• Plasmodium (Malarial Parasite) belongs to the subphylum Sporozoan of the phylum Protozoa. All sporozoites are parasites. They are intracellular or extracellular parasites of both vertebrates and invertebrates.
• Among sporozoans, Plasmodium is the best known and most important species that causes malaria fever. Of these, about 60 species are known among humans and other animals. It is called malaria parasite because it causes malaria disease.
• Its four species are found in humans and cause different types of malaria fever – Plasmodium vivax, P. ovale, P. malariae and P. falciparum.
• These four species inhabit human red blood cells, multiply there, and eventually destroy them. Mosquitoes are carriers or vectors of these malaria parasites.
• Malaria parasite species are widely distributed among reptiles, birds, and mammals, giving them a very wide geographic range.
• Among the species found in humans, P. vivax is generally widespread, but mostly confined to temperate latitudes. Plasmodium falciparum is confined to hot regions of the world.
• The distribution of P. malariae is similar to that of P. vivax, but of these four species, P. ovale is the least widespread, though not the most widespread.
• Vertebrates such as humans and birds are the primary hosts for malaria parasites, with mosquitoes as intermediate hosts or vectors.
• Among mosquitoes, Cuilicines female mosquitoes spread malaria to birds and Anopheles female mosquitoes spread malaria to humans.
Among malaria parasites, Plasmodium vivax is more commonly found in humans. Causes 3-day or benign 3-day or 3-day malaria. In this type of malaria, fever always recurs after 48 hours. Its life cycle is complex and has many stages. It is a digenetic endoparasite because its life cycle is completed in two hosts with equal fronts.
(1) The primary host of Plasmodium vivax, as the adult stage of Plasmodium, known as the primary or primary or final host human trophozoite, is found in his RBC humans. Plasmodium causes malaria fever in humans. It is converted to its sexual form in human red blood cells. These sexual forms are known as gametocytes.
(2) Vector or Secondary or Intermediate Host-Female Anopheles mosquito is carrier secondary host of P. vivax, because the parasite completes sexual cycle gametogony and sporogony in it, resultantly the infective forms parasite sporozoites are formed. In this way female Anopheles mosquito works a vector, host which inseminate Plasmodium among humans.
Reservoir host- Monkeys are reservoir hosts of malariaparasites, but they themselves never get diseased by malarialfever. These parasites in their blood. They are transmitted from monkey to humans by mosquitoes.
This is the actual adult stage of the malaria parasite found in human red blood cells. Its unicellular body is round or disk-shaped surrounded by a two-layered cell membrane called the plasma membrane.
The following structures are found in its cytoplasm:
(1) a well-developed nucleus,
(2) endoplasmic reticulum in the form of small vesicles with ribosomes in their walls;
(3) free ribosomes in the cytoplasm;
(4) presence of mitochondria, Golgi apparatus and vacuoles;
(5) As a result of digestion, there are phagosomes where broken pieces of hemoglobin are found in the form of hemosin. Contractile vacuoles are completely absent. It obtains its liquid nutrition from human blood by pinocytosis.
P. vivax is a digenetic endoparasite. Its life-cycle completes in two hosts. Man is its primary host in which it completes its asexual cycle by schizogony. The female Anopheles mosquito is its vector or secondary host in which it completes its sexual cycle.
ASEXUAL CYCLE OF PLASMODIUM IN MAN
(1) Infection of Plasmodium in man-In man, the Plasmodium infection takes place by a small infective stage, which is known as sporozoites. These sporozoites are found in the salivary glands of female Anopheles.
• Human blood is the food of female Anopheles. Thuswhen the female Anopheles pierces the skin of a healthy man by its piercing-sucking mouthparts it pours a drop of its saliva in the blood circulation before sucking the blood.
• Along with this drop of saliva, thousands of minutesporozoites enter in the blood of man.
(2) Sporozoites-Sporozoites are the infective forms of Plasmodium. These are small spindle-shaped or sickle-shaped and uninucleated animals. Its length varies from 10 to 15 µ and width from 0-5 to 1p.
(3) Liver Schizogony– After entering in the human blood sporozoites travel in the blood for about half an hour and then enter in the liver with blood flow. As they reach in the liver they secrete lytic enzymes from their glandular organelles and pierce the cells of liver to enter in them. Inside the liver cells they do asexual multiplication by schizogony.
