Plasmodium belongs to the class of telospora which comes under phylum protozoa.
There are many species of plasmodium present. Amongst them plasmodium falciparum and plasmodium vivax are main. These are responsible for causing malaria in human body.
In this lecture, I would explain about this malaria parasite in detail. I would also explain the life cycle of plasmodium.
Some basic knowledge on malaria parasite’s brief history, cause, treatment, prevention would also be explained.
Let us now dive into this interesting topic.
Introduction to Plasmodium
Most people have heard the name of malarial fever and many have suffered from it.
Malarial fever spreads by a particular kind of mosquito which is called Anopheles.
Actually, there is nothing such in mosquito by which malarial fever can be started but in its body there is a parasitic protozoon which is known as Plasmodium.
Some of the phases of the life-cycle of this parasite are found in mosquito and others in men.
The mosquito introduces the phases of parasite into the blood of man when it bites and sucks its blood. It is by these phases that the malaria is caused.
Thus Plasmodium Or malarial parasite completes its life-cycle in two hosts. Man is its primary host and female Anopheles is its secondary host.
Mosquitoes transmit the parasite from one man to another, therefore, they are called the carrier of malaria.
Plasmodium reproduces by asexual method in man while sexually in mosquito. Four species of Plasmodium are known to cause different types of malaria fever in man.
In this article description of Plasmodium vivax is given.
Classification of Plasmodium
Life Cycle of Plasmodium
The life cycle of malarial parasite is completed in two phases:
- Asexual or Endogenous phase,
- Sexual or Exogenous phase.
Life Cycle of Plasmodium Vivax in Man
This phase takes place in man. It is further divided into three phases:
- Pre-erythrocytic phase,
- Exo-erythrocytic phase,
- Erythrocytic phase.
(a) Pre-erythrocytic phase
When an infected female Anopheles mosquito (whose main food is the blood of man) in which malarial parasites are present, bites a healthy man. It leaves some sprozoites in the blood of man along with its saliva.
Each sporozoites is sickle-shaped and about 14 micra long. These sporosozite come into the blood stream of man.
After about half an hour they leave the circulatory system and reach parenchymatous cells of the liver.
In parenchymatous cells of liver, sporozoites grow and form schizonts. After some time the nucleus of schizont divides into about 1,000 small nuclei. A little cytoplasm gathers around
each nucleus and thus 1,000 merozoites are formed which are also called cryptozoites. The nucleus of schizont divides by multiple fission. This division is known as liver schizogony.
Now the wall of schizont and liver cell bursts due to great number of cryptozoites in them, and all the cryptozoites go into the sinusoides of the liver.
There is no effect of medicines on these cryptozoites which also remain unaffected by the resistance of the host.
(b) Exo-erythrocytic phase
The cryptozoites of the pre-erythrocytic phase which are released in the liver sinusoides attack new liver cells and they again form schizonts.
These again undergo multiple fission and form many cryptozoites which are called metacryptozoites. This phase is known as exo-erythrocytic phase.
After two such phases metacryptozoites come into the blood circulation and attack the red blood corpuscles.
(C) Erythrocytic cycle
This cycle occurs in RBCs and begins when a cryptomerozoite or micrometa-cryptomerozoite enters into an RBC.
Meta crylomerozoites start growing in the red blood corpuscles obtaining their food from them. They are round in shape and soon a non-contractile vacuole is formed in them. This stage is called signet ring stage.
With further growth the vacuole disappears and it assumes Amoeba like form.
It starts feeding on hemoglobin and grows rapidly. This stage is known as trophozoite stage. After feeding on hemoglobin they form a yellowish brown substance called haemozoin.
As the trophozoite increases in size, it becomes round and completely fills the red blood corpuscle. It is now known as schizont.
The red blood corpuscle becomes weak and loses its normal shape due to the growth of the schizont within it. It swells in size and becomes light in colour with its regular shape.
In its remaining cytoplasm several small, orange or yellow coloured granules are formed which are known as Schuffner’s dots.
Multiple fission takes place in schizont and its nucleus divides into many small nuclei around each of which a little cytoplasm gathers.
In this manner 6 to 36 meozoites are formed in each schizont. After some time erythrocyte bursts liberating the merozoites in the blood.
These merozoites attack fresh red blood corpuscles. This process is repeated many times and innumerable red corpuscles are destroyed.
Haemozoin is a toxic substance formed as a waste product in R.B.C. by the destruction of haemoglobin.
When it is released in the blood it dissolves in the plasma due to which malarial fever starts with chills and shiverings.
Life Cycle of Plasmodium In Mosquito
Sexual or Exogenous phase
The sexual phase takes place in female Anopheles. It is divided into two stages:
(a) Gamogony, or Gamogamy or Gametogenesis
After many asexual cycles some merozoites increase in size within the red blood corpuscles and form gametocytes.
Some of these are small and known as microgametocytes while others are large and called macrogametocytes.
Further development of these gametocytes is not possible in man because of the high body temperature.
Female or macrogametocytes are round and their cytoplasm is laden with food. Their nucleus is eccentric, i.e., placed on one side. Male or microgametocytes have clear cytoplasm with a central nucleus.
If a female Anopheles mosquito bites an infected man having this stage of Plasmodium then along with merozoites some gametocytes are also sucked up with blood and reach the stomach of mosquito, where their further development takes place.
The nucleus of microgametocytes divides into 6 to 8 small nuclei around each of which a little cytoplasm collects forming long, flagellated known as microgametes. This process is called exflagellation.
Macrogametocyte undergoes little changes. It increases in size and its nucleus divides into two. One of these nuclei moves out along with some cytoplasm and forms the polar body.
In the remaining larger part a small conical structure is formed which is known as reception zone.
