Toxoplasma gondii

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Toxoplasma gondii - Ke Hu and John Murray
Toxoplasma Sporulated Oocyst - Wikimedia Commons
Toxoplasma Tacchyzoites - Wikimedia Commons
Toxoplasma Life Cycle Diagram - Dennis Jacobs & Mark Fox RVC
Toxoplasma gondii - Courtesy of the Laboratory of Parasitology, University of Pennsylvania School of Veterinary Medicine
  • Major pathogenic species called Toxoplasma gondii
  • Causes disease in a wide range of animal species including humans
  • Important cause of abortion in sheep
  • Zoonotic
    • Can cause abortion
    • Can cause congenitally aquired defects
  • Forms a sporulated oocyst which is only 10μm
    • Contains 2 sporocysts with 4 sporozoites
  • Transmission through ingesting the intermediate host or via the faecal-oral route

Life Cycle

  • Complex
  • Usually indirect
    • Referred to as facultatively heteroxenous
    • Intermediate host is not essential for completion of the life cycle
  • Gametogony (sexual stage) is host specific for felids
  • Any warm blooded animal can act as a facultative intermediate host
    • Asexual reproduction occurs in the intermediate host forming tissue cysts
    • Intermediate host swallows sporulated oocysts or tissue cysts
    • Can be transferred between intermediate hosts by carnivorism
  • Cats
    • Sporulation occurs in 2-3 days
    • Cats either swallow infective (sporulated) oocysts where Toxoplasma gondii has a prepatent period of 3 weeks
    • Or eat the tissues of an infected intermediate host where Toxoplasma gondii has a prepatent period of 3-10 days
    • Self-limiting infection
    • Oocysts are shed for 1-2 weeks
      • Shedding can occur later if immunity wanes or cat is immuno-compromised
  • Intermediate host
    • 3 sources of infection
      • Oocysts from environment contaminated by cat faeces
      • Eating cysts in tissues of other infected hosts through carnivorism or undercooked meat
      • Transplacental transmission in some host species during the acute phase of infection
  • Acute phase of infection
    • After infection of the intermediate host the organism undergoes a phase of rapid division and dissemination throughout the body
      • Parasite enters cell and asexual reproduction occurs by endodyogeny (budding) producing 8-16 tachyzoites
      • Tachyzoites are released when host cell bursts
      • Haematogenous spread as more cells are infected
      • Infection continues until the animal develops immunity (around 2 weeks) at which point the infection enters the chronic phase
  • Chronic phase of infection
    • Occurs once the host's immune response has become effective
      • Groups of slow growing intracellular bradyzoites become walled off forming infective cysts
      • Bradyzoites inside cysts are protected from the host immune response whereas extracellular tachyzoites are killed
      • Cysts remain viable for months to years and are particularly numerous in muscle and nervous tissue
      • If immunity is suppressed the infection can revert to the acute form
  • Meat animals
    • Significant proportion of cattle, sheep, pigs and rabbits can tissue cysts

Pathogenesis

  • Cattle and horses
    • Sometimes infectious causing opthalmitis
  • Dogs
    • Complication of canine distemper
    • Causes pneumonia and encephalitis


Cat Toxoplasmosis

Sheep Toxoplasmosis

Human Toxoplasmosis