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Synonyms: trichinosis, trichinelliasis
Trichinellosis is caused by nematodes (roundworms) of the genus Trichinella. Trichinellosis is a parasitic disease caused by eating raw or undercooked pork and wild game products infected with the larvae of Trichinella spiralis.
Several other species of Trichinella are now recognised, including T. pseudospiralis (mammals and birds worldwide), T. nativa (Arctic bears), T. nelsoni (African predators and scavengers), and T. britovi (carnivores of Europe and western Asia).
- Trichinellosis is acquired by eating meat containing encysted larvae of Trichinella spp.
- The larvae are released from the cysts and invade the small bowel mucosa where they develop into adult worms (the female is just over 2 mm in length, and the male just over 1 mm in length).
- The lifespan in the small bowel is approximately 4 weeks.
- After 1 week, the females release larvae that migrate to skeletal muscle where they encyst. The encysted larvae may remain viable for several years. The encysted larvae are then ingested to complete the cycle.
- Rats and other rodents maintain the prevalence of infection. Animals, such as pigs or bears, feed on infected rodents or meat from other animals.
- Humans are infected when eating improperly processed or contaminated meat from infected animals.
- The most important source of human infection worldwide is the domestic pig, but meats of horses and wild boars have also played a significant role during outbreaks.
- Distribution is worldwide but trichinellosis is endemic in central and eastern Europe, the whole of the Americas, parts of Africa and Asia.
- Mild infections are usually asymptomatic.
- Heavy infection causes gastrointestinal symptoms (diarrhoea, abdominal pain, vomiting) within 1-2 days of infection.
- Larval migration into muscle tissues (one week after infection) can cause periorbital and facial oedema, conjunctivitis, headaches, fever, joint and muscle pains, petechiae and pruritus.
- Symptoms usually subside within a few months. Larval encystment in the muscles causes myalgia and weakness, followed by subsidence of symptoms.
- Occasionally, infection may be life-threatening with myocarditis, central nervous system involvement (eg ataxia or respiratory paralysis), and pneumonitis.
- FBC shows eosinophilia in virtually all patients.
- Creatine kinase (CK) is elevated in most patients.
- Parasite-specific indirect immunoglobulin G (IgG) enzyme-linked immunosorbent assay (ELISA) titres and anti-newborn larvae antibodies:
- These are recommended but may not be positive initially and there is some cross-reactivity with other parasitic disorders and so specificity is less when results are weakly positive.
- Antibody levels peak in the second or third month post-infection and then decline slowly for several years.
- Stool studies can identify adult worms, with females being about 2 mm long and males about half that size.
- A muscle biopsy is the definitive diagnostic test.
- Other tests may be indicated depending on the presentation and possible differential diagnoses, eg CT/MRI scan for suspected central nervous system involvement, ECG, lumbar puncture, electromyography.
- Symptoms can be treated with aspirin and corticosteroids (steroids are used for infections with severe symptoms).
- Mebendazole, albendazole or tiabendazole can kill adult worms in the intestine and are therefore effective if used within 1 week of eating contaminated meat to kill worms in the intestine and prevent systemic spread of larvae.
- There is no antihelminthic treatment that kills the larvae but albendazole may be be marginally effective. The mainstay of treatment once larval migration occurs is therefore bedrest, analgesics and antipyretics. Corticosteroids are used for severe infections.
- Long-term sequelae of the central nervous system, eg decreased mental power, numbness of hands and feet, decreased stress tolerance, loss of initiative, and depression.
- Prolonged weakness and myalgias may occur.
- Adrenal insufficiency.
- Obstruction of blood vessels.
- Patients with mild infection are usually asymptomatic. Those with mild symptoms improve in 2-3 weeks. Symptoms associated with heavy infections may persist for 2-3 months.
- Severe disease develops in only 5%-20% of patients during epidemics.
- Usually, full recovery occurs after cardiac or pulmonary involvement. Mortality is uncommon but may occur as a result of exhaustion, pneumonia, pulmonary embolism, encephalitis, or cardiac failure and/or arrhythmia. Death from trichinellosis usually occurs in 4-8 weeks but may occur as early as in 2-3 weeks.
- Cooking meat products until the juices run clear or to an internal temperature of 62°C.
- Freezing pork kills larval worms. Freezing wild game meats, even for long periods of time, may not effectively kill all worms.
- Cooking wild game meat thoroughly.
- Cooking all meat fed to pigs or other wild animals.
- Cleaning meat grinders thoroughly when preparing ground meats.
Further reading and references
Parasites A-Z; Centers for Disease Control and Prevention
Murray C; Trichinosis, eMedicine, Jan 2010
Gottstein B, Pozio E, Nockler K; Epidemiology, diagnosis, treatment, and control of trichinellosis. Clin Microbiol Rev. 2009 Jan22(1):127-45, Table of Contents.