Trichinosis (Trichinellosis) Workup

Updated: May 01, 2018
  • Author: Darvin Scott Smith, MD, MSc, DTM&H; Chief Editor: Burke A Cunha, MD  more...
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Workup

Laboratory Studies

CBC count

Obtain a CBC count.

Leukocytosis occurs in 65% of patients, with cell counts of up to 24,000/µL.

Eosinophilia typically rises 10 days after infection, with total eosinophil counts of up to 8700/µL (40%-80% of total WBC). The counts peak in 3-4 weeks and resolve over the next few months.

Nearly all patients with trichinosis, either symptomatic or asymptomatic, exhibit eosinophilia. The only exception is in severe cases, when the eosinophil count may be severely depressed. A low eosinophil count indicates an increased mortality rate.

Erythrocyte sedimentation rate

Erythrocyte sedimentation rates are usually within the reference range.

Creatine kinase

Obtain creatine kinase (CK) levels. [16]

CK levels are elevated to 17,000 U/L.

CK (isoenzyme myocardial band [MB]) elevations may indicate myocardial involvement; however, as many as 35% of patients without cardiac involvement may have elevated CK-MB levels.

Lactate dehydrogenase

Levels of lactate dehydrogenase isoenzymatic forms (ie, lactate dehydrogenase fraction 4 [LD4] and lactate dehydrogenase fraction 5 [LD5]) are elevated in 50% of patients.

Immunoglobulin E

Immunoglobulin E levels are typically elevated.

Serology

Serology results are not positive until 2-3 weeks after infection. They peak around the third month and may persist for years.

Serology ratios do not correlate with the severity of disease or the clinical course. However, a strong positive test result usually indicates an early infection.

Perform indirect hemagglutination.

Bentonite flocculation results are usually not positive for more than 1 year after infection.

Perform indirect immunofluorescence.

Latex agglutination results are usually not positive for more than 1 year after infection.

Enzyme-linked immunosorbent assay (ELISA) is 100% sensitive on day 50, with 88% of results remaining positive 2 years after infection.

Hypersensitivity skin test

The immediate hypersensitivity skin test is no longer commercially available. Reactions results are positive (5 mm) at approximately day 17 and remain positive for life.

Molecular techniques

Molecular techniques are being developed but have not been validated.

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Imaging Studies

In patients with CNS involvement, CT scanning and MRI with contrast enhancement may reveal 3- to 8-mm nodular or ringlike lesions.

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Other Tests

Electrocardiography may show the following:

  • Premature contractions

  • Prolongation of the PR intervals

  • Small QRS complexes with intraventricular block

  • Flattening or inversion of the T waves, especially lead II and precordial leads

Polymerase chain reaction is useful for isolating the parasite and subsequent genetic typing. it is used primarily as a research tool.

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Procedures

Electromyelography

Electromyelography may be helpful in diagnosing moderate-to-severe infection, but no pathognomonic findings exist. The test result may reveal acute myositis or diffuse myopathic dysfunction.

Changes usually resolve 2-3 months after infection but may persist for 1-8 years.

Lumbar puncture

Lumbar puncture is used to evaluate for suspected neurologic disease.

Results are normal in 50%-75% of patients.

Larvae are found in 8%-24% in patients.

Eosinophilic meningitis may be present.

Muscle biopsy

Muscle biopsy provides a definitive diagnosis; however, it is rarely recommended except in difficult cases when serology tests are unhelpful.

Obtain a 0.5- to 1-g muscle biopsy specimen from the deltoid or gastrocnemius muscle because these are most easily accessible. The yield increases if the biopsy site is swollen or tender. Stain the specimen with hematoxylin and eosin (H&E) and examine multiple sections. Occasionally, larvae can be found after the muscle has been digested enzymatically.

If a biopsy is performed prior to larvae coiling (beyond day 17 of infection), worm tissue can be confused with muscle tissue.

A negative result does not necessarily exclude infection.

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Histologic Findings

A histologic examination may reveal destruction of skeletal muscles, including a basophilic degeneration of the fibers observed on H&E-stained sections. Dead, nonencapsulated parasites can be observed. Muscle cells contain small hemorrhages and an accumulation of inflammatory cells (eg, eosinophils, lymphocytes, macrophages).

The results of a histologic examination in myocardial muscle are consistent with an immune-mediated reaction. Parasites migrate through the myocardium but do not encyst; however, a strong inflammatory reaction occurs, with numerous eosinophils, erythrocytes, fibrin deposits, and foci of necrotic myocardium. A mild-to-moderate pericardial effusion may also be present. Perivascular collections of eosinophils, lymphocytes, macrophages, and polymorphonuclear leukocytes develop in the CNS and are associated with areas of ischemia. Larvae may be surrounded by astrocytes and microglial cells.

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Staging

Case definitions for human trichinosis include possible cases (not applicable), probable cases (patients who meet the clinical criteria and with an epidemiological link [below]), and confirmed cases (patients who meet the laboratory criteria and clinical criteria within the past 2 months). [13]

Clinical criteria - At least 3 of the following: (1) fever, (2) muscle soreness and pain, (3) gastrointestinal symptoms, (4) facial edema, (5) eosinophilia, or (6) subconjunctival, subungual, and retinal hemorrhages

Laboratory criteria - At least 1 of the following: (1) demonstration of Trichinella larvae in tissue obtained by muscle biopsy or (2) demonstration of Trichinella -specific antibody response by indirect immunofluorescence, ELISA, or Western blot

Epidemiological criteria - At least one of the following: (1) consumption of laboratory-confirmed parasitized meat, (2) consumption of potentially parasitized products from a laboratory-confirmed infected animal, or (3) epidemiological link to a laboratory-confirmed human case by exposure to the same common source

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