Medical term:

trichinelliasis

Trichinosis

Definition

Trichinosis is a disease caused by a roundworm (nematode) called Trichinella spiralis. An individual worm of this species is called a trichina, from the Greek word meaning "hairlike." Trichinae can be readily avoided by proper handling and cooking of certain meats, particularly pork products.

Description

The life cycle of T. spiralis includes several different stages. The adult trichina lives in the intestinal lining of such meat-eating animals as swine, bears, walrus, and rodents. After mating, the male worm dies while the female goes on to produce the offspring.

Roundworms have a stage of development called the embryonic stage, which in many species occurs after birth. In trichinae, however, this embryonic stage occurs within the uterus of the female, so the offspring that are ultimately discharged into the host's intestinal lining are in the larval second stage of life. These larvae—about 1500 from each female worm—travel through the circulatory system to the heart, then through the blood vessels leading to striated muscle (the muscle of the skeletal system and the heart). Most larvae that cannot find suitable locations in striated muscle will die.

Those larvae that reach striated muscle will grow to a length of about one millimeter, coil themselves, and enclose themselves within a protective wall called a cyst. This process is referred to as encysting. The worms in the cysts can live for up to ten years in this form.

A pig that has been infected with T. spiralis, then, has thousands of cysts lying dormant within its muscles—the very muscles that humans look forward to consuming in the form of pork chops, ham, barbecued ribs, etc. When humans sit down to a delicious meal of undercooked, trichina-infected pig dinner, they are ingesting T. spiralis cysts. The cyst walls are broken down by the usual process of food digestion in the stomach, allowing the larvae to escape into the new host's intestines. There the larvae mature to become adult worms, capable of producing a new crop of larvae. When these new larvae hatch, they begin their migration throughout the human host's bloodstream to his or her muscles, where they live for a short while before encysting.

Causes and symptoms

Human hosts who eat meat infested with trichinae may experience symptoms in varying degrees. If the meat ingested has only a few cysts, then the human host's load of parasites (worm burden) is said to be relatively small, and symptoms will be moderate. In fact, many trichinosis infections are subclinical, which means that the symptoms are so mild that the infection remains undiagnosed. In a host with a greater worm burden, the initial symptoms will be caused by the presence of the adult worms in the intestine. These symptoms usually include fever, diarrhea, abdominal pain, and perhaps vomiting. The symptoms begin about one to two days after eating the contaminated meat, and may last for a week or so.

When the larvae begin their migration through the blood vessels, the host will begin to experience symptoms that affect the whole body (systemic symptoms), such as fever; swelling of the face and the area around the eyes; rash; bleeding into the nail beds, retina, and whites of the eyes; and cough. In very severe cases of trichinosis, inflammation of the heart muscle (myocarditis), lungs (pneumonitis), or brain (encephalitis) may occur. These symptoms can lead to the few deaths caused by trichinosis.

The larvae begin to burrow into the host's muscles and form cysts within two to three weeks of the initial infection. This encysting produces signs of muscle inflammation (myositis) including swelling of the affected muscle groups, pain, and weakness. The most frequently affected muscles are the muscles outside the eye (extraocular muscles) that control eye movements; the muscles of the jaw, neck, and upper arm (biceps muscle); the muscles of the lower back (lumbar region); and the diaphragm, which is the muscle that separates the abdominal and chest cavities and aids in breathing.

The symptoms of trichinosis are at their most severe at about three weeks after infection, and decrease very slowly in their severity. Recovery is extremely gradual, and symptoms may last for as long as three months. Fatigue and muscle pain (myalgia) may take several more months to subside.

Diagnosis

An initial diagnosis of trichinosis relies heavily on the presence of its classic symptoms—swelling around the eyes, muscle inflammation, fever, and high levels of a certain type of white blood cell (eosinophils)—coupled with the patient's history. If the patient reports having eaten undercooked meat from an animal known to be a potential carrier of trichinosis, the doctor may order a muscle biopsy to confirm the diagnosis. By the third or fourth week of infection, muscle biopsies usually indicate the presence of larvae. Stool tests rarely reveal adult worms, although larvae can sometimes be found in blood or duodenal washings after the second week of infection. The blood test that is the most specific for trichinosis is the bentonite flocculation (BF) test.

T. spiralis can infect a number of different animal species used for food. The most common food culprit in the United States has been pork sausage, while outbreaks in Europe have caused by wild boar and horse meat. Outbreaks of trichinosis in Asia and Africa have been traced to dog meat, and outbreaks in Northern Canada have resulted from consumption of walrus and bear meat.