Liver schizogony can be explained like this :
(A) Pre-erythrocytic Phase-In this stage after entering in the liver cells each sporozoite feeds on the components of liver cells and grows for few days and finally converts into a colourless schizont. Schizonts divide into around 1,000 small oval, uninucleated cells by schizogony.
• By the pressure of innumerable cryptozoites thus formed, the schizont and liver cells burst out and the cryptozoites become free in small blood sinusoids of liver.
• This first stage of asexual reproduction or schizogony in liver is known as pre erythrocytic phase.
(B) Exo-erythrocytic Phase-The cryptozoites of pre-erythrocytic stage now enter separately into new liver cells, grow in schizont stage like before and form innumerable merozoites by schizogony. These merozoites which are formed in the second generation of liver schizogony are known as metacryptozoites or phanerozoites and this phase is known as exoerythrocytic phase or phanerozoic schizogony. It has been already discovered that the metacryptozoites formed in exo-erythrocytic schizogony are of two types:
(1) Micro-metacryptozoites These are relatively small and more in number,
(ii) Macro-metacryptozoites-They are large but less in number.
(4) Erythrocytic Schizogony or Phase-The micro-metacryptozoites or merozoites after coming in the blood circulation attack on the RBCs or erythrocytes, enter in the and start erythrocytic phase. In this phase the following stages are there :
(i) Young Trophozoite Stage-After entering in the RBCs or erythrocytes each micro-metacryptozoite transforms into a small disc-like structure which is known as young trophozoite and the stage is known as young trophozoite stage.
(ii) Signet Ring Trophozoite Stage-As the young trophozoite grows in size a central non-contractile vacuole develops in it due to which its nucleus shifts towards its periphery.
• This stage is known as signet trophozoite stage, because as its nucleus shifts towards its periphery it looks like a stoned ring.
• Signet ring trophozoite ingests maximum protein of the cytoplasm of RBC and forms a food vacuole inside which it secretes digestive enzymes.
• These enzymes proteolyse the blood haemoglobin, as a result of which the haemoglobin breaks into its component protein globin and red hematin pigment.
• Trophozoite digests the globin protein and use it as food, whereas unused hematin forms a poisonous malarial pigment or granule called haemozoin, which accumulates in the cytoplasm of trophozoite.
(iii) Amoeboid Stage and Growth Period-After some time, the signet ring trophozoite transforms into a amoeboid trophozoite. Due to active feeding it grows rapidly and its pseudopodia gradually disappears. In about 1/2 to 2 days it converts into an adult trophozoite which occupies most of the part of RBC.
(iv) Schizont and Schizogony Stage The amoeboid trophozoite after active feeding increases in size, becomes rounded in shape and occupies most of the part of RBC. This form of trophozoite is known as schizont.
• As the schizogony proceeds towards its completion, the schizont and human RBCs become weak and burst out.
• A complete erythrocytic cycle of P. vivax in man completes in 48 hours.
Malaria fever in man-When the schizonts present inside the RBCs burst and release the merozoites and haemozoin granules (also called malaria pigment) in the blood plasma, the man suffers from the malarial fever. It has been known that after about four erythrocytic cycles, on the completion of each cycle the patient feel chills, shivers and feverish, because all the schizonts present in the RBCs burst together.
• Haemozoin is called as poisonous substance and because of the presence of this substance the patient suffers from high fever and shivering.
• Each cycle takes a fixed time of 48 to 72 hours (48 hours in P. vivax), depending on the species. It proves that in the metabolism of this low graded parasite a virtual ‘biological clock works and due to the consecutive cycles innumerable RBCs of man get destroyed.
Post-erythrocytic Schizogony-Sometimes the merozoites formed in the erythrocytic schizogony leave the blood flow and re-enter in the liver. Here they enter in the liver cells and repeat the liver schizogony. This cycle of schizogony in liver cells which starts from the merozoites from blood is known as post-erythrocytic cycle or schizogony.
(1) Formation of Gametocytes in the RBCs of Man-Due to the recurrence of erythrocytic cycles again and again in the human blood a stage comes in which the merozoites reduces or loses its ability or impulse to reproduce asexually or by schizogony and the merozoites stop to repeat the erythrocytic cycle.
• In this adverse condition after entering in the RBCs merozoites grow differently, in which they become rounded and are transformed into gametocytes or gamonts.
• Its nucleus is big and cytoplasm is more dense, because it contains much more haemozoin granules.
