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 Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever, chills, myalgia, and periorbital edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). 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 Diaphragm The diaphragm is a large, dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm consists of muscle fibers and a large central tendon, which is divided into right and left parts. As the primary muscle of inspiration, the diaphragm contributes 75% of the total inspiratory muscle force. Diaphragm
      • Tongue Tongue The tongue, on the other hand, is a complex muscular structure that permits tasting and facilitates the process of mastication and communication. The blood supply of the tongue originates from the external carotid artery, and the innervation is through cranial nerves. Oral Cavity: Lips and 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 Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). 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 Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain (midabdomen)
    • Diarrhea Diarrhea Diarrhea is defined as ≥ 3 watery or loose stools in a 24-hour period. There are a multitude of etiologies, which can be classified based on the underlying mechanism of disease. The duration of symptoms (acute or chronic) and characteristics of the stools (e.g., watery, bloody, steatorrheic, mucoid) can help guide further diagnostic evaluation. Diarrhea
    • Nausea
    • Vomiting
  • Systemic phase:
    •  1–2 weeks after ingestion
    • Can last up to 8 weeks
    • Fever and chills
    • Periorbital edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema/palpebral edema Edema Edema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity). Edema
      • Proptosis
      • Chemosis
      • Conjunctivitis Conjunctivitis Conjunctivitis is a common inflammation of the bulbar and/or palpebral conjunctiva. It can be classified into infectious (mostly viral) and noninfectious conjunctivitis, which includes allergic causes. Patients commonly present with red eyes, increased tearing, burning, foreign body sensation, and photophobia. Conjunctivitis
    • Myalgia:
      • Face
      • Chest
    • Weakness
    • Dry cough
    • Splinter hemorrhage and/or retinal hemorrhage
    • Petechial rash/ urticaria Urticaria Urticaria is raised, well-circumscribed areas (wheals) of edema (swelling) and erythema (redness) involving the dermis and epidermis with associated pruritus (itch). Urticaria is not a single disease but rather is a reaction pattern representing cutaneous mast cell degranulation. Urticaria (Hives)
    • Headache
    • Hepatomegaly

Complications

  • Myocarditis Myocarditis Myocarditis is an inflammatory disease of the myocardium, which may occur alone or in association with a systemic process. There are numerous etiologies of myocarditis, but all lead to inflammation and myocyte injury, most often leading to signs and symptoms of heart failure. Myocarditis:
    • Transitory passage leads to infiltration of inflammatory cells.
    • Life-threatening arrhythmias are the most common cause of death.
  • Meningitis Meningitis Meningitis is inflammation of the meninges, the protective membranes of the brain, and spinal cord. The causes of meningitis are varied, with the most common being bacterial or viral infection. The classic presentation of meningitis is a triad of fever, altered mental status, and nuchal rigidity. Meningitis/ encephalitis Encephalitis Encephalitis is inflammation of the brain parenchyma caused by an infection, usually viral. Encephalitis may present with mild symptoms such as headache, fever, fatigue, and muscle and joint pain or with severe symptoms such as seizures, altered consciousness, and paralysis. Encephalitis
  • Pulmonary:
    • Secondary bacterial pneumonia Pneumonia Pneumonia or pulmonary inflammation is an acute or chronic inflammation of lung tissue. Causes include infection with bacteria, viruses, or fungi. In more rare cases, pneumonia can also be caused through toxic triggers through inhalation of toxic substances, immunological processes, or in the course of radiotherapy. Pneumonia
    • Respiratory myositis (involving the diaphragm)
  • Renal failure
  • Thromboembolic disease

