Introduction to foodborne parasites

A.A. Gajadhar    Canadian Food Inspection Agency, Saskatoon, SK, Canada

1.1 Introduction

A safe food supply is essential to the survival of humankind. As people have multiplied and technology has advanced, the food supply has evolved from a base of local consumer production to large-scale integrated production and distribution practices. Consumers, particularly those in urban regions, are largely unaware of the origin and processing of their food. Similarly, most producers are unaware of the variety of potential public health hazards that are transmissible by food. Both consumers and producers rely on government regulations and oversight for guidance and protection. Although information regarding foodborne hazards is widely available from many sources, especially regarding hazards such as bacteria and chemical residues, there is limited information on foodborne parasites. This is becoming an urgent matter as the food supply web is now global and expertise in parasitology in developed countries has waned. This complacency has been driven primarily by improved hygiene standards and the increasingly effective control of zoonotic parasites in food animals in those regions. This chapter introduces the diversity and unique characteristics of foodborne parasites, the food vehicles by which they are transmitted, and their significance to consumers. A ranking of these foodborne parasites by various socioeconomic factors and information relevant to their control are presented.

The world’s population currently exceeds 7 billion consumers living in regions where foodborne parasites are endemic or where there is risk of infection from imported food. Yet, relatively little is known about parasites, and their identity and characteristics are often confused with those of bacteria and viruses. Parasites are eukaryotic organisms that are classified within diverse taxa ranging from single-cell protozoa to complex hermaphroditic helminths. Foodborne parasites that are prevalent globally or regionally include the flagellated protozoon Giardia, the coccidian protozoa Cryptosporidium and Toxoplasma, the roundworms (nematodes) Trichinella and Anisakis, the tapeworms (cestodes) Taenia and Echinococcus, and the flukes (trematodes) Opisthorchis, Clonorchis, and Fasciola. The diversity of their structural and life-cycle characteristics that are employed in astounding strategies for survival, transmission, host adaptation, and replication makes parasites difficult to control but also fascinating to study.

1.2 Parasites transmitted by food

Human parasitic infections originate from many sources and are acquired by various modes of transmission. Oral transmission via food and water represents the most common route of infection. Because food is produced globally in a wide range of microenvironments, and because it originates from a variety of aquatic or terrestrial animals and plants, there is a diverse range of strategies for parasite survival, replication, and transmission. A list of various food products and examples of the parasites they transmit is provided in Table 1.1. Although many waterborne parasites such as Giardia and Entamoeba have simple, direct life cycles, foodborne parasites often have life cycles that involve multiple stages and various host species that exploit both the exogenous and endogenous environments and food habits of consumers. Generally, meatborne parasites replicate and attain infectivity in protective structures such as cysts within muscle, established during the animal’s development (Table 1.1). Examples include Toxoplasma gondii, Sarcocystis hominis, and Trichinella spiralis. Other parasites such as Giardia and Cryptosporidium can infect consumers by contaminating surfaces of ready-to-eat fruits and vegetables.