Toxoplasmosis - Overview

Toxoplasmosis is a zoonotic protozoan disease characterized by a chronic course, polymorphism of clinical manifestations, and predominant damage to the central nervous system, organs of vision, liver, and lungs.

Toxoplasmosis is a widespread disease caused by the intracellular parasite Toxoplasma gondii. Humans are primarily infected through food contaminated with toxoplasma or through contact with infected cats.

In most cases, toxoplasmosis is asymptomatic. If clinical signs are present, they may be similar to those of mononucleosis or flu-like illnesses. However, even a subclinical infection may subsequently lead to signs of damage to individual organs, such as neurological diseases, pathology of the visual organ, etc.

Toxoplasmosis is an opportunistic AIDS-associated infection. Against the background of immunodeficiency, HIV-infected patients may develop acute encephalitis, which often causes the death of the patient.

Congenital toxoplasmosis is acquired by transplacental transmission of the parasite to the fetus when a woman is primarily infected with toxoplasmosis during pregnancy. This can lead to serious consequences (miscarriage, CNS damage, developmental defects, etc.).

It has been established that severe forms of fetal damage occur during primary infection of a pregnant woman during the period of organogenesis.

According to literature data, in women infected or who had toxoplasmosis before pregnancy, transmission of toxoplasma to the fetus does not occur.

The absence of specific symptoms in toxoplasmosis does not allow diagnosing this disease without laboratory test results.

The guidelines present the main methods of serological diagnostics, the most accessible and used in clinical laboratories, as well as a list of drugs and their administration regimens for the treatment of toxoplasmosis.

Timely implementation of preventive measures, examination of women before a planned pregnancy, and timely initiation of specific treatment can help avoid toxoplasmosis.

ICD-10 codes

• B58. Toxoplasmosis. Includes: infection due to Toxoplasma gondii. Excludes: congenital toxoplasmosis (P37.1).
• B58.0. Toxoplasmic oculopathy.
• B58.1. Toxoplasmic hepatitis (K77.0).
• B58.2. Toxoplasmic meningoencephalitis (G05.2).
• 858.3. Pulmonary toxoplasmosis (J17.3).
• B58.8. Toxoplasmosis with involvement of other organs.
• B58.9. Toxoplasmosis, unspecified.

Epidemiology of toxoplasmosis

Toxoplasmosis is a primary natural focal invasion, i.e. a zoonosis of wild animals (Zasukhin D.N., 1952; Jirovec, 1952). However, toxoplasmosis should currently (from the epidemiological point of view) be considered a zoonosis of agricultural and domestic animals. Humans are usually involved in the circulation of the pathogen in a populated area, i.e. in a sanitary focus of toxoplasmosis. There is reason to believe that the most frequent factor in the transmission of the pathogen to humans is the meat of infected agricultural animals. The geo-oral mechanism of transmission of the invasion to humans is realized, apparently, much less frequently. However, this should not underestimate the epidemiological significance of cats.

It has recently been established that in areas where there are no cats, such as on some Pacific islands, toxoplasmosis is also not found (Wallace et al., 1972). This undoubtedly indicates not only that cats are the most important direct source of human infection, but also that without them, farm animals, from which humans are subsequently infected indirectly, remain uninfected. At the same time, in settlements where there are cats, toxoplasmosis affects a wide variety of groups of residents, including strict vegetarians, who cannot become infected except from a cat. Thus, there is every reason to believe that cats occupy a central place in the epidemiology of toxoplasmosis. If we could prevent the contamination of the external environment with oocysts, the invasion of farm animals would gradually cease, and the synanthropic focus would be doomed to extinction. It is appropriate to note that humans as a source of infection in the epidemiological sense do not play any significant role. Rare, in general, cases of transplacental transmission of the invasion (no more than 1% of all cases of human infection with toxoplasmosis) and, undoubtedly, even rarer cases of infection during obstetric and surgical care (if they occur at all), during blood transfusion, and also during organ transplantation do not change the validity of this assumption.

The incidence of toxoplasmosis, due to the difficulties in recognizing clinically expressed, and especially subclinical toxoplasmosis, remains unknown in all countries, including, for now, in Ukraine.

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Routes of infection with toxoplasmosis

Oral

The main route of infection is through oocysts when consuming vegetables, berries, raw water, contact with cats and soil, and through cysts or tachyzoites when consuming undercooked meat and raw milk.

Contact

Occurs through mucous membranes and damaged skin, and is possible with close contact with sick animals.

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Congenital (transplacental)

Intrauterine infection of the fetus occurs through the placenta. The source of infection is a pregnant woman with a fresh infection, when parasitemia is observed, and the pathogen enters through the placenta, where the primary focus of infection is formed, from where toxoplasmas enter the fetus by hematogenous route.

