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Trichinosis

(Trichiniasis)

by Richard D. Pearson, MD

Trichinosis is infection with Trichinella spiralis or related Trichinella species. Symptoms include initial GI irritation followed by periorbital edema, muscle pain, fever, and eosinophilia. Diagnosis is clinical and with serologic tests. Muscle biopsy may be diagnostic but is seldom necessary. Treatment is with mebendazole or albendazole and, if symptoms are severe, with prednisone.

Trichinosis occurs worldwide. In addition to the classic agent Trichinella spiralis, trichinosis can be caused by T. pseudospiralis, T. nativa, T. nelsoni, and T. britovi in different geographic locations.

Pathophysiology

The life cycle is maintained by animals that are fed (eg, pigs, horses) or eat (eg, bears, foxes, boars) other animals whose striated muscles contain encysted infective larvae (eg, rodents). Humans become infected by eating raw, undercooked, or underprocessed meat from infected animals, most commonly pigs, wild boar, or bear. Larvae excyst in the small bowel, penetrate the mucosa, and become adults in 6 to 8 days. Females are about 2.2 mm long, and males are about 1.2 mm long. Mature females release living larvae for 4 to 6 wk and then die or are expelled. Newborn larvae migrate through the bloodstream and body but ultimately survive only within striated skeletal muscle cells. Larvae fully encyst in 1 to 2 mo and remain viable for several years as intracellular parasites. Dead larvae eventually are resorbed or calcify. The cycle continues only if encysted larvae are ingested by another carnivore.

Symptoms and Signs

Many infections are asymptomatic or mild. During the 1st wk, nausea, abdominal cramps, and diarrhea may occur. One to 2 wk after infection, systemic symptoms and signs begin: facial or periorbital edema, myalgia, persistent fever, headache, and subconjunctival hemorrhages and petechiae. Eye pain and photophobia often precede myalgia.

Symptoms due to muscle invasion may mimic polymyositis. The muscles of respiration, speech, mastication, and swallowing may be painful. Severe dyspnea may occur in heavy infections.

Fever is generally remittent, rising to 39° C or higher, remaining elevated for several days, and then falling gradually. Eosinophilia usually begins when newborn larvae invade tissues, peaks 2 to 4 wk after infection, and gradually declines as the larvae encyst.

In heavy infections, the inflammation may cause complications: cardiac (myocarditis, heart failure, arrhythmia), neurologic (encephalitis, meningitis, visual or auditory disorders, seizures), or pulmonary (pneumonitis, pleurisy). Death may result from myocarditis or encephalitis.

Symptoms and signs gradually resolve, and most disappear by about the 3rd mo, when the larvae have become fully encysted in muscle cells and eliminated from other organs and tissues. Vague muscular pains and fatigue may persist for months. Recurrent infections with T. nativa in northern latitudes can cause chronic diarrhea.

Diagnosis

  • Enzyme immunoassay

  • Rarely muscle biopsy

No specific tests to diagnose the intestinal stage are available. After the 2nd wk of infection, a muscle biopsy may detect larvae and cysts but is seldom necessary. Diffuse inflammation in muscle tissue indicates recent infection.

A number of serologic tests have been used, but enzyme immunoassay (EIA) using T. spiralis excretory-secretory (ES) antigen seems to be the quickest way to detect the infection and is used in the US. Antibodies are often not detectable for the first 3 to 5 wk of infection, so tests should be repeated at weekly intervals if results are initially negative. Because antibodies may persist for years, serologic tests are of most value if they are initially negative and then positive. Serologic tests and muscle biopsy are complementary tests: Either one can be negative in a given patient with trichinosis. Skin testing with larval antigens is unreliable.

Muscle enzymes (creatine kinase and LDH) are elevated in 50% of patients and correlate with abnormal electromyograms.

Trichinosis must be differentiated from

  • Acute rheumatic fever, acute arthritis, angioedema, and myositis

  • Febrile illnesses such as TB, typhoid fever, sepsis, and undulant fever

  • Pneumonitis

  • Neurologic manifestations of meningitis, encephalitis, and poliomyelitis

  • Eosinophilia due to Hodgkin lymphoma, eosinophilic leukemia, polyarteritis nodosa, or disease caused by other migrating nematodes

Treatment

  • Symptomatic treatment

  • Mebendazole or albendazole to eliminate adult worms

Anthelmintics eliminate adult worms from the GI tract but probably have little effect on encysted larvae.

Albendazole 400 mg po bid for 8 to 14 days or mebendazole 200 to 400 mg po tid for 3 days, followed by 400 to 500 mg tid for 10 days can be used.

Analgesics (eg, NSAIDs, opioids) may help relieve muscle pains. For severe allergic manifestations or myocardial or CNS involvement, prednisone 20 to 60 mg po once/day is given for 3 or 4 days, then tapered over 10 to 14 days.

Prevention

Trichinosis is prevented by cooking meat thoroughly until brown (71° C [160° F] throughout). Larvae can usually be killed by freezing the meat at 17° C (−1° F) for 3 wk or 30° C (22° F) for 6 days, but T. nativa is relatively resistant. Smoking, microwave cooking, or salting meat does not reliably kill larvae.

Domestic swine should not be fed uncooked meat products.

Key Points

  • Humans become infected by eating raw, undercooked, or underprocessed meat from infected animals—most commonly pigs, wild boar, or bear.

  • Larvae excyst in the small bowel, penetrate the mucosa, and become adults that release living larvae; the larvae migrate through the bloodstream and encyst within striated skeletal muscle cells.

  • Symptoms begin with GI irritation followed by periorbital edema, muscle pain, fever, and eosinophilia.

  • Manifestations gradually resolve by about the 3rd mo, when the larvae have become fully encysted, although vague muscular pains and fatigue may persist for months.

  • Diagnose using enzyme immunoassay.

  • Treat symptoms (eg, with analgesics for pain and prednisone for allergic manifestations or CNS or myocardial involvement); antihelminthics kill adult worms but have little effect on encysted larvae.

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