By Department of Zoology P.G.G.C.G. - 42, Chandigarh

Kingdom:Protista Group:Protozoa Phylum:Sporozoa Class:Coccida Family:Plasmodidae Genus:Plasmodium Species:vivex

 Pathogen of malaria.  Have two hosts : (a) an invertebrate (female Anopheles mosquito), (b) a vertebrate (Human).  P. vivax, P. ovale, P. malariae, P. falciparum.  P. vivax & P. falciparum are more common.  Plasmodium is a wide distribution in many tropical or sub-tropical regions of the world.  Causes malaria in human, but is practically harmless to the mosquito.  Mosquito contains the sexual phase of the parasite (Primary, definitive or final host), whereas Human contains asexual phase of the parasite (Secondary or intermediate host).  Anopheles mosquito is also called a Vector (carrier of infective stage of Plasmodium, sporozoite).  Common species of Anopheles spreading malaria in India are A. stephensi, A. maculatus, A. fluvitalis, A. culicifacies, A.minimus, etc.

Life history of Plasmodium vivax is digenetic, i.e., involves two generations consisting of asexual and sexual phases. The three main phases are: (a)Schizogony: It involves phase of growth and asexual multiplication. Occurs in human. (b) Gamogony: It is a sexual phase which starts in human and completed in the mosquito gut. (c)Sporogony: Phase of sexual multiplication which takes place in the mosquito gut.

 Schizogony: It involves two cycles :- (i) exo-erythrocytic or hepatic schizogony (in liver cell), (ii) eythrocytic schizogony (in R.B.C.). (I) Exo-erythrocytic Schizogony involves infection of man and multiplication of parasites. It comprises of: (a) Inoculation or infection of Human by parasite (Plasmodium sp.) when an infected female Anopheles mosquito bites (injecting saliva containing about 1000 infective sporozoites) while sucking blood meal for her egg production. (Sporozoites has a motile, slender, slightly curved, sickle-like body about 15µm in length tapering at both ends). (b) Multiplication of parasite (sporozoites) takes place in the liver cells where they assumes various forms such as Cryptozoites, Cryptoschizonts, Cryptomerozoites, Metacryptozoites, Metacryptoschizonts and Metacryptomerozoites as depicted ahead in the flow-chart. (Contd.)

(c) Metacryptomerozoites are of further two types:- (Contd.) (i) micro-metacryptomerozoites (smaller in size but larger in number) and; (ii) macro-metacryptomerozoites (larger in size but fewer in number). The whole of the above process takes 12 days to complete (incubation period) and after that the parasite takes two course of action: the macro-metacryptomerozoites again infects the fresh liver cells and forms a reserve stock of the infection (post-erythrocytic schizogony), while, the micro-metacryptomerozoites takes another route to infect the R.B.Cs. and start erythrocytic schizogony. (II) Erythrocytic Schizogony: A single parasite usually enters one corpuscle where it feeds and grow into another form called Trophozoite where it develops a large vacuole in its cytoplasm and now termed as Signet-ring stage. Lateron, this ring disappears and trophozoite assumes an irregular shape called Amoeboid stage. It further eats up the cytoplasm of R.B.C. and grow to its size forming erythrocytic Schizont stage, which finally divides by fission into tiny, uninucleate bodies called Merozoites arranging themselves into Rosette stage inside the R.B.C. wall. These merozoites were released into the plasma after 48 hrs. of the infection of R.B.C. along with residual cytoplasm, haematin and toxins.

Resting posture of mosquitoes A – Anopheles; B – Culex Adult trophozoites as shown by electron microscope

 Gamogony: To avoid their host from death after weakning its immune system which also indirectly means their self-destruction the parasite need a new healthy host. For this merozoites grow slowly and undergoes another forms of sexual phase called gametocytes or gamonts:- (i) Male or microgametocytes (smaller and fewer). (ii) Female or megagametocytes (larger and numerous). Both these forms are unable to develop further inside the body of humans and the parasite now waits for being sucked up by a new Anopheles mosquito along with blood-meal. If not, these gametocytes are destroyed due to high body temperature of the host. They were also get destroyed if sucked up by other genera of mosquitoes (e.g., females of Aedes or Culex).

In female Anopheles mosquito gametocytes withstand the action of gastric juices, break open the corpuscles and becomes free in the mosquito gut. The low body temperature of mosquito permits gametocytes to develop further into respected anisogametes- male or microgametes and female or megagametes. (A single microgametocyte produces 4-8 motile microgametes, whereas, a single megagamete or ovum will be produced from each megagametocyte). Syngamy of dissimilar gametes will results into the formation of zygote (rounded and motionless). It soon becomes elongated and worm-like (Ookinete or Vermicule) which pierces through the wall of the stomach upto just beneath the outermost layer and forms a cyst around itself, now called as Oocyst or Sporont. There may be upto 50 oocysts on the stomach of one infected mosquito. These nuclei in the oocyst lateron divides repeatedly to form infectious Sporozoites. One oocyst may produce 50,000 sporozoites which finally migrated to the salivary gland of the mosquito which already have 200,000 sporozoites at one time.

Stomach of an infected mosquito showing encysted zygotes or oocysts

Life-history of Plasmodium vivax – the malarial parasite

 Sporogony and Schizogony represents the Asexual generation in the life cycle of Plasmodium vivex as in both these phases the parasite reproduces by an asexual method of multiple fission.  Gamogony on the other hand represents the Sexual generation as it involves the formation of male and female gametes and their subsequent fusion.  The two generations follow each other quite nicely in the two different hosts (Human being and mosquito) in the life cycle of Plasmodium vivex and complements each other very well for the successful establishment of parasite and its pathogenicity. This phenomenon is called alternation of generation and regular change of host with change in generation is called alternation of host.

Early trophozoite (ring form) 1 red nucleus on the ring-like light blue cytoplasm; single infection in a cell. Late trophozoite It is irregular shape like ameboid form with pseudopodia; within cytoplasm, brown pigment granules (malarial pigment- haemozoin) appear.

Immature schizont Oval in shape; nucleus divided into 2-4 or more; malarial pigment begins to concentrate in a mass Mature schizo nt Nucleus divided into 12-24; cytoplasm also divided; each nucleus surrounded by a portion of cytoplasm to form merozoites; malarial pigment clumped

Male gametocyte Oval in shape; 1 loose nucleus in centre of it; malarial pigments diffuse Female gametocyte Oval in shape; 1 compact nucleus not in centre of it.

Chlorquine and quinine : anti-erythrocytic stage drugs. Primaquine and pyrimethamine : anti- exoerythrocytic stage drugs. Reconstruction of environment : eradicate the breeding places of moquitoes. Use mosquito nets, screen, or mosquito repellents to protect the person from mosquito bites. Spray insecticides like DDVP and so on.