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Transplantation

Occurs during organ transplantation from a donor with toxoplasmosis. Transmission of the pathogen during blood or leukocyte transfusion has also been proven.

The incidence rate of toxoplasmosis in the population in various countries (France, Holland, USA, etc.) often depends on the degree of consumption of undercooked meat dishes in culinary practice (up to 90% in France, in Holland - 45-80%, USA - 18-20%).

To better understand the epidemiology of infection caused by T. gondii, it is necessary to establish the source of infection, possible routes and factors of infection. These data will allow developing a specific strategy for prevention and, if necessary, treatment of toxoplasmosis in a specific case.

It is quite logical to assume that the differences in the intensity of immunity in the indicated cases are associated, first of all, with the different social status of the examined groups and the different risks of their infection and reinvasion with toxoplasmosis.

What causes toxoplasmosis?

Toxoplasmosis is caused by Toxoplasma gondii (subkingdom Protozoa, phylum Apicomplecxa, order Coccidia, suborder Eimeriina, family Eimeriidae).

In the body of humans and animals, T. gondii goes through several stages of development: trophozoite (endozoite, tachyzoite), cysts (cystozoite, bradyzoite) and oocysts. Trophozoites are 4-7x2-4 µm in size and resemble a crescent in shape. Cysts are covered with a dense membrane, up to 100 µm in size. Oocysts are oval in shape, 10-12 µm in diameter.

According to genotyping data, three groups of toxoplasma strains are distinguished. Representatives of the first group cause congenital toxoplasmosis in animals. Strains of the second and third groups of toxoplasma are detected in humans, and representatives of the last group are more often found in patients with HIV infection. The antigen structure of various stages of toxoplasma development has been determined and it has been established that trophozoites and cysts have both common and specific antigens for each of them.

Pathogenesis of toxoplasmosis

From the site of introduction (most often - the hollow organs of digestion) toxoplasma with the lymph flow enter the regional lymph nodes, where they multiply and cause the development of lymphadenitis. Then the parasites in large quantities enter the blood and are carried throughout the body}, as a result of which foci of damage occur in the nervous system, liver, spleen, lymph nodes, skeletal muscles, myocardium, eyes. Due to the reproduction of trophozoites, infected cells are destroyed. Specific granulomas are formed around the foci of necrosis and accumulation of toxoplasma. With a normal immune response of the body, trophozoites disappear from the tissues and the process of cyst formation begins (the inflammatory reaction around them is weak). The disease toxoplasmosis passes from the acute phase to the chronic, and even more often - to chronic carriage with the preservation of cysts in the tissues of organs.

What are the symptoms of toxoplasmosis?

The incubation period of toxoplasmosis lasts from several weeks to several months. Depending on the mechanism of infection, acquired and congenital toxoplasmosis are distinguished.

Acquired toxoplasmosis usually occurs without symptoms (up to 99% of all cases of T. gondii infection). These are healthy people, usually with a low level of antitoxoplasma antibodies, who do not need either medical supervision or treatment. In people with a normal immune response, toxoplasmosis in most cases occurs in a latent form. Manifest toxoplasmosis occurs in acute (up to 0.01% of infected) and chronic (1-5%) forms, with a tendency to relapse and exacerbation.

How is toxoplasmosis diagnosed?

Toxoplasmosis is diagnosed based on epidemiological risk factors for infection and clinical and laboratory diagnostic data.

Parasitological methods (examination of biopsy specimens of lymph nodes and other organs) are not widely used due to their complexity and labor intensity. Specific antibodies of the IgM and IgG classes to toxoplasma antigens are detected in repeated serological studies: ELISA, RNGA and RIF (but they are not informative enough in AIDS patients): an intradermal test with toxoplasmin (native or recombinant) is performed. When analyzing and interpreting the results of serological diagnostics, it is necessary to take into account the "immunological" incubation - the appearance of antibodies to parasite antigens only after a certain latent period - and evaluate the results of studies in dynamics. The skin test indicates infection with toxoplasma, but does not provide information on the nature of the course of the disease. Pregnant women with positive serological reactions undergo an ultrasound of the fetus in dynamics.

How is toxoplasmosis treated?

Acquired chronic toxoplasmosis with a latent form does not require treatment. The effectiveness of etiotropic drugs for chronic toxoplasmosis is low, since chemotherapy drugs and antibiotics have virtually no effect on endozoites located in tissue cysts. In chronic toxoplasmosis, treatment is indicated only in the case of an exacerbation of the process and in case of miscarriage (treatment is carried out outside the period of pregnancy).

What is the prognosis for toxoplasmosis?

Acquired toxoplasmosis has a favorable prognosis, since the latent form without clinical manifestations predominates. Septic forms observed in AIDS patients and in patients with immunodeficiency of other etiologies are severe and can end in death.