(NewsNation) — The U.S. outbreak of diarrhea-triggering cyclospora raises questions about how human fecal matter is intersecting with imported produce, a Johns Hopkins University healthcare researcher says.

"I've heard a lot of things about, 'Are people using human sewage as fertilizer?' I think just the conditions on the farms we need to monitor much, much more carefully," Dr. Neal Shah told "Katie Pavlich Tonight" on Friday.

Federal officials have said there is a link between iceberg lettuce from Mexico and the rash of stomach parasite cases across the U.S., but health experts, including Shah, caution there could be more than one food source that needs to be identified.

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Shah said Americans don't know enough about how some of their food is grown abroad. In some countries, he said, "humanure" is used as a fertilizer, especially with rising fertilizer prices.

In the U.S., treated sewage sludge known as "biosolids" can be used to "condition the soil or fertilize crops or vegetation grown in the soil," according to the U.S. Environmental Protection Agency.

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