PLASMODIUM (Malarial parasite)
In olden times the malarial fever was believed to have been caused by inhaling foul air and hence the term (mala = foul; aera = air). In 1880 a French Biologist Laveran pinned the cause for malaria on a protozoan parasite, Plasmodium vivax, occurring in the blood of man. It is a sporozoan occurring in the blood and hence belongs to haemosporidia.
The life-cycle is completed in 2 hosts namely (1) man and (2) female Anopheles mosquito.
Life Cycle of Plasmodium vivax one in
Life-cycle in Man: The life Cycle consists of two phases one in the liver and other in the erythrocyte.
Pre-erythrocytic cycle: A man gets infected when bitten by female Anopheles mosquito that librates sporozoite into his blood. Each sporozoite enters the liver and transforms into cryptozoite, grows, fills up the entire cell and is termed undergoes asexual multiplication resulting in cryptomerozoites may either which enter the are blood or liberated into circulation when the schizont bursts (schizogony). The cryptomerozoites invade the RBC or enter fresh liver cells to continue the exo-erythrocytic cycle.
Exo-erythrocytic cycle: The cryptomerozoites producing a number of metacryptozoites. This may be repeated several times and each time liver cells are infected. A few metacryptozoites after escaping into the blood stream, invade the erythrocytes to begin the erythrocytic cycle. After the entry of sporozoites into blood, Plasmodium is seen back in the blood only after 7 to 17 days after completing its cycle in the liver (incubation period).
The cryptomerozoite entering into R.B.C. becomes amoeboid and it is the feeding stage termed trophozoite. Soon a vacuole appears pushing the cytoplasm and nucleus outwards forming the signet- ring stage. The trophozoite makes use of the haemoglobin as its food. When it is full grown, the vacuole disappears, it becomes disc-shaped, ready to divide asexually and is termed schizont which has a brown pigment haemozoin, the digested haemoglobin. Its nucleus divides into 6 to 24 bits, each bit surrounded by cytoplasm, and these are the merozoites. They break open the R.B.C. to infect a fresh R.B.C. When they are released, the toxin is set free resulting in fever. This cycle is asexual and hence a gamogony. Since schizont produces merozoites, it is schizogony or merogony. This part of the life-cycle was worked out by Golgi and named after him as cycle of Golgi.
In man, the life-cycle is asexual, exo-erythrocytic cycle in liver and endo-erythrocytic cycle in the R.B.C.
By repeated asexual cycle, almost all R.B.C. may be destroyed and for the continuance of the species, the Plasmodium will have to get into the mosquito. For this purpose it produces two special types of gamete-producing cells called macrogametocyte with small nucleus and reserve food in cytoplasm and microgametocyte with large nucleus and no reserve food.
Life-cycle in mosquito: When the mosquito sucks the blood of a malarial patient, all the asexual stages and the two types of gametocytes enter its stomach where gametocytes alone survive. They come out of R.B.C. In the macrogametocyte, a bit of nucleus and cytoplasm forming the polar body get thrown out and the larger cell forms the egg. In microgametocyte, the nucleus divides a number of times and slender finger-like outgrowths develop from the margin. öne of the nuclei enters it forming the male gamete. Cytoplasmic process is formed and the slender nucleated bodies exhibit lashing movement and therefore the process is referred to as exflagellation. The male gametes separate off and one enters the ovum resulting in fertilization, forming a zygote. The male and female gametes are dissimilar and so are termed anisogametes or heterogametes. The zygote exhibits wriggling movement forming the ookinete which penetrates into the stomach wall, develops a cyst wall forming oocyst. It grows absorbing the nourishment from the host. The nucleus once again divides a number of times and around each nuclei, small amount of cytoplasm gets collected. Thus, inside the oocyst a number of cells develop. They are slender, sickle shaped called sporozoites. mature cyst ruptures liberating the content into the haemocoel of mosquiW The sporozoites being motile pierce the salivary glands and collect themselves in the tube
Female Anopheles mosquito infected in such a way bite more individuals and inoculate the sporozoite into their blood.
life-cycle in mosquito is sexual and hence gamoony. Since it is sporogony. This cycle was worked out first by Sir Ronald Ross.
Malarial control: Many fertile agriculturally rich places had to be given up due to the cases of malaria. During the past few years malaria was kept under control and in recent it is on the increase. To eradicate malaria, malarial control measure are followed.
Malarial control always goes hand in hand with mosquito control. It is achieved by curative methods. Prophylactic method involves preventing the malarial parasite from entering the blood of man. Since the mosquitoes breed in stagnant waters, the surroundings of habitations should be drained of the stagnant water. Dark comers of houses are to be sprayed with insecticides. Sleeping inside mosquito curtains will keep the mosquitoes away. Smearing the body with mosquito repellents like Citronella oil, or by using fumes of repellents, mosquitoes can be avoided. Oil is to be poured on stagnant water since it forms a thin film preventing the mosquito larvae from breathing. D.D.T. (Dichloro Diphenyl Trichloroethane) was effective for sometime but now mcsquitoes are resistant to it by converting D.D.T. into D.D.E. (Dichloro Diphenyl Ethane) in their body Dragonfly adults feed mainly on mosquito adults and their larvae feed only on mosquito larvae. Certain fishes like Gambusia feed on mosquito larvae and they are termed larvicidal fishes. They are introduced into wells, open tanks, etc.
Curative method involves treating the patient after malaria has a firm hold in his body. Quinine derived from the bark of Cinchona tree was in wide use since it was able to kill the parasites. Now, numerous synthetic formulations of Quinine are in the market for treatment of malaria, common being paludrine, mepacrine, camoquin, etc.
3. PARAMECIUM
This belongs to the class Ciliata of the Phylum Protozoa.
Occurrence: It is a common protozoan occurring in ponds and pools containing organic decay. It can be cultured in an infusion of hay.
Structure: The common species is P. caudatum. The anterior end is rounded and the posterior end is pointed. or the outside of that ectoplasm is a distinct pellicle. The oral groove has a depression extending backward for about two-thirds of the length of the body leading into a cytostome or cell mouth. it is connected with a short cytopharynx which opens in the cytoplasm. The cytoplasm contains food vacuoles. The surface of the body is covered with cilia arranged in a definite pattern. Near each end of the endoplasm is a contractile vacuole and into each of these, a variable number of radiating canals empty.
Paramecium — Stages in conjugation
Endomixis: It is a nuclear reorganisation in one and the same individual macronucleus divides and disappears, the micronucleus establishes future macro and nuclei.
Behaviour: Paramecium is sensitive to many kinds stimuli contact, temperature, electric current, chemicals, gravity and others. The response is negative to strong light ultraviolet rays. It is positive for water currents. They are attracted by weak acetic acid repulsed by strong acid. If a Paramecium strikes against an obstacle, it gets back and swims away. The anterior end is more sensitive than the posterior end.