Parasites: Cryptosporidium, Giardia and

Cyclospora as foodborne pathogens

Dr Rosely Nichols and Professor Huw Smith, Scottish Parasite

Diagnostic Laboratory, UK

17.1 Introduction

Giardia, Cryptosporidium and Cyclospora are intestinal protozoan parasites that

parasitise both human and non-human hosts. Increasing evidence since 1970 has

implicated these organisms as significant contaminants of food. Their life cycles

consist of reproductive stages, which infect the intestine, and transmissive stages

(cysts of Giardia and oocysts of Cryptosporidium and Cyclospora [(oo)cysts])

which are excreted in the faeces of infected hosts. Of great importance is that

(oo)cysts are environmentally robust, being capable of prolonged survival in moist

dark environments. Whereas cysts of Giardia and oocysts of Cryptosporidium are

infectious to susceptible hosts immediately following excretion, oocysts of

Cyclospora are not infectious when excreted and require a period of maturation in

the environment before they become infective to other hosts. Of the various species

of Giardia, Cryptosporidium and Cyclospora, Giardia duodenalis, Cryptosporidium

parvum and Cyclospora cayetanensis are significant pathogens of humans.

17.2 Description of the organisms

17.2.1 Life cycles

Giardia

The genus Giardia consists of five species: G