Trichinella spiralis: Encysted Larva in Striated Muscle
Bench-card visuals for Trichinella spiralis diagnosis: the coiled larva encapsulated in a nurse cell within striated muscle, seen on compressed muscle squash preparation and on H&E tissue section.
Trichinella spiralis: nurse cell capsule in striated muscle
Muscle squash: coiled larva visible within an oval nurse cell capsule in the muscle fiber. H&E section: spindle-shaped fibrous capsule with coiled larva, surrounded by eosinophilic inflammatory infiltrate. Deltoid muscle is the pr...
Compressed muscle squash (between two slides): oval to spindle nurse cell capsule containing a tightly coiled (1-2 turns) larva. Larva is bright curved worm within the clear capsule space. Examine fresh or after HCl-pepsin digestion. The capsule forms from the host's modified muscle fiber cells. Deltoid muscle is the preferred biopsy site - taken from the most tender area.
H&E tissue section: spindle-shaped fibrous capsule (pale pink) surrounding a coiled larva in cross or longitudinal profile. Eosinophilic inflammatory infiltrate during acute infection; fibrosis and eventual calcification in chronic/resolved infection. Adult worms are in the small intestinal mucosa during weeks 1-2 of infection - rarely recovered in tissue biopsy. Calcified cysts may be incidentally noted on X-ray in old resolved infections.
Compressed muscle squash (between two slides): oval to spindle nurse cell capsule containing a tightly coiled (1-2 turns) larva. Larva is bright curved worm within t...
H&E tissue section: spindle-shaped fibrous capsule (pale pink) surrounding a coiled larva in cross or longitudinal profile. Eosinophilic inflammatory infiltrate duri...
What to look for in muscle biopsy
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Oval nurse cell capsule with coiled larva in muscle fiber | T. spiralis - diagnostic | Muscle squash is most sensitive; HCl-pepsin digestion releases larvae for identification and larval burden count (LPG) |
| Spindle fibrous capsule, coiled larva, eosinophilic infiltrate | T. spiralis on H&E | Dense inflammatory cuff in early/heavy infection; fibrosis develops over weeks; calcification begins ~6-18 months |
| Coiled larva 1 mm x 36 um | Trichinella larva dimensions | Count larvae per gram (LPG) in digested muscle - guides severity assessment; >200 LPG = severe infection |
| No eggs or larvae in stool | Trichinella - not a stool-detectable infection | Stool O&P is NOT appropriate for trichinosis diagnosis; muscle biopsy or serology required |
| Calcified spindle capsule, no viable larva | Old/resolved T. spiralis infection | Remote infection; larvae may remain viable for years before calcification; not necessarily indicative of active disease |
Trichinella spiralis diagnosis guide
| Pattern | What to call it | How to think about it |
|---|---|---|
| Coiled larva in oval nurse cell capsule in muscle fiber | T. spiralis - positive | Confirm with serology; report larval burden (LPG); anthelmintic + corticosteroids as clinically indicated |
| Positive anti-Trichinella IgG ELISA | Probable T. spiralis infection | Serology is most practical; muscle biopsy if high suspicion and serology negative; serology may be negative in first 3-5 weeks |
| Calcified spindle capsule, no viable larva | Old/resolved T. spiralis infection | Remote infection; clinical context required; not necessarily indicative of active disease requiring treatment |
- Coiled larva in oval nurse cell capsule within striated muscle = Trichinella spiralis - diagnostic.
- Routine stool O&P is NOT appropriate - Trichinella does not pass eggs in stool; use muscle biopsy or serology.
- Clinical context: periorbital edema + myalgia + eosinophilia + undercooked meat ingestion = trichinosis until proven otherwise.
Coiled worm in oval muscle capsule = Trichinella. Not in stool. Muscle squash + serology. Deltoid biopsy.