Trichinellosis

Definition, Etiology, PathogenesisTop

Trichinellosis (trichinosis) is a systemic inflammatory parasitic zoonosis that involves the skeletal muscles and causes allergic vasculitis.

1. Etiologic agent: Nematodes of the Trichinella genus. At least 12 species have been identified based on their geographic, biologic, genetic, and biochemical characteristics, as well as host susceptibility (T spiralis, T nativa, T britovi, T pseudospiralis, T murrelli, T papuae, and T zimbabwensis). Among them, T spiralis has a worldwide distribution and is responsible for most cases of infection. Larvae present in contaminated meat consumed by the patient enter the intestinal mucosa, within ~48 hours transform into the adult form, and after subsequent 5 to 6 days start producing new larvae (for ~5 weeks). The larvae enter the intestinal blood vessels and lymphatic vessels and are carried by bloodstream to various organs (mainly skeletal muscles with a high metabolic rate: tongue, diaphragm, as well as intercostal muscles, masseter muscle, extraocular muscles, neck muscles, and flexor muscles of the extremities). In the muscles the larvae encyst in a capsule produced by the host (termed a “nurse cell”), where they may survive up to a few years. In cell types other than the skeletal muscle the larvae are not able to encyst and continue to migrate, thus causing inflammation and necrosis. (An illustrative presentation of Trichinella life cycle is available at cdc.gov.)

2. Reservoir and transmission: All carnivorous and omnivorous animals (mammals, birds; domestic and wild). Transmission occurs via the oral route as a result of ingesting raw or undercooked meat of infected animals.

3. Epidemiology: Trichinellosis is prevalent worldwide in all climate zones. The number of registered outbreaks of trichinellosis varies from a few to several hundred per year; these are usually caused by consumption of infected pork or bear or walrus meat.

4. Risk factors: Consumption of undercooked meat of domestic animals (pork, horse meat) or wild animals (most frequently game meat) that has not been subject to veterinary inspection.

5. Incubation period: Incubation period is 2 to 45 days (usually 10-14 days), depending on the severity of invasion (a shorter incubation period suggests severe invasion). The patient is not contagious.

Clinical Features and Natural historyTop

Depending on severity, invasion may be asymptomatic, subclinical, or associated with mild, moderate, or severe symptoms, including:

1) Acute diarrhea, which occurs in the early stage of invasion, when the adult forms of T spiralis invade the small intestine. Diarrhea is accompanied by anorexia, upper abdominal pain, and vomiting, which on average persist for 1 to 2 days.

2) Typical symptoms of trichinellosis, which are caused by penetration of larvae to muscle cells and a severe local inflammatory reaction:

a) High-grade fever (often >40 degrees Celsius), myalgia (predominantly of the extraocular muscles, muscles of the neck, and flexor muscles of the extremities), malaise.

b) Signs and symptoms of parasitic tissue damage: Periorbital or (rarely) facial edema, petechiae in the conjunctiva and nail beds (in severe cases similar hemorrhages are also found in the brain, lungs, pericardium, and endocardium). A variety of concomitant cutaneous lesions may be observed.

c) Involvement of other organs (in more severe invasions): see Complications, below.

Signs and symptoms persist for 3 to 4 weeks (in heavy invasions even for 2-3 months), then slowly resolve, usually without sequelae. Fatigue, asthenia, and diarrhea may persist for a few months.

DiagnosisTop

Trichinellosis should be suspected if the epidemiologic history suggests multiple cases of morbidity related to consumption of uninspected meat and meat products. The amount of consumed meat and the time between consumption and onset of signs and symptoms are relevant for prognosis.

Diagnostic Tests

1. Identification of the organism:

1) Serology (enzyme-linked immunosorbent assay [ELISA], Western blot): Positive specific IgM and IgA (as early as in the second week of invasion) and IgG (rarely earlier than 2 weeks following invasion; they may persist for many years). Serology is the key method of confirming the diagnosis.

2) Polymerase chain reaction (PCR) has been developed in a variety of settings but is not widely available for clinical use.

3) Muscle biopsy: Parasitology: Larvae identified on microscopic examination (this is the definitive confirmation of infection for epidemiologic and legal purposes).

2. Ancillary tests:

1) Complete blood count (CBC): Eosinophilia (up to 70%) and leukocytosis are early signs observed prior to the onset of clinical manifestations; they may persist for up to 3 months.

2) Biochemical studies: Significant increases in serum levels of muscle enzymes: creatine kinase (CK) and lactate dehydrogenase (LDH). These tests are very useful when combined with eosinophilia, clinical manifestations, and epidemiologic history, allowing for early implementation of treatment.

