Trichinella

• Trichinella Spiralis

  Also known as Trichina Worm
• First described by Tiedemann


  1822
• James Paget and Richard Owen demonstrated trichinella in human cadaver in London


  1835
• German investigators proved that eating raw or insufficiently cooked meat like pork causes human trichinellosis
• Recognized species of Trichinella

  • Trichinella spiralis
  • Trichinella britovi
  • Trichinella nativa
  • Trichinella murrelli
  • Trichinella nelsoni
  • Trichinella papuae
  • Trichinella zimbabwensis
• Trichinella spiralis


  Most important cause of trichinellosis in humans, most adapted to domestic or wild pigs
• Trichinella britovi


  Second most common species affecting humans, most widely distributed species among wild animals, can also infect domestic pigs
• Trichinella nativa


  Infects primarily wild carnivores in the frigid zone
• Other species affecting humans

  • T. murrelli
  • T. nelsoni
  • T. papuae
  • T. zimbabwensis
• Trichinellonosis

  Primarily a zoonotic infection
• Adult female Trichinella

  Viviparous (Larviparous), gives birth to larvae
• Mode of transmission

  Ingestion of raw or undercooked meat of pig and other animals with encysted larvae
• Infection is maintained in pig-to-pig, or pig-to-rat-to-pig cycle
• Trichinella species implicated in human disease

  • T. spiralis
  • T. pseudospiralis
  • T. nativa
  • T. murelli
  • T. nelsoni
  • T. britovi
  • T. papuae
• Adult worms and encysted larvae develop within a single vertebrate host, and an infected animal serves as a definitive host and potential intermediate host

  
• The domestic cycle most often involved pigs and anthropophilic rodents, but other domestic animals such as horses can be involved
• In the sylvatic cycle, the range of infected animals is great, but animals most often associated as sources of human infection are bear, moose and wild boar
• Trichinellosis


  Cosmopolitan infection, documented on at least 55 countries worldwide
• Approximately 10,000 cases per year, with 0.2% resulting in mortality
• Trichinella Spiralis

  • Smallest parasitic nematode to humans
  • Thread-like
  • Ivory white in color when unstained

  
• Male Trichinella Spiralis

  0.62 - 1.58 mm by 0.025 - 0.033 mm, single testis near the posterior end, no spicule, cloaca with 2 conspicuous conical papillae (claspers) for copulation
• Female Trichinella Spiralis

  1.26 - 3.35 mm by 0.029 - 0.038 mm, single ovary in posterior part, vulva for copulation located at anterior fifth, ventral side
• Trichinella Spiralis Larva

  At birth: 80 - 120 um x 5.6 um, Encysted: 0.65 - 1.45 mm x 0.026 - 0.040 mm, Anterior end spear-like for burrowing, Digestive tract similar to adult, Reproductive organ not fully developed but sex can be identified

  
• Life cycle in humans

  1. Ingest undercooked meat containing encysted larvae

  2. Larvae released from cyst, invade small intestine, develop into adults

  3. Adult male and female copulate, female gives birth to larvae

  4. Newborn larvae burrow into intestinal wall, enter circulation, encyst in striated muscles

  
• Humans can be definitive host, intermediate host, or final host in the life cycle of Trichinella spiralis
• The role of definitive, intermediate and final host is also applicable to animals to complete the life cycle of Trichinella spiralis

  
• Encysted larvae are both the diagnostic stage and infective stage of trichinellosis

  
• Trichinellosis is caused by ingestion of undercooked meat containing encysted larvae, except for T. pseudospiralis and T. papuae which do not encyst

  
• Adult female Trichinella

  2.2 mm in length
• Adult male Trichinella

  1.2 mm in length
• Life span of adult Trichinella in small bowel is about 4 weeks
• After 1 week, female Trichinella release larvae that migrate to striated muscles and encyst
• Diagnosis is usually made based on clinical symptoms, and confirmed by serology or identification of encysted or non-encysted larvae in biopsy or autopsy specimens
• Light infection (10 larvae)


  Usually asymptomatic
• Moderate infection (50-500 larvae)


  Symptomatic, shows gastroenteritis, diarrhea or abdominal pain
• Severe infection (>1,000-3,000 larvae)


  Severe disease
• Phases of Trichinellosis infection

  • Enteric Phase
  • Invasion Phase
  • Convalescent Phase
• Enteric Phase


  • Corresponds to incubation and intestinal invasion
  • Symptoms may resemble those of an attack of a acute food poisoning
• Invasion Phase


  • Corresponds to larval migration and muscle invasion
  • Migrating larvae in resulting metabolites lead to immunological, pathological, and metabolic reactions
• Convalescent Phase


  Corresponds to encystment and encapsulation