Toxoplasma Gondii Life Cycle

Toxoplasmosis

• Caused by the parasite Toxoplasma gondii, which is transmitted through carnivorism
• The life cycle of Toxoplasma gondii involves:
  • Ingestion of tissue cysts or oocysts by a cat
  • Release of viable organisms that invade epithelial cells of the small intestine
  • Asexual and sexual cycles in the small intestine, followed by formation of oocysts
  • Excretion of oocysts, which take 1-5 days to sporulate and become infective
• In humans, the parasites form:
  • Tissue cysts (bradyzoites) in skeletal muscle, myocardium, and brain
  • Blood tachyzoites that can infect the fetus through the bloodstream

Infection and Symptoms

• Asymptomatic in normal individuals
• Congenital toxoplasmosis can result in:
  • Abortion
  • Serious brain and eye damage to the fetus
  • Retino-chorditis or mental retardation in childhood
• In immunocompromised individuals, infection can result in:
  • Generalized parasitemia
  • Involvement of brain, liver, lung, and other organs
  • Often death
• Acute infection may produce flu-like symptoms, sometimes associated with lymphadenopathy

Diagnosis and Treatment

• Suspected toxoplasmosis can be confirmed by:
  • Isolation of the organism from blood
  • Serological testing
  • PCR
• Treatment involves:
  • Pyrimethamine in combination with sulfadiazine
  • No treatment is needed for healthy individuals

Trypanosomiasis

• Caused by the parasite Trypanosoma brucei
• The life cycle of Trypanosoma brucei involves:
  • Ingestion of infected blood by a tsetse fly
  • Transformation into procyclic trypomastigotes, which multiply by binary fission
  • Transformation into epimastigotes, which reach the fly's salivary glands and multiply by binary fission
  • Transformation into metacyclic trypomastigotes
• The cycle in the fly takes approximately 3 weeks
• Humans are the main reservoir for Trypanosoma brucei gambiense, but it can also be found in animals
• Wild animals are the main reservoir of T.b. rhodesiense

Stages of Infection

• Bite reaction: A painful, itchy chancre forms within 1-3 weeks after the bite and lasts 1-2 weeks, leaving no scar
• Hemolymphatic stage: Marked by fever, malaise, insomnia, headache, and lymphadenopathy
• CNS stage: Marked by lack of interest, mental retardation, and uncontrollable urge to sleep, leading to death from coma or cardiac failure

Diagnosis and Treatment

• Detection of parasite (trypomastigotes) in the bloodstream, lymph secretions, and enlarged lymph node aspirate provides a definitive diagnosis in early stages
• Cerebrospinal fluid must always be examined for organisms
• Immuno-serology (enzyme-linked immune assay, immunofluorescence) may be indicative but does not provide definite diagnosis
• The blood stage of African trypanosomiasis can be treated with reasonable success with Pentamidine isethionate or Suramin

Malaria

• Caused by four Plasmodium species: P.falciparum, P.vivax, P.ovale, and P.malariae
• Vector: Anopheles mosquito
• Life cycle:
  • Sporozoites infect liver cells and mature into schizonts
  • Schizonts rupture and release merozoites
  • Merozoites infect red cells and mature into trophozoites
  • Trophozoites undergo schizogony and merogony in red cells, which ultimately burst and release daughter merozoites
  • Some merozoites transform into male and female gametocytes
  • Gametocytes are ingested by an Anopheles mosquito during a blood meal
  • The parasites multiply in the mosquito, forming oocysts, which rupture and release sporozoites

Symptoms and Diagnosis

• Fever is the most constant symptom of malaria, occurring in paroxysms when lysis of red cells releases merozoites
• Typical paroxysms occur every 2nd day or more frequently in P.falciparum, every 3rd day in P.vivax and P.ovale, and every 4th day in P.malariae
• Other symptoms include:
  • Vomiting
  • Diarrhea
  • Convulsions
  • Pallor, resulting from the lysis of red blood cells and reduced synthesis of red blood cells in the bone marrow