Members of the genus Plasmodium are collectively known as malarial parasites because they cause a febrile disease by the bite of the malarial parasite infected female anopheles mosquitoes called malaria. The term malaria is an Italian word and is composed of two words, mal = bad, and aria = air; so malaria means bad air. The name ‘malaria’ was given by Italian author Macculoch in 1827.
Systematic position (According to Honigberg, 1964 and Corliss, 1967):
Phylum – Protozoa
Subphylum – Sporozoa
Class – Telosporea
Subclass – Coccidia
Order – Eucoccida
Genus – Plasmodium
Species – P. vivax, P. falciparum,
P. malariae, P. ovale, etc.
Host:
The life cycle of human malarial parasites (Plasmodium spp.) is completed by two hosts (digenetic). The asexual cycle of the parasite is completed within man and the sexual cycle is completed within the female anopheline mosquito. Here man is the intermediate or secondary host and female Anopheles mosquito is the definitive or primary host.
Definitive host or Primary host:
The host which provides nutrition to the parasites and in which the parasite attains sexual maturity and undergoes sexual reproduction is termed the primary or the definitive host.
Intermediate host or Secondary host:
The Intermediate or Secondary host is one in which some development of the parasites takes place and the sexual reproduction does not happen.
Considering the reproductive condition of the malarial parasites (Plasmodium spp.), the female anopheline mosquito is the primary or definitive host because sexual reproduction takes place within the mosquito and humans, or other vertebrates are the intermediate host because asexual reproduction takes place within them.
In other sense the definitive host like female anopheles mosquitoes or other insects, is one which transmits parasites from one host to another, called vector or carrier host.
Vector is of two types:
(i) Biological vector:
Biological vector in which some stage of the life cycle of the parasite is completed within host; e.g., female anopheles mosquito.
(ii) Mechanical vector:
Mechanical vector is one by which the parasites are transmitted from one host to another passively; e.g., fly, rat, etc. The life cycle of the parasites is not completed within mechanical vector’s body and only the pathogens or infections are transmitted mechanically and passively with the help of different organs of the individual.
The word ‘Anopheles’ has been derived from Greek. ‘An’ means in Greek a prefix to a word which gives it a negative sense and ‘opheles’ means in Greek use or advantage. About 91 anopheline mosquitoes are reported to be vectors of malaria in the world and 10 species have been reported as vectors of malaria in India. A list of Indian insect vectors of different diseases is given in Table No. 10.2.
Historical Background:
(i) Charles Louis Alphonse Laveran (1880):
Charles Louis Alphonse Laveran (1880), a physician with the French army first observed some amoeba-like organisms termed them Plasmodium, in the red blood cells of a patient in Algeria, N. Africa suffering from malaria. He was awarded Nobel Prize in 1907 for the role of plasmodium in causing malaria.
(ii) Patrick Manson (1894):
Patrick Manson (1894) suggested that the Plasmodium is carried by the mosquitoes.
(iii) Sir Ronald Ross:
Sir Ronald Ross, a surgeon with the British army in India discovered in 1897 that the female Anopheles mosquitoes are the carrier of human malarial parasites (Plasmodium spp.) and also discovered the developmental stages of the parasite in mosquito.
This discovery was done in Presidency General Hospital, Calcutta (Recently it is called S.S.K.M. Hospital, Kolkata). He was awarded Nobel Prize for Medicine in 1902 for this discovery.
(iv) Bignami, Bastianelli and Grassi (1898-1899):
Bignami, Bastianelli and Grassi (1898-1899) discovered the life cycle of human malarial parasite within Anopheles maculipennis.
Geographical Distribution:
They are found worldwide extending from 60° N to 46° S of the equator and covering a larger portion of the tropical, subtropical and warmer temperate regions. They have recorded at a height of 9348 ft in Tudzhik, Russia.
