Trichinella/Trichinellosis

Trichinellosis is an illness caused by infection with Trichinella. The most common causative parasite is Trichinella spiralis, which is usually found in pigs and transmitted to humans through the ingestion of undercooked meat. Once ingested, the parasite grows and matures within the intestinal walls. The adult forms mate, and the larvae produced spread through the bloodstream, reaching striated muscles. Symptoms occur during larval migration. Patients may have GI symptoms within a few weeks after consumption of the infected meat, and systemic symptoms such as fever, chills, myalgia, and periorbital edema may follow. Diagnosis can be made by serologic examination and confirmed by the presence of cysts or larvae in a muscle biopsy. Mild infections are self-limited, but systemic disease is managed with antiparasitic medications and corticosteroids. Infection can be prevented by proper meat handling and cooking techniques.

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General Characteristics

Basic features of Trichinella

  • Intestinal and tissue nematode (roundworm)
  • Intracellular parasite
  • Associated disease: trichinellosis

Clinically relevant species

  • Most notable and worldwide: Trichinella spiralis
  • Causes of trichinellosis:
    • Encapsulating/encysting (within host muscles):
      • Trichinella britovi (Europe, Asia, and northern and western Africa)
      • Trichinella murelli (North America)
      • Trichinella nativa (Arctics)
      • Trichinella nelsoni (Eastern Africa)
    • Nonencapsulating:
      • Trichinella papuae (Papua New Guinea)
      • Trichinella pseudospiralis (worldwide)
      • Trichinella zimbabwensis (Tanzania)

Epidemiology

  • Worldwide, an estimated 10,000 cases of trichinellosis occur every year.
  • In the United States: 
    • 90 cases from 2008 to 2012
    • Decline in cases due to improved swine production
  • High prevalence in:
    • China
    • Former Soviet Union
    • Romania and other parts of Central Europe
    • Thailand
    • Mexico
    • Argentina
    • Bolivia

Pathophysiology

Reservoirs and transmission

  • Reservoir:
    • Trichinella spiralis: swine/pigs
    • Other animals: wild boars, bear, rodents, horses, moose
  • Transmission: raw or undercooked meat (especially pork and bear) containing larvae

Pathogenesis

  • Encysted larvae live within the striated muscles of the reservoir animal (some species are not encysted).
  • Larvae are ingested when undercooked meat is consumed.
  • GI phase:
    • After exposure to gastric acid and pepsin, larvae are released from the cysts.
    • Larvae invade small-bowel mucosa and then become adult worms.
    • Mating occurs; life span in the small bowel lasts approximately 4 weeks.
  • Systemic phase:
    • Females release larvae that enter circulation.
    • Larvae migrate to striated muscles.
    • Tissue migration can last up to 1 month, causing symptoms.
    • Highly active muscles affected:
      • Diaphragm
      • Tongue
      • Masseter
      • Intercostals
      • Extraocular muscles
    • Can reach myocardium and the brain and lead to cardiac and cerebral inflammatory reaction.
    • As larvae encyst, tissue infiltration (by PMNs and eosinophils) and edema occur.
    • Calcification follows after a period of months, and after that, they remain in that state for years, but symptoms decrease after months.
Trichinella spiralis life cycle

The life cycle of Trichinella spiralis in humans:
There are two phases in humans: the intestinal phase and the systemic phase (larvae get into the circulation and can reach myocardium and the brain, leading to cardiac and cerebral inflammatory reactions).

Image by Lecturio.

Clinical Presentation

  • General principles:
    • Incubation period: 1–6 weeks
    • Severity of symptoms depends on amount of larvae (infectious dose) consumed.
    • Two phases: intestinal and systemic
  • Intestinal phase:
    •  2–7 days after exposure
    • Can be asymptomatic
    • Abdominal pain (midabdomen)
    • Diarrhea
    • Nausea
    • Vomiting
  • Systemic phase:
    •  1–2 weeks after ingestion
    • Can last up to 8 weeks
    • Fever and chills
    • Periorbital edema/palpebral edema: 
      • Proptosis
      • Chemosis
      • Conjunctivitis
    • Myalgia:
      • Face
      • Chest
    • Weakness
    • Dry cough
    • Splinter hemorrhage and/or retinal hemorrhage
    • Petechial rash/urticaria
    • Headache
    • Hepatomegaly

Complications

  • Myocarditis:
    • Transitory passage leads to infiltration of inflammatory cells.
    • Life-threatening arrhythmias are the most common cause of death.
  • Meningitis/encephalitis
  • Pulmonary:
    • Secondary bacterial pneumonia
    • Respiratory myositis (involving the diaphragm)
  • Renal failure
  • Thromboembolic disease

