7.1.2 Protozoan diseases

. . Protozoan diseases About genera of protozoans live as parasites within the human body and cause diseases. Nipah virus is a zoonotic virus (transmitted from animals to humans) and also transmitted through contaminated food. In infected people, it causes a range of illness from asymptomatic infection to acute respiratory illness and fatal encephalitis. XII Std Biology-Zoology Chapter- XII Std Biology-Zoology Chapter- Human Health and Diseases and female Anopheles mosquito as the primary host. The life cycle of Plasmodium involves three phases namely schizogony, gamogony and sporogony (Fig. . ) . The parasite first enters the human blood stream through the bite of an infected female Anopheles mosquito. As it feeds, the mosquito injects the saliva containing the sporozoites . The sporozoite within the blood stream immediately enters the hepatic cells of the liver. Further in the liver they undergo multiple asexual fission (schizogony ) and produce merozoites . After being released from liver cells, the merozoites penetrate the RBC’s. Inside the RBC, the merozoite begins to develop as unicellular trophozoites. The trophozoite grows in size and a central vacuole develops pushing them to one side of cytoplasm and becomes the signet ring stage . The trophozoite nucleus then divides asexually to produce the schizont. The large schizont shows yellowish - brown pigmented granules called Schuffners granules. The schizont divides and produces mononucleated merozoites. Eventually the erythrocyte lyses, releasing the merozoites and haemozoin toxin into the blood stream to infect other erythrocytes. Lysis of red blood cells results in cycles of fever and other symptoms. This erythrocytic stage is cyclic and repeats itself approximately every to hours or longer depending on the species of Plasmodium involved. The sudden release of merozoites triggers an attack on the RBCs. Sometimes, merozoites differentiate into macrogametocytes and microgametocytes . When these are ingested by a mosquito, they develop into male and female gametes respectively. In the mosquito's gut, the infected erythrocytes lyse and male and female gametes fertilize to form a diploid zygote called ookinete . The ookinete migrates to the mosquito's gut wall and develop into an oocyte . The oocyte undergoes meiosis by a process called sporogony to form sporozoites . These sporozoites migrate to the salivary glands of Free flagellum Attached flagellum Pellicle Basal granule Nucleus Reserve food granules Kinetoplast Undulating membrane Fig. . . Trypanosoma gambiense . T. rhodesiense is transmitted by Glossina morsitans causing Rhodesian or East African sleeping sickness. . T. cruzi is transmitted by a bug called Triatoma megista and causes Chagas disease or American trypanosomiasis. Kala – azar or visceral leishmaniasis is caused by Leishmania donovani, which is transmitted by the vector Phlebotomus (sand fly) . Infection may occur in the endothelial cells, bone marrow, liver, lymph glands and blood vessels of the spleen. Symptoms of Kala azar are weight loss, anaemia, fever, enlargement of spleen and liver. Malaria is caused by different types of Plasmodium species such as P. vivax, P. ovale, P. malariae and P. falciparum (Table . ). Plasmodium lives in the RBC of human in its mature condition it is called as trophozoite . It is transmited from one person to another by the bite of the infected female Anopheles mosquito.