The USDA advises us: “Make half your plate fruits and vegetables.” Similarly, Canada’s Food Guide suggests choosing more fruits and vegetables than any other food group. But at the same time, we’re told to always wash these healthy foods before eating them.
We’ve heard about outbreaks of E. coli 0157:H7 infections associated with salad greens, and Salmonella spp. contaminating various fruits; the CDC lists many such outbreaks, large and small. But we haven’t heard nearly as much about the possibility of fresh fruits and vegetables transmitting parasites – although it is a risk.
Parasitic Contamination of Salad Greens
Parasites of various kinds rely on being swallowed in order to infect a human host. Three protozoa in particular, however, are responsible for the majority of documented outbreaks of foodborne parasitic illness: Cryptosporidium spp., Giardia duodenalis (also known as Giardia lamblia) and Cyclospora cayetanensis. Such outbreaks are less common than those caused by bacteria, and it’s tempting to think that the presence of protozoa on our produce is a rare exception, but recent evidence suggests that it may be far more common than we think.
A 2009 -2010 study by Dixon et al at Health Canada sampled 544 packages of pre-cut ready-to-eat salad greens purchased from Ontario grocery stores and found protozoan parasites in 9%, nearly one in ten. Also surprising, all of the contaminated greens were grown in either the United States or Canada. Cryptosporidium was the most common contaminant, followed by Giardia and Cyclospora in roughly equal numbers. Two products contained both Cryptosporidium and Giardia.
These microscopic organisms can contaminate salad greens and other vegetables in a handful of ways. They might be present in the soil that crops are grown in, in animal manure used for fertilizer, or the water used to irrigate crops. They might be present on the hands of field workers or people working in processing plants, or in water used to pre-wash produce. They might be introduced in packaging or somewhere during transport, or even the spray used to keep produce fresh on grocery store shelves. The explanations for the protozoa detected in the Health Canada study are unknown, but may include any or all of the above.
Foodborne Parasitic Protozoa
Giardia duodenalis, Cryptosporidium spp., and Cyclospora cayetanensis are well-known for causing foodborne and waterborne illness, with Cryptodporidium perhaps the most infamous.
- Cryptodporidium spp: In 1993, this tiny coccidian the size of a yeast cell sickened an estimated 403,000 people in Milwaukee, after a water treatment plant malfunction. The organism has also contaminated apples, apple cider, coleslaw, and various vegetables, causing diarrheal illness.
- G. duodenalis: We know G. duodenalis better for attacking travelers, and hikers who drink from mountain streams, but it’s caused community water supply problems as well. Documented outbreaks have revealed the protozoan’s presence in oysters, chicken salad, vegetables and salad greens.
- Cyclospora cayetanensis: Cyclospora cayetanensis, the rarest of the three, at least in industrialized countries, tends to turn up in imported produce from the tropics. Past outbreaks have involved raspberries, snow peas, fresh herbs and salad greens.
Giardia duodenalis, Cryptosporidium spp., and Cyclospora strike with nausea, abdominal pain, and often severe diarrhea, a week to two weeks after we swallow them. The symptoms eventually resolve on their own in the majority of cases, but the wait may be long and uncomfortable.
Cyclospora cannot be passed directly from person to person, but the other two can, so infected individuals can pass their parasites along if they are not meticulous about personal hygiene, particularly hand washing.