Habit and Habitat:
The human malarial parasites live as intracellular parasite. The asexual stage is found in the reticulo-endothelial system of man and sexual stage spends in the digestive system in the female anopheline mosquito. But E.M. study has revealed that Plasmodium lives within the invagination created on the dorsal surface of RBC. So Plasmodium is intercellular parasite, not intracellular.
Life history of Plasmodium vivax Grassi and Feletti, 1890
(Causative organism of benign tertian malaria) [benign = not dangerous; tertian = recurring (fever) every other day]
Historical background:
Grassi and Feletti first propose the scientific name Plasmodium vivax in 1890. Schaudin (1902) discovered the developmental stages of Plasmodium vivax in detail.
Geographical distribution:
They are worldwide in distribution, mainly in the temperate zone. They are wide prevalent in India, Sri Lanka and Bangladesh.
Life cycle of Plasmodium vivax:
The life cycle of Plasmodium vivax is divided into following stages—(i) Human cycle and (ii) Mosquito cycle (Fig. 10.27).
The asexual cycle of Plasmodium vivax occurs within human beings and sexual cycle is passed in the female anopheline mosquitoes.
A. Asexual cycle in man:
Classically the asexual life cycle of P. vivax is divided into 3 stages—(i) Pre- erythrocytic cycle (ii) Erythrocytic cycle and (iii) Exo-erythrocytic cycle but Cheng (1986) has divided into two stages—(i) Exoerythrocytic schizogony including pre- erythrocytic and exoerythrocytic cycles and (ii) Erythrocytic schizogony.
For the convenience of discussion we follow here classical division:
I. Pre-erythrocytic cycle:
Infection or inoculation:
The parasites are inoculated as sporozoites by the bite of the infected female Anopheles mosquito in the blood stream of healthy person.
Sporozoites (Under light microscope):
(i) The sporozoites represent the infective stage of the parasite.
(ii) They are slender and spindle-shaped individuals.
(iii) Each sporozoite measures 10-14 µm in length and 0.5-1 µm in diameter.
(iv) The body is covered with a plasma- membrane.
(v) An oval-shaped nucleus is situated at the centre of the body.
(vi) The cytoplasm does not contain pigments.
E. M. Study:
Garnham, Bird and Baker (1963) have reported the following structures of the sporozoites (Fig. 10.28).
(i) The outer firm and elastic pellicle consists of 3 layers which contain longitudinally arranged 11 hollow microtubules.
(ii) The microtubules are contractile in nature and help in wriggling movement of the body.
(iii) At the anterior end of the sporozoite there is a cup-like depression with 3 concentric rings which forms a complex structure called apical complex.
(iv) A pair sac-like bodies with a narrow part called rhoptries opening to the anterior end, are thought to produce proteolylic enzymes aid in the penetration of the host body cells.
(v) The cytoplasm contains a single vesicular nucleus and a mitochondrion and several convoluted tubules.
Within half an hour of their entry into the blood stream of man, the sporozoites take refuge in the liver parenchyma cells. The sporozoites remain within parenchyma cells for about 7 days. During this period each sporozoite develops into a round shaped biologically organised structure, called Schizont.
The nucleus of each schizont is peripherally situated and is about 42 µm. The schizont carries on multiple fission (asexual reproduction) or schizogony. This phase of multiplication within parenchyma cells is called Pre-erythrocytic cycle.
Each schizont produces over 12000 genetically identical merozoites or cryptozoites within 8-9 days. The cryptozoites are regarded as exo-erythrocytic merozoites of first generation where only exo-erythrocytic schizogony is considered instead of pre-erythrocytic schizogony.
Characteristics of pre-erythrocytic merozoite:
(i) Each pre-erythrocytic merozoite is oval in shape and is bounded externally by pellicle.
(ii) The nucleus is distinct and centrally placed.
(iii) It is about 1.5 µm in length and 0.5 µm in diameter.
