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Invasive Tissue Parasites 201

parasitology concerned with the transmission of parasites These roundworms or nematodes have a stage of
between different animal species and the importance of development called the embryonic stage, which occurs in
predator–prey relationships in such transmissions. A num- many species of parasites and other biological organisms.
ber of good accounts by many investigators from more than In trichinae, however, this stage occurs within the uterus
a century ago relating to the history of trichinosis exist, as of the female. The offspring are then released in the larva’s
trichinosis is one of the oldest and most documented para- second stage of life into the host’s intestinal lining. Up to
sitic infections found in humans. 1500 larvae may be produced from each female worm and
The condition where the muscle tissues are infested these travel through the circulatory system to the heart,
by the larval form of T. spiralis is not identifiable in gen- and from there to striated skeletal muscle. Those larvae
eral by examination of stool specimens, as is the case for that reach striated muscle will grow to a length of about
a number of other species of parasites. The most specific 1 mm before coiling themselves into a cyst (encysting)
identification for the infection is examination of biopsies for protection. Encysted worms may live up to 10 years
of the muscles in which the larvae are encysted. It appears in this stage. Humans are considered a dead-end host, as
that the only way the organism is transmitted to humans few animals have the opportunity to feed on humans.
is through ingestion of raw or undercooked meat, primar- In order to reproduce, larvae are released from the
ily pork, but several other types of animal meat are also be encapsulated cysts of the muscle tissue that was eaten
capable of causing the contraction of the disease. and mature to the adult stage in the intestines. Females
then produce larvae that are able to penetrate the intesti-
Symptoms nal mucosa and then enter into the blood circulatory sys-
tem. These organisms have an affinity for striated skeletal
The first stage of the infection is the intestinal phase, muscle and form multiple cysts in the fibers of skeletal
where the ingested larvae invade the intestinal mucosal muscle. They grow and mature, reaching adulthood in
tissues. The first symptoms develop within a day and approximately a month. As the encysted larvae grow they
the dose of worms ingested relates directly to the sever- coil into the cavity in the muscle and may remain alive for
ity of these initial symptoms. The victim may experience a number of years before the cyst calcifies and the larvae
symptoms similar to influenza or a similar viral illness. within die (Figure 9-1). The encysted larvae may cause a
Some mistakenly believe they are suffering from acute great deal of pain but during the intestinal phase of devel-
food poisoning as nausea and vomiting, diarrhea, general opment few if any symptoms are experienced, except for
malaise, acute edema of upper eyelids, abdominal cramps vague abdominal discomfort and perhaps slight diarrhea.
and pain, and fever soon emerge as major complaints. After producing larvae, the adult dies, so reinfection is
More serious symptoms may ensue for heavy in- required for a continuous cycle of reproduction.
fections but often occur after the initial crisis when only
vague muscular pains arise that may persist for weeks. Disease Transmission
The prognosis for the patient is positive and the severity
and length of illness depends on the number of worms The contracting of trichinosis requires ingesting raw
ingested. There may also be a generalized appearance of or uncooked meat containing Trichinella larvae, lead-
poor health during the initial phase of the illness. ing to human infection. The most common type of meat
containing the Trichinella parasites that infect humans
Life Cycle is pork and, in the past, feeding scraps of meat and
other foods to pigs provided a ready source of the larvae
The development of the T. spiralis organism appears to that infected the animals. The larvae are released from the
be quite simple but the life cycle of T. spiralis includes muscles of the food source where they go on to mature and
several different stages. Once larvae are ingested through reproduce in order to encyst in the host’s muscle tissues.
the eating of infected meat, the adult organism lives in
the intestinal lining of such meat-eating animals. Follow- Laboratory Diagnosis
ing the mating of a pair of T. spiralis organisms, the male
worm dies while the female proceeds to produce the Trichinella organisms are rarely found in a patient’s stool,
offspring. blood, or cerebrospinal fluids. Clinical chemistry tests