Multicellular Parasites

Figure 24.30 Cutaneous Larva Migrans, an Occupational Hazard of
Plumbers and Others Who Crawl Under Buildings Where Dog or Cat Feces
Have been Deposited Larval hookworms from the feces penetrate and wander
under the skin, causing serpentine tracts of inflammation before dying.
100 µm
Figure 24.31 Strongyloides stercoralis Larva in Feces The adult worm is
only about 2 mm long, and has a life cycle similar to hookworms. Unlike other
intestinal roundworms, however, S. stercoralis can multiply in the soil and in the body
of a single host.
period of treatment than for hookworm, and even so, relapses
may occur.
Trichinosis (Trichinellosis)
Trichinosis, also known as trichinellosis, differs from all the roundworm
diseases discussed previously in that it is contracted by eating
inadequately cooked meat. Since pork is a leading source of
infection, the incidence of the disease has fallen markedly in the
United States over the years due largely to federal regulations on
how pigs raised commercially are fed. Nevertheless, the potential
for large outbreaks of the disease still exists because the meat of a
number species of animals can be infectious.
Symptoms
Trichinosis is characterized by abdominal pain and diarrhea, followed
in about a week by fever, muscle pain, swelling around
the eyes, and sometimes a rash. Occasional cases are fatal
because of damage to the heart or brain. The disease occurs
worldwide except for Australia and some Pacific Islands.
Causative Agent
The cause of trichinosis is usually Trichinella spiralis, a 1 to 4 mm
long roundworm that lives in the small intestine of meat-eating
animals, especially rats, pigs, bears, dogs, and humans. Its life
cycle is shown in figure 24.32. The female worm discharges her
living young into the lymph and blood vessels of the host’s
intestine without an intervening egg stage, and those larvae are
carried to all parts of the body. Most of the larvae are killed by
body defenses, but some survive in the muscles of the host
where they become encased with scar tissue. The worms then
stay alive for months or years within the muscle. If the flesh of
an infested pig or other carnivorous animal is eaten by humans
or other animals, the digestive juices of the new host release the
larvae, permitting them to burrow into the new host’s intestinal
lining. The larvae then mature, and the females begin producing
larvae in the new host to complete the life cycle of the
worm. Each female adult Trichinella may live 4 months or more
and produce 1,500 young.
Pathogenesis
The penetration of larval worms into the host’s tissues is responsible
for the symptoms of trichinosis. Abdominal pain and diarrhea
begin within the first week after eating meat containing T.
spiralis, when the worms mature and begin discharging their
larvae. Fever, muscle pain, rash, and facial swelling result from
inflammatory and immune reactions to larvae lodged in various
tissues. Only larvae that penetrate skeletal muscle survive.
Epidemiology
In trichinosis, the same animal generally serves as both the
definitive and the intermediate host, human beings one hopes
being the exception. Almost all warm-blooded carnivores can
be hosts for T. spiralis. Many cases of the disease have been contracted
from the meat of wild animals, including bear, wild boar,
walrus, and cougar.
Prevention and Treatment
Prevention of trichinosis depends on thorough cooking of meat
so that all parts reach at least 170°F (77°C). Pigs raised commercially
must by law be fed only cooked garbage to ensure that
they do not receive meat scraps containing viable Trichinella larvae.
Government inspection of meat does not detect Trichinella
infestation. Larvae in pork are generally killed at 5°F (:15°C) or
lower for 3 weeks if the meat is less than 15 cm thick. Wild
game, however, often harbors Trichinella species that are not
killed by freezing. Medication is available that can probably prevent
the disease in persons who have eaten meat shown to contain
Trichinella, but there is no effective treatment.
MICROCHECK
Parasitic worms are divided into the roundworms
(nematodes) and the flatworms. Flatworms are further
divided into tapeworms (cestodes) and flukes