FILE: Rows of romaine lettuce growing on a Central Coast farm.

The source of the nation’s worst cyclosporiasis outbreak remains a mystery after the U.S. Food and Drug Administration announced it discovered a false positive over the weekend.

Initially, the FDA said a lettuce sample supplied by Taylor Farms de Mexico, which is headquartered in Salinas, tested positive for the parasite cyclospora on July 18. However, FDA laboratory scientists then reviewed the sample to confirm the finding. They “have concluded that the finding does not represent true amplification and should be considered a false positive,” the FDA said in an update on Sunday. Currently, there are “no confirmed positive sample results” among products tested for........

© SFGate