Toxoplasma gondii
Disease Agent:
- Toxoplasma gondii
Characteristics:
- Protozoa, 2.5 ¥ 5.0 mm
- Order: Eucoccidiorida
- Family: Sarcosystidae
- Humans harbor only asexual, replicating stages including tachyzoites, which can occur in blood cells and bradyzoites in tissues.
- Obligate intracellular parasite
Three forms
Trophozoites
- It can invade any nucleated cell (i.e. not RBQ and replicate by endodyogeny or internal budding.
- This rapidly multiplying trophozoite is known as tachyzoites.
- It can be seen extracellularly also.
- Crescentic parasites distend the cells which are called as pseudocyst or pseudo colony.
- It is differentiating from true tissue cyst by its staining property.
- Stained by Giemsa.
- It is non-infectious.
- It is formed during the acute phase.
Tissue cyst
- It is formed during chronic phase in various organs
- Persists principally in central nervous system and muscles.
- Cyst contains slowly multiplying rounded parasite called Bradyzoites.
- It is stained by silver stains.
Oocyst
- It develops only in intestine of definitive host.
- It contains two sporocysts with sporozoites inside.
Life Cycle
- Cats play a vital role in T. gondii’s life cycle and the spread of toxoplasmosis to people.
- A cat becomes infected with T. gondii after eating birds or small mammals infected with the parasite.
- The parasites then reproduce in the cat’s small intestine and form oocysts, which are thick-walled cysts containing zygotes.
- The cat eventually “sheds” the oocysts in its faeces.
- Most cats shed oocysts only once in their lifetime, but they do so continually for 1 to 2 weeks.
- Within 5 days, the oocysts undergo a process called sporulation, in which they become infectious.
- Other animals become infected when they ingest soil, water, or plants contaminated with oocysts.
- Human, sheep, pig etc intermediate host.
- Shortly after being ingested, the parasites develop into tissue cysts and remain in their intermediate host for the rest of the animal’s life.
Transmission
- The freshly passed oocyst is not infectious.
- It becomes infectious only after development in soil or water for few days–
- Infective forms
- Sporulated oocyst from contaminated soil with cat faeces.
- Tissue cyst containing bradyzoites in undercooked pork, beef or meat.
Transmission Routes
- Ingestion of sporulated oocyst (Sporocyst) or tissue cyst containing bradyzoites (most common) oral route.
- Blood transfusion.
- Kidney or heart transplant.
- Transplacental transmission.
First trimester-
- The incidence of transplacental infection is lowest (15%
- But the disease in the neonate is most severe.
Third trimester,
- Incidence of transplacental infection is greatest (65%)
- But the infant is usually asymptomatic at birth.
Exam Question
- The parasites reproduce in the cat’s small intestine and form oocysts, which are thick-walled cysts containing zygotes.
- Human, sheep, pig etc intermediate host.
Transmission
- The freshly passed oocyst is not infectious.
- It becomes infectious only after development in soil or water for few days-
- Infective forms
- Sporulated oocyst from contaminated soil with cat faeces.
- Tissue cyst containing bradyzoites in undercooked pork, beef or meat.
- Transmission Routes
- Ingestion of sporulated oocyst (Sporocyst) or tissue cyst containing bradyzoites (most common) oral route.
- Blood transfusion.
- Kidney or heart transplant.
- Transplacental transmission.
First trimester-
- The incidence of transplacental infection is lowest (15%
- But the disease in the neonate is most severe.
Third trimester,
- Incidence of transplacental infection is greatest (65%)
- But the infant is usually asymptomatic at birth.
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