Lecture Notes on Invertebrate Zoology

Content Module 1. Introduction. Major Invertebrate Taxa: Sarcomastigophora, Apicomplexa, Microsporidia, Myxozoa.

Lecture Topic 2. Characteristics of Sporozoans. Adaptations to Parasitism in Their Organization

Characteristics of Coccidia and Associated Diseases. Life Cycle of Eimeria. Alternation of Reproductive Modes. Blood Sporozoans. Plasmodium Species as Agents of Malaria. Control Measures for Infectious Diseases. Concepts of Disease Vectors, and the Parasitism of Intermediate and Definitive Hosts. Structural Features and Life Cycle of Microsporidia and Myxosporea. Their Practical Significance.

Sporozoans are obligate endoparasites localized in various animal cavities (body cavity, gut lumen) or intracellularly, infecting both vertebrates and invertebrates. The body surface is densely covered with short flagella; they exhibit a complex developmental cycle involving an alternation of sexual and asexual generations, culminating in spore formation. In Blood sporozoans, all stages occur internally within the Organism. Gregarines are distinguished by their relatively large size, worm-like body shape, gamete formation through pairwise encystment of individuals, and the absence of schizogony (in the vast majority of species).

Gregarines parasitize the intestines of invertebrates, including cockroaches, where representatives such as Gregarina blattarum can be found. The body of an adult gregarine, termed a gamont, is clearly divided into two sections: the protomerite and the deutomerite, the latter containing a single Nucleus. These sections are separated by a specialized partition known as the septum, formed by cytoskeletal microfilaments. In young gregarines, another body region is present—the epimerite, which appears as a rounded knob and serves for attachment to the intestinal epithelium. Coccidia cause severe disease in rabbits characterized by bloody diarrhea and mortality. Upon ingestion with food, sporozoites reach the intestine and penetrate the epithelial Cells. Here they feed, grow, develop, and reach the mature schizont stage, capable of reproduction. The Nucleus undergoes multiple divisions and becomes enveloped by distinct portions of Cytoplasm. Thus, through schizogony, several merozoites are formed, leading to an increase in parasite numbers within the host's body and subsequent auto-infection.

Following several rounds of schizogony, preparation for the sexual process—gametogony—begins. The schizont breaks down into several individuals known as gametocytes, which are sexually differentiated. Female individuals, or macrogametocytes, grow, accumulate nutrient reserves, and each develops into a macrogamete. Male individuals, or microgametocytes, undergo division to produce several flagellated microgametes. Pairwise copulation via the active penetration of a microgamete into a macrogamete

leads to The formation of a zygote, which transforms into an oocyst and exits the gut with the host's excrement. Outside the host's body, during sporogony, the oocyst nucleus divides twice to form sporoblasts, which become coated with tough walls and transform into spores. The embryo within the spore divides to produce several sporozoites (typically two). When oocysts are ingested by a subsequent host, the cyst ruptures, releasing the sporozoites and thereby restarting The life cycle. The presence of multiple sporozoites within a single oocyst accelerates the infection of the host individual by numerous parasites.

Sporozoites of the malaria parasite enter the human bloodstream through the bite of a mosquito of the genus Anopheles. Initially, they invade Liver cells, where they reproduce via schizogony. This stage is designated as the tissue, or pre-erythrocytic, stage. Merozoites produced in the tissue cells enter the Blood Plasma and rapidly invade erythrocytes, within which the maturation of merozoites (trophozoites) takes place. They feed on the substance of the erythrocyte and deposit metabolic waste toxic to the host in the form of dark melanin pigment. Undergoing multiple divisions, the merozoites transform into schizonts. This process is termed schizogony.

Upon the release of merozoites, the erythrocyte ruptures, and the daughter Generation of the parasite invades new blood corpuscles. Following several rounds of schizogony, preparation for the sexual phase begins. Merozoites develop into gamonts—sexually differentiated forms incapable of reproduction within the human blood. Further Development of the parasite proceeds exclusively in the gut of the mosquito, which it enters during blood-feeding. Here, the macrogamont transforms into a macrogamete, while the microgamont produces several flagellum-like microgametes. Following copulation, a zygote is formed, which is termed an ookinete due to its motility. Burrowing through the gut wall, the ookinete migrates to its outer surface, where it becomes encysted and transforms into an oocyst that divides into thousands of crescent-shaped sporozoites. The sporozoites accumulate in the Salivary Glands and, upon the mosquito biting a human, are injected into the blood of a new host, thereby renewing the cycle.