There are two subspecies in Plasmodium vivax. These are P. v. vivax and P. v. hibernans. Some of the sporozoites remain in liver parenchyma cells either in normal condition or in dormant condition up to 250 days. Normal sporozoites are called tachysporozoites and found in P. v. vivax and dormant sporozoites are called hypnozoites or bradysporozoites and are found in P. v. hibernans.
Newly produced cryptozoites or merozoites go to liver sinusoids from where they invade fresh parenchyma cells or red blood corpuscles.
II. Exo-erythrocytic cycle:
The pre-erythrocytic merozoites or cryptozoites enter the fresh hepatic cells (hepatocytes) and multiply by schizogony which produces about 1,000 exo-erythrocytic merozoites. These merozoites of the second generation which are produced in the hepatic cells are also called metacryptozoites or phanerozoites. These merozoites invade fresh RBC, In the RBC the merozoites carry on exo-erythrocytic cycle.
This exo-erythrocytic cycle was recorded in monkey, where malaria is caused by Plasmodium cynomolgi, As P, cynomolgi is very much similar to P. vivax, it is assumed that an identical exo-erythrocytic cycle also occurs in man.
III. Erythrocytic cycle:
The erythrocytic cycle starts when the RBC of blood are attacked by the pre- erythrocytic merozoites. The merozoites can penetrate the red blood corpuscles with mediated receptor sites.
Inside the RBCs the merozoites assume the shape of a rounded disc-like structure with a single large nucleus, called the trophozoite stage. The trophozoite measures about 1/3rd of the red blood corpuscle and is approximately 2.5-3 µm in diameter. The trophozoite shows the following features under the electron microscope (Fig. 10.29).
1. The body of the trophozoite is covered by a double-layered plasmalemma.
2. The cytoplasm contains small particles, probably containing nucleoproteins.
3. A large nucleus with granular material in the nucleoplam is seen.
4. A single nucleolus is situated eccentrically.
5. The endoplasmic reticulum appears vesicle-like and the surface may be either smooth or rough.
6. Several double-membrane bound mitochondria with cristae are seen in the central area of the cytoplasm.
7. Golgi bodies are vesicle-like and one or two food vacuoles containing haemozoin granules are present.
8. A double-layered concentric body seen in the cytoplasm, remains attached to the plasmalemma.
The young trophozoite grows at the expense of haemoglobin of blood corpuscles. It absorbs haemoglobin both by general body surface and pseudopodia. Soon after the entry of the merozoite into the corpuscle, a non- contractile vacuole develops in its body and nucleus is pushed to one side.
The shape of the trophozoites resembles a ring at this stage, and is called signet-ring stage (signet, L. signatus = private seal for use). The young trophozoite possesses pseudopodia and shows amoeba-like movement. The vacuole disappears with further growth.
A full grown trophozoite assumes a round shape and occupies the whole of RBC. During the signet ring stage of Plasmodium, the parasites ingest the haemoglobin of the RBC and the haemoglobin is decomposed into amino acids and haematin.
The amino acids are used by the trophozoite stage of the parasite and the unusual yellow-brown or black coloured haematin part, a kind of toxic malarial pigment is stored in the cytoplasm as haemozoin pigments.
Another kind of closely packed fine granules are seen on the surface of the RBCs. These granules are seen under light microscope after Romanovsky’s staining, called Schuffner’s dots named after the discoverer. By E. M. they can be seen as small vesicles in the membrane of the RBC containing antigens.
In about 48 hrs. a trophozoite becomes full grown, almost completely fills the enlarged corpuscle and forms the round shaped schizont. The schizont multiplies asexually by schizogony or merogony. When the schizont bursts a number of erythrocytic merozoites (approx. 16) are set free.
These merozoites are found to be arranged towards the periphery like the petals of a rose-flower and this appearance represents the rosett stage. The merozoites enter fresh red blood corpuscles and the cycle is repeated (Fig. 10.30). Erythrocytic cycle is also known as cycle of Golgi and is completed in 48 hrs.
