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Sporozoa lifecycle.

The life cycle of apicomplcxans typically involves an asexual and a sexual phase. An infective stage, called a sporozoite, invades the host and undergoes asexual m ultiplication by fission, producing individuals called merozoites. Merozoites can continue schizogamy but eventually form gametes (gamogony) that fuse to form a zygote. The zygote undergoes meiosis to form sporozoites.

The nature and life cycle of apicomplexan sporozoans can be illustrated by the coccidians, which include the parasites that cause malaria in humans. Malaria continues to be one of the worst scourges of mankind. About 300 million people are believed to be infected each year. The untreated disease can be long lasting and terribly debilitating. Malaria has played a m ajor and often unrecognized role in human history. The name means literally "bad air" because the disease was originally thought to be caused by the air of swamps and marshes. Although malaria had been recognized since ancient times, the causative agent was not discovered until 1880, when a physician with the French army in North Africa identified the coccidian parasite Plasmodium in the blood cells of a malarial patient. In 1887 the mosquito was recognized to be the vector.

The introduction of the parasite into a human host is brought about by the bite of certain species of mosquitoes, which inject the sporozoites along with their salivary secretions into the capillaries of the skin (Fig. 2-23). The parasite is carried by the bloodstream to the liver, where it invades a liver cell. Here further development results in asexual reproduction through multiple fission. These daughter cells invade other liver cells and continue to reproduce. After a week or so there is an invasion of red blood cells by parasites produced in the liver. Within the red cell the parasite increases in size and undergoes multiple fission. The individuals (merozoites) produced by fission within the red cells escape and invade other red cells. The liberation and reinvasion are not continuous but occur simultaneously from all infected red blood cells. The timing of the event depends on the period of time required to complete the developmental cycle within the host's cells. The release causes chills and fever, the typical symptom s of malaria. Eventually, some of the parasites invading red cells do not undergo fission but become transformed into gametocytes. The gametocyte remains within the red blood cell. If such a cell is ingested by a mosquito, the gametocyte is liberated within the new host's gut. After some further development, a male gametocyte (microgametocyte) fuses with a fem ale gametocyte (macrogametocyte) to form a zygote. The zygote penetrates the stomach wall and gives rise to a large number of spore stages (sporozoites). It is these stages, which migrate to the salivary glands, that are introduced into the hum an host by the bite of the mosquito.

The asexual stage of other coccidians occurs in blood cells or in gut cells. A number of diseases of domesticated animals are caused by coccidians. The genus Eimeria, for example, affects chickens, turkeys, pigs, sheep, and cattle (Fig. 2-22).

Another common group of apicomplexans contains the gregarines, which attain the largest size among the sporozoans. They are parasites of invertebrates, especially annelids and insects, and therefore not of economic importance. Intracellular par¬asitic species are only a few microns long, but those that inhabit the body or gut cavities of the host may reach 10 mm in length. The body of a gregarine trophozoite is elongate (Fig. 2-24), and the an-terior part sometimes possesses hooks, a sucker or suckers, or a simple filament or knob for anchoring the parasite into the host's cells. The host becomes infected through ingesting spores containing sporozoites of the parasites (Fig. 2-25). Depending on the species, the liberated gregarine sporozoites either remain in the gut of the host or penetrate the gut wall to reach other areas of the body. The life cycle commonly lacks schizogony.

Members of the class Piroplasmea are another small group of sporozoans that also attack the red blood cells of vertebrates. Spores and gametes are not produced, and the parasites are transmitted by ticks. Pathogenic infections in cattle and other do¬mesticated animals are of considerable economic importance.

The phylum Microspora contains a smaller number of intracellular parasites, but they are found in most animal groups, especially arthro¬pods. They lack the apical complex of other spo¬rozoans, and the sporelike stage is characterized by a polar filament that is extruded when this stage is taken into the host. The filament appears to be in¬volved in some way with the invasion of the host's cell. As parasites of the honeybee and silkworm, the microsporidians are of economic importance. One of the early studies of sporozoans was that of Pasteur in 1870, on Nosema bombycis in silk¬worms.

Figure 2-22 Life cycle of an eimcriid coccidian, a destruc¬tive intracellular parasite of the gut epithelium of many verte¬brates, including domesticated buds and mammals.

Figure 2-23 The life cycles of Plasmodium in a mosquito and in man. Reinvasion of liver cells in the tissue cycle does not occur in Plasmodium falciparum.

Figure 2-24 Trophozoites of the gregarinc Gregarina gamhami attacking the midgut epithelium of a locust.

Figure 2-25 Life cycle of a gregarine, Stylocephalus longicollis, an intestinal parasite of a beetle. There is no schizogony in this species.