Toxoplasmosis - Causes and Pathogenesis

Causes of toxoplasmosis

The cause of toxoplasmosis is Toxoplasma gondii (subkingdom Protozoa, type 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.

T. gondii is an obligate intracellular parasite that penetrates the intestinal epithelial cells and reproduces in them by endodyogeny. Then, trophozoites (tachyzoites) enter other organs and tissues (lymph nodes, liver, lungs, etc.) with the blood and lymph flow, where they actively penetrate cells. In the affected cells, clusters of endozoites of one generation appear, surrounded by a membrane of a parasitophorous vacuole (the so-called pseudocysts). As a result of the host's immune response, the parasites disappear from the blood and cysts covered with a dense membrane are formed in the infected target cells. In chronic cases of the disease, T. gondii in the form of intracellular cysts remain viable indefinitely. Cysts are localized mainly in the brain, cardiac and skeletal muscles, uterus, and eyes.

The main hosts of T. gondii are representatives of the Felidae family (cats) and can simultaneously be intermediate hosts, since in their bodies toxoplasmas are able to move from the intestine to the cells of various organs. By means of merogony, the parasite reproduces in the epithelial cells of the intestine; as a result, merozoites are formed. Some of them give rise to male and female reproductive cells - gamonts. After leaving the enterocytes, male gamonts divide repeatedly, forming microgametes ("spermatozoa"); macrogametes ("eggs") are formed from female gamonts. After fertilization, an immature oocyst is formed, which is excreted into the environment with feces. Under favorable conditions, the maturation of oocysts (sporogony) lasts from 2 days to 3 weeks. Mature cysts are resistant to adverse environmental factors and can remain viable for up to a year or longer.

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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 lesions 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). 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. In unfavorable conditions for the body (acute diseases and stressful situations that have an immunosuppressive effect), the cyst membranes are destroyed; the released parasites, multiplying, affect intact cells and enter the bloodstream, which is clinically manifested by an exacerbation of chronic toxoplasmosis. Inflammatory infiltrates and necrosis are found in skeletal muscles, myocardium, lungs and other organs. Inflammatory foci with subsequent necrosis appear in the brain, which sometimes leads to the formation of petrifications. Productive necrotic inflammation occurs in the retina and choroid. Toxoplasmosis acquires a malignant course against the background of a detailed picture of AIDS, while a generalized form of the disease develops, in some cases causing the death of patients.

In response to toxoplasma antigens, specific antibodies are produced and an immune reaction similar to DTH develops.

In congenital toxoplasmosis, as a result of parasitemia, the pathogen is introduced into the placenta, forming a primary focus, and from there it enters the fetus with the bloodstream. The fetus becomes infected regardless of the presence of clinical manifestations in the pregnant woman, but the outcome depends on the stage of pregnancy at which the infection occurred. Infection in the early stages of embryogenesis ends in spontaneous miscarriage, stillbirth, causes severe, often incompatible with life, developmental disorders (anencephaly, anophthalmia, etc.) or leads to the development of generalized toxoplasmosis. When infected in the third trimester of pregnancy, asymptomatic forms of the course predominate, the late clinical signs of which appear months and years later.

Life cycle of toxoplasma

The causative agent of toxoplasmosis is an obligate intracellular parasite; the possibility of intranuclear parasitism of toxoplasma has been proven. The causative agent was discovered in 1908 independently of each other by the Frenchmen Nicoll and Manso in Tunisia in gondi rodents and by the Italian Splendore in Brazil in rabbits. The generic designation of toxoplasma reflects the crescent shape of the asexual stage of the parasite ("taxon" - arc, "plasma" - form), the species designation - the name of the rodents (gondi).

From a general biological point of view, T. gondii is characterized by features that allow it to be considered a parasite with very deep adaptations. It is found on all continents and at all geographic latitudes, can parasitize and reproduce in hundreds of species of mammals and birds, and is capable of affecting a wide variety of tissues and cells of its hosts.

In 1965, Hutchison experimentally demonstrated for the first time that cats participate in the transmission of T. gondii. In 1970, scientists from England, Denmark and the USA almost simultaneously and independently discovered oocysts in the feces of cats infected with toxoplasmosis, very similar to those of coccidia. This proved that toxoplasma belongs to coccidia, and soon the life cycle of the parasite was completely deciphered, consisting of two phases: intestinal and extraintestinal, or non-tissue.

The intestinal phase of the life cycle of Toxoplasma involves development in the cells of the intestinal mucosa of the definitive host, which are the domestic cat and other felines (wildcat, lynx, Bengal tiger, ocelot, snow leopard, jaguarundi, eir).

The complete development cycle (from oocyst to oocyst) of T. gondii can be carried out only in the body of representatives of the cat family. The life cycle of toxoplasma includes 4 main stages of development: schizogony, endodyogeny (internal budding), gametogony, sporogony. These stages occur in different ecological environments: schizogony, gametogony and the beginning of sporogony occur only in the intestines of representatives of the cat family (the final hosts of toxoplasma), sporogony is completed in the external environment, and endodyogeny occurs in the cells of the tissues of the intermediate host (including humans) and in the cells of the main host - cats.

