Essay on Protozoa

In this essay we will learn about Protozoa:- 1. Meaning of Protozoa 2. Characteristics of Protozoa 3. Morphology 4. Classification 5. Nutrition 6. Reproduction 7. Significance.

Contents:

1. Essay on the Meaning of Protozoa
2. Essay on the General Characteristics of Protozoa
3. Essay on the Morphology of Protozoa
4. Essay on the Classification of Protozoa
5. Essay on the Nutrition in Protozoa
6. Essay on the Reproduction in Protozoa
7. Essay on the Significance of Protozoa

Essay # 1. Meaning of Protozoa:

Protozoa (Sing., protozoan; Gk. protos and zoan, meaning “first animal”) include over 65,000 species and their size, in majority of cases, ranges between 5 and 250 µm in diameter. Protozoa represent a diverse group of eukaryotic protists.

They are mostly unicellular, but a few form colonies in which individual cells are joined by cytoplasmic threads or are embedded in a common matrix. Thus colonies of protozoa are essentially aggregates of independent cells.

The algal protists, on one hand, are far more plant-like but the protozoan protists, on the other hand, are far more animal-like. The latter are entirely without chlorophyll and their nutrition is accomplished variously by osmophilic heterotrophy, phagotrophy, and by symbiosis.

Out of 65,000 described species of protozoa, slightly more than 50% are fossil forms; of the remaining 50%, some 22,000 are free-living species, while 10,000 species are parasitic. It is estimated that even now there are hundreds of thousands of species yet to be described. Protozoa are distributed in diverse moist habitats. They commonly occur in sea, in fresh water, and in soil.

Free-living protozoa have even been found in the polar region and at very high altitudes. Various protozoan species are parasitic and are found in association with many of the animals and plants.

Parasitic protozoa have ability to modify their morphology and physiology to cope with a change in the host. For convenience, Plasmodium (the malaria parasite) produces male gametes in response to a drop in temperature when it is transferred from a worm-blooded mammalian host to a mosquito.

Although it is difficult to determine the detailed ancestry of protozoans, it is generally considered that they have conceivably evolved from leucophyte-type of algal ancestors. Leucophytes are those organisms that have lost their photosynthetic ability.

Various algal protists belonging to dinoflagellates (also to some other algal groups) are known where the photosynthetic pigments have been lost and they possess phagotrophic mode of nutrition, e.g., Polykrikos, Noctiluca, Dinamoebidium.

It is advocated that these leucophytes might have undergone modifications to give rise to nonphotosynthetic organisms. In this way, the leucophytes offer very valuable clues for tracing the protozoan ancestry.

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Essay # 2. General Characteristics of Protozoa:

1. Protozoa are mostly free living; the latter being mostly aquatic, inhabiting fresh and marine waters and damp places. Parasitic and commensal protozoa live over or inside the bodies of animals and plants.

2. They are small, usually microscopic and ordinarily not visible without the help of a microscope.

3. Protozoa are unicellular or cellular. They contain one or more nuclei and organelles, with little or no histological differentiation into tissues and organs.

4. They occur singly or form loose colonies in which individuals remain alike and independent.

5. Vegetative body of protozoa is naked or bounded by a pellicle and provided often with simple to elaborate cells or exoskeletons.

6. The shape of the cell usually constant but changes with changing environment or age in many.

7. The single cell performs all the essential vital activities hence there is little or no physiological division of labour.

8. Locomotary organs are finger-like pseudopodia or whip-like flagella or hair-like cilia or they may be absent.

9. Nutrition in protozoa may be holozoic (animal-like) halophytic (plant-like), saprophytic or parasitic. They are with or without definite oral and anal apertures. Digestion occurs intracellular inside food vacuoles.

10. Respiration in protozoa takes place through general outer surface of body.

11. Excretion occurs through general surface or through contractile vacuoles which also serve for osmo­regulation.

12. Protozoa reproduce asexually by binary or multiple fission and budding; and sexually by conjugation of the adults or by fusion of gametes.

13. Life histories often complicated by alternation of generations as well as the hosts.

14. Encystment commonly takes place to help in dispersal as well as to resist unfavourable conditions of food, temperature and moisture.

