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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.