Before we move on to a detailed examination of the life cycle of toxoplasma, it is necessary to touch upon the issue of terminology of the parasite stages. Given that the development cycle of toxoplasma was deciphered only in 1970, and many details are still unclear, issues of toxoplasma terminology are in the process of clarification, and different authors offer their own terms for the same stages of the parasite.

Thus, to designate the tissue (extraintestinal phase of toxoplasmosis development, asexual stage - endodiogeny) in case of acute invasion, the following terms are used: "proliferative form", "endodizoite", "endozoite", "trophozoite", "tachyozoite", and the stage characteristic of the chronic course of invasion is designated by the terms - "cystic form", "zoite", "cystozoite" and "bradyzoite". At the current level of knowledge about the life cycle of toxoplasma, according to the majority of domestic studies, the most acceptable terms are: endozoite - asexual tissue stage of toxoplasma, usually rapidly multiplying, localized in toxoplasma or in cell vacuoles, characteristic of acute infection; the onset of cystozoite - tissue forms localized inside the cyst and characteristic of the chronic course of infection.

All other terms used to designate tissue stages of the Toxoplasma life cycle should be considered synonymous with "endozoite" and "cystozoite".

The terminology of the stages of development of Toxoplasma in the intestinal epithelium of the main host is similar to that of typical coccidia.

Intestinal phase of toxoplasma development

Intestinal phase of toxoplasma development in the body of the final host. The intestinal stage of development begins with infection (orally) of felines - the main hosts of the parasite with both oocysts with sporozoites and vegetative forms - endozoites and cystozoites, swallowed with the tissues of intermediate hosts. Cystozoites enter the intestine in tissue cysts, the membrane of which is quickly destroyed by proteolytic enzymes. Endozoites and cystozoites freed from the membrane penetrate the cells of the intestinal mucosa and multiply intensively by asexual reproduction (endodyogeny and schizogony).

After approximately 2 days, as a result of repeated cycles of asexual reproduction (schizogony), a special type of schizonts is formed - merozoites, which give rise to the next stage of the parasite's development - gametogony.

When mature toxoplasma oocysts freed from their membranes enter the cat's intestine, the sporozoites penetrate the cells of the intestinal ciliated epithelium and also begin to reproduce by schizogony. As a result of asexual reproduction, 4 to 30 merozoites are formed from one schizont. Submicroscopic studies have shown that the schizont is surrounded by a pellicle, which consists of an inner and outer membrane. One or more mitochondria, a ribosome, a nucleus, a well-developed endoplasmic reticulum, and a conoid at the anterior end are found. Subpellicular tubules are absent.

Unlike coccidia, during Toxoplasma schizogony, merozoites are formed near the nucleus, not on the periphery of the schizont. In the intestine of cats, Toxoplasma undergoes several successive schizogony, after which the merozoites give rise to the sexual stage of the parasite's development (gametogony). Gametocytes (immature sex cells) are found approximately 3-15 days after infection throughout the small intestine, but most often in the ileum of the cat. Gametogony begins with the formation of microgametocytes, which occurs in the lower part of the small intestine and in the large intestine of the main host. The development of microgametocytes is accompanied by a series of successive divisions of the egg. Along the periphery of the macrogametocyte, 12-32 microgametes are formed by exvagination of its membrane. They have the shape of a strongly elongated crescent with sharp ends and, together with the flagella, reach 3 µm in length, and also have 2 flagella (the third is rudimentary), with the help of which they move in the lumen of the intestine and move to the macrogamete.

The development of the macrogametocyte occurs without division of the nucleus. In this case, the gametocyte increases in size (from 5-7 to 10-12 µm in length), the large nucleus with the nucleolus becomes compact, a large amount of glycogen accumulates in the cytoplasm, many ribosomes, mitochondria and endoplasmic reticulum are found.

Fertilization, i.e. the fusion of macro- and microgametes, occurs in an epithelial cell, resulting in the formation of a zygote, which forms a dense membrane and turns into an ookinete, and then into an oocyst. The shape of the oocysts is round-oval with a diameter of 9-11 to 10-14 μm. For some time, the oocysts remain in the epithelial cells, but then fall into the lumen of the intestine, and toxoplasma enters the next stage of development - sporogony, which continues in feces and in the external environment. Mature oocysts have a dense colorless two-layer membrane, due to which they are resistant to the effects of various environmental factors, including a number of chemical agents. With sufficient humidity, temperature and oxygen access, after a few days, two sporocysts with four banana-shaped sporozoites in each are formed inside the oocyst. Sporocysts, in turn, have a dense two-layer membrane. Their sizes are on average from 6-7 x 4-5 to 8 x 6 µm. Sporozoites are similar in structure to endozoites and cystozoites - tissue stages of toxoplasma. Mature oocysts with sporozoites are invasive stages of the parasite for both the final host (cats) and intermediate hosts, including humans. In humid conditions, sporozoites in oocysts remain invasive for up to 2 years.