15. The single-celled individual not differentiated into somatoplasm and germplasm; therefore, exempt from natural death which is the price paid for the body.

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Essay # 3. Morphology of Protozoa:

The protozoan cell is devoid of cell wall. The outer-most boundary is made up of a cell unit membrane called plasmalemma. The plasmalemma not only protects the cell from external factors and controls exchange of substances, but it also acts as the site of perception of mechanical or chemical stimuli as well as the establishment of contact with other cells.

Sometimes, the protozoan cell is surrounded by a non-rigid cuticle composed of chitin or chitin-like substances.

Various protozoa produce an exoskeleton in the form of shells, which are made up of siliceous, calcareous, or proteinaceous material. A characteristic type of covering called cyst occurs commonly in various protozoa.

The cyst protects the protozoan cell from environmental factors, such as high light intensity, high temperature, desiccation, food exudation, and anaerobiosis. The cyst is also involved in the transmission of protozoan diseases.

Like other eukaryotic cells, the cytoplasm of a protozoan cell is more or less homogenous substance and the various structures (endoplasmic reticulum, Golgi bodies, mitochondria, ribosomes, blepharoplasts, food vacuoles, contractile vacuoles, and nucleus) are embedded within it. Protozoa possess a more or less continuous network or canals and lacunae that give rise to endoplasmic reticulum.

Their nucleus is eukaryotic and may be either one (in most of the protozoa) or more than one (almost in all cilliates) in each cell. Cilliates possess two dissimilar nuclei, a macronucleus (polyploid) and a micronucleus (diploid). The macronucleus controls the metabolic activities and regeneration, whereas the micronucleus is concerned with reproductive activity.

Essay # 4. Classification of Protozoa:

1. Conventional Scheme:

Broadly, four phyla based primarily on organelles of locomotion, i.e., the means of molality have been recognised in the early days for the purpose of classification of protozoa. These phyla are: the Mastigophora flagellates), the Rhizopoda or Sarcodina (pseudopodiates), the Ciliophora (ciliates), and Sporozoa (sporeformers).

Mastigophora (Flagellates):

This phylum consists of flagellate protozoa (few produce pseudopodia in addition to flagella) many of which are plant-like (dinoflagellates, euglenoids and some other algal forms) and others animal-like (zooflagellates).

These protists are generally regarded as the most primitive of all protozoa. The zooflagellates now in existence are largely free-living and phagotrophic, and they generally resemble colourless flagellate algal protists.

Free- living zooflagellates undoubtedly gave rise to many symbiotic forms. They are highly specialized and adapted to specific hosts.

For example, Trichonympha, a wood digesting symbiont living in the gut of termites; Trichomonas, a commensal in the gut of man and other vertebrates; Trypanosoma, different species of which live parasitically in the bloods of various vertebrates and are transmitted from host through insects; and Astasia (Fig. 10.5 A).

Rhizopoda (Amoeboid Protozoa):

These protists, also called Sarcodina, have an amoeboid locomotion generally by pseudopodia, although some may possess flagella. It is considered that their likely ancestors probably include free-living zooflagellates and algal groups such as Chrysophyta and Dinophyceae. Since these protozoans generally move by pseudopodia (“false legs”), their body contour, therefore, is ever changing in movement.

Most of the rhizopods are free- living, whereas some are parasitic. Amoeba is a typical representative of this class. Entamoeba histolytica, the causative organism of amoebic dysentry in man is an example of parasitic rhizopod.

Some rhizopods develop a protective covering, called ‘shell’, composed of calcium carbonate. Many deposits of ‘chalk’ in nature are said to be due to the deposition of these shells. Arcella, Foraminiferans are other examples (Fig. 10.5B).

Ciliophora (Cillate Protozoa):

These are undoubtedly the most advanced and structurally, the complex protozoan protists. This large class consists of aquatic phagotrophic protozoans that are characterized by the presence of short hair-like structures called cilia, the organs of locomotion. The cilia originate from kinetosomes which are components of a complex system of intracellular conductile fibrils.