Diagnosis

Diagnostic tests Diagnostic tests Diagnostic tests are important aspects in making a diagnosis. Some of the most important epidemiological values of diagnostic tests include sensitivity and specificity, false positives and false negatives, positive and negative predictive values, likelihood ratios, and pre-test and post-test probabilities. Epidemiological Values of 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 Hypokalemia Hypokalemia is defined as plasma potassium (K+) concentration < 3.5 mEq/L. Homeostatic mechanisms maintain plasma concentration between 3.5-5.2 mEq/L despite marked variation in dietary intake. Hypokalemia can be due to renal losses, GI losses, transcellular shifts, or poor dietary intake. 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 Gastroenteritis Gastroenteritis is inflammation of the stomach and intestines, commonly caused by infections from bacteria, viruses, or parasites. Transmission may be foodborne, fecal-oral, or through animal contact. Common clinical features include abdominal pain, diarrhea, vomiting, fever, and dehydration. 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 Polymyositis Polymyositis (PM) is an autoimmune inflammatory myopathy caused by T cell-mediated muscle injury. The etiology of PM is unclear, but there are several genetic and environmental associations. Polymyositis is most common in middle-aged women and rarely affects children. 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 Antibodies Immunoglobulins (Igs), also known as antibodies, are glycoprotein molecules produced by plasma cells that act in immune responses by recognizing and binding particular antigens. The various Ig classes are IgG (the most abundant), IgM, IgE, IgD, and IgA, which differ in their biologic features, structure, target specificity, and distribution. Immunoglobulins and anti-Mi2 aid in diagnosis. Management is usually with immunosuppressants Immunosuppressants Immunosuppressants are a class of drugs widely used in the management of autoimmune conditions and organ transplant rejection. The general effect is dampening of the immune response. Immunosuppressants and glucocorticoids Glucocorticoids Glucocorticoids are a class within the corticosteroid family. Glucocorticoids are chemically and functionally similar to endogenous cortisol. There are a wide array of indications, which primarily benefit from the antiinflammatory and immunosuppressive effects of this class of drugs. Glucocorticoids.
  • Ascariasis Ascariasis Ascariasis is most often caused by A. lumbricoides. If symptomatic, characteristics typically follow 2 phases, which correlate with the migration of the parasite through the body. The early phase may include cough, dyspnea, and wheezing. The late phase typically includes abdominal discomfort, bloating, nausea, and intermittent diarrhea. Ascaris/Ascariasis: infection caused by the parasitic roundworm Ascaris Ascaris Ascaris is a genus of parasitic nematodes. The infection, ascariasis, is most often caused by A. lumbricoides. Transmission occurs primarily via ingestion of water or food contaminated with Ascaris eggs. Most patients with ascariasis are asymptomatic. Ascaris/Ascariasis 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 Iron Deficiency Anemia Iron deficiency anemia is the most common type of anemia worldwide. This form of anemia is caused by insufficient iron due to a decreased supply, an increased loss, or an increased demand. Iron deficiency anemia is seen across all ages, sexes, and socioeconomic strata; however, children, women of childbearing age, and patients from lower socioeconomic strata are at higher risk. Iron Deficiency Anemia and failure to thrive Failure to Thrive Failure to thrive (FTT), or faltering growth, describes suboptimal weight gain and growth in children. The majority of cases are due to inadequate caloric intake; however, genetic, infectious, and oncological etiologies are also common. Failure to Thrive. Diagnosis involves inspection of human feces for larvae and eggs.
  • Strongyloidiasis Strongyloidiasis Strongyloidiasis is a common parasitic disease caused by infection with the roundworm Strongyloides stercoralis. Transmission occurs through skin penetration. Strongyloides has a unique life cycle that can be entirely completed in the human host, migrating from the skin to the pulmonary system and then to the GI system. Strongyloidiasis: a disease caused by the roundworm (nematode) Strongyloides. Strongyloidiasis Strongyloidiasis Strongyloidiasis is a common parasitic disease caused by infection with the roundworm Strongyloides stercoralis. Transmission occurs through skin penetration. Strongyloides has a unique life cycle that can be entirely completed in the human host, migrating from the skin to the pulmonary system and then to the GI system. 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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