Morphology and life cycle forms
The causative agent has a complex life cycle involving several morphological forms:
- Oocysts. Formed in the intestine of definitive hosts (feline family) as a result of sexual reproduction. These are oval structures (10–12 µm) containing two sporocysts, each with four sporozoites. They become infectious in the external environment within 3 days and can survive for up to a year.
- Tachyzoites. Rapidly dividing forms characteristic of the acute stage of infection. Externally, they resemble an orange wedge or a crescent, measuring 4–7 µm. When stained with Romanowsky–Giemsa, their cytoplasm turns blue, and the nucleus becomes ruby red.
- Pseudocysts. Intracellular clusters of tachyzoites (endozoites), for instance, within macrophages. They lack a true dense wall and are surrounded by the host cell vacuole membrane. A single cell may contain from 10 to 30 individuals.
- Cysts. Formed during the transition to the chronic stage. They have a dense wall enclosing hundreds of slowly dividing parasites (bradyzoites). Cysts can reach up to 1000 µm in size and persist in body tissues for decades.
Epidemiology and transmission routes
The definitive host of Toxoplasma gondii is members of the feline family—they are the ones that shed oocysts into the external environment. Intermediate hosts can include humans, as well as many species of wild and domestic birds and mammals.
Main routes of human infection:
- Alimentary (main). Eating undercooked meat containing tissue cysts and pseudocysts, or swallowing oocysts with unwashed vegetables, fruits, and water.
- Contact. The pathogen is capable of penetrating through damaged skin and mucous membranes.
- Vertical (transplacental). Transmission from mother to fetus during primary infection in pregnancy.
- Iatrogenic. Rare cases of transmission via blood transfusion or organ transplantation.
Pathogenesis and clinical presentation
Upon entering the human intestine, sporozoites emerge from the oocysts, penetrate the lymphatic vessels, and reach regional lymph nodes, where they actively multiply (tachyzoite stage). Subsequently, the pathogen breaks into the bloodstream and is disseminated throughout the body, invading cells of the reticuloendothelial system in various organs (nervous tissue, liver, eyes, heart).
In approximately 90% of cases, acquired toxoplasmosis runs asymptomatically. In manifest cases (about 10%), the disease presents nonspecifically: enlargement of occipital lymph nodes, myalgia, fever, rash, and hepatosplenomegaly are characteristic. Severe complications are possible: myocarditis, pneumonia, meningoencephalitis.
Congenital toxoplasmosis
An extremely dangerous form that occurs during primary infection of a woman while carrying a child (the risk is about 3 cases per 1,000 pregnant women). The most severe consequences are observed when infection occurs in the first trimester.
The infection exhibits a marked teratogenic effect. It can lead to fetal death, spontaneous abortion, or stillbirth. Surviving newborns are frequently diagnosed with severe damage to the central nervous system, liver, and spleen, along with profound intoxication, resulting in high perinatal mortality or progression of the infection into a severe chronic form.
Diagnostics and treatment
Immunity in toxoplasmosis is non-sterile, with the development of cellular and humoral responses, as well as delayed-type hypersensitivity.
Algorithm for laboratory diagnosis:
- Serological method (main). Determination of IgM antibodies (acute phase) and IgG. A specific test is the Sabin–Feldman dye test: under the action of antibodies in the patient's serum, the cytoplasm of toxoplasmas changes its staining with methylene blue.
- Microscopy. Examination of blood smears, cerebrospinal fluid, or punctates stained with Romanowsky–Giemsa.
- PCR. Of key importance for detecting congenital toxoplasmosis and severe forms in HIV patients.
- Allergic skin test. Intradermal test with toxoplasmin (positive from the 4th week).
The baseline treatment regimen includes a combination of pyrimethamine and sulfonamides. However, pyrimethamine is toxic to pregnant women and is therefore replaced with spiramycin, which does not cross the placental barrier and is safe for the fetus.
Briefly: answers to topical questions
What is the mechanism of the Sabin–Feldman reaction in the diagnosis of toxoplasmosis?
The Sabin–Feldman reaction (dye test) is based on the alteration of toxoplasma cytoplasm staining with methylene blue under the influence of serum antibodies.
Why is the avidity index of IgG antibodies determined in the serological diagnosis of toxoplasmosis in pregnant women?
The avidity index of IgG antibodies is determined to detect recent primary infection. Low IgG avidity indicates a recent primary infection. This is important during pregnancy because congenital toxoplasmosis occurs when a woman is primarily infected during pregnancy, which is especially dangerous for the fetus.
To remember the Romanowsky–Giemsa staining of tachyzoites, imagine a "blue sky over a ruby crescent"—blue cytoplasm, ruby-red nucleus, and the crescent shape of the parasite.
Class and exam questions
Primary infection during pregnancy (especially in the 1st trimester) causes a teratogenic effect, leading to fetal developmental defects, miscarriages, or severe CNS damage in the newborn.
Primary infection during pregnancy (especially in the 1st trimester) has a teratogenic effect by crossing the placenta. This can lead to severe developmental defects, fetal death, or the birth of a child with lesions of the central nervous system and internal organs.
How do humans most commonly contract Toxoplasma?
The primary route is alimentary. Infection occurs by consuming undercooked meat containing tissue cysts, or by ingesting oocysts with unwashed vegetables, fruits, or unwashed hands after contact with cat feces.
Which stage of Toxoplasma serves as a marker of chronic infection?
The marker of chronic infection is tissue cysts, which have their own dense wall and can persist in the body for decades.
What else to study on this topic
- Antigenic structure of the pathogen: markers of tachyzoites (SAG1, SAG2) and bradyzoites (SAG4, BSR4)
- Principles and mechanism of the Sabin–Feldman dye test
- Features of cellular and humoral immunity in toxoplasmosis
- Cultivation of Toxoplasma gondii in chicken embryos and cell cultures
- Differential diagnosis of congenital toxoplasmosis
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