All ciliates are also uniquely characterized by the possession of two kinds of nuclei, a small ‘micronucleus’ which functions principally in sexual processes, and a large ‘macronucleus’ that controls metabolism, development and all other cellular processes.

Paramecium is a typical representative of this class whereas Tetrahymena is one of the simplest ciliate. There are other examples like Stentor, Vorticella, etc., of this phylum which inhabit fresh-water (Fig. 10.5C).

Sporozoa (Spore-forming Protozoa):

This class includes parasitic Protozoan protists, which are generally immotile, develop spores, and reproduce by multiple fission. Plasmodium is undoubtedly the most familiar sporozoan genus.

Four species of Plasmodium, namely, P. vivax, P. malariae, P. falciparum, and P. ovale, cause human malaria; the female Anapheles mosquito is the specific intermediate host of the protozoan and transfers the latter, the pathogen, to the humans. Other example is Leishmania that causes lishmaniasis disease in humans (Fig. 10.5D).

2. Modern Scheme:

An expert committee of the Society of Protozoologists constituted to study the systematics and evolution proposed a scheme of classification of the protozoans in 1980. They regarded the Protozoa as a subkingdom of the kingdom Protista and classified it into seven phyla.

This classification scheme of subkingdom protozoa into phyla is based primarily on types of nuclei, mode of reproduction, and mechanism of locomotion. However, the proposed seven phyla under this scheme are: Sarcomastigophora, Labyrinthomorpha, Apicomplexa, Microspora, Ascetospora, Myxozoa, and Ciliophora. This classification scheme is shown in Table 10.1.

Giardia:

Giardia is a flagellate protozoan belonging to subphylum Mastigophora of phylum Sarcomastigophora and is commonly nick named as the “Grand Old Man of Intestine.” Giardia lamblia (also called G. duodenalis, G. intestinals) was discovered by Antony van Leeuwenhoek in 1681 when he examined his own stools.

Giardia particularly G. lamblia, commonly occurs in the intestine of man and many other vertebrates such as rats mice rabbits, guinea-pigs, dogs, cats, frogs, herons, etc. G. lamblia is the only protozoan that inhibits the small intestine of man, especially, children, and is worldwide in distribution.

G. lamblia exists in a feeding vegetative form, known as trophozoites or as cysts. The trophozoites measure about 14 µm in length and 7 µm in breadth and have eight flagella and two prominent nuclei (Fig. 10.6). There is also a large characteristic sucking organ by which they attach to the intestinal wall.

They grow generally in the small intestine of humans and other animals. Cysts are slightly smaller, oval and thick walled. Infection occurs by ingestion of cysts through food and water. The cysts migrate with ingested food to the small intestine where they produce trophozoites.

After 4 to 7 days, the trophozoites are transformed into cysts and are excreted in faces. Thus, Giardia has a simple life-cycle having only two types of growth form-an active trophozite stage and an inactive cyst stage.

G. lamblia causes giardiasis, a diarrhoeal disease in man. The epidemiology, pathogenesis and the treatment of giardiasis has intensified since G. lamblia waterborne outbreaks were reported in Europe and USA during 1960s and 1970s. Worldwide, the majority of patients infected with G. lamblia are asymptomatic.

However, clinical symptoms of giardiasis usually begin 1-3 weeks after ingestion of cysts and are marked by diarrhoea, malaise, flatulence, greasy stools, and abdominal cramps. Other symptoms commonly include bloating, weight loss, and anorexia.

Illness can last several months if untreated and can be characterized by continued exacerbations of diarrhoeal symptoms. With chronic illness, malabsorption of fat, lactose, vitamin A, and vitamin B₁₂ are reported, and failure of children to thrive has been noted.

Plasmodium:

Plasmodium, a sporozoan protozoa, belongs to phylum Apicomplexa. It is an intracellular, blood-inhabiting obligate parasite. It lives in the red blood corpuscles and parenchyma cells of liver of man and other vertebrates and in the alimentary canal and salivary glands of mosquitoes.

In the mature stage, it does not possess any locomotion organelles hence is non-motile. About 60 species of Plasmodium are known to cause malaria in reptiles, birds, and mammals.

