Story by Marc Glass; photo by Bill Mindak Photography

Dr. Joan (Hager) Shields ’86, the resident parasitologist in the U.S. Food and Drug Administration’s Division of Virulence Assessment, succinctly describes her job as “looking at what it is in food-borne organisms that causes people to become ill and how to detect those pathogens in food.”



Shields, who earned a Ph.D. in environmental health science and policy at the University of California, Irvine, and majored in biology at UMF, pays particular attention to Cyclospora cayetanensis, an intestinal parasitic protozoan that causes diarrheal diseases in humans. The pathogen is sometimes conveyed by hard-to-scrub fruits and vegetables like raspberries and salad greens. How it progresses from a host and through the food chain is the subject of Shields’ scientific sleuthing. And, she says, the transmission mystery is compounded by the fact that Cyclospora cayetanensis is noninfectious when it first leaves its host.

One hypothesis is that the infectious form of the protozoan (activated post-host by “some unknown environmental pressure”) spreads via water used for irrigation. But, Shields explains, “raspberries grow on tall canes, not the ground, so it’s hard to believe contaminated water splashes up that far.”

The reality is the protozoan could spread at any point in food’s farm-to-table journey, including picking, processing, packing, shipping, and, ultimately, kitchen preparation. “The question is, where along that train are these pathogens entering?” says Shields, who focused her doctoral dissertation on Cyclospora cayetanensis. “We look at what’s happening pre-harvest and post-harvest. Poor hygiene at the consumer level can sometimes be the source.”

The search for an answer has Shields continually developing better, more cost-effective methods of detecting the organism by its DNA. Since only the species Cyclospora cayetanensis is harmful to humans, detection of the bug begins with genetic testing for the tell-tale presence of “the right set of sequences,” she says.

However, finding enough DNA material to test is a challenge. Shields, who was invited to write a chapter on the bug’s genome for a forthcoming American Society of Microbiologists book on food- and water-borne pathogens, says separating Cyclospora cayetanensis DNA material from the environmental organic matter to which it clings is difficult. And because very small quantities of the organism can sicken, valid genetic assessments must be highly sensitive.

“The goal is to make sure that the tests we’re recommending improve chances of finding the pathogen if it’s there,” says Shields. “And the methods have to be cost effective so regional labs and state health agencies will actually use them in the field.”

The stakes are high, far beyond protecting Americans and ensuring their confidence in the food supply.

“We have to be absolutely certain with test results,” she explains, “especially if we trace the source to another country.”

Prior to joining the FDA in January, Shields was a research scientist with the Centers for Disease Control and Prevention in Atlanta, where she studied Cryptosporidium. The hardy, gastrointestinal disorder-causing parasite sickened 1,600 Utah residents in 2007, causing state department of health epidemiologists to consider swimming pools and sprinkler attractions as likely disease vectors. Among other investigations at the CDC, Shields conducted experiments on disinfection rates for Cryptosporidium and the efficacy of various chemical and “engineering-based” strategies like using ultraviolet light to inactivate the organisms to safe levels in pool water.

For Shields, the transition to the FDA is part of an ongoing sojourn to keep science meaningful.

“I get the most satisfaction from applied science and knowing that we’re doing something to keep people safe,” she says.