INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

ALVEOLATES

PHYLUM APICOMPLEXA

CLASS COCCIDEA

Predominantly intracellular parasites of vertebrates, including humans, and more rarely of invertebrates. About 2,500 species are known.

Coccidia are microscopic organisms measuring only a few micrometers. The coccidian Cell is spherical and lacks the regional differentiation seen in gregarines. Their life cycle alternates between asexual reproduction, sexual reproduction, and sporogony, often accompanied by a change of host.

Well-known species belong to the genus Eimeria, which parasitize the intestines of young rabbits, cattle, and poultry. The diseases they cause are termed coccidioses.

The typical coccidian life cycle can be traced using E. stiedae as an example—a parasite of the rabbit intestine, which becomes infected by ingesting oocysts along with food (Fig. 47). Each oocyst contains four spores, with two sporozoites in each spore. In the rabbit's intestine, they shed their membranes and penetrate the Cells of the Small Intestine or Liver wall, where they transform into trophozoites that feed on cell contents via pinocytosis. The Nucleus of the trophozoite divides multiple times, producing a multinucleated form known as a schizont, which subsequently fragments into dozens of merozoites. The infected cell ruptures, releasing merozoites into the intestinal lumen where they invade healthy epithelial cells, and the cycle repeats.

After four to five successive generations, the merozoites in the intestinal cells differentiate into gamonts. Some of these—microgamonts—undergo mitotic divisions to produce numerous biflagellated microgametes; others—macrogamonts—do not divide, each developing directly into a single macrogamete. Following copulation, the zygote encysts and transforms into an oocyst, which is eliminated from the host's body with excrement.

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Fig. 47. Life Cycle of the coccidian Eimeria (from Dogiel):

I, II, III - first, second, and third generations of schizonts, respectively; IV - gametogony; V - sporogony;

1 - sporozoites; 2 - trophozoite; 3 - schizont; 4, 5 - merozoites; 6 - development of macrogamete; 7 - development of microgametes;

8 - microgamete; 9 - oocyst; 10 - oocyst passed out of the rabbit's intestine;

11 - oocyst with sporoblasts; 12 - development of spores; 13 - oocyst containing spores and sporozoites

In the external environment, sporogony takes place within the oocysts: spores and sporozoites are formed, capable of infecting a new host. Thus, The life cycle of Eimeria proceeds without alternating hosts, with schizogony and the sexual process occurring inside the host, while sporogony takes place in the external environment.

Among the coccidia, Toxoplasma gondii poses a significant threat to humans as the CAUSATIVE AGENT OF Toxoplasmosis, a disease distributed worldwide. The life cycle of T. gondii involves two hosts: a definitive and an intermediate host (Fig. 48). The definitive host is the Organism in which the parasite reproduces sexually, whereas the intermediate host is where it reproduces asexually.

The definitive hosts of Toxoplasma include various species of the cat family (domestic cats, leopards, tigers, etc.), within the intestinal cells of which the parasite reproduces via schizogony, forming merozoites and subsequently gamonts. The sexual process takes the form of oogamy; the zygote becomes encysted to form an oocyst, which is shed via feces. In the external environment, two spores develop within it, each containing four sporozoites. Intermediate hosts of Toxoplasma include birds, various mammals (rodents, ungulates, etc.), and humans. They become infected by ingesting oocysts through contaminated food or Water. In the intermediate host's intestine, sporozoites emerge from the oocysts, penetrate the intestinal wall, and enter the bloodstream, spreading throughout the entire body.

Toxoplasma infects cells of various Organs, primarily the Brain and the reticuloendothelial system. Within the cells of the intermediate host, merozoites multiply by endodyogeny (Fig. 48, b). Clusters of merozoites become enclosed in a membrane, forming pseudocysts. Felines become infected by feeding on infected meat or Organs of the intermediate host. The definitive host (felines) can also be infected by ingesting oocysts.

Toxoplasmosis can be contracted not only via oocysts but also through the consumption of animal products. Humans can likewise become infected through careless handling of individuals or animals suffering from toxoplasmosis (merozoites may be excreted in urine, mucus, etc.), by eating insufficiently cooked or undercooked meat and Internal Organs of infected animals, or from infected cats (via oocysts).

According to some estimates, between 40% and 80% of the global population is infected with this parasite, though the majority are asymptomatic carriers. It is these carriers who ensure the spread and transmission of the parasites. Acute disease (toxoplasmosis) is much rarer and manifests quite severely in humans, affecting the Lymph Nodes, Muscles, Nervous system, and visual organs. Pregnant women can transmit toxoplasmosis vertically across the Placenta to the fetus, frequently leading to Miscarriage or the birth of children with developmental defects.

Fig. 48. Life cycle of Toxoplasma gondii:

a - trophozoite in the host cell; b - stages of endodyogeny; c - definitive host; d - sporogony in the external environment; e - intermediate hosts: 1 - trophozoites in internal organs; 2 - schizogony; 3 - gametogony in intestinal epithelial cells

Coccidia also include Muscle spores (genus Sarcocystis), which are widely known parasites inhabiting the muscles of mammals. Their life cycle involves a change of hosts: definitive hosts are carnivorous mammals and humans, while intermediate hosts include various mammals (cattle, sheep, etc.) and birds. In cases of heavy infection, muscle spores can cause severe diseases in humans and domestic animals.