In humans, malaria is caused by four species of Plasmodium (P. vivax, P. falciparum, P. ovale, P. malariae). Out of these four, P. vivax and P. falciparum are the two that most commonly cause infections. P. vivax is the most widespread while the P. falciparum is the most serious.

Malaria parasite carries out part of its life cycle in human reservoir and part in the mosquito (Anopheles) vector. It is the mosquito vector that transmits the parasite from one human host to the other. Only female Anopheles mosquitoes transmit malaria. Anopheles mosquitoes inhabit warmer parts of the world; therefore, malaria occurs predominantly in the tropics and sub-tropics.

Malaria is worldwide. Each year hundreds of million people get infected by the disease and many millions of these victims die of. In the middle of the 20th century, there were 700 million malarial patients, about 3 million of whom died. In 1952, 350 million contracted the disease and approximately one million died. The same morbidity was registered in 1959 and 1964, and 140 million and 115 million died, respectively.

Malarial incidence was very high in India, Pakistan, and Africa. Extensive measures to reduce malarial morbidity in the whole world are conducted since 1955 according to World Health Organization (WHO) programme. In 1971, 52.5 thousand patients were registered throughout the world, with a mortality rate of 6%.

Spread of malaria is again encountered recently in areas where it was thought to be exterminated. In many regions of Africa and Asia the disease is extremely menacing and is marked by a very high mortality rate.

Life Cycle:

The life-cycle of Plasmodium, the malaria parasite, is complex (Fig. 10.7). First, the human host is infected by plasmodial sporozoites, small, elongated cells produced in the mosquito, which localize in the salivary gland of the insect.

The mosquito injects saliva (containing an anticoagulant) along with the sporozoites. The sporozoites travel through the bloodstream to the liver, where they may remain quiescent, or they may replicate and become enlarged in a stage known as a schizont.

The schizonts then segment into a number of small cells called merozoites, and these cells are liberated from the liver into the blood-stream. Some of the merozoites then infect red blood cells (erythrocytes).

The cycle in erythrocytes proceeds as in the liver and, in the case of P. vivax, usually repeats at synchronized intervals of 48 hours. During this 48 hour period, the defining clinical symptoms of malaria occur, characterized by chills followed by fever of up to 40°C (104°F).

Not all protozoal cells liberated from red blood cells are able to infect other red blood cells. The protozoal cells that cannot infect red blood cells are called gametocytes and are ineffective only for the mosquito. These gametocytes are ingested when another Anopheles mosquito bites the infected person; they mature within the mosquito into gametes.

Two gametes fuse, and a zygote is formed. The zygote then migrates by ameboid motility to the outer wall of the insect’s intestine where it enlarges and forms a number of sporozoites. These are released and some reach the salivary gland of the mosquito, from where they can be inoculated into another human, and the cycle begins again.

Entamoeba:

Entamoeba belongs to the subphylum Sarcodina of the phylum Sarcomastigophora. It is non-flagellated amoeba and moves with the help of pseudopodia. Though there are many species of entamoeba that occur in humans, E. histolytica is the most pathogenic and causes amoebiasis (amoebic dysentery).

It is a monogenetic (Gk. monos = one, genos = race) because the whole life-cycle is completed within a single host. Man and monkeys are its natural hosts. Besides there are a number of other hosts such as dogs, rats, pigs, rabbits, cats, etc. Wild cats and dogs are supposed to be the reservoir host and possible source of primary infection to man.

E. histolytica is a very common parasite and is endemic in warm climates where adequate sanitation and effective personal hygiene is lacking. However, this amoeba is a major cause of parasitic death worldwide; about 500 million people are infected and as many as 100,000 die of amoebiasis every year.

The symptoms of amoebasis are highly variable, ranging from an asymptomatic infection to fulminating dysentery, exhaustive diarrhoea accompanied by blood and mucous, appendicitis, and abscesses in the liver, lungs, or brain.

Life Cycle:

The life cycle (Fig. 10.8) of E. histolytica completes with the help of three distinct forms (or phases):

(1) Trophozoite or magna form,

(2) Precystic or minuta form, and

(3) Cystic form.