Diagnosis

Diagnostic tests

  • Serologic evidence:
    • Only detectable after 3 weeks of incubation
    • Methods used:
      • Western blotting
      • ELISA
      • Indirect immunofluorescence
      • Latex agglutination test
  • Muscle biopsy:
    • Leads to a definitive diagnosis, but done only if initial tests are inconclusive
    • Symptomatic muscle tissue is biopsied.
    • Examination would show Trichinella spiralis cyst embedded in muscle tissue.
Trichinella spiralis cyst

Trichinella spiralis cyst embedded in a muscle tissue specimen in a case of trichinellosis:
Trichinellos is acquired by ingesting meat containing cysts (encysted larvae) of the roundworm parasite.

Image: “10180” by Dr. I. Kagan. License: Public Domain

Additional tests

  • Laboratory tests:
    • CBC:
      • Eosinophilia
      • Leukocytosis
    • Metabolic panel and muscle tests:
      • Hypokalemia
      • ↑ Creatine kinase
      • ↑ Aminotransferases
      • ↑ Lactate dehydrogenase
      • ↑ Aldolase
  • Plain X-ray: intramuscular calcification

Management

Treatment

  • Mild infection: Most infections have no complications and are self-limited.
  • For infection with systemic symptoms, treatment consists of: 
    • Antiparasitic drugs:
      • Albendazole
      • Mebendazole
    • Corticosteroids (especially for severe complications)
  • Prevention:
    • Cook meat up to 77°C
    • Freeze meat at –15°C (not applicable to Arctic species)
    • Proper meat handling
    • Postexposure prophylaxis with mebendazole may be effective if given within 6 days after exposure.

Prognosis

  • Self-limited
  • Complete recovery within 2–6 months
  • Increased morbidity with cardiac or CNS involvement

Differential Diagnosis

  • Gastroenteritis: intestinal phase of trichinellosis can be mistaken for food poisoning owing to the constitutional GI symptoms: Viral and bacterial causes of gastroenteritis must be considered and ruled out using history, physical exam, and stool analysis.
  • Myopathies: Autoimmune disorders such as polymyositis or dermatomyositis present with myalgia and vasculitis-like symptoms and signs, similar to the systemic signs of trichinellosis. These cases usually involve the proximal muscle groups, unlike in trichinellosis. Investigations for specific autoantibodies such as anti–signal recognition protein (anti-SRP) antibodies and anti-Mi2 aid in diagnosis. Management is usually with immunosuppressants and glucocorticoids.
  • Ascariasis: infection caused by the parasitic roundworm Ascaris lumbricoides: Transmission occurs via ingestion of contaminated water or food. Most patients are asymptomatic. If symptoms do occur, they can be mild, with only abdominal discomfort, or severe, causing intestinal obstruction. Other symptoms, such as cough, are due to migration of the worms through the body.
  • Hookworm infection: caused by the helminths Necator americanus and Ancylostoma duodenale: Patients will present with iron deficiency anemia and failure to thrive. Diagnosis involves inspection of human feces for larvae and eggs.
  • Strongyloidiasis: a disease caused by the roundworm (nematode) Strongyloides. Strongyloidiasis has various clinical manifestations, including GI symptoms and eosinophilia. Diagnosis is via serology.

References

  1. Gottstein, B., Pozio, E., Nöckler, K. (2009). Epidemiology, diagnosis, treatment, and control of trichinellosis. Clin Microbiol Rev 22(1):127–145. https://pubmed.ncbi.nlm.nih.gov/19136437/
  2. Faber, M., et al. (2015). Outbreak of trichinellosis due to wild boar meat and evaluation of the effectiveness of post exposure prophylaxis, Germany, 2013. Clin Infect Dis 60(12):e98–e104. https://pubmed.ncbi.nlm.nih.gov/25770171/
  3. Rawla, P., Sharma, S. (2020). Trichinella spiralis. StatPearls. Retrieved April 2, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK538511/
  4. Riedel, S., et al. (Eds.). (2019). Medical parasitology. Jawetz, Melnick, & Adelberg’s Medical Microbiology, 28th ed. McGraw-Hill.
  5. Taher, E.E., et al. (2017). Modified dot-ELISA for diagnosis of human trichinellosis. Exp Parasitol 177:40–46. https://pubmed.ncbi.nlm.nih.gov/28438521/
  6. Weller, P., Leder, K. (2020). Trichinellosis. UpToDate. Retrieved April 2, 2021, from https://www.uptodate.com/contents/trichinellosis

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