Toxoplasma

• Toxoplasma gondii


  Worldwide distribution, infect humans and many species of animals, disease is manifested in immunocompromised individuals

  
• Definitive hosts


  • Cat family
• Intermediate hosts

  • Mouse, bird, pigs, cows, sheep
• Life Cycle of Toxoplasma gondii

  1. Male microgamete fertilizes female macrogamete producing unsporulated oocyst shed in feces

  2. Unsporulated oocyst undergoes sporulation in the environment

  3. Sporulated oocyst ingested by intermediate host, sporozoites become tachyzoites

  4. Tachyzoites localize in neural and muscle tissue, encyst as bradyzoites

  5. Encysted bradyzoites in muscle ingested by cat, become micro and macrogamete in cat's intestinal cells

  6. Cycle of definitive host continues

  7. Cats directly ingest sporulated oocyst

  
• Oocyst


  Infective form, similar to Isospora but smaller, contain 2 sporocysts each with 4 sporozoites
• Tachyzoites


  Crescent-shaped, 3-7 x 2-4 um, single central nucleus, actively dividing forms
• Bradyzoites


  Slow-growing forms, similar to tachyzoites but smaller, hundreds of thousands enclosed in a cyst 12-100 um in diameter

  
• Infective Stages


  • Sporulated oocyst (muscular)
  • Encysted bradyzoites (intestinal)
  • Motile tachyzoites (urinary/urogenital, blood)

  
• Motile tachyzoites


  1. Enter blood circulation, recognized by immune system and become bradyzoites encysting in organs

  2. When immune system weakens, tachyzoites excyst and cause disseminated toxoplasmosis
• Human Transmission


  • Ingestion of mature oocyst from infected cat feces
  • Ingestion of contaminated undercooked meat with encysted bradyzoites
  • Placental transmission by tachyzoites
  • Blood transfusion by tachyzoites
  • Transplantation by bradyzoites and tachyzoites

  
• Human Infection


  1. Sporulated oocyst contaminates food/water and is ingested

  2. Intermediate host (pigs, sheep) becomes encysted with bradyzoites, human ingests undercooked meat

  3. Blood unit infected with tachyzoites is transfused

  4. Infected mother transmits tachyzoites transplacentally

  5. Infected organ/bone marrow transplant contains bradyzoites or tachyzoites

  
• Pathogenesis in immunocompetent individuals

  • Asymptomatic
• Pathogenesis in immunocompromised patients


  • At greater risk for toxoplasmosis, tissue cysts in skeletal muscle, myocardium, brain, and eyes, acute symptoms of fatigue, lymphadenitis, chills, fever, headache, myalgia, chronic symptoms of encephalomyelitis, myocarditis, hepatitis, retinochoroiditis
• Congenital toxoplasmosis


  • Severe and fatal, TRIAD of hydrocephalus, intracranial calcification, chorioretinitis, also microcephaly, mental retardation, convulsions, psychomotor abnormalities, severe visual impairment or blindness
• Cerebral toxoplasmosis in HIV-AIDS


  • Chronic headache, focal neurologic deficits, seizure, fever, altered mental status, lethargy, multiple ring enhancing lesions on imaging
• Diagnosis


  • Demonstration of tachyzoites in blood or encysted bradyzoites in tissue
  • PCR
  • Serologic testing for IgM, IgG antibodies, Sabin Feldman dye test
• Sabin Feldman dye test

  1. Methylene blue stains Toxoplasma gondii cytoplasm, incubated with patient's serum at 37°C for 1 hour

  2. If patient has antibodies, they lyse the parasite membrane, no stained organism (positive result)

  3. If no antibodies, parasite remains intact, stained organism (negative result)
• Treatment


  • Pyrimethamine plus sulfadiazine with folinic acid
  • For pregnant women: Spiramycin first 4 months, then pyrimethamine plus sulfadiazine with folinic acid
• Prevention and Control


  • Avoidance of contact with cat feces
  • Pregnant women avoid contact with cats
  • Meat and eggs should be well cooked
  • Unpasteurized milk should be avoided
  • Lab workers should be careful handling parasites
  • Vaccination of cats