Toxoplasma gondii

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Toxoplasma gondii

• cosmopolitan distribution
• seropositive prevalence rates vary
• generally 20-75
• generally causes very benign disease in
  immunocompetent adults
• tissue cyst forming coccidia
• predator-prey life cycle
• felines are definitive host
• infects wide range of birds and mammals
  (intermediate hosts)

• Definitive Host
• adult forms
• sexual reproduction
• Intermediate Host
• immature forms
• asexual reproduction

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Typical Isospora Life Cycle in Felines
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Typical Isospora Life Cycle in Felines

• fertilization within infected host cells
• immature oocysts in feces
• sporulation in environment (1-4 d)

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Tachyzoite Stage

• sporozoites ? merogony
• rapid replication
• dissemination via macrophages
• reticuloendothelial cells
• acute stage infection

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Endodyogony
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Tachyzoite Stage

• sporozoites ? merogony
• rapid replication
• dissemination via macrophages
• reticuloendothelial cells
• acute stage infection

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Tachyzoite Stage

• sporozoites ? merogony
• rapid replication
• dissemination via macrophages
• reticuloendothelial cells
• acute stage infection

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Bradyzoite Stage

• dormant, slowly replicating
• due to host immune response
• chronic or latent infection
• tissue cysts in brain and muscle

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Bradyzoite Stage

• dormant, slowly replicating
• due to host immune response
• chronic or latent infection
• tissue cysts in brain and muscle

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Fatal Attraction in Rats Infected with
Toxoplasma gondii
From Berdoy et al (2000) Proc. R. Soc. (Biol.)
2671591
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Human Transmission

• ingestion of sporulated oocysts (cat feces
  incubation)
• ingestion of zoites (undercooked meat)
• congenital infection (only during acute stage)
• organ transplants
• chronic infection in donor
• immunosuppression
• blood transfusions (only during acute stage)

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Acquired Postnatal Toxoplasmosis

• 1-2 week incubation period
• acute parasitemia persists for several weeks
  until development of tissue cysts
• often asymptomatic (gt80)
• a common symptom is lymphadenopathy without fever
• occasionally mononucleosis-like (fever, headache,
  fatigue, myalgia)
• likely persists for life of patient
• immunosuppression can lead to reactivation (eg.,
  organ transplants)

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Toxoplasmic Encephalitis

• common complication associated with AIDS during
  the 1980's
• recrudescence of latent infection
• multifocal disease associated with
  immunosuppression
• lesions detectable with CT or MRI
• little spread to other organs
• symptoms include lethargy, apathy,
  incoordination, dementia
• progressive disease ? convulsions

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Congenital Toxoplasmosis

• 1o infection must occur during pregnancy
• can only occur once
• 1/3 will pass infection to fetus
• incidence 1 per 1000 births
• severity varies with age of fetus
• move severe early in pregnancy
• more frequent later in pregnancy
• infection can result in spontaneous abortion,
  still birth, premature birth, or full-term
  overt disease

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• typical disease manifestations include
  retinochoroiditis, psychomotor disturbances,
  intracerebral calcification, hydrocephaly,
  microcephaly

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Ocular Toxoplasmosis

• retinochoroiditis likely due to both active
  parasite proliferation and immune
  hypersensitivity
• generally a recrudescence--rarely from primary
  infection
• congenital infection
• 20 exhibit ocular symptoms at birth
• 82 by adolescence
• most lesions are focal and self-limiting
• rapidly destructive in AIDS patients

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Ocular Toxoplasmosis

• retinochoroiditis likely due to both active
  parasite proliferation and immune
  hypersensitivity
• generally a recrudescence--rarely from primary
  infection
• congenital infection
• 20 exhibit ocular symptoms at birth
• 82 by adolescence
• most lesions are focal and self-limiting
• rapidly destructive in AIDS patients

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Diagnosis

• seldom by direct parasite demonstration
• biopsy
• inoculation into mice or cell culture (only acute
  stage)
• various serological tests
• active (acute) vs chronic infection
• compare samples at 2 week intervals
• IgM gt IgG ? Ab titers

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Prevention
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