CHAPTER 8
Non-bacterial Agents of Foodborne
Illness
We have seen that foods may act as a vehicle for viable bacteria such as
Salmonella, or for pre-formed bacterial toxins such as botulinum toxin
which can cause disease or illness once introduced into the body.
Foods may also act as vehicles for other disease causing agents such as
helminths, nematodes, protozoa and viruses as well as toxic metabolites
of fungi and algae. Each of these is a specialist area, and cannot be dealt
with in the same detail as bacteria, but a food microbiologist should be
aware of the occurrence and significance of these nonbacterial agents of
foodborne illness.
8.1 HELMINTHS AND NEMATODES
The flatworms and roundworms are not normally studied by microbiol-
ogists but amongst these groups there are a number of animal parasites
which can be transmitted to humansviafood and water. These complex
animals do not multiply in foods and they cannot be detected and
enumerated by cultural methods in the way that many bacteria can.
Their presence is normally detected by direct microscopic examination
often following some form of concentration and staining procedure.
8.1.1 Platyhelminths: Liver Flukes and Tapeworms
In the context of foodborne parasites the two most important classes of
the Platyhelminths (flatworms) are the Trematoda, which includes the
liver flukeFasciola hepatica, and the Cestoda which includes tapeworms
of the genusTaenia. These organisms have complex life cycles which may
include quite unrelated hosts at different stages. Thus the mature stage of
the liver fluke develops in humans, sheep or cattle which may be referred
to as the definitive host (Figure 8.1). It is a leaf-like animal, growing up
to two and a half centimetres in length by one centimetre in width, which