INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

PROTISTS - PROTISTA

PROTOZOA

ALVEOLATES

PHYLUM APICOMPLEXA

CLASS HAEMATOZOEA

CLASS HAEMATOZOEA

Blood parasites of various vertebrates. About 100 species of the genus Plasmodium infect erythrocytes in mammals, birds, and reptiles. Four species of plasmodia (Plasmodium vivax, P. malariae, P. falciparum, P. ovale) are the causative agents of one of the most widespread human diseases, malaria, which has been known since ancient times.

The life cycle of the malaria parasite involves the alternation of Selection/8.html">Asexual and sexual generations as well as a change of hosts (Fig. 49). The definitive host is the anopheline mosquito of the genus Anopheles, while the intermediate host is man. When an infected female malaria mosquito bites a human, sporozoites enter the blood along with saliva. Carried by the blood, the sporozoites enter the parenchymal Cells of the Liver, where they multiply by schizogony, giving rise to merozoites. The latter penetrate erythrocytes, undergoing the trophozoite stage followed by the multinucleated schizont stage, thereby producing a new generation of merozoites. The infected erythrocytes are destroyed, and the merozoites are released into the Blood Plasma to invade other erythrocytes, initiating subsequent generations of merozoites.

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Fig. 49. Life Cycle of Plasmodium vivax (after Hausmann, modified):

1 - sporozoite; 2-5 - schizogony in the liver parenchyma; 6-9 - schizogony in erythrocytes; 10-11 - gamonts; 12 - macrogamont;

13 - formation of microgametes; 14 - copulation; 15 - ookinete; 16-18 - development of sporozoites;

19 - penetration of sporozoites into the mosquito Salivary Glands

Along with merozoites, their Metabolic waste products enter the blood plasma, causing human intoxication, the manifestation of which is elevated BODY Temperature AND a fever attack. The destruction of erythrocytes and the release of merozoites occur synchronously; therefore, the concentration of toxins in human blood rapidly increases. After several cycles of erythrocytic schizogony, gamonts are formed from merozoites within the erythrocytes, and their further development is possible only in the gut of the malaria mosquito.

During blood-suturing (feeding), gamonts from the blood of an infected human enter the mosquito's Stomach. Macrogamonts transform into non-motile macrogametes, whereas microgamonts divide to produce eight motile flagellum-like microgametes. Following copulation, a zygote capable of amoeboid movement (ookinete) is formed. It penetrates the mosquito gut wall, where it becomes covered with a thin membrane, transforming into an oocyst. Sporogony takes place inside the oocyst, resulting in The formation of several thousand sporozoites. The oocyst wall ruptures, and the sporozoites migrate to the salivary glands, where they accumulate until the next blood meal. Unlike coccidia, blood-dwelling Sporozoans do not form spores, as none of the life cycle stages are released into the external environment.

Human malaria, caused by species of the genus Plasmodium, is a severe vector-borne human disease without a natural reservoir. The Plasmodium species that cause it parasitize exclusively in humans, and therefore no natural focus is formed for this disease. Infection of erythrocytes by the malaria parasite leads to anemia, enlargement of The Liver and Spleen, and periodic fever attacks. The course of the disease is determined by the species of the parasite.

The most common species, Plasmodium vivax, is the CAUSATIVE AGENT OF tertian malaria, in which fever attacks occur every 48 hours, accompanied by the destruction of infected erythrocytes.

Humans are also parasitized by P. malariae, the causative agent of quartan malaria, prevalent in southern latitudes (erythrocytic schizogony lasts 72 hours); P. falciparum, the causative agent of the most severe form of malaria—tropical or malignant malaria—in which fever attacks occur very frequently because the release of parasites from erythrocytes is unsynchronized; and P. ovale, which causes a tertian-type malaria but is quite rare.

The next two phyla, Microsporidia and Myxozoa, are traditionally considered within Protozoa, although modern molecular genetic studies have proven that microsporidia are related to Fungi, while myxozoa—whose multicellular nature is considered established—are, according to recent concepts, a group sharing common ancestors with modern Cnidarians.