Extraintestinal (tissue) phase of toxoplasma development in the body of intermediate hosts

In the cells of various tissues of intermediate hosts, including humans, asexual reproduction occurs by endodyogeny, i.e. the formation of two daughter cells inside the mother cell. In 1969-1970, a method of multiple internal budding was discovered, for which the term endopolygeny was proposed. These two methods of asexual reproduction, along with schizogony, were also discovered in the intestines of the parasite's main host - the cat.

The tissue phase of toxoplasma development begins when either the sexual stages of the parasite - oocysts with sporozoons, or asexual stages (endozoites and cystozoites) with tissues of invaded animals enter the intestines of animals and humans (intermediate hosts). In the small intestine, under the influence of proteolytic enzymes, sporozoites released from oocysts, or cystozoites or endozoites from cysts penetrate the epithelial cells of the intestinal mucosa, where asexual reproduction - endodyogeny and endopolygeny - begins.

Endozoites appear as a result of reproduction. Within 2-10 hours from the moment of introduction of the sporozoite (endozoite) into the cell, 12-24-32 daughter endozoites emerge from the destroyed host cell. The newly formed endozoites actively penetrate into neighboring cells. Local necrotic foci are formed in the small intestine of the host, from where endozoites can enter the blood and lymphatic vessels and then into various tissues. Dissemination of endozoites throughout the body of the intermediate host is also facilitated by phagocytosis of the parasite by cells of the reticuloendothelial system. At this stage, rapid asexual reproduction by endodyogeny is repeated cyclically. Endozoites are outside the cell during the period after leaving the destroyed cell and before penetrating into a new cell. They reproduce only in living cells, where their accumulation resembles a cyst. But these clusters of endozoites are localized directly in the cytoplasm or in the cytoplasmic vacuole. The delicate membrane around such clusters of parasites is formed by the host cell in the acute stage of toxoplasma. These clusters do not have their own membrane, so in reality they are pseudocysts. If endozoites are localized in cytoplasmic vacuoles, then such vacuoles are called parasitophorous.

Gradually, a parasitic membrane forms around the clusters of endozoites, and toxoplasma passes into a new stage - a true tissue cyst. The parasites themselves participate in the formation of a complex cyst membrane, and this occurs in chronic toxoplasmosis. Such membranes are impermeable to antibodies and ensure the viability of the parasite for many years, and sometimes for life. As a rule, cysts are located inside the cell, although extracellular localization has also been proven. The diameter of cysts is from 50-70 to 100-200 µm. With the formation of a cyst, endozoites in it turn into a new stage - cystozoites. A mature cyst can contain several thousand cystozoites.

The biological purpose of tissue cysts is very great. First of all, cysts ensure the survival of the parasite in the immune organism and thus increase the chances of infection with toxoplasmosis of both the final and new individuals of intermediate hosts. The formation of the cyst stage is an important stage in the life cycle of toxoplasma, since the cyst stage - cystozoites - is much more resistant to external factors. Thus, if swallowed endozoites die under the action of gastric juice after one or two minutes, then cystozoites remain viable in this environment for 2-3 hours, although the cyst membrane is destroyed almost instantly under the action of pepsin. It has been experimentally proven that from cystozoites in the intestine of a cat with greater constancy and faster, i.e. sooner, the intestinal phase of toxoplasma development in the body of the final host is completed.

Thus, from the description of the life cycle of toxoplasma it follows that intermediate hosts (wild and farm animals, as well as humans) are carriers of the vegetative (tissue) stages of the parasite, which are endozoites in cysts. It is with them that doctors, veterinarians and parasitologists have to deal when diagnosing toxoplasmosis.

The ultrastructure of endozoites and cystozoites is identical to that of coccidia merozoites. From the point of view of a parasitologist-epidemiologist and clinician, it is very important to know a number of features of Toxoplasma biology. Primarily, Toxollasma is a parasite of cats, in whose organism it is capable of completing both the intestinal and extraintestinal (tissue) phases of development without the participation of other hosts. Thus, cats can simultaneously perform the functions of intermediate and definitive hosts and ensure the phase development of Toxoplasma from oocyst to oocyst. But Toxoplasma is not a monoxenous parasite: intermediate hosts take part in its life cycle, although their participation is optional; therefore, Toxoplasma is characterized by facultative heterogeneity. Moreover, endozoites and cystozoites - stages from intermediate hosts - can infect not only definitive hosts, but also new intermediate hosts (carnivores and humans). Here, a kind of transit or transfer takes place without the participation of the final host and without the release of toxoplasma into the external environment.

In many animals (mice, rats, guinea pigs, hamsters, rabbits, dogs, sheep, pigs) and in humans, transplacental transmission of toxoplasma at the endozoite stage has been noted, thereby causing congenital toxoplasmosis.

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