Plasmodium ( The Malarial Parasite)

Plasmodium is the best known protozoan parasite of man, which causes malaria. malaria is one of the widespread tropical diseases among human being. About one-third of the entire population of many hot countries suffer from it every year. It is a serious economic and social problem as it causes deaths and affects majority of population. It is responsible for persistent sickness, loss of working ability and slowing down of mental and physical development among children. In 1943, Russel estimated that there were three hundred million cases of malaria in the world. In 1957 it was estimated that more than 200,000,000 (about 10 percent) of world population had malaria of grater or lesser severity. A significant role was played by malaria in military history. It has decided the fates of many army men. There are several examples of army man being defeated by malaria instead of enemies. During the war of 1914-18, malaria caused death of several army men. Among British troops there were 160,000 admissions of malaria cases to hospitals in Macedonia; 35,000 in Egypt; 107,000 in East Africa and 120,000 in Mesopotamia. Due to large number of patients suffering from malaria all soldiers did not receive treatment in hospitals. During 1918 the british forces in Macedonia alone lost 2,000,000 during service days.


History:
Malaria [Gr. malo: bad; aria: air]. In ancient times, it was believed that malaria was caused by harmful air of marshy places. In 1880 Charles Laveran, a French military surgeon, for the first time, reported Plasmodium in the blood of malaria patient. The occurence of Plasmodium in the blood suffering from malaria was further confirmed by an Italian scientist Golgi in 1885. In 1892, Richard peiffer, a German doctor suggested that some blood sucking insects also carried the malaria organism. Laveran, working in Algeria proved that the mosquito spreads malaria. In 1894, Sir Ronald Ross while in London came in contact with Sir Patrick Manson. Manson explained his theories to Ross and suggested him to work with the parasites and to follow the life history in the mosquito. Back in India, Ross started to work and in 1897 he succeeded in his efforts. He explained that the germs of malaria develop are sucked by the female Anopheles from the body of infected human. The germs of malaria develop within the body of female Anopheles up to infective stage and when the infected mosquito bites a healthy man, the germs are inoculated in his blood and spread the disease. For this excellent work, Sir Ronald Ross was awarded the Nobel prize for medicine in 1902. Ross had done his detail work in the case of parasites of birds. It was Grassi (1917) who studied the human malaria and followed the complete cycle of the development of the three human malaria parasites in mosquitoes.

Systematic Position:
Phylum: Protozoa
Subphylum: Sporozoa
Class: Telosporea
Order: Eucoccidia
Suborder: Haemosporidia
Genus: Plasmodium
Species: vivax

Distribution and Habitat: Plamosium is commonly known as "malarial parasite" as it causes material fever in human. The species of Plasmodium is widespread in tropical and temperate countries. It is an intracellular parasite in man, living in the RBCs and liver cells, while extracellular in mosquito, living in its alimentary canal and salivary glands. Plasmodium comprises several species, four of them attack man and cause malaria of four different kinds. These are P. vivax, P. malaria, P. ovale and P. falciparum. Here Plasmodium vivax is taken as a typical example, as it is the most widespread species causing malaria in man.

Structure:

Plamodium shows different shape of the body in different stages of its life cycle. It may be elongated, oval, spherical, spindle shaped or amoeboid (irregular) in shape. A fully grown matured parasite is amoeboid and uninucleate structure. It is known as trophozoite. Garnham (1966) and Zinska (1969) studied the ultra structure of trophozoite under electron microscope. The trophozoite is surrounded by a double layered plasmalemma. The cytoplasm contains dark Palade's granules. The endoplasmic reticulum is not well developed. It is formed of smooth or rough vesicles scattered in the cytoplasm. The ribosomes are found either free in the cytoplasm or remains attached with the rough endoplasmic reticulum and the outer layer of nuclear membrane. Mitochondria are very few in number with peripheral cristae. The number of mitochondria varies with the age of the parasite - single in nerozoites and numerous in tropozoites. A double layered concentric body is found in the cytoplasm attached with the plasmalemma. THe plasmalemma probably performs the function of mitochondria. The golgi body is composed of small vesicles arranged in several rows. Different sizes of vacuoles are also found in cytoplasm. The nucleus contains eccentrically placed nucleolus and granular nucleoplasm. The cytoplasm also contains food vacuoles having haemozin and some pigment granules.

Life Cycle: The life cycle of Plasmodium vivax is completed into two hosts i.e. it is digenetic. In man and other vertebrates parasites live inside the liver cell and the Red Blood Corpuscles (RBCs), multiply asexually (Schizogony) and cause malaria. Therefore, man is designated as the secondary or intermediate host of the malarial parasite. The sexual cycle involving gametogamy and sporogony occurs in the stomach of the female Anopheles. The female Anopheles acts as a vector because it feed on blood and transmits the parasite from one human being to the other. As the sexual cycle occurs in female Anopheles, it represents the primary or definitive host of the malarial parasite.