Thus, a single macrogamete is formed from each macrogametocyte.
A small cytoplasmic bulge called cone of reception or fertilization cone forms on one side of ovum.
Microgamete is attracted towards the macrogamete and enters the latter in its reception zone. The nucleus and cytoplasm of both the gametes fuse and form a zygote. This process is known as fertilization.
The zygote remains inactive for some time after which it elongates and becomes motile like a worm. This motile zygote is known as ookinete or vermicule.
The ookinete penetrates the wall of the stomach and after passing through its muscles settles in the outermost layer where it forms a protective covering or cyst around itself.
Such structures are called oocyst and this process is known as oocystation. Fifty to five hundred oocysts are found in the stomach of a female mosquito.
(b) Sporogony and Oocyst
Encysted zygote or the oocyst, now increases about five times its size and its nucleus divides into many small nucleus.
Simultaneously several vacuoles are formed in the cytoplasm and the nuclei gather around them.
A little cytoplasm collects around each nuclei forming elongated structures called sporozoites, or single oocyst may contains about 1,000 sporozoites. This process of formation of sporozoites in the wall of the stomach is known as sporogony.
After some time the wall of oocyst bursts due to much pressure and the sporozoites are set free in the body cavity of the mosquito.
Soon after, sporozoites enter the salivary gland of the mosquito which becomes infective. When this mosquito bites a healthy man the sporozoites enter his blood along with the saliva.
Thus, the life history of malarial parasite is completed.
What is Parasitism?
The parasites are dependent on their hosts and cannot exist without them. Plasmodium is a parasitic animalcule. Its structure is very simple.
The existence of different stages in the life-cycle of the Plasmodium is far increasing its number so that the race of parasite is not destroyed at the death of the host.
Malarial parasite does not cease to exist when a patient of malaria dies because its different stages have already reached the mosquito’s body due to which its life cycle continues.
The presence of secondary host is also essential, because without it several reproductive cycles cannot be completed and the parasite cannot be transmitted from one man to another.
Different Species of Plasmodium
Malarial fever is caused by four species of Plasmodium. The life cycle of these species is almost similar. The difference is in the structure and the time taken for completion of schizogony.
1. Plasmodium ovale
This parasite is found in western and southern Africa. It causes mild tertian fever which recurs after every 48 hours.
2. Plasmodium malariae
This parasite causes quartan malaria in which fever comes after every 72 hours. This species is common in Africa, Shri Lanka and Southern India.
3. Plasmodium vivax
This species causes benign tertian malaria in which the fever comes after every 48 hours. It is found in China, Africa, Central America and other hot countries.
4. Plasmodium falciparum
From this species the fever can come any time after 36 to 48 hours. Its fever is also called subtertian fever.
History of Malaria and Its Treatment
Malaria is a highly fatal disease. Every year millions of people are killed by it. French surgean Laveran was the first to discover parasite in man in Algeria.
In 1898 Sir Ronald Ross found that malaria is transmitted from one person to another by female Anopheles mosquito.
Later in 1899 Graessi studied the life cycle of malarial parasite in Anopheles mosquito. Its life cycle in man was studied by Shortt Tate, Huff and James.
Symptoms of Malaria
Malaria fever comes with chill and sbivering after which the body temperature of the patient rises up to 103°F and 104°F. Cramps start in the body. The fever comes at noon or midnight. It remains for 6 to 8 hours and then gradually comes to normal.
Control of Malaria
There are several methods for prevention and control of malaria. For complete eradication of malaria three methods are used:
1. Destruction of Mosquitoes and Their Eggs
Malaria is spread by mosquitoes. Therefore, if we check the mosquitoes and the development of their eggs, the possibility of spreading of malaria can be controlled to a large extent. The following are the methods used:
(a) The ditches where mosquitoes breed should be filled up with sand or their water be drained out.
(b) If above is not possible then lime powder or kerosene oil be sprayed on the surface of the water so that eggs, pupa or larvae of mosquito may not respire and die.
(c) Gambusia fishes which feed on the larvae of mosquito be introduced in ponds and ditches.
(d) D.D.T. or other insecticides should be sprayed in houses.
2. Measures to Escape Mosquitoes
(a) Mosquito repellent preparations or mustard oil should be applied on the exposed parts of the body before sleeping at night.
(b) Mosquito nets should be used.
Destroying the Malarial Parasite in Human Body
There are several medicines by which we can kill the parasite in our body. Commonly used drugs are quinine, palurine, and camoprima.
Ayurvedic and Unani medicines are also useful.
Chemotherapy is the treatment of infection. The treatment of malaria is not possible by inoculation of vaccination as the parasite does not produce toxins.
The following allopathic oral medicines are given to the patients of malaria, such as Quinine, Camaquin, Atebrin, Chloroquine, Daraprin, Resochin, etc.
Quinine rapidly kills the schizonts but has no effect on gametocytes. Plasmochin is more effective on gametocytes.
Paludrine and Resochin are considered to be more effective, they kill all stages of the parasites. Daraprin is considered more promising but slow in action. If Darapin is taken regularly, it suppresses malaria completely.
It is prevention from infection. It is said that ‘prevention is better than cure‘. Prevention from the infection can be done by two methods:
(a) Protection from Mosquito Bites-Protection from mosquito bites should be taken. At the time of sleeping mosquito nets should be used, otherwise the exposed parts of the body should be covered by clothes. Application of repellents such as anti mosquito creams, mustard oil, citrinella oil, coconut oil, dimethyl carbate etc. should be used before sleeping.
(b) Chemoprophylaxis, i.e., Prevention of Infection by Medicines– Small regular doses of preventive medicines should be taken to prevent the clinical development of malarial fever. Atebrine, Chloroquine, Paludrine, Daraprin are good prophylactic drugs.