Treatment

Supportive care

Treatment of trichinosis is primarily aimed at decreasing the severity of the symptoms. Symptomatic relief includes bed rest and medications to relieve fever and muscle pain. The medications most commonly given are aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs). Steroids such as prednisone (Deltasone, Meticorten) are reserved for the most severe cases of muscle inflammation, or for complicated cases that include myocarditis.

Anthelminthic medications

In addition to medications for pain relief, trichinosis can be treated with drugs that are called anti-worm medications or anthelminthics. Two related anti-worm medications, mebendazole (Vermox) and thiabendazole (Mintezol), have been reported to be effective against intestinal larvae, but not against larvae encysted in the muscles. In particular, thiabendazole has worked best when given to patients who knew within 24 hours that they had eaten infested meat. Thiabendazole has, however, anti-inflammatory properties that can relieve some of the pain during the muscle stage of trichinosis.

Prognosis

The prognosis for recovery from trichinosis is generally good. Most people with the disease are unaware that they have even been infected. It is estimated that between 150,000 and 300,00 people in the United States become infected yearly, so that at any given time, 1.5 million people have T. spiralis infections. Most of these people have such light cases that trichinosis is never identified. Worm burden is measured in larvae per gram of muscle tissue; people with 10 or fewer larvae per gram of muscle tissue usually have no significant symptoms. When the number climbs to 100 larvae per gram of muscle tissue, the symptoms become noticeable. People with over 1000 larvae per gram of muscle tissue are usually extremely ill, and often die. The mortality rate of trichinosis is about 1%.

Prevention

Prevention of trichinosis is relatively simple. Swine should be fed only grain or cooked garbage because uncooked garbage may contain contaminated pork scraps. Meat from animals prone to trichinosis infection should be cooked or smoked thoroughly until it is no longer pink. Freezing meat at an adequately low temperature (5°F/-15°C for three weeks) can kill most encysted larvae, except for species which infect such arctic mammals as walrus or bear.

Resources

Organizations

Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311. http://www.cdc.gov.

Key terms

Anthelminthic — A type of medication that is given to destroy or eliminate parasitic worms.

Cyst — In the life cycle of the round worm, a protective, walled-off capsule in which the larvae lie dormant.

Embryonic — In the life cycle of the round worm, a very early life stage occurring within the uterus of the female round worm.

Host — The animal within which a parasite lives, and from which the parasite receives its nutrition.

Inflammation — A reaction within the body to an invader (virus, bacteria, fungus, worm, etc.) or to tissue injury. The classic signs of inflammation include redness, heat, pain, and loss of function.

Larva — In the life cycle of the round worm, the second stage of life, sometimes considered the "adolescent" stage.

Nematode — A type of roundworm with a long, unsegmented body, usually parasitic on animals or plants.

Striated muscle — Also known as striped muscle; it includes muscles of the skeletal system and of the heart.

Trichina — An individual example of Trichinella spiralis.

trichinosis

[trik″ĭ-no´sis]

infection with the parasitic roundworm Trichinella spiralis. The worm larvae live embedded in tiny capsulelike cysts of muscle tissue of infected pork. When the meat is properly cooked, the larvae are killed by the high temperature; however, if the pork is undercooked, they survive. Then when the meat is eaten digestive fluids dissolve the cyst capsules and free the larvae in the intestines, where they grow to maturity.

Trichinosis is found in most parts of the world with the exception of Australia and the Pacific islands. It is relatively common in Europe and the United States. In recent years there has been a decline in incidence owing mainly to enforcement of laws requiring the cooking of garbage fed to hogs. Other factors influencing the decline include storage of meat at low temperatures and education of the general public in the proper cooking of pork.

Symptoms and Development. Trichinosis develops in stages that correspond to the worms' development in the intestine and, later, the movement of their larvae into other parts of the body. Symptoms and the severity vary according to the stage of the disease, the tissues invaded, and the total number of invading parasites.

During the initial stage, when larvae are developing into mature worms in the intestinal tract, there may be diarrhea, nausea, abdominal pain, and fever. Symptoms that make identification more certain appear generally after an incubation period of 1 to 2 weeks. By this time, the mature worms have deposited their young in the intestinal wall and these new larvae are beginning to move through the body, causing a variety of reactions in one or several organs and areas. Edema may develop in the eyelids. This may be followed by hemorrhage of the retina, pain in the eyes, and extreme sensitivity to light. As the larvae invade other parts of the body, there may be muscle soreness and pain, fever, thirst, chills, profuse sweating, and edema in the infected areas.

For most people there is more discomfort than danger in this most serious phase of trichinosis, when the larvae are active in various parts of the body. Often the infection is mild enough for patients to be treated at home. The occasional fatal cases (fewer than 5 per cent of known cases) usually involve additional infections or disorders in organs weakened by the parasites.