Life Cycle in Man (Asexual Reproduction)

Infection: The sporozoites are the infective stage of the parasite. It is minute, slender, spindle-shaped uninucleate organism. It measures about 11-12 micron long and 0.5 to 1 micron wide. When an infected female Anopheles bites a healthy person, several thousands of sporozoites (present in the salivary glands) get inoculated into the human blood along with the saliva. This is called inoculation. Male mosquito does not feed on blood, hence only female serves as vector. The sporozoites circulate for about half an hour and then enter the cells of the liver to start the life cycle. The asexual life cycle in man is completed in four phases: (i) Pre-erythrocytic cycle (Schizogony), (ii) Exo-erythrocytic cycle (Schizogony), (iii) Erythrocytic cycle (Schizogony) and (iv) Post- erythrocytic cycle (Schizogony).

i) Pre-erythrocytic cycle: The sporozoites rapidly grow in size absorbing nourishment to form a large round schizont. The schizont divide by schizogony- a type of asexual reproduction in which the schizont divides by multiple fission resulting in the formation of a number of small, spindle-shaped uninucleate cells called merozoites. The schizonts rupture and the merozoites are liberated into the sinusoids or vemous passages of the liver. This phase of asexual reproduction is called as pre-erythrocytic schizongony ant the merozoites of this phase are called cryptozoites. The cryptozoites are immune to medicines and natural resistant of the host.

(ii) Exo-erythrocytic cycle: The cryptozoites attack new liver cells and grow into schizonts. The schizont divides by schizogony to form merozoites. This phase of asexual reproduction is called exo-erythrocytic schizogony and the merozoites of this second generation are called metacryptozoites. The exo-erythrocytic cycle may continue for a number of times and each time a new liver cell may be attacked and thus liver becomes a reservoir of these merozoites.

(iii) Erythrocytic Cycle (Schizogony): At least after two cycles of schizogony, some merozoites re-enter the RBC or erythrocytes. The merozoites attack young and immature corpuscles, but the merozoites of P. malariae attack only old corpuscles and those of P. falciparum attack only old corpuscles and man till the intiation of erythrocytic cycle is called pre-patent period. It is about 8-10 days. During this period the patient shows no symptoms of malaria.

The merozoites feed on erythrocytes, become rounded and modified into a trophozoite. As the merozoites grow, a vacuole appears in the centre and the nucleus is pushed to one side which gives it a ring-like appearance. This is known as signet ring stage as it resembles a signet ring. It feeds upon the red blood corpuscles and forms a food vacuole. The food vacoule secrete some digestive enzymes, which break down haemoglobin into protein and hematin. The protein is used as food and hematin forms a toxic malarial pigment, the haemozoin. The trophozoite enlarges and soon the vacuole dissapears. It becomes amoeba shaped. This is known as amoeboid stage. Schizont appears after a period of growth of about 36 to 40 hours and represents the full-grown trophozoite, ready to divide. The parasites become rounded in shape and has lost all amoeboid activities. The larger schizont makes the erythrocyte very large. At this stage the red blood corpuscles acquire reddish-orange granules called schuffner's dots. The nucleus of schizont divides in the next 6 to 8 hours to form 12 to 24 daughter nuclei. The nuclei get arranged at the periphery and become surrounded by a little cytoplasmic mass. Each cytoplasmic mass with a nucleus forms oval merozoite. The merozoites arrange themselves in the form of a "rosette" (usually in two rows) with the pigment granules at the centre. This is known as rosette stage. This phase of asexual multiplication is known as erythrocytic schizogony. One erythrocytic cycle is completed in 48 hours.

(iv) Post erythrocytic Cycle: The merozoites may again go tot the liver cells and carry on another cycle of asexual multiplication. This phase is known as post erythrocytic schizogony. The haemozoin toxins are liberated into the blood along with the liberation of merozoites. The toxins are then deposits in the liver, spleen and under the skin, so that the host gets a sallow colour. The accumulated toxins cause malaria fever in which the patient suffers from chills, shivering, sweating and high temperature. The fever lasts for six to ten hours and then it comes again after every 48 hours with the liberation of new generation of merozoites.

Incubation Period: In erythrocytic schizogony the parasite multiplies in geometrical progression and on reaching a sufficient concentration in the blood, brings about the onset of fever. The period between infection and the appearance of the first symptoms is known as incubation period. The duration of the incubation period varies with the species of the parasite. It is about 10 to 14 days in P. Vivax.

