TRICHINOSIS
Trichinosis is caused by the roundworm Trichinella spiralis. When raw or incompletely cooked pork or a pork product containing living trichina larvae is eaten, the walls of the cysts are digested, liberating the microscopic larvae. Within the small intestine of the host, the freed larvae maturate during the next 2 days to adult male or female worms, and copulate. Beginning 5 days after ingestion of the infected meat and, in humans, continuing for as long as 12 weeks, the gravid adult female worms, which are partially embedded in the mucosal wall of the small bowel, give birth to larvae. The latter successively enter the lacteals of the intestinal villi, right ventricular cavity, pulmonary capillaries, and systemic circulation. By predilection the larvae penetrate skeletal muscle fibers, where they enlarge progressively, simultaneously causing degeneration of the fibers and provoking a local reaction characterized by edema, hyperemia, and granulomatous and eosinophilic inflammation. Approximately 30 days after invasion of the muscle fibers, the larvae, which now have assumed a characteristic coiled appearance, attain their maximal size and become encapsulated. In nature, the life cycle of the parasite is completed when a carnivorous mammal becomes infected with the parasite, as the result of ingesting the flesh of another animal containing living larvae (see Plate 6-146).
The incubation period usually ranges
between 7 and 10 days but may be as short as 1 day or as long as 28 days. As a
rule, the first symptoms are intestinal; the small bowel mucosa is irritated by
partial penetration of the larvae, then excysted from the digested infected
meat, and the adult worms develop. Symptoms include nausea, vomiting, abdominal
cramps, and diarrhea or constipation and usually last about 1 week. Symptoms of
the muscular phase generally begin about 1 week after infection. As a rule, the
first symptom noted by the patient is edema of the eyelids, followed by pain,
tenderness, and swelling of the face, then pain on movement of any voluntary
muscle. The severity of this phase depends on the number of second-generation
larvae invading the tissues of the host. In a severe or moderately severe
infection, this phase lasts from 4 to 8 weeks and
is accompanied by fever up to 104° F (40° C) and weakness. Incidental involvement
of other organs may cause bronchitis or bronchopneumonia, encephalitis or
meningismus, a cutaneous rash, the sensation of insects or worms crawling
beneath the skin, myocarditis, or arterial thrombosis. Mortality in patients
hospitalized because of trichinosis is approximately 5%. The stage of
convalescence usually is rapid. The most frequent serious complication of
trichinosis is granulomatous myocarditis, attended by congestive heart
failure or terminated by sudden unexpected death during the sixth to eighth
week of the infection. The heart is regularly involved, with young larvae
invading the myocardium, although they never encyst there. As a rule, ECG
changes appear during the second month of the infection. Occasionally, large
arteries of the brain or an extremity become thrombosed, apparently from
hypercoagulability of the blood. Recovery from myocarditis is usually complete.
Eosinophilia begins about 10 days
after trichina infection, reaches its peak during the third week, and usually disappears after 6 months. Although
moderately severe infections are often attended by eosinophilia of 35% or as
much as 70% or more, the degree of eosinophilia does not parallel the severity
of infection. An abrupt fall from a high level to 1% or zero is ominous. In the
diagnosis, a skin test is performed by intradermal injection of an antigen
prepared from an extract of powdered larvae. A positive immediate reaction
(evoked beginning 17 days after infection) consists of a soft wheal, 7 to 10 mm
in diameter, appearing within 15 to 20 minutes;
it is surrounded by an area of bright erythema and disappears within 1 hour.
Other diagnostic techniques include precipitin, flocculation, hemagglutination,
complement fixation, and fluorescent-antibody tests, most of which yield
positive reactions beginning 17 to 30 days after infection. Biopsy is
diagnostic if encysting larvae accompanied by a characteristic myositis are
found.
Management of the patient with
trichinosis includes bed rest, symptomatic treatment, and steroid therapy.
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