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FOODBORNE ANIMAL PARASITES - AN INTRODUCTION

Although it has been estimated that globally there are more than 100 known species of parasite that can be transmitted by food, the number that are of particular significance in the UK is much smaller. Although the contribution of protozoan parasites to overall foodborne illness appears to be small, it is highly likely that the numbers are underestimates.

Introduction
The risk of acquiring a foodborne parasite in the developed world has been increasing due to:

changes in patterns of food consumption towards eating fresh (i.e. raw and minimally cooked) foods such as fruit and vegetables, thereby avoiding the available food safety steps (e.g. cooking) required for the safe processing of other foods
the substantial increase in the globalisation of the food trade towards the end of the 20^th Century, with the consequence that foods can be rapidly transported between distant countries
the increase in tourism and travel

Consequently, since the 1970s, there has been increasing evidence that foodborne parasites such as Giardia, Cryptosporidium and Cyclospora can be significant contaminants of food.

Parasite Life Cycle
In contrast to foodborne bacteria, parasites do not live and grow in foods. They always need a living host, with some requiring more than one animal host in which to progress from an 'egg' (larval or juvenile form) to the mature adult parasite. Development usually takes place over a number of stages.

The definitive host is the animal in which the adult parasite carries out its sexual cycle.
The intermediate host is the animal where the larval or juvenile forms develop.

In some instances, there is only one definitive host (e.g. Cryptosporidium parvum; in others, more than one animal can serve as the definitive host (e.g. Diphyllobothrium latum); and in still further cases, both larval and adult forms can live in the same host (e.g. Trichinella).

Classification
The animal parasites that can be contracted by eating certain foods belong to three distinct groups:

Protozoa

Giardia lamblia
Cryptosporidium parvum
Cyclospora cayetanensis

Flatworms

Fasciola hepatica (liver fluke)
Clonorchis sinensis
Opisthorcis felineus
Taenia saginata and Taenia solium
Echinococcus granulosus
Diphyllobothrium latum

Roundworms

Anasakis simplex and Pseudoterranova decipiens
Trichinella spiralis

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