Toxoplasma gondii: Life Cycle, characteristics, morphology

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

Morphology of toxoplasma gondii.

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

Tachyzoites

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

Bradyzoites

• The bradyzoites are formed during the chronicphase of the infection.
• They multiply within tissue cyst, so they arecalled cystozoites.
• They are found in the muscle and various othertissues and organs including the brain.
• Bradyzoites (Gk. Brady, slow) divide slowly andhence the name.
• Their nucleus is situated toward the posteriorend of the organism.

Oocyst.

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

Life cycle of Toxoplasma gondii

• The life cycle of T. gondii consists of twostages asexual and sexual.
• The asexual stage takes place in theintermediate hosts, which are mammals or bird.
• During asexual phase, rapid intracellular growthof the parasite as tachyzoite takes place.
• Tachyzoites can infect and multiply in almostany nucleated mammalian or avian cell.
• Following accumulation (64 – 128), tachyzoitesare secreted into the blood stream, and spread in the body, leading todevelopment of an acute disease (toxoplasmosis).
• The acute phase transforms into chronicinfection as a result of immune response which transforms the trachyzoite intoa cyst enclosing the bradyzoites.
• The cysts are formed mainly in neural andmuscular tissues especially brain, skeletal and cardiac muscles and canpersist, inactivated, in the body for a very long time.
• In the immune-compromised patient, the releaseof bradyzoites from the cyst, may cause acute encephalitis.
• The sexual stage takes place in intestine of thedefinitive host.
• When bradyzoites or oocysts are ingested by afeline, formation of oocysts proceeds in the epithelium of the small intestine.
• Several millions of unsporulated oocysts may bereleased in the faeces of a single cat over a period of 3 – 18days, dependingon the stage of T. gondii ingested.
• Oocysts have been found to be very stable,especially in warm and humid environment, and resistant to many disinfectingagents but survive poorly in dry, cold climates.

Ways of transmission

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

Diagnosis of toxoplasmosis

• Any of the following methods or combination isused;
• 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 bloodyfluids
• Clinical signs and symptoms are observed.
• The most reliable general techniques and the onemost widely used for diagnosis of T. gondii is by finding antibodies in theserum of the host.

Serological methods that have been sodeveloped 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 meatand/or meat product.
• Freezing of meat at -15^oC for 3 daysor at -20^oC for 2 days or heating it at 65^oC for 4 –5minutes to kill the cysts.
• Preparing meat/meat products with salt andnitrates.
• Number of free ranging cats should bedrastically reduced.
• Over familiarity especially by pregnant womenwith cats (as pets) should be avoided since they may be passing oocysts.
• Cats should be tested for antibodies before theyare allowed near the home. Domestic cats should be fed canned meat rather thanraw meat and should not be allowed to hunt mice for food.
• Pregnant women and infants should undergoimmune-diagnostic test to detect the presence of antibodies to toxoplasma.

Treatment of toxoplasmosis

• Most healthy people recover from toxoplasmosiswithout treatment.
• Persons who are ill can be treated with acombination of drugs such as pyrimethamine and sulfadiazine, plus folinic acid.