Differential Diagnosis

1. Early phase of trichinellosis: Food poisoning, infectious diarrhea.

2. Typical symptoms of trichinellosis: Viral infection with influenza-like symptoms (eg, influenza, viral hepatitis, infectious mononucleosis, mumps), typhoid fever, sepsis, leptospirosis, rheumatic fever.

3. Signs and symptoms of parasitic tissue damage: Dermatomyositis, polyarteritis nodosa, serum sickness, angioedema, drug allergy.

TreatmentTop

Antiparasitic Treatment

The first-line drug in adult patients is oral albendazole 400 mg every 12 hours for 14 days. Given the potential for extensive morbidity, including death in severe cases, the drug should be administered as soon as possible after ingestion of infected meat (it will not be active against tissue cysts).Evidence 1Strong recommendation (benefits clearly outweigh downsides; right action for all or almost all patients). Low Quality of Evidence (low confidence that we know true effects of the intervention). Quality of Evidence lowered due to imprecision, indirectness, and observational nature of data. Watt G, Saisorn S, Jongsakul K, Sakolvaree Y, Chaicumpa W. Blinded, placebo-controlled trial of antiparasitic drugs for trichinosis myositis. J Infect Dis. 2000 Jul;182(1):371-4. doi: 10.1086/315645. Epub 2000 Jun 30. PMID: 10882628. Siriyasatien P, Yingyourd P, Nuchprayoon S. Efficacy of albendazole against early and late stage of Trichinella spiralis infection in mice. J Med Assoc Thai. 2003 Jun;86 Suppl 2:S257-62. PMID: 12929998. Alternatively oral mebendazole 200 to 400 mg every 8 hours for 10 to 14 days may be considered. These agents are active only against parasite stages prior to encystment.

To date, there have been no randomized studies on the impact of albendazole or mebendazole during pregnancy. Currently, albendazole and mebendazole are not recommended in pregnant patients, especially during the first semester. The use of those drugs could be considered only in consultation with appropriate specialists.

Symptomatic Treatment

1. Glucocorticoids may be indicated in severe disease (to limit the inflammation caused by larvae as well as reactions to drug-induced parasite disintegration). Prednisone 40 to 60 mg/d for 3 to 5 days may reduce symptoms.Evidence 2Weak recommendation (benefits likely outweigh downsides, but the balance is close or uncertain; an alternative course of action may be better for some patients). Low Quality of Evidence (low confidence that we know true effects of the intervention). Quality of Evidence lowered due to the observational nature of data. Shimoni Z, Klein Z, Weiner P, Assous MV, Froom P. The use of prednisone in the treatment of trichinellosis. Isr Med Assoc J. 2007 Jul;9(7):537-9. PMID: 17710786.

2. Anti-inflammatory and analgesic agents: Nonsteroidal anti-inflammatory drugs (NSAIDs).

3. Correct for fluid, electrolyte, and protein disturbances as necessary.

Follow-UpTop

Monitor CBC (eosinophils, leukocytes) and activity of the muscle enzymes (CK, LDH) on a regular basis.

Certain clinical features may resolve as late as after several months. Until then, the patient has to be regularly followed-up by a physician. Patients with specific organ complications should be referred to appropriate specialists (neurologist, cardiologist, ophthalmologist).

ComplicationsTop

1. Central nervous system: Meningitis, encephalitis, encephalopathy.

2. Eye: Orbital pain, photophobia, scotoma.

3. Respiratory system: Pneumonia, pulmonary edema.

4. Cardiovascular system (affecting up to 20% of patients): Arrhythmias, myocarditis, heart failure, hypoalbuminemia, peripheral edema.

5. Skeletal muscles: Contractures, weakness.

PrognosisTop

In patients with severe trichinellosis (massive invasion) death can occur as a result of myocarditis, heart failure, meningitis, or encephalitis. In milder cases the prognosis is good and usually associated with a complete resolution of signs and symptoms.

PreventionTop

1. Avoidance of consumption of uninspected meat, particularly pork and bear meat.

2. Meat from unreliable supply should be heat treated before consumption (cooked, fried, or roasted until the red or pink color disappears). Larvae are relatively resistant to freezing and can be destroyed only after several days of deep freezing; freezing at home is not a reliable method of preventing infection.

3. Adherence to food safety guidelines involves ensuring adequate conditions for livestock as well as postslaughter veterinary inspection of meat in reference centers.

4. Epidemiologic reporting to health authorities. Trichinella is a reportable disease under the Health of Animals Act in Canada but not a notifiable disease when diagnosed in humans.