Trichinella spp.

Severe muscle infections
Classic Case: Trichinellosis

Middle-aged male presents with a fever (38.5°C for one week), significant fatigue, chills, headache, terrible muscle aches and pains, and swelling around the eyes. He is an avid hunter, and eats his home-prepared meat. Labs show elevated eosinophils.

Causative Agent of:
  • Trichinosis
    • Also known as Trichinellosis
  • Multiple species:
    • T. spiralis is the classic species
    • T. murrelli, nativa, britovi also
Physical Features
  • Helminth
  • Nematode
  • Small: 1-2 mm long
Epidemiology
  • World-wide: Various species of Trichinella
  • U.S.: 26 cases in 2016 (CDC; MMWR)
    • Often seen in small clusters
    • People who ate same animal meat
Reservoir and Hosts
  • N. America:
    • Many wild animals
    • Bear, boar, moose, cougar, walrus
  • GloballyPigs
    • (In N. America, pigs are screened)
Transmission and Risk Groups

Transmission:

  • Ingestion of undercooked meat that contain larvae

Risk Groups:

  • World-wide: Consumption of undercooked pork
  • U.S.: Usually hunters, consumption of wild game
    • Undercooked (or smoked)
Life Cycle
  • Cyst ingested, degraded by pepsin/gastric acid in stomach
  • Releases larvae into sm intestine, larvae invade mucosa
  • Males + females mate, survive 4 weeks, make ~1500 larvae
  • Larvae penetrate intestine, spread thru blood/lymphatics
  • Larvae invade/encyst in active, striated skeletal muscle cells
  • Here, Trichinella forms a “Nurse Cell”: a protective cyst
    • Mature nurse cell: distinct from all other cell types
    • Can survive for the entire life of host
  • Ingestion of infected muscles by another animal continues cycle

Clinical

Trichinosis
Symptoms
  • Incubation period: 3-7 days
  • Prodrome (GI phase):
    • Gastroenteritisdiarrheanausea
  • Classic Signs (Systemic Phase):
    • Myalgiamuscle weaknessrashes common
    • Conjunctivitis, upper eyelid edema,
    • Splinter hemorrhages in fingernails
    • Usually occur 1-2 weeks after prodrome
    • biphasic illness
Complications
  • Death: Rare, but can occur 3-6 days after serious symptoms
  • Myocarditis: Most common parasitic cause of myocarditis
    • Larvae do not encyst in cardiac muscle
    • Due to eosinophil-induced inflammatory myocarditis
  • CNS involvement (meningitis/encephalitis)
  • Primary or secondary pneumonia
Pathogenesis
  • GI Phase:
    • Larvae in GI tract, mate, produce larvae
    • Larvae penetrate GI lining = GI symptoms
  • Muscle/Systemic Phase:
    • Larvae migrate through body tissues
    • Encyst in muscle = damage/inflammation
    • Causing muscle/systemic symptoms
Diagnosis
  • Serology: anti-Trichinella IgGs seen by 3 weeks post-infection
    • Most common test for suspected cases
  • Tissue Biopsy: Microscopic detection of encysted larvae
    • Heavily infected people: 50+ larvae/gram of muscle
    • Invasive, and infrequently used, but is also definitive Dx
  • Patient History: Consumption of undercooked meat
  • Lab FindingsEosinophilia
    • Most cases, as early as 2 weeks post-infection
    • Elevated creatine kinase, aldolase, LDH
      • (Muscle tissue damage)
Prevention
  • Cook meat (Pork, wild game)
    • U.S. commercial pork is screened
  • T. spiralis: killed at -20°C for 3-5 days
    • But other sp. of Trichinella are freeze-resistant 
  • Humans are a “dead-end” host
Treatment
  • Albendazole, mebendazole

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