The Food and Drug Administration on Sunday withdrew an earlier positive test that had tied Taylor Farms’ lettuce to the cyclospora outbreak, even as health officials said the supplier remains a focus of their investigation.
The false positive doesn’t change the scope of the current recall, but it could prevent it from expanding. The FDA had said Saturday that it had found a positive test linking cyclospora to Taylor Farms’ lettuce that wasn’t part of the current recall, suggesting it could widen.
By Sunday evening, the agency said it had re-examined the sample and determined the result was a false positive.
“Due to the complexity in detection of cyclospora, FDA laboratory experts re-reviewed the sample results and have concluded that the finding does not represent true amplification,” the FDA said.
Taylor Farms said the “FDA has not identified a single positive product test result for cyclospora.”
Health officials are using other methods to trace the outbreak back to Taylor Farms produce, given the short shelf-life of lettuce and the specific challenges of testing for cyclospora.
Cyclosporiasis, a gastrointestinal disease caused by ingesting a microscopic parasite, has infected people in the US across 34 states, according to the US Centers for Disease Control and Prevention. Michigan, the state with the highest case count, has recorded more than 5,000 illnesses as of July 17. State counts typically lag behind national numbers.
The disease causes severe diarrhea and nausea for a few days or up to a couple months. Symptoms appear up to two weeks after someone has ingested the parasite, making it challenging for federal health authorities to trace back to a source.
The CDC has said the three types of data it uses to investigate foodborne outbreaks are epidemiologic, traceback, and food and environmental testing.
Former deputy FDA Commissioner Stephen Ostroff said a lack of positive tests didn’t clear a supplier in an outbreak like cyclospora, especially when dealing with lettuce.
Unless the FDA can check samples from the specific lots of lettuce thought to have started the outbreak, “you can’t fall back on ‘you don’t have any positive samples, it’s not us’,” he said. With cyclospora in particular, “it’s notoriously difficult to find in the environment and notoriously difficult to find in food items,” he said.
Taylor Farms did not immediately respond to a request for comment on some health officials’ continued focus on its produce.
Earlier, Taylor Farms pulled products from a Mexico farm that were sent to 27 states. Over the weekend, Walmart Inc. said it has removed four bagged iceberg lettuce salad products manufactured by Taylor Farms.
Health officials have previously said a lettuce supplier to Taco Bell locations in the Midwest was the source of most infections.
“We moved immediately to adjust our supply chain and within 72 hours had voluntarily removed all affected Taylor Farms lettuce from every Taco Bell restaurant in the country,” Taco Bell, which is owned by Yum! Brands Inc., said.
This story was originally featured on Fortune.com