Parasites and food contamination

A regular cause of traveller’s diarrhoea for those visiting developing countries, Cyclospora cayetanensis is increasingly causing disease in the US.

Currently more than 212 people in the US have been infected with Cyclospora parasites after eating pre-packaged Del Monte Fresh Produce vegetable trays containing broccoli, cauliflower, carrots and dill dip.

Incidences of cyclosporiasis have been increasing in the US in the last few years and now the CDC and the US FDA have issued a health alert to doctors to consider a diagnosis of cyclosporiasis in patients who experience prolonged or remitting-relapsing diarrhoea. Since 1990, at least 11 foodborne outbreaks of cyclosporiasis, affecting approximately 3600 persons, have been documented in the United States and Canada. The outbreaks have been linked to various types of imported fresh produce, including raspberries, basil, snow peas, mescalin lettuce and coriander; no commercially frozen produce has been implicated to date.

Infections tend to be more prevalent in late spring and summer — a time frame that correlates with increased import of fruits and vegetables into the US from its more southern neighbours.

Cyclospora cayetanensis, the cause of cyclosporiasis, is a one-celled, microscopic parasite. Currently little is known about this organism, although cases of cyclosporiasis are being reported across the globe with increasing frequency.

The only hosts for C. cayetanensis are humans. The protozoan lives out its life cycle intracellularly within the host’s epithelial cells and gastrointestinal tract.

Cyclospora infects the bowel and usually causes watery diarrhoea, with frequent, sometimes explosive, bowel movements. Other common symptoms include loss of appetite, weight loss, stomach cramps/pain, bloating, increased gas, nausea and fatigue. The time between becoming infected and becoming sick is usually about one week.

Most people who have healthy immune systems will recover without treatment. If not treated, the illness is usually self-limiting and resolves within 10–12 weeks. To date, the most effective drug for the treatment of the protozoan is a seven-day course of oral trimethoprim-sulfamethoxazole (TMP-SMX). No vaccine against this pathogen is available.

People get cyclosporiasis by consuming food or water contaminated with C. cayetanensis oocysts (the infective stage of the organism).

When freshly passed in stools, the oocyst is not infective (thus, direct faecal-oral transmission cannot occur; this differentiates Cyclospora from another important coccidian parasite, Cryptosporidium). In the environment, sporulation occurs after days or weeks at temperatures between 22 and 32°C, resulting in division of the sporont into two sporocysts, each containing two elongate sporozoites. Fresh produce and water can serve as vehicles for transmission and the sporulated oocysts are ingested (in contaminated food or water). The oocysts excyst in the gastrointestinal tract, freeing the sporozoites which invade the epithelial cells of the small intestine. Inside the cells they undergo asexual multiplication and sexual development to mature into oocysts, which will be shed in stools. The potential mechanisms of contamination of food and water are still under investigation.

Other foodborne parasites

The most common foodborne parasites are protozoa such as Cryptosporidium spp., Giardia intestinalis, Cyclospora cayetanensis, and Toxoplasma gondii; roundworms such as Trichinella spp. and Anisakis spp.; and tapeworms such as Diphyllobothrium spp. and Taenia spp.

Illness caused by foodborne parasites is much more common in developing countries, a wide variety of helminthic roundworms, tapeworms and flukes are transmitted in foods such as:

• Undercooked fish, crabs and molluscs.
• Undercooked meat; raw aquatic plants such as watercress.
• Raw vegetables that have been contaminated by human or animal faeces.

Some foods are contaminated by foodservice workers who practise poor hygiene or who work in unsanitary facilities. So wash your hands.