Toxoplasma gondii

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

• Toxoplasma can cause acute interstitial pancreatitis in systemic toxoplasmosis

• Toxoplasma gondii causes myositis
  • Cats as final hosts
  • Intermediate hosts, birds and mammals have tachyzoites throughout body, infrequently in skeletal muscle
  • Histologically:
    • Multifocal necrosis of myofibres
    • Lymphocyte and plasma cell infiltration
    • In later stages bradyzoite cysts

Cat Toxoplasmosis

Sheep Toxoplasmosis

Human Toxoplasmosis