Trichinella spiralis
A. Classification
Phylum: Nematoda
Family: Trichinellidae
Genus/Species: Trichinella spiralis
Common Name: Trichina
There is now thought to be four "varieties", or subspecies, of the Trichinella spiralis species that exist
worldwide: Trichinella spiralis spiralis, Trichinella spiralis nelsoni, Trichinella spiralis nativa, and
Trichinella spiralis pseudospiralis.
B. Morphology
The adult Trichinella spiralis male is approximately 1.2 mm long by 60μm in diameter, while the
female is twice as long and 90μm in diameter. The females have with a vulva near the middle of the
esophagus and one uterus. The uterus is filled with developing eggs in the posterior section and
developing hatched juveniles in the anterior section.
Like all nematodes, Trichinella spiralis has a tough transparent cuticle and the longitudinal muscles of
the roundworm contract and produce a whipping motion.
The larvae are .8 mm to 1 mm long and are coiled in a lemon-shaped capsule when in the muscle tissue
of the host.
Figure 1. Male and female Trichinella spiralis.
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Figure 2. Coiled Trichinella spiralis larva freed from the cyst.
C. Lifecycle and Epidemiology
Trichinella spiralis has no stages outside of the definitive host which can be hogs, humans, bears, dogs,
cats, rats, and a variety of other mammals. The life cycle begins when a potential definitive host feeds
on another definitive host that contains the infective larvae. The mammal ingests the J1 larvae, which
are encysted in the muscles of the host. After exposure to gastric acid and pepsin, the larvae are
released from the cysts and enter the small intestine mucosa where they develop into adult worms.
During this phase, the adult worms mate and then the male worms die. The adult females penetrate the
mucosa of the intestine and produces larvae. The females will produce approximately 1,500 larvae
over the next four to sixteen weeks and then die. The larvae enter a muscle fiber and form a fibroblast
cyst. In time the cyst becomes surrounded by a calcarious layer.. The larvae are most frequently found
in very active muscles, for example the diaphragm, abdominal walls, tongue, biceps, eyes, larynx, and
deltoid muscle fibers. The larvae in the cyst develop into the infective first stage-larva in about 3
weeks. Thus, the definitive host serves as the intermediate host. If a human ingests raw or
undercooked pork, rat, bear, etc., the they can become infected with Trichinella spiralis. Within cysts,
larvae remain viable for many years; up to 25 years in man and 11 years in pigs.
Figure 3. Lifecycle of Trichinella spiralis.
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Figure 4. Internal portion of the lifecycle
D. Geographic Distribution
Trichinella spiralis is found all over the world. However, it is more prevalent in Europe, the United
States, and Asia. Trichinella spiralis spiralis is found in temperate regions. Trichinella spiralis
nelsoni is commonly found in the tropical regions. Trichinella spiralis nativa is found in the Artic,
while Trichinella spiralis pseudospiralis is found in New Zealand.
Figure 5. Geographic Distribution
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E. Pathology and Symptoms
Trichinosis, also called trichinellosis, is the infection caused by Trichinella spiralis. Symptoms can
range from none at all to very serious symptoms depending on the number of infectious worms present.
The first symptoms of trichinosis may occur 1-2 days after infection. It is often misdiagnosed because
of the vagueness of symptoms. These first symptoms include nausea, diarrhea, vomiting, fatigue, fever,
and abdominal discomfort. When the newborn larvae are released into tissues and start to migrate,
usually five to six weeks after infection, the symptoms may include pneumonia, pleurisy, encephalitis,
meningitis, nephritis, deafness, and peritonitis.
As the larvae start to penetrate muscle cells, muscular pain, breathing difficulties, swelling of masseter
muscles, a weak pulse, and a low blood pressure may occur. The average case of trichinosis is not
severe and produces no noticeable discomfort. Death may occur in severe cases.
Complications of trichinosis are secondary symptoms of the infection. Some of these complications
may include skeletal muscle cysts, heart complications, inflammation of the heart walls, lung
complications, pneumonitis, pleurisy, central nervous system complications, meningitis, encephalitis
(inflammation of the brain), vision disorders, and hearing disorders.
F. Diagnosis
Within the first couple of weeks of infection, Trichinella spiralis may be identified using fecal samples.
Trichinosis may be diagnosed using blood tests, serum eosinophils, muscle biopsy, or microscopic
examination of muscle tissue. If trichinosis is present the blood test will show an increase in the
number of eosinophils. Microscopic examination of muscle tissue will show larvae. Trichinosis may
be misdiagnosed as food poisoning or muscular rheumatism. Muscular rheumatism is characterized by
stiffness, pain, or swelling in the muscles.
Figure 6. Photomicrograph of Trichinella spiralis in a human muscle tissue.
G. Treatment
There is no treatment for the trichinosis infection, but treatment may be given to help relieve the
symptoms. Thiabendazole may be used only in the early stages of trichinosis, during the incubation
state. Corticosteroids may be used with thiabendazole if it is in the larval migration stage, but steroids
may prolong the intestinal phase of the infection. Pain medications, bed rest, fluids, and hospitalization
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are also recommended for treatment. Standard antiparasite drugs may be used to dislodge some of the
worms.
H. Public Health and Eradication
Legislation in the United States has required that pigs should be fed with meat which is thoroughly
cooked. To prevent infection it is recommended to heat pork to at least 170ºF and to cook wild game
meat thoroughly. Meat grinders should be cleaned thoroughly to prevent contamination. Curing,
drying, smoking, or microwaving does not effectively kill infective worms.
Slaughter testing is one preventative measure that may be used. Small pieces of pork are collected from
the diaphragm and are then examined microscopically for the presence of worms. Countries have
imposed trade restrictions that do not use this test.
Freezing the meat at –10ºF will kill the Trichinella spiralis instantaneously. Pork that is intended for
use in processed products is required by the U.S. Department of Agriculture to be frozen for at least a
half hour at –30ºF. Irradiation is also another preventative measure.
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Angela Chung
Drew Dickinson
Carrie Barker
March 18, 2005
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