Toxoplasma gondii: Properties, Life Cycle, and Diagnosis

By Acharya Tankeshwar •  Updated: 02/22/22 •  8 min read

Toxoplasma gondii is a protozoan parasite of many vertebrates including humans and causes the disease Toxoplasmosis. Its name was derived from the crescent shape of the tachyzoite stage of the parasite (taxon- ‘bow’; plasma- ‘form’). The parasite was first discovered in 1908, by Charles Nicolle and Louis Manceaux at the Pasteur Institute, in the North African rodent called the gundi, hence the species name gondii.

If a pregnant mother has a history of stillbirth or miscarriage she might be tested with ‘TORCH’ panel test. TORCH is an acronym representing congenital infections caused by Toxoplasma gondii, other agents, rubella, cytomegalovirus (CMV), and herpes simplex virus (HSV).

Properties

Source of infection

Humans can become infected with Toxoplasma gondii via any of several routes:

Morphological Forms

Toxoplasma gondii exists in three forms. All of these stages are infectious.

Oocyst

Tachyzoites

Bradyzoites

Life Cycle

An infected cat shed unsporulated oocysts in its feces for 1-2 weeks. After 1-5 days oocysts sporulate in the environment and become infective. Intermediate hosts in nature (including birds, rodents, and even humans) become infected after ingesting soil, water, or plant material contaminated with sporulated oocysts. Humans are also infected by the ingestion of raw meats, particularly pork, lamb, or venison.

Oocysts transform into tachyzoites shortly after ingestion. These tachyzoites localize in the neural and muscle tissue of intermediate hosts and develop into tissue cyst bradyzoites.

Life Cycle of Toxoplasma gondii (image source: CDC)

Cats become infected after

In cats, some merozoites are transformed into the sexual stages, initiating gametogony. After sexual fusion of micro and macrogametes, oocysts develop, exit from the host cell into the gut lumen, and pass out via feces.

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.

Signs and symptoms

Tachyzoites of the parasite Toxoplasma gondii may be found in circulating blood. They invade cells within lymph nodes and other organs, including the lungs, liver, heart, brain, and eyes. The resulting cellular destruction accounts for the manifestation of toxoplasmosis.

Immunocompetent Patients

Usually asymptomatic or very mild infection in approximately 90% of serologically positive individuals.

Common symptoms

Immunocompromised Patients

Infection in immunocompromised patients can lead to severe complications depending on the presence of underlying diseases such as malignancies, AIDS, and organ transplantation. In immunocompromised patients, the central nervous system (CNS) is primarily involved

Important symptoms

Congenital Infection

If a mother acquired infection before becoming pregnant unborn child is protected by the mother’s immunity. Mother’s blood contains IgG antibodies against T. gondii which can cross the placenta and reach the fetus.

If a mother is primarily infected with Toxoplasma during pregnancy or just before pregnancy, she can pass the infection on to the fetus. Congenital infections may be particularly severe if the mother acquires the infection during the first or second trimester of pregnancy. The mother may not have any symptoms from the infection or mild morbidity (flu-like illness) but the infant may develop serious symptoms later in life, such as blindness or mental disability.

A girl with hydrocephalus due to congenital Toxoplasma infection
A girl with hydrocephalus due to congenital Toxoplasma infection

Time determines the fate

Laboratory Diagnosis

Diagnosis of toxoplasmosis is usually achieved by serology. Test for the presence of specific IgG or IgM is used to determine if a person has an acute infection with Toxoplasma or immunity due to prior infection. Other procedures include performing PCR, examining biopsy specimens, buffy coat cells or CSF fluid, and isolating the organism in tissue culture or in laboratory animals.

Laboratory diagnosis methods

  1. Seroloy
  2. Giemsa stain and examining biopsy specimens, buffy coat cells, or cerebrospinal fluid
  3. Isolating the organism in tissue culture or in laboratory animals.
  4. Molecular methods: Performing PCR

Serology

Toxoplasmosis: risk management in pregnancy
Toxoplasmosis: risk management in pregnancy
(Chart source: European Journal of Obstetrics and Gynecology and Reproductive Biology)

Sabin-Feldman dye test

Microscopy

Culture

T. gondii can be isolated by intraperitoneal inoculation of body fluid or tissue in infection-free laboratory mice. Peritoneal fluid and spleen smears may show the tachyzoites after 7-10 days.

Molecular methods

Detection of a specific region of DNA of Toxoplasma in the sample (eg. Blood, CSF, etc). Prenatal diagnosis of congenital toxoplasmosis from amniotic fluid.

References and further readings

Acharya Tankeshwar

Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please email at microbeonline@gmail.com

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13 responses to “Entamoeba histolytica: Life Cycle, Diseases, Lab Diagnosis”

  1. chile faya says:

    Hello Sir, first of all i am like to congrate you with a big task of educating us. What i like to know Sir, i like to know well on Vibrio Cholera, how to make culture, how to read the culture, how to identify from the plate. All in all i like to know the all process/procedure what to do after taking rectal swab

  2. Anonymous says:

    Great job sir!!

  3. Faryal says:

    Hello sir…..I want to thank you for doing such good work…..I learn alott from here…..sir I want to know about different techniques of striking and how can we apply them on a media….

  4. fareeza says:

    I need a brief explanation about enatamoeba histolytica morphology

  5. Heeren Roy says:

    Symptoms of extra intestial disease

  6. ankeetkumar says:

    Hello sir thank you so much for making this topic so easy I want to know how this parasite basically it’s a protozoan (unicellular organism) . So how this entamoeba histolytica it’s able to ingest red blood cells?

  7. satyanarayana numarsu says:

    what are charcot Leyden crystals?

  8. peter says:

    thanks for clarifying the cycle of entamoeba

  9. suvarna says:

    Sir the given information is very good
    I want this type of information (Lab Diagnosis) of following
    1 Pulmonary tuberculosis
    Enteric Fever
    Syphilis
    Leptospirosis
    Culture of virus
    HIV
    Ascaris
    Hepatitis B virus

    • Acharya Tankeshwar says:

      Dear Suvarna
      We have already updated information about the queries you have. Please check relevant sections or use the site search to get information.

  10. Emmanuel says:

    Thank you sir for providing such a platform for people to learn.
    Sir I want to know why the mode of transmission of hookworm is described as active, while that of Ascaris lumbricoides is passive.

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