Some of the merozoites behave differently from those which repeat schizogony in RBCs. At the end of their trophic phase they do not divide but come out into the plasma by rupturing the red blood cells. Some of these individuals change into rounded structures, called gametocytes or gamonts.
Within 96 hours, the haploid gametocytes become full grown and reach into superficial blood vessels. There are two forms of full-grown gametocytes—the female or macrogemetocyte and male or microgametocyte.
The macro-gametocytes are larger in size (10-12 µm) than the microgemetocytes, take deep stain and contain many pigment granules. The nucleus is round and is situated marginally. The microgametocytes are smaller in size (9-10 µm) and less numerous. It has a centrally placed, large diffused nucleus. It takes faint stain. Further development of the gametocytes does not take place within human body.
Ultimately the sexual cycle of the Plasmodium sp. starts in the blood of the human host with the formation of gametocytes. The rest of the sexual cycle occurs within female anopheline mosquito. If the blood cells containing gametocytes are not ingested by a mosquito, they will destroy within several weeks.
B. Sexual cycle in female Anopheles mosquito:
In order to complete the life cycle the parasite requires another definite or primary host- the female Anopheles mosquito within which the sexual cycle is completed.
Transfer to mosquito:
When a female Anopheles mosquito bites an infected man, the ingested blood fills her stomach containing gametocytes.
Gametogony:
All other stages of the parasites are destroyed in the stomach of the mosquito by the digestive juices except the gametocytes-sexual forms which survive and transform in the next stages of the gametocytes.
Development of microgametes or male gamates:
The changes in the haploid microgametocytes are more marked and five minutes after ingestion by the mosquito the microgametocytes become spherical. The nucleus divides thrice endomitotically and forms 8 nuclei which are arranged around the nuclear membrane.
The centriole is also replicated and each is associated with a nucleus. Later the centriole forms the basal body of each flagellum-like microgamete or male gamete. Ultimately 8 filamentous motile appendages emerge. This process is called ex-flagellation. The motile appendages are flagella-like structures with the 9 + 2 fibrils, and a nucleus with each flagellum are called male gametes or microgametes.
The difference in temperature range between human blood and the mosquito stomach helps the formation of motile microgametes. The microgametes become free and start moving towards female gametes for fertilization.
Development of macrogametes or female gametes:
In the stomach the haploid macro-gametocytes become spherical and a single macrogamete or a female gamete develops from a single macro-gametocyte. The macrogametes are less active and are ready to be fertilized.
Fertilization:
The microgametes are attracted towards macrogametes by a process of chemotaxis and fusion or fertilization takes place in the stomach of the insect. The other name of fertilization is called syngamy which happens to occur 10 minutes after the ingestion of blood. The syngamy is anisogamous.
Ookinete:
After fertilization, the fertilizing product is called zygote which is diploid (2n) and non-motile in nature. After 24 hours the non-motile zygote becomes elongated in shape and motile in habit. It performs gliding movements and in this new state it is called the ookinete or active vermicule, measuring 15 µm – 22 µm in length and is about 3 µm in breadth.
Electron Microscopic study:
(i) The pellicle of ookinete consists of two unit membranes.
(ii) Below the pellicle is a row of microtubules, and the number is about 55-65.
(iii) The microtubules are thought to help in locomotion.
(iv) Anterior end bears a slit-like cytosome.
(v) It contains several aggregations which are thought to be crystalloids and aid in the penetration of the gut wall of mosquito (Read, 1972).
Migration of ookinete:
The ookinete penetrates through the internal lining of the stomach of the mosquito and comes to rest in the sub-epithelial tissue.
Formation of oocyst or sporont:
The ookinete in the sub-epithelial tissue becomes round and encased in a covering or cyst derived partly from stomach tissue and partly from its own secretion. The encysted ookinete is now termed the oocyst or sporont. Oocyst formation becomes complete after 48 hours of ingestion.
The oocyst increases in size and measures 6-7 µm. The oocyst bulges on the outer wall of the stomach towards the haemocoel and the stomach wall blistered. In severe infections as many as 5,000 cysts may be seen.
