SUB PHYLUM : SPOROZOA
CLASS : TELOSPORIA
SUB CLASS : COCCIDIA
ORDER : EUCOCCIDIA
GENUS : PLASMODIUM
SPECIES : VIVOX
Plasmodium vivax is a cytozoic , pathogenic , digenetic and most widely distributed protozoan parasite. It occurs in the liver cells and RBCs of human beings causing benign tertian malaria. Its primary host is the female Anapheles Mosquito and the secondary host is man. Reservoir host is monkey. The infective stage is sickle shaped sporozoite and the mode of infection is inoculation.
Four species of Plsmodium cause four types of malaria in man. They are
1.Plasmodium vivox – benign tertian malaria.
2.Plasmodium falciparum – malignant tertian malaria or cerebral malaria.
3.Plasmodium ovale – mild tertian malaria.
4.Plasmodium malariae – quartan malaria.
Off all these four species , Plasmodium vivox is the most common and most widely distributed malaria parasite. Hence its life cycle along with pathogenicity , treatment and preventive measures are briefly described here.
HISTORY OF MALARIA
1.The term Malaria is taken from two Italian words Mala and aria which means “bad air” as it was thought that malaria was due to foul air produced in marshy lands.
2.The word malaria was coined by Macculoch in 1827.
3.Charles Laveron , a French military doctor ,first observed the parasite in the blood of malaria patient in 1880.
4.Camilo Golgi an Italian scientist , observed the malarial parasite in the red blood cells of malarial patients in 1885.
5.Richard Pfiffer , a German doctor said that the malaria may spread by blood sucking insects.
6.Sir Patric Manson, a Scottish doctor discovered that the filariasis is spread through female culex mosquitoes in 1894.
7.Sir Ronald Ross , british army doctor working at secunderabad discovered oocysts of plasmodium in the crop wall of female anopheles mosquito on 20th august 1897. For this discovery, he was awarded Nobel prize in the year 1902.
In recognition of this famous work of ross, 20th august of every year is celebrated as world malaria day or world mosquito day.
8.G.B Grassi and others have described the life cycle of Plasmodium vivax in the female Anopheles mosquito in 1898.
9.Garnham published a detailed monograph on Plasmodium vivax in 1966.
Structure of sporozoite:
1 1.The infective stage of malaria parasite to humanbeings is sporozoite.
2.The sporozoite occurs in the salivary glands of female anopheles mosquito.
3.The structure of sporozoite was described by Garnham 1963.
4.The sporozoite is sickle shaped with a swollen moddle part and pointed at both ends of its body.
5.It measures about 15 microns in length and 1 micron in width.
6 6.The body is covered by an elastic pellicle with Microtubules which help in the wriggling movements of the sporozoite.
7.The cytoplasm is granular with cell organelles such as Golgi complex , endoplasmic reticulum, mitochondria and a nucleus.
8.The cytoplasm also shows many convoluted tubules of unknown function throughout the length of the body.
9.It shows a cup like depression called apical structure at the anterior end which helps in adhesion.
10. A pair of narrow tubular structures called secretory organelles open into the apical cup. They secrete a cytolytic enzyme, which helps in the penetration of sporozoite into the liver cells.
LIFE CYCLE OF PLASMODIUM VIVAX
The life cycle of plasmodium vivax includes two hosts viz., the primary and secondary hosts.
The primary host is the female Anopheles mosquito, in which the parasite completes the sexual part of the life cycle without causing any disease. This part of the life cycle is called cycle of ross in the honour of sir Ronald Ross. Mosquito is also the vector host.
The secondary host is the human being. In human being , asexual part of the lifecycle is completed by causing disease. Thispart of the life cycle is called Golgi cycle in the honour of Camillo Golgi .
The reservoir host of malarial parasite is monkey.
In man, the plasmodium vivax reproduces asexually by multiple fission called schyzogony. It occurs in the livercells (hepatocytes) as well as in R.B.C. In liver cells, it is called hepatic schyzogony and in RBC it is called erytrocytic schyzogony.
The hepatic schyzogony of plasmodium vivax was discovered by Shortt and Garnham. Whenever , a mosquito infected by plasmodium bites a man , nearly 2000 sporozoites are released into the blood of man through its saliva . Within half an hour , they reach the hepatocytes where they undergo pre-erythrocytic and erythrocytic cycles.
Pre-erythrocytic cycle :
1. Sporozoites are the infective stages of plasmodium to man.
2 . Whenever the sporozoites reach the liver cells , they transform into trophozoites.
3. They feed on the contents of the heparic cells and attain the maximum size. This stage is called schizont.
4. The nucleus of schizont divides several times mitotically, followed by the cytoplasmic divisions reaulting in approximately 12,000 daughter individuals called cryptozoites or the 1st generation merozoites.
5. The cryptozoites enter the sinusoids of liver by rupturing the cell membrane of the schizont and the liver cells.
6. This entire process is completed approximately in 8days.
7. Now these cryptozoites have two options i.e., they can enter either fresh liver cells and continue exo-erythrocytic cycle or they can enter RBC and continue erythrocytic cycle.
1 If the cryptozoites enter the fresh liver cell, it transforms into a trophozoiteand feeds on liver cell.
1. After attaining maximum size it undergoes multiple fission and results the second generation merozoites called matacryptozoites.
2. Metacryptozoites are of two types – the smaller micro-meta cryptozoites and larger macro-metacryptozoites.
3. This entire process is completed approximately in two days.
4. The macro-metacryptozoites attack fresh liver cells and continue another exo-erythrocytic cycle. Where as the micro-metacryptozoites always enter blood stream and attack fresh RBC to continue erythrocytic cycle.
The interval between the first entry of plasmodium into the blood in the form of sporozoites and the second entry of plasmodium in the form of cryptozoites is called prepatent period. It lasts approximately 8 days During this period, the host does not show any clinical symptoms of the disese . It is only a means of multiplication.
1. The erythrocyic cycle of plasmodium vivax was first described by Camillo Golgi.Hence it is also called Golgi cycle.
4. The trophozoite develops a small vacuole which gradually enlarges in size , pushing the cytoplasm and nucleus to the periphery. Now the Plasmodium looks like a finger ring. Hence this stage is called signet ring stage.
5. Soon it looses the vacuole, develops pseudopodia and becomes amoeboid stage.
7. The malaria parasite digests the globin part of the ingested haemoglobin and converts the soluble haem into insoluble crystalline haemozoin. It is called the malaria pigment which is a disposable product.
8. During this stage , small red cooured dots appear in the cytoplasm of the RBC known as Schuffner’s dots.These are believed to be the antigens released by the antigens released by the parasite.
12. This cycle is completed approximately in 48 hours.
The incubation period between the entry of Plasmodium into the blood in the form of sporozoite and first appearance of symptom of malaria in man is called incubation period. It is approximately 10-14 days.
FORMATION OF GAMETOCYTES :
In human beings as erythrocytic cycles are repeated several times, a decrease in the number of RBCs sets in.
Due to this, parasites may not be able to survive further. Hence some of the merozoites instead of continuing the erythrocytic cycle transform into gametocytes. The gametocytes are formed in the spleen and matured in the bone marrow.Gametocytes are of two types , namely
Microgametocytes are small in size, with a large central nucleus and form the male gamete. Macrogametocytes are
larger in size with a small eccentric nucleus an granular cytoplasm.
In human beings, the lifecycle does not continue after the formation of
gametocytes. The gametocytes donot develop as the conditions like body temperature and PH of blood in man are no favourable. These conditions favour their further development in mosquito. Hence if they fail to reach mosquito , they degenerate and die in about one week time.