Toxoplasma gondii: Life Cycle, characteristics, morphology

  • It is an obligate, intracellular, parasitic protozoan which causes toxoplamosis.
  • It’s known to infect about 200 species of mammals and birds.
  • There are usually two types of hosts; the definitive host and the intermediate host.
  • Sexual development occurs in the definitive host while asexual development occurs in the immediate host.
  • The definitive hosts of T. gondii include domestic cats and other felids while the intermediate hosts are birds and mammals including humans.
  • Prevalence of infection is higher in warm moist areas than in areas that are cold and dry.
  • In humans, T. gondii has its greatest effect on the developing foetus.
  • If a woman becomes infected during pregnancy, there is 40% chance that the foetus will be infected via congenital transmission.
  • The disease could result in: abortion, stillbirth, neonatal death, choroiretinitis, hydrocephaly, microcephaly and cerebral calcification.

Morphology of toxoplasma gondii.

  • It occurs in three forms:
  • Trophozoite (also called tachyzoites or endodyzoites or endozoites)
  • Bradyzoites within tissue cyst (also called cystozoites
  • Oocyst.
  • Tachyzoites and Bradyzoites are asexual stages and multiply by endodyogeny.
  • Oocyst is the sexual stage formed by gametogony or sporogony.
  • The three forms occur in the domestic cat and other felines.
  • All the stages are infective by ingestion.

Tachyzoites

  • The tachyzoite is crescent shaped with one end pointed and the other rounded.
  • It is measured approximately 2-3µm by 4-7µm.
  • The nucleus is ovoid and centrally located.
  • Tachyzoite (GK. Tachos, speed) is the most rapidly multiplying stage in any cell of the intermediate hosts and in non-intestinal epithelial cells of the definitive host.
  • Aggregates of numerous tachyzoites are called clones, terminal colonies, or groups.

Bradyzoites

  • The bradyzoites are formed during the chronic phase of the infection.
  • They multiply within tissue cyst, so they are called cystozoites.
  • They are found in the muscle and various other tissues and organs including the brain.
  • Bradyzoites (Gk. Brady, slow) divide slowly and hence the name.
  • Their nucleus is situated toward the posterior end of the organism.

Oocyst.

  • Oocyst is a zygote surrounded by extremely resistant wall.
  • It is spherical or ovoid, about 10 – 12µm in size and contains a sporoblast.
  • Cats shed millions of oocysts per day in faeces for about two weeks during the primary infection.
  • The freshly passed oocyst is not infectious.
  • It becomes infectious only after the development in soil or in water for 1-5 days depending on aeration and temperature.
  • During this state of sporulation, the sporoblast divides into two sporocysts, each having four sporozoites.
  • Oocyst is very resistant to environmental conditions and can remain infective in soil for about a year.
  • When the infective oocyst is ingested, it releases sporozoites in the intestine, which initiate infection.

Life cycle of Toxoplasma gondii

  • The life cycle of T. gondii consists of two stages asexual and sexual.
  • The asexual stage takes place in the intermediate hosts, which are mammals or bird.
  • During asexual phase, rapid intracellular growth of the parasite as tachyzoite takes place.
  • Tachyzoites can infect and multiply in almost any nucleated mammalian or avian cell.
  • Following accumulation (64 – 128), tachyzoites are secreted into the blood stream, and spread in the body, leading to development of an acute disease (toxoplasmosis).
  • The acute phase transforms into chronic infection as a result of immune response which transforms the trachyzoite into a cyst enclosing the bradyzoites.
  • The cysts are formed mainly in neural and muscular tissues especially brain, skeletal and cardiac muscles and can persist, inactivated, in the body for a very long time.
  • In the immune-compromised patient, the release of bradyzoites from the cyst, may cause acute encephalitis.
  • The sexual stage takes place in intestine of the definitive host.
  • When bradyzoites or oocysts are ingested by a feline, formation of oocysts proceeds in the epithelium of the small intestine.
  • Several millions of unsporulated oocysts may be released in the faeces of a single cat over a period of 3 – 18days, depending on the stage of T. gondii ingested.
  • Oocysts have been found to be very stable, especially in warm and humid environment, and resistant to many disinfecting agents but survive poorly in dry, cold climates.

Ways of transmission

  • Consuming raw or undercooked meat containing T. gondii tissue cysts.
  • Ingesting anything contaminated with oocysts shed in the faeces of an infected animal.
  • Blood transfusion or organ transplant.
  • Transplancental transmission.

Diagnosis of toxoplasmosis

  • Any of the following methods or combination is used;
  • Oocysts are looked for in faeces
  • Antibodies are looked for in serum
  • Tissue cysts are examined in the tissues
  • Tachyzoites are isolated from blood or bloody fluids
  • Clinical signs and symptoms are observed.
  • The most reliable general techniques and the one most widely used for diagnosis of T. gondii is by finding antibodies in the serum of the host.

Serological methods that have been so developed include:

  • Sabin Feldman dye test
  • Complement fixation test
  • Haemagglutination test
  • Intradermal skin test
  • Inflorescence antibody test.

Prevention and control of toxoplasmosis

  • Avoidance of eating raw or undercooked meat and/or meat product.
  • Freezing of meat at -15oC for 3 days or at -20oC for 2 days or heating it at 65oC for 4 – 5minutes to kill the cysts.
  • Preparing meat/meat products with salt and nitrates.
  • Number of free ranging cats should be drastically reduced.
  • Over familiarity especially by pregnant women with cats (as pets) should be avoided since they may be passing oocysts.
  • Cats should be tested for antibodies before they are allowed near the home. Domestic cats should be fed canned meat rather than raw meat and should not be allowed to hunt mice for food.
  • Pregnant women and infants should undergo immune-diagnostic test to detect the presence of antibodies to toxoplasma.

Treatment of toxoplasmosis

  • Most healthy people recover from toxoplasmosis without treatment.
  • Persons who are ill can be treated with a combination of drugs such as pyrimethamine and sulfadiazine, plus folinic acid.