Life Cycle Toxoplasma Gondii Flashcards
What is the classification of Toxoplasma Gondii?
Obligate intracellular protozoa
How is Toxoplasma Gondii transmitted?
Feces to meat or contaminated water or cat litterbox
What are the steps of the Toxoplasma Gondii lifecycle in the cat?
- Intestinal epithelium of cat: sexually mature merozoites multiply in the epithelium and sexually reproduce to produce sporozoites which are released in membrane enclosed oocysts by the millions. Oocysts are secreted into the lumen of the intestine where they pass in the faecal matter of the cat for about 3 weeks.
- The oocysts are unsporulated when they exit the cat and mature over the course of 1-5 days to become infective and can persist for months in the environment.
What are the lifecycle steps of Toxoplasma Gondii lifecycle in the human (or rodent, bird, pork, sheep) host?
- The oocysts are unsporulated when they exit the cat and mature over the course of 1-5 days to become infective and can persist for months in the environment. When an animal consumes plants or water infected with oocysts the wall of the oocyst is broken down in the stomach by proteases and acid activating the sporozoites that develop rapidly into tachyzoites which are highly mobile and replicative.
- They burrow through the epithelium in the intestines where they replicate within the cell in a parasitophorous vacuole (PV) made from the host membrane until the cell bursts releasing the tachyzoites into the bloodstream and spreading to every organ in the body. This stage is associated with clinical symptoms and normally lasts for about a week. The tachyzoites localize in neural and muscle tissue and develop into tissue cyst bradyzoeites.
- When a cat consumes the flesh of an infected host bradyzoeites pass through the stomach and then reactivate to become merozoites in cats where they sexually produce and complete the life cycle.
In what routes can humans get infected with Toxoplasma Gondii?
- Eating undercooked meat of animals (pork or lamb) harboring tissue cysts
- Consuming food or water contaminated with cat feces or by contaminated environmental samples (such as fecal-contaminated soil or changing the litter box of a pet cat)
- Blood transfusion or organ transplantation
- Transplacentally from mother to fetus during active infection
What happens to the life cycle when Toxoplasma gondii ends up in the human host?
In the human host, the parasites form tissue cysts, most commonly in skeletal muscle, myocardium, brain and eyes; these cysts may remain throughout the life of the host.
What type of life cycle has Toxoplasma Gondii?
Indirect life cycle
What is the definitive, intermediate and/or Accidental host for Toxoplasma Gondii?
Definitive host: Felids or domestic cats
Intermediate host: all warm blooded vertebrae like birds and mammals but mainly rodents and birds
Accidental hosts: Humans & farm animals
What causes the symptoms of Toxoplasma Gondii?
Immunocompetent adults are usually asymptomatic.
But 10-20% can experience mild flu-like symptoms due to the immune response. As immune pressure mounts the dominant phenotype switches to very slowly replicating bradyzoites which form persistent cysts which last the lifetime of the patient. These are unreactive and not associated with symptoms.
In immunocompromised hosts and children with building immune pressure there is not this switch and tachyzoites which enter the central nervous system can cause growing neurotic abscesses which results in neurological damage. Bradyzoite tissue cysts can reactivate into tachyzoites phenotype should an individual become immunosuppressed.
It localizes to the central nervous system because in its major intermediate hosts (birds and rodents) it alters the epigenome of the host after it enters the tissue cyst phase reducing the hosts aversion to felids (cats). This ensures that infected hosts are more likely to become prey animals
Where is Toxoplasma Gondii endemic?
Worldwide but infection rates are highest in areas with hot, humid climates and lower altitudes because oocysts survive better in these types of environments
How is Toxoplasma Gondii diagnosed?
Serology for IgG and IgM antibodies. If both IgG and IgM are positive it means early infection. If IgG is positive but IgM is negative it is a late infection and with IgG negative and IgM positive it’s a negative test.
Additionally microscopy of cyst biopsies and cerebrospinal fluid can be performed but this is not preferred due to it’s invasive nature.
Finally PCR of amniotic fluid is used for in utero diagnosis
