their environment. Cysts will only develop further if they are swallowed
by an appropriate definitive host, usually cattle or sheep in which
infection can cause serious economic loss, or more rarely in humans
after eating raw or undercooked watercress on which the cysts have
become attached.
In people the symptoms are fever, tiredness and loss of appetite with
pain and discomfort in the liver region of the abdomen. The disease is
known as fascioliasis and can be diagnosed by finding the eggs in the
faeces or body fluids such as biliary or duodenal fluids.
Of the Cestoda, the tapewormsTaenia soliumassociated with pork
andT. saginataassociated with beef, are best known. These long, ribbon-
like flatworms have humans as their definitive host but they differ in their
secondary host. The larval stages of the beef tapeworm have to develop
in cattle and finally infect humans through the consumption of under-
cooked beef. However, the larval stages ofTaenia soliumcan develop in
humans or pigs. These larval stages, known as cysticerci, develop in a
wide range of tissues including muscle tissue where they can cause a
spotted appearance. The mature tapeworm of these species can only
develop in the human intestine where it causes more severe symptoms in
the young and those weakened by other diseases, than in healthy adults.
The effects are fairly general and may include nausea, abdominal pain,
anaemia and a nervous disorder resembling epilepsy, as well as mechan-
ical irritation of the gut. If the latter is so severe as to cause a reversed
peristalsis, so that mature segments of the tapeworm (proglottids) enter
the stomach and release eggs (onchospheres), there may be an invasion of
the body tissues (cysticercosis). The resulting bladder worms, or cystic-
erci, can especially invade the central nervous system, a situation which is
often fatal. There are some species of tapeworm, such as Dip-
hyllobothrium latum, which have complex life cycles involving crustacea
and fish. Humans may become infected through the consumption of raw
or under cooked fish.
8.1.2 Roundworms
Perhaps the most notorious of the nematodes in the context of foodborne
illness, and the only one which will be dealt with here, isTrichinella
spiralis, the agent of trichinellosis which was first recognised as a cause of
illness in 1860.
This parasite has no free-living stage but is passed from host to host
which can include quite a wide range of mammals including humans and
pigs. Thus trichinellosis in the human population is usually acquired
from the consumption of infected raw or poorly cooked pork products.
Trichinellahas an intriguing life history for it is the active larval stages
which cause discomfort, fever and even death. Infection starts by the
272 Non-bacterial Agents of Foodborne Illness