Howard’s [Howard, L. (1960):
Howard’s [Howard, L. (1960) studies on the mechanism of infection of mosquitoes by malaria parasites, D.P.H. Thesis, John Hopkins University.] is of the opinion that only the zygotes that remain in the periphery of the blood meal of the mosquito become lodged between the cells of the mosquito stomach wall to develop as oocysts.
Those zygotes who fail to get shelter in the stomach wall of the mosquito develop into ookinete. The ookinete is voided with the faeces and as such they are nothing but dying parasites. Thus according to Howard “The long-held assumption that ookinete is motile invasive zygote is an example of a concept based on observations which were never made”.
Sporogony:
After about 7 days a number of lobes are formed in the oocyst, and sporogony occurs. During which the nucleus of the mature oocyst divides repeatedly to form a number of daughter nuclei. The first division of the nucleus of the oocyst is believed to be meiotic in nature (Bano, 1959).
Portions of protoplasm of the oocyst collect round each nucleus and thousands of daughter individuals forming a bundle of spindle shaped haploid sporozoites are formed. About 10,000 sporozoites are produced asexually from a single zygote. The muscular wall enveloping the oocyst bursts and cluster of sporozoites are liberated in the haemocoel of mosquitoes.
Then the sporozoites find their way into the salivary glands of the mosquito. In mosquito the whole cycle is completed in 10-20 days depending upon the temperature. The infective sporozoites are introduced into a healthy person when the infected mosquitoes bite a person and the life cycle is repeated again.
Life history of Plasmodium falciparum, Welch, 1897
[Causative organism of malignant (harmful) tertian malaria]
Historical background:
Welch (1897) gave the name Plasmodium falciparum.
Geographical distribution:
Worldwide in distribution. The parasite had been reported in the southern part of South America, South Africa, Europe, Southern parts of Russia, Southern part of China, India, Sri Lanka, Bangladesh, Vietnam, etc.
The life cycle of P. falciparum like P. vivax is completed through two hosts—man and female anopheline mosquito. It causes the fatal malignant tertian fever.
Asexual cycle in man:
The asexual cycle is completed by three phases:
(i) Pre-erythrocytic schizogony
(ii) Erythrocytic schizogony and
(iii) Gametogony.
In P. falciparum, exo-erythrocytic schizogony is completely absent.
Pre-erythrocytic schizogony:
The parasites are inoculated as sporozoites into the peripheral blood stream of man by the bite of infected female anopheline mosquito and take refuge in the liver. The sporozoites remain in the parenchyma cells of liver and each develops into a schizont. The schizont is multiplied by schizogony (multiple fission) and merozoites are produced.
The duration of pre-erythrocytic schizogony is about 6 days. The growing schizonts are found in liver parenchyma and are 15 µm in diameter. The mature schizonts are 60 µm in length and 30 µm in breadth. Each fully grown schizont produces about 30,000 merozoites. By the 7th day the merzoites are liberated from the liver and enter the RBCs and start the erythrocytic cycle.
Erythrocytic schizogony:
Two to six merozoites enter a single erythrocyte, and erythrocytic schizogony takes place in the blood capillaries of spleen, liver and bone marrows. The growing schizonts represent only ring forms and are found in the peripheral blood circulation. The cytoplasmic ring forms are smaller than P. vivax.
The wall of the ring is thick and is about 1 µm in diameter. The ring is situated at the margin of the erythrocyte. The nucleus of the partly grown ring or early schizonts often rod-shaped and divided into two granules lie at two opposite poles of the ring. Two or more rings (2-6) are found in an erythrocyte.
All schizonts disappear from the peripheral blood circulation when the schizonts become mature and are exclusively found in the capillaries of the internal organs. As schizont become mature, the diameter is about 4.5 µm-5.5 µm and Maurer’s dots appear on the infected erythrocytes. Maurer’s dots are brick coloured granules and are thought to produce from the outer covering of the parasite.
