Chapter 29
Foodborne Animal Parasites
The animal parasites that can be contracted by eating certain foods belong to three distinct groups:
protozoa, flatworms, and roundworms. Several of the more important members of each group of
concern in human foods are examined in this chapter along with their classification.
In contrast to foodborne bacteria, animal parasites do not proliferate in foods, and their presence
must be detected by direct means, as they cannot grow on culture media. Because all are larger in size
than bacteria, their presence can be detected rather easily by the use of appropriate concentration and
staining procedures. Because many are intracellular pathogens, resistance to these diseases is often by
cellular phenomena similar to that for listeriosis (see Chapter 25). Finally, another significant way in
which some animal parasites differ from bacteria is their requirement for more than one animal host
in which to carry out their life cycles. The definitive host is the animal in which the adult parasite
carries out its sexual cycle; the intermediate host is the animal where larval or juvenile forms develop.
In some instances, there is only one definitive host (e.g., cryptosporidiosis); in others, more than one
animal can serve as definitive host (e.g., diphyllobothriasis); and in still other cases, both larval and
adult stages reside in the same host (e.g., trichinosis).
PROTOZOA
The protozoa belong to the kingdom Protista (Protoctista), which also comprises the algae and
flagellate fungi. They are the smallest and most primitive of animal forms, and the five genera of
concern in foods are classified as follows:
Kingdom Protista
Phylum Sarcomastigophora
Phylum Sarcomastigophora
Class Zoomastigophorea
Order Diplomonadida
Family Hexamitidae
Genus Giardia
Subphylum Sarcodina
Superclass Rhizopoda
Class Lobosea
679
Foodborne Animal Parasites
The animal parasites that can be contracted by eating certain foods belong to three distinct groups:
protozoa, flatworms, and roundworms. Several of the more important members of each group of
concern in human foods are examined in this chapter along with their classification.
In contrast to foodborne bacteria, animal parasites do not proliferate in foods, and their presence
must be detected by direct means, as they cannot grow on culture media. Because all are larger in size
than bacteria, their presence can be detected rather easily by the use of appropriate concentration and
staining procedures. Because many are intracellular pathogens, resistance to these diseases is often by
cellular phenomena similar to that for listeriosis (see Chapter 25). Finally, another significant way in
which some animal parasites differ from bacteria is their requirement for more than one animal host
in which to carry out their life cycles. The definitive host is the animal in which the adult parasite
carries out its sexual cycle; the intermediate host is the animal where larval or juvenile forms develop.
In some instances, there is only one definitive host (e.g., cryptosporidiosis); in others, more than one
animal can serve as definitive host (e.g., diphyllobothriasis); and in still other cases, both larval and
adult stages reside in the same host (e.g., trichinosis).
PROTOZOA
The protozoa belong to the kingdom Protista (Protoctista), which also comprises the algae and
flagellate fungi. They are the smallest and most primitive of animal forms, and the five genera of
concern in foods are classified as follows:
Kingdom Protista
Phylum Sarcomastigophora
Phylum Sarcomastigophora
Class Zoomastigophorea
Order Diplomonadida
Family Hexamitidae
Genus Giardia
Subphylum Sarcodina
Superclass Rhizopoda
Class Lobosea
679