Food contamination is invisible to the naked eye, which is why outbreaks often aren't discovered until people become sick across multiple states. During a recent briefing by the American Association for the Advancement of Science, food safety experts explained how public health investigators piece together the puzzle of foodborne illness outbreaks, using epidemiology, supply chain traceback, and laboratory science to identify contaminated products weeks after people have already consumed them.

Why Can't We Just Test Our Way to Food Safety?

One of the most common questions consumers ask is straightforward: if food safety programs exist, why does contaminated produce reach grocery stores? The answer reveals a fundamental limitation of food testing.

"We cannot visually see contamination by bacteria or by Cyclospora or norovirus," explained Kalmia Kniel, PhD, professor of microbial food safety at the University of Delaware.

Kalmia Kniel, PhD, Professor of Microbial Food Safety at the University of Delaware

Contamination often remains undetected until multiple illnesses appear and public health officials recognize an unusual pattern.

Environmental monitoring programs can occasionally identify contamination before illnesses occur, but testing alone cannot guarantee food safety.

"We can't test our way really to food safety. If we tested all the food, we'd have nothing to eat," Kniel stated.

Kalmia Kniel, PhD, Professor of Microbial Food Safety at the University of Delaware

Instead, prevention depends on strong worker hygiene programs, sanitation, environmental monitoring, pest management, and continual employee education throughout the food production process.

How Do Investigators Actually Trace a Foodborne Outbreak?

When illnesses are reported, epidemiologists begin what experts describe as the fundamental detective work of outbreak investigation. Craig Hedberg, PhD, professor at the University of Minnesota, explained that investigators start by talking to people who became sick, asking detailed questions about symptoms, illness onset, restaurants visited, grocery purchases, and foods consumed during the days or weeks before illness developed.

The challenge is determining which foods truly stand out from normal eating patterns.

"If you are interviewing 10 people and 7 of them say they ate lettuce, that might seem like a lot. But in fact, that's about the rate at which we generally consume lettuce," Hedberg explained.

Craig Hedberg, PhD, Professor at the University of Minnesota

Investigators therefore collect increasingly detailed information, including restaurant names, grocery stores, product varieties, and brands. They may also compare patients with healthy control groups to determine whether certain foods were consumed more frequently among those who became ill.

Once investigators identify likely food vehicles, FDA traceback specialists begin reconstructing the product's journey through the supply chain. However, modern food distribution rarely follows a single, uncomplicated path. Distributors frequently receive products from multiple processors, while prepared foods such as salads or salsas may contain several ingredients originating from different suppliers. Every additional supplier introduces more uncertainty, making traceback investigations both labor intensive and time consuming.

Why Is Cyclospora Especially Difficult to Investigate?

Cyclospora cayetanensis is a protozoan parasite, not a bacterium, which makes it fundamentally different from pathogens like Salmonella or E. coli.

"Cyclospora is really unique. It is a single-celled protozoan parasite. It's not a bacterium," Kniel said.

Kalmia Kniel, PhD, Professor of Microbial Food Safety at the University of Delaware

The parasite requires a human host to reproduce and is shed only by infected individuals through feces. Newly shed oocysts are not immediately infectious; instead, they require approximately 7 to 14 days in the environment before they mature and become capable of causing illness.

This delayed maturation distinguishes Cyclospora from many other foodborne pathogens and influences how contamination occurs. Fresh produce remains particularly vulnerable because it is frequently consumed raw, eliminating the cooking step that would normally destroy many pathogens. At the same time, modern agricultural distribution systems combine products from numerous farms before shipping them nationwide, making traceback investigations considerably more difficult.

Laboratory technology has revolutionized outbreak investigations for many bacterial pathogens. Whole genome sequencing now allows investigators to determine whether bacterial isolates from patients are genetically related, strengthening epidemiologic evidence. However, because Cyclospora cannot be cultured easily, laboratories have limited material available for genetic analysis. Few laboratories possess the specialized expertise required to perform advanced molecular characterization.

"We just don't have the ability to say that this group of 5 Cyclospora cases are all related," Hedberg explained.

Craig Hedberg, PhD, Professor at the University of Minnesota

Instead, investigators rely more heavily on illness timing, geography, and patient interviews than they would for bacterial outbreaks.

How to Strengthen Food Safety Investigations: Key Factors Experts Emphasize

• Workforce Investment: Outbreak investigations depend heavily on trained epidemiologists, laboratorians, and public health personnel. Recent reductions in federal and state public health funding threaten both laboratory capacity and outbreak response capabilities.
• Advanced Laboratory Techniques: Clinical laboratories increasingly use multiplex PCR panels that simultaneously detect more than 20 gastrointestinal pathogens, dramatically improving diagnosis and reporting. Many illnesses previously would have gone undiagnosed without these tools.
• Electronic Traceability Systems: Food companies maintain sophisticated electronic traceability systems documenting product movement throughout the supply chain, which helps investigators reconstruct supply chains more efficiently.
• Worker Training and Education: Prevention depends on strong worker hygiene programs, sanitation, environmental monitoring, pest management, and continual employee education. Understanding why these measures matter is as important as implementing them.

Both experts noted that foodborne disease surveillance has improved substantially during the past 2 decades. However, technology alone cannot replace experienced investigators.

"Our workforce is frequently a limiting factor," Hedberg warned, noting that recent reductions in federal and state public health funding threaten both laboratory capacity and outbreak response.

Craig Hedberg, PhD, Professor at the University of Minnesota

If public health agencies want to improve their capability of responding to outbreaks, significant investment in trained personnel will be essential.

The ongoing Cyclospora outbreak illustrates both the strengths and limitations of today's food safety system. While modern surveillance, laboratory techniques, and traceability systems have improved dramatically, the fundamental challenge remains unchanged: contamination is invisible, supply chains are complex, and preventing illness requires coordination among local and state health departments, the CDC, the FDA, clinical laboratories, growers, processors, and epidemiologists working together to identify and remove contaminated products from the food supply.