Schuffner’s dots are not found in P. falciparum and presence of Maurer’s dots indicate the trophozoite stage. Haemozoin granules are dark brown or black coloured pigments which remain in a compact mass and are not found scatterly as seen in other malarial parasites.
When the diameter of the ring increases up to 4 µm, the vacuole of the ring disappears and the parasite assumes a compact form with a single nucleus and dark pigments. When the diameter of mature schizont becomes 5 µm the nucleus is divided repeatedly and 8-24 or more merozoites are produced. Each merozoite measures about 0.5-0.7 µm in diameter.
Gametogony:
Some of the merozoites are transformed into gametocytes. The mature gametocytes measure about 10-12 µm x 2-3 µm and are seen in the peripheral blood circulation after 10 days of infection by the sporozoites. The gametogony takes place in the capillaries of spleen and bone marrow. The gametocytes are sausage-shaped, also called crescents.
Some of the gametocytes are changed into small-sized, blunt shaped structures called microgametocytes and large size and elongated narrow with pointed ends gametocytes called macro-gametocytes. The microgametocytes measure about 9-11 µm in length and stain light blue or reddish colour.
The nucleus is large, and haemozoin granules are smaller and are scattered throughout the cytoplasm. The macro-gametocytes measure about 12-14 µm in length and haemozoin granules are grouped around nucleus like a wreath of flowers. The single erythrocytic schizogony completes within 48 hours.
The exoerythrocytic schizogony is totally absent in Plasmodium falciparum. So malaria fever never relapses by the infection of P. falciparum.
Sexual cycle in mosquito:
When a female anopheline mosquito sucks the blood from a falciparum infected person, asexual forms, and mature and immature gametocytes pass into the stomach of the mosquitoes. All other stages except mature gametocytes are destroyed by the gastric juices of the insect.
The haploid gametocytes are transformed into two types of gametes. Four flagella-like male gametes or micro- gametes are produced from a single micro- gametocyte by the process of ex-flagellation and a single macrogamete is produced from a single macro-gametocyte by extruding the polar bodies. The female gametes are fertilized after 1/2 hour of the blood meal taken by the mosquitoes.
After fertilization the diploid, non-motile zygote is formed and then the zygote becomes an elongated structure and acquires the power of motility. At this state the elongated, motile zygote is called ookinete or vermicule.
The ookinete secrets a proteolytic enzyme which helps to move through the stomach of the mosquitoes and comes to lie at the basement membrane of the stomach where the ookinete is covered by a cyst, called oocyst.
The nucleus of the oocyst divides several times of which first one is meiotic. Portions of the protoplasm of the oocyst collect round each nucleus and thousands of daughter individuals are formed.
These daughter individuals mature and called sporozoites which are haploid and infective. The cyst wall of the oocyst bursts and the sporozoites migrate into the salivary glands. From the salivary glands the sporozoites are introduced into the human host by the bite of mosquitoes.
Pathogenicity:
The basic epidemiology of malaria is the feeling of feverish condition first several days after the infection of Plasmodium.
The interval between the time of infection of the parasite and the appearance of symptoms of the malaria is called incubation period which varies 10-40 days and in P. vivax it varies 13-17 days, and 9-12 in P. falciparum. The pre-patent period follows after incubation period which is the interval between the infection of the parasites and appearance of parasites in the red blood corpuscles.
The symptoms of the infection at the end of the incubation period are head-aches, loss of appetite, limb pains, nausea, vomiting and sweating. Finally, the disease is characterized by paroxysm which is divisible into 3 stages, the cold stage or chill, the hot or fever stage and the sweating stage. In the hot or fever stage, the temperature rises as high as 106°F.
The benign tertian malaria is caused by the infection of P. vivax and the most fatal malaria is malignant tertian malaria caused by the infection of P. falciparum.
