Introduction to Human Nutrition

Food Safety 335

number of water-borne outbreaks but there has been
no association with food. One food-borne outbreak
of parvovirus linked to consumption of cockles has
been reported.

14.5 Food-borne parasites

Food-borne parasitic diseases are a major public
health problem affecting millions of people, predomi-
nantly in nonindustrialized countries. The incidence
of parasitic disease associated with the consumption
of foods of animal origin has declined in industrial-
ized countries in recent years, where improvements in
animal husbandry and meat inspection have led to
considerable safety and quality gains. The situation in
nonindustrialized countries is very different, in that
these diseases are associated with poor standards of
sanitation and hygiene, low educational standards,
and extreme poverty.
Parasites are organisms that live off other living
organisms, known as hosts. They may be transmitted
from animals to humans, from humans to humans,
or from humans to animals. Food-borne parasitic
disease occurs when the infective stages of parasites
are eaten in raw or partially cooked protein foods, or
in raw vegetables and fruits that are inadequately
washed before consumption. These organisms then
live and reproduce within the tissues and organs of
infected human and animal hosts, and are often
excreted in feces. The parasites involved in food-borne
disease usually have complex life cycles involving one
or two intermediate hosts (Figure 14.1). The food-
borne parasites known to cause disease in humans are
broadly classifi ed as helminths (multicellular worms)
and protozoa (single-celled microscopic organisms).
These include the major helminthic groups of trema-
todes, nematodes, and cestodes, and some of the
emerging protozoan pathogens, such as cryptospo-
ridia and cyclospora. The illnesses they can cause
range from mild discomfort to debilitating illness and
possibly death.
These infections occur endemically in some 20
countries, where it is estimated that over 40 million
people worldwide, mainly in eastern and southern
Asia, are affected. Of major concern are the fi sh-borne
trematode infections. The trematode species con-
cerned all have similar life cycles involving two inter-
mediate hosts. The defi nitive host is man and other
mammals. Food-borne infection takes place through

the consumption of raw, undercooked, or otherwise

underprocessed freshwater fi sh or crustaceans con-

taining the infective stages (metacercariae) of these

parasites. Table 14.4 summarizes the distribution,

the principal reservoirs, and freshwater fi sh or crus-

taceans involved in the transmission of these parasites

in the food chain. The most important parasites with

respect to the numbers of people affected are species

of the genera Clonorchis, Opisthorchis, and Paragoni-

mus. The diseases caused by food-borne trematodes

include cholangiocarcinoma, gallstones, severe liver

disease, and gastrointestinal problems.

Nematodes

The food-borne roundworms of primary importance

in humans belong to the phylum Nematoda and are

known as nematodes. Undercooked or raw fi shery

products and pork meat are the usual foods

involved.

Where fi shery products are the food vector, the

defi nitive hosts of roundworms causing disease in

humans are piscivorous marine mammals such as

seals. Marine invertebrates and fi sh are the two inter-

mediate hosts and humans are infected when they

consume raw or minimally processed products. Fish

are the secondary hosts and are infected when they

consume the invertebrate primary host or fi sh that are

already infected. There are many species of nematodes

and a very large number of species of fi sh, worldwide,

that are known to act as intermediate hosts. The most

common species of nematode causing disease in

humans is Anisakis simplex, sometimes referred to as

Figure 14.1 Life cycle of Diphyllobothrium latum.