• In the cytoplasm of RBCs containing gametocytes Schuffner’s granules are also more. These gametocytes of Plasmodium clearly demarcate sexual diamorphismthat means they are of two types:
(i) Male or Microgametocytes– These are small. Their nucleus is big and centrally located. Cytoplasm is light brown in colour, because haemozoin granules are scattered in it. It lacks stored food materials.
(ii) Female or Megagametocytes-These are big in size. Its nucleus is small and located towards the periphery. Cytoplasm is more dense and dark brown in colour due to the stored food material in them, the haemozoin granules are more and centrallyaccumulated.
This gametocytic stage of Plasmodium is infective for its secondary host the female Anopheles mosquito. The further life cycle is possible in mosquito only, thus in the blood of man the gametocyte does not undergo any change.
SEXUAL CYCLE OF PLASMODIUM IN MOSQUITO
Infection of female Anopheles mosquito-When the female Anopheles mosquito sucks the blood of an infected human, along with the blood it takes in the RBCs with different stages of erythrocytic cycle of Plasmodium in its alimentary canal, which also contains RBCs with gametocytes. Except the gametocytes, Red Blood Cells and all other stages of malaria parasite get digested in the stomach of mosquito.
The gametocytes are not at all effected by the digestive juices. They are released in the stomach of mosquito and start the stages of sexual cycle actively. Their activation mainly depends on the temperature. For the activation of merozoites and trophozoites higher temperature is needed, whereas for the activation of gametocytes low temperature is needed. Thus gametocytes are unable to start their sexual cycle in Man, as Man is warm-blooded and its body temperature remains high
than the body temperature of cold-blooded mosquito. The sexual-cycle of Plasmodium can be divided into following stages:
(1) Gametogony– This process completes in the stomach of mosquito. After releasing in the stomach of female Anopheles mosquito, gametocytes form gametes of sex cells by gametogony, which are as follows:
(i) Spermatogenesis-In this process, the nucleus of each microgametocyte divides by meiotic division into 6 to 8 haploid daughter nuclei which arrange themselves to the periphery. Now from the cytoplasm of microgametocytes long, thin and flagella like cytoplasm comes out in which one daughter nuclei enters. This process is known as flagellation.
(ii) Oogenesis-As a result of meiotic division in each female or megagametocyte its reformation takes place, under which two polar bodies remove out and it transforms into a haploid ovum. It is also known as megagamete.
(2) Fertilization-The process of fertilization also completes in the stomach of mosquito. In this process a small cytoplasmic projection is formed on the surface of each ovum which is known as reception cone or fertilization cone. The nucleus inside the ovum also comes near this reception cone. When any microgamete or sperm comes in contact with the ovum, it sticks to the reception cone of the ovum and enters in it. The cytoplasm of both sperm and ova mixes inside the ova and the haploid nucleus of sperm and egg fuses and forms a diploid nucleus which is known as zygote or synkaryon.
(3) Post-zygotic or Motile Zygotic or Ookinite Stage-These immobile zygotes formed in the stomach of mosquito remain in this stage for some times, but then they soon become long, worm like and motile. They does writhing or gliding movements in the gastric cavity, these motile zygotes are called vermicules or ookinite or motile zygote. Each of them is 15 μ to 22 u long and 3 μ wide.
(4) Encystment-Motile zygote or ookinite pierces the peritrophic membrane of stomach of mosquito from inside by its light coloured end. At the same end it secretes a jelly-like substance which helps it in piercing the wall of stomach. When the ookinite reaches just below the outermost peritonium layer of its stomach wall it again becomes a rounded zygote and encloses itself in a cyst of 8-10 u diameter secreted by itself. The wall of cyst is thin, flexible and semi transparent. This encysted zygote is known as oocyst. It can be seen as transparent rounded structures on the outer surface of stomach. On the stomach of an infected female Anopheles its number varies from 50 to 500.
(5) Sporogony– The process of formation of sporozoites from oocysts is known as sporogony. This whole process takes place below the peritoneum membrane on the Outerwall of stomach of female Anopheles mosquito. Sporogony is an asexual multiplication which can be understood as follows:
At this stage, the mosquito becomes infective. In the salivary gland of an infected mosquito about 2,00,000 sporozoites can be found. When it sucks the blood of a healthy human being, thousands of sporozoites along with its larva enter in the blood flow of man and starts the asexual cycle again.