In the usual course of the disease, there is a gradual decrease in symptoms as the larvae in muscle tissue become encysted and dormant and those in other types of tissue are destroyed. After about 3 months, most symptoms disappear, although vague muscular pain and fatigue may still continue for several months. Trichinosis almost never leaves any permanent disability.

Treatment and Prevention. With its varying symptoms, trichinosis is sometimes difficult to diagnose, but a skin test has been developed that makes identification of the disease certain in most cases. Chest x-rays and microscopic examination of muscle tissue also can be useful in diagnosis. There is no specific treatment for trichinosis; recovery is a matter of rest and time, allowing the body's natural defenses to overcome the parasites. Medications, including thiabendazol, cortisone, and ACTH, may be prescribed to relieve muscular pain and other symptoms.

The only certain safeguard against trichinosis is the thorough cooking of all meat products to ensure destruction of any encysted Trichinella larvae. Pork should be cooked at a temperature of 150°C (350°F), with a roasting time of 35 minutes per pound, until the meat is gray in color; if it is pink it is underdone. Cooking in this manner ensures that all parts of the roast will be heated to well above 68°C (140°F), the thermal death point of trichinae.

trich·i·no·sis

(trik'i-nō'sis),

The disease resulting from ingestion of raw or inadequately cooked pork (or bear or walrus meat) that contains encysted larvae of the nematode parasite Trichinella spiralis. The initial symptoms of human disease are abdominal pain, cramping, and diarrhea, associated with the development of the parasites in the small intestine. After the resultant larval parasites migrate and invade muscular tissue, a second set of symptoms is manifest, including facial and periorbital edema, myalgia, fever, pruritus, urticaria, conjunctivitis, and signs of myocarditis.

Synonym(s): trichinelliasis, trichinellosis, trichiniasis

[Trichinella (trichina) + G. -osis, condition]

trichinosis

(trĭk′ə-nō′sĭs)

A disease caused by eating raw or undercooked meat, usually pork, that contains trichinae, which develop as adults in the intestines and as larvae in the muscles, causing intestinal disorders, fever, nausea, muscular pain, and edema of the face. Also called trichinellosis.

trichinosis

Parasitology Infection by Trichinella spiralis due to ingestion of inadequately cooked pork, polar bear, wild carnivores Transmission Ingestion of inadequately cooked pork with encysted larvae in the muscle; once the cyst-laden meat passes into the lower small intestine, the larvae decloak and develop into viviparous adults which temporarily pitch tent in the ileum and reproduce; the resulting larvae penetrate the capillaries of the small intestinal mucosa, disseminate, eventually encyst in the muscle of the new host Clinical The 2 'waves' of Sx correspond to the stages of the life cycle of T spiralis in a host–the 1^st wave occurs in the small intestine when larvae are liberated from cysts, evoking colicky pain and diarrhea; the 2^nd occurs when the larvae penetrate capillaries and encyst, causing hypersensitivity reactions–facial and periorbital edema, eosinophilia, itching, urticaria, low-grade fever, inflammation–eg, myocarditis, myositis, muscle pain; early Sx of infestation are usually GI–cramping, diarrhea, larvae migrate to muscle, causing inflammation and myalgia, especially on movement; muscles in constant motion–eg, of respiration–diaphragm and intercostal muscles are painful; during migration of larvae through tissue, infected Pts often develops swelling in the face and around the eyes; larvae can invade myocardium, causing arrhythmias, myocardial damage Management Thiabendazole, Mebendazole. See Trichinella spiralis.

trich·i·no·sis

(trik'i-nō'sis)

The disease resulting from ingestion of raw or inadequately cooked pork or other meat that contains encysted larvae of the nematode parasite Trichinella spiralis. The initial symptoms are abdominal pain, cramping, and diarrhea, associated with the development of the parasites in the small intestine. After the larval parasites invade muscular tissue, a second set of symptoms is manifest, including facial and periorbital edema, myalgia, fever, pruritus, urticaria, conjunctivitis, and signs of myocarditis.

[Trichinella (trichina) + G. -osis, condition]

trichinosis

Infection with the roundworm Trichinella spiralis usually from eating undercooked pork containing the tiny cysts of the dormant forms of the worm. Digestion releases the larvae which attach to the intestinal lining, reproduce, and spread throughout the body settling in the muscles to form cysts. During this migration there may be fever, swelling of the face, cough, and pain, stiffness and tenderness in the muscles. Large infections may be fatal.

trich·i·no·sis

(trik-i-nō'sis)

The disease resulting from ingestion of raw or inadequately cooked pork (or bear or walrus meat) that contains encysted larvae of the nematode parasite Trichinella spiralis.

[Trichinella (trichina) + G. -osis, condition]

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