The damage of the malignant malaria is caused by the blocking of the capillaries in the heart, intestine and brain, etc. by the infected red blood cells. The other names of malignant malaria are pernicious malaria, aestivo-autumnal malaria and oubtertian or tropical malaria.
The malaria fever occurs due to release of a toxic substance in the plasma of blood, the haemozoin pigments with the rupture of schizonts in the red blood cells. The haemozoin pigments induce high fever and shivering.
Prophylaxis (Prevention of infection):
The malaria can be controlled under following categories:
1. Prophylactic use:
Certain antimalarial drugs such as quinine, paludrine, daraprim and chloroquine should take small doses regularly as per doctor’s advice which may be effective as a precaution before infection for the mosquito bites.
2. Use of antimalarial drugs:
The most effective drugs are Quinine, Mepacrine, Chloroquine, Amodiaquine, Primaquine, Pamaquine, Qaraprim, Paludrin, Resochin, Proguanil, Suphones which may be used to suppress the symptoms of various stages.
For malignant malaria Pamaquine and Primaquine drugs should be taken as per doctor’s advice.
3. Protection against the bites of mosquitoes:
i. Mosquito nets preferably insecticide treated nets should be used during sleep at night which prevent mosquito bites.
ii. Antimosquito creams such as bamber oil, odomos, mylol, dibutyl phthalate (DBP), mustard oil, is also effective which is smeared on the exposed parts of the body.
iii. Various kinds of mosquito coils containing cyclothrin, pyrethrum may be effective to protect the body from mosquito bites.
iv. Camphor, Oil of Citronella and Dimethyl phthalate are used as mosquito repellents, causing insects to move away from their sources.
(i) Elimination of breeding sites of mosquitoes:
The breeding places of the mosquitoes are clean water that stagnates or flows slowly, must be eliminated in such ways:
a. Marshes, nullahs, stagnant water bodies, ditches must be drained off.
b. Drains and sceptic tanks must be cleared.
c. All water containing vessels and tanks must be covered with lids and clean at least once a week.
(ii) Destruction of larvae and pupae:
a. The larvae and pupae are killed by spraying kerosine oil, crude oil, petroleum on the surface of water which forms a film on the water by which the larvae and pupae die for wanting of breathing. The use of Panama larvicide and Paris green in water is helpful to kill the larvae of mosquitoes.
b. Certain chemicals such as DDT, BHC are used as larvicides which are sprayed in the water, which kill the pupae and larvae by suffocation.
(iii) Biological control:
a. Certain fishes such as Guppy (Poecilia reticulatus), Stickle backs, Minnows, Trout, Gambusia (Gambusia affinis), Northo sp. Gold fish (Carassius auratus), Tilapia (Tilapia sp.) eat the larvae and pupae of mosquitoes (WHO Report).
b. Several types of virus, bacteria (Bacteria thuringiensis), Nosema (Protozoa) do harm the vectors of malarial parasites help in checking mosquito population.
(iv) Destruction of adult mosquitoes:
a. Spraying of some of insecticides such as DDT, BHC, malathion, dieldrin, pyrethrum, etc.
b. Naptha, Sulphur and Cresol, etc. are used as fumigants which kill the mosquitoes. The gas enters the body through the spiracles and kills the adult mosquitoes.
c. Use of chemosterilants on male mosquitoes causing sterility may be effective to minimise the reproductive potential of mosquitoes.
d. The application of vaccine for the control of adult mosquito population is on the trial stage. The works of Holbrook (1980), Diggs (1980) and others may be mentioned.
Other Malarial Parasites:
The members of the genus Plasmodiumm are recognised as the causative agent of malaria in man. They are P. vivax, P. malariae, P. falciparum and P. ovale. They cause Benign tertain, Quartan, Malignant tertian and Ovale tertian fever respectively.
In Benign tertian outbreak of fever occurs every other day, in quartan fever the outbreak sets in every third day while malignant malaria is irregular occurring almost daily and is often fatal. Chief differences between the four species are tabulated in Table 10.3—Protozoa. 