in other states. 

JoNel Aleccia is a senior health reporter with NBC News. Reach her on Twitter at @JoNel_Aleccia or send her an email.