Sexual Cycle:

Formation of Gametocytes: After many generations in the blood, some merozoites increase in size to form two types gametocytes, the macrogametocytes and microgametocytes. The macrogametocytes (female) are large, round with the food laden cytoplasm and a small eccentric nucleus. The microgametocytes (male) are small having clear cytoplasm and a large central nucleus. Both gametocytes contain large amount of haemozoin. They enlarge the erythrocytes. The gametocytes do not develop further until they get sucked by the appropriate species of mosquito. If this does not happen, they degenerate and die, because they require lower temperature for further development.

Life Cycle in Mosquito:

Ingestion by mosquito: When a female Anopheles sucks the blood of a malaria patient, the gametocycles also enter along with blood. They reach the stomach and formation of gametes takes place. All other stages of the parasite as well as the erythrocytes are digested, while the gametocytes survive inside the stomach. These continue sexual cycle in following stages:

(i) Gametogenesis (Gamogony): It is the stage in which two types of gametes are formed:

a) Formation of male gamete: The microgametocytes become active and their nucleus divides to produce 6 to 8 haploid daughter nuclei. The nuclei arrange at the periphery. The cytoplasm gives out same number of falgella-like projections. A daughter nucleus enters in each projection. These projections separate from the cytoplasm. This process of the formation of microgametes is called exflagellation. Thus, from each microgametocyte 6 to 8 flagella like active microgametes are formed.

b) Formation of Female gamete: The megagametocyte undergoes some reorganization and form megagamete or female gamete. It is now ready for fertilization.

(ii) Fertilization: A small cytoplasmic projection, the fertilization cone is given out by megagamete. The male gamete enters the female gamete at the fertilization cone. The male and female gamete fuse together to form a round zygote or sybkaryon. The fusion of two dissimilar gametes is called anisogamy. The zygote remains inactive for some time and then elongates into a worm-like ookinete or vermicule. The ookinete penetrates the wall of the stomach and comes to lie below its outer epithelial layer. It gets enclosed in a cyst formed partly by the zygote and partly by the stomach of mosquito. The encysted zygote is called oocyst. The oocyst absorb nourishment and grow to about five times in size. They protrude from the surface of the stomach as transparent rounded structures.

(iii) Sporogony: The nucleus of oocyst divides repeatedly to form a large number of haploid daughter nuclei, the first division being meiosis. At the same time the cytoplasm develops vacuoles forming numerous cytoplasmic mass. The daughter nuclei arrange themselves along with the cytoplasmic masses. The cytoplasm forms slender spindle like processes and a daughter nucleus passes into each of them to form sporozoite. Thus each oocyst may contain thousands of sporozoites and group of sporozoites get arranged around the vacuoles. This phase of asexual multiplication is known as sporogony. In mosquito whole sexual cycle is completed in 10 to 21 days. Lastly the oocyst bursts and sporozoites are liberated into the haemolymph of the mosquito. They spread throughout the haemolymph and eventually reach the salivary glands and enter the duct of the hypopharynx. The mosquito now becomes infective and sporozoites get inoculated or injected to the human blood when the mosquito bites. It is estimated that a single infected mosquito may contain as many as 200,000 sporozoites.

Symptoms of Malaria: Malaria id first characterised by recurring attacks of fever. Chill, high temperature and fever are the main symptoms of malaria. Due to deposition of toxins the host gets a sallow (pale and yellowish) colour. Due to loss in RBC anaemia is caused. The liver and spleen get infected and so enlarge.

Types of Malaria: Different types of human malaria caused by four different species of Plasmodium are as follows:

1) Tertian or Benign or Vivax Malaria: This type of malaria is caused by P. vivax. The incidence of this disease is worldwide, but is more common in temperate regions. It is characterized by fever after 48 hours. Its incubation period is 10 to 14 days. It is very mild disease and does not result in the death of the patient.

2) Quartan Malaria: This is caused by P. malariae. It is more common in tropical and subtropical regions, although the disease has worldwide incidence. Fever recurs after every 72 hours. Its incubation period is 18 to 40 days. In general it is not a fatal disease but sometimes chronic infections may give rise to lethal kidney conditions.

3) Ovale or Mild Tertian Malaria: It is caused by P. ovale. It is confined in tropical Africa. Fever recurs after 48 hours. Its incubation period is 14 to 18 days. It closely resembles with tertian malaria.

4) Aestivo-autumnal or Malignant Tertian Malaria or Pernicious Malaria: The causative agent is P. Falciparum and fever recurs after every 24 to 48 hours or almost continously. Its incubation period is 9 to 14 days. It is often fatal to the patient as it effects the brain.