Class Sporozoa

The fourth class of Protozoa is the Sporozoa, all of which are parasitic. They are characterized by the lack of cilia of flagella in adult forms and by their complex life cycle. A single sporozoan undergoes multiple fission, dividing into numerous smaller cells (the spores) simultaneously.

The sporozoans are parasitic protozoans which were formerly united within one class, the Sporozoa, because of the presence of sporelike infective stages in some members of both groups. The presence of flagellated gametes, the ability to move by gliding or body flexions, and the possession of pseudopodia as feeding organelles in different groups suggest a relationship to the flagellates and to the sarcodines.

The subphylum Sporozoa contains the most familiar of the sporozoan parasites—the gregarines and the coccidians. The coccidians infect the intestinal or blood cells, and include the parasites causing human malaria and the coccidioses of domestic animals. Species of Sporozoa are widespread and parasitize vertebrates and most invertebrate phyla.

The life cycle is complex. It can be divided into three phases: schizogony, an asexual multiplication of the parasite following infection of the host; gamogony, the development of gametes; and sporogony, also a multiplication and an infective stage, typically a spore.

The best known sporozoans are members of the genus Plasmodium, which cause malaria. Plasmodium is passed back and forth between man and the Anopheles mosquito. The female mosquito requires blood for the development of her eggs. If the female draws her blood from a person with malaria, she will pick up Plasmodium cells which multiply within her body, although they do not harm her, and travel in her salivary glands. When she bites her next victim, she injects a droplet of salivary fluid under his skin. This fluid seems to anesthetize the victim against the bite; it also keeps the blood from clotting so that it will flow freely through her fine proboscis. Ultimately, through an inflammatory reaction, it raises the familiar welt. This drop of salivary fluid also carries Plasmodium, which eventually enter the blood cells and begin to multiply. The Plasmodium break out of the blood cells at regular intervals – usually every 48 or 72 hours, depending on the species – that is why malaria is characterized by recurrent bouts of chills and fever.

Date: 2014-12-22; view: 1096