Introduction to Human Nutrition

338 Introduction to Human Nutrition

the herringworm. The other species involved in anisa-
kiasis in North America, Europe, and Japan is Pseudo-
terranova decipiens (the codworm or sealworm).
Nematodes are commonly present in fi sh caught in
the wild, most frequently in the liver and belly cavity,
but can also occur in the fl esh. Anisakiasis is an
uncommon disease because the parasite is killed by
heating (55°C for 1 min), and by freezing (−20°C for
24 h). There is a risk of illness from fi shery products
consumed raw, for example sushi, or after only mild
processing, such as salting at low concentrations or
smoking. Many countries now require that fi sh used
for these mildly processed products be frozen before
processing or before sale.
Trichinella spiralis is the cause of trichinosis in
humans. This most commonly results from the con-
sumption of contaminated raw or undercooked pork
or pork products. Since the mid-1980s outbreaks have
been associated with raw and undercooked horse-
meat. Isolated cases have been reported from the con-
sumption of bear meat and ground beef in the USA.
The incidence of trichinosis can be controlled by
avoiding feeding infected waste foods to pigs or by
fully cooking pig swill. Freezing pork products (−15°C
for 20 days) or thorough cooking (78°C at the thermal
center) before human consumption will destroy tri-
china larvae.

Cestodes

Cestodes are tapeworms and the species of major
concern associated with consumption of fi sh is the
fi sh tapeworm, Diphyllobothrium latum. Humans are
one of the defi nitive hosts, along with other fi sh-
eating mammals. Freshwater copepods and fi sh are
the intermediate hosts. The plerocercoid is present in
the fl esh of the fi sh and infects humans following the
consumption of raw or minimally processed fi sh. The
recorded epidemiology of D. latum shows it to be
prevalent in many countries worldwide. The inci-
dence is relatively high in Scandinavia and the Baltic
region of Europe. Diphyllobothriasis in humans can
be prevented by cooking or freezing fi sh before con-
sumption. Infections with tapeworms are also associ-
ated with eating undercooked or raw pork and beef.
Taenia saginata (the beef tapeworm) and Taenia
solium (the pork tapeworm) are unique among para-
sites in that they have no vascular, respiratory, or
digestive systems. Humans are their defi nitive hosts
and they rely solely on the human body for all of their

nourishment. Infections can be prevented by sanitary

disposal and treatment of human waste and by

thorough cooking and freezing of contaminated pork

and beef.

Protozoa

The protozoal human parasites are unicellular organ-

isms that colonize the intestinal epithelium and form

cysts. These are excreted and may survive for long

periods in the environment. There are fi ve genera of

concern in foods: Giardia, Entamoeba, Toxoplasma,

Cyclospora, and Cryptosporidium.

Table 14.5 summarizes the distribution, principal

reservoirs and route of transmission of these parasites

in the food chain.

14.6 Transmissible spongiform

encephalopathies and food

Transmissible spongiform encephalopathies (TSEs)

are fatal degenerative brain diseases which include

BSE in cattle; scrapie in sheep; kuru, Creutzfeldt–

Jakob disease (CJD), and new variant CJD (vCJD) in

humans. They are characterized by the appearance in

the brain of vacuoles – clear holes that give the brain

a sponge-like appearance – from which the condi-

tions derive their name.

Several theories have been proposed to explain the

nature of the agents that cause TSE. Prusiner was

awarded the Nobel Prize in 1997 for the prion theory,

which postulates that the agent is a proteinaceous

infectious particle (PrP) that is capable of replication

without the need of an agent-specifi c nucleic acid.

The disease-associated prion (PrPSc) has been shown

to have a different helical shape to normal cellular

prion protein (PrPC) found on neuronal cells and

some other cells, for example lymphoid cells. However,

while it is widely acknowledged that PrPSc is very

closely associated with the causative agent, there is a

reluctance by some to accept PrPSc as the sole agent

responsible for transmission. Another hypothesis

suggests the agent is an unconventional virus, while a

third suggests that it is a virion that has similar prop-

erties to a virus but uses host proteins to coat its

nucleic acid.

Bovine spongiform encephalopathy

BSE, sometimes referred to as “mad cow disease,” was

fi rst identifi ed in the UK in 1986. The disease is fatal