1. Trophozoite or magna form:

Trophozoit or magna form of E. histolytica is the most active, motile and feeding form which is pathogenic to man. Each trophozoite usually measures 20 to 30 µ in diameter and, more or less, resembles the common amoeba in all structural details.

The outermost body covering or plasmalemma is a thin, elastic and semi-permeable membrane. The cytoplasm is differentiated into the outer clear and non-­granular ectoplasm and the central more fluid and granular endoplasm.

The trophozoite lives in the mucous and sub-mucous layers of the large intestine of man, and multiplies asexually by binary fission resulting in the formation of two daughter amoebae. However, some of the daughter trophozoites rapidly grow in size, attack and penetrate fresh host cells, while others remain small and free and become the precystic or minuta form.

2. Precystic or minuta form:

Precystic or minuta form of E. histolytica is small, spherical, non-motile, and non-feeding form. It measures 12 to 15 µ in diameter. In structural details it resembles the trophozoite except that it is smaller in size and the food vacuoles are absent. It lives in the lumen of large intestine and in non-pathogenic to man. But when resistance of host’s body is low, it changes into magna form and invades the tissues of intestine.

3. Cystic form:

The precystic or minuta form undergoes encystation. It rounds up and secrets a thin, refractile, flexible and resistant cyst wall around it. At this stage, the cyst is uninucleate.

Soon the single nucleus divides mitotically twice and the cyst becomes tetra- or quadrinucleate. The whole process of encystation completes within a few hours. The mature quadrinucleate cysts are the most resistant and ineffective forms of the parasite.

They are unable to develop in the host in which they are produced and require a fresh susceptible host. The cysts are voided with the faeces of the host and reach the new human host via food, vegetables, or drinking water contaminated with faecal matter.

On being swallowed, cysts pass unchanged through the stomach to small intestine where the cyst wall dissolves by enzymatic action releasing a single quadrinucleate amoeba called the excystic or metacystic form.

A metacyst undergoes a series of nuclear and cytoplasmic divisions, producing 8 little uninucleate daughter amoebulae called metacystic trophozoites. The latter being actively motile, makes their way into the large intestine, invade its mucous and sub-mucous layers, and grow into mature trophozoites.

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Essay # 5. Nutrition in Protozoa:

The protozoan nutrition is accomplished by osmophilie heterotrophy, phagotrophy, and symbiosis. Osmophilic heterotrophy is considered to be the commonest mode of nutrition in majority of protozoa. In this mode the liquid organic foods are absorbed osmophilically by the organisms.

Phagotrophy is the mode of ingestion of food materials in many protozoa and in this respect they resemble animals. Food is captured by means of specialized structures and the protozoa engulf the captured food or suck its contents. In some protozoa, which live symbiotically with other organisms, one may see autotrophy.

A good example of this is the facultative symbiosis between Paramecium burs-aria (a protozoan) and Chlorella (photosynthetic microalga); the symbiosis is called facultative because both the organisms may live independently in nature. Paramecium cells harbour large numbers of Chlorella in their cytoplasm, a fact that accounts for their intermittent greenish appearance.

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Essay # 6. Reproduction in Protozoa:

Protozoa reproduce by both asexual and sexual means, but the most protozoa reproduce asexually while only some carry out sexual reproduction.

Asexual Reproduction:

Protozoa reproduce asexually by binary fission, multiple fission and budding.

1. Binary fission:

In binary fission, which is the most common method of asexual reproduction in protozoans the body divides producing two equal or sometimes unequal daughter bodies.

The binary fission is usually transverse, it is longitudinal in certain protozoa. For convenience, the binary fission in Amoeba is transverse (fig.10.3). Before fission, the pseudopodia are withdrawn and the body becomes almost rounded. The nuclear membrane persists and the nucleus undergoes mitosis.

Then follows the cytokinesis. Amoeba elongates and a fissure or constriction appears from outside in the middle between the two daughter nuclei. The constriction deepens more and more ultimately bisecting the body into two similar haves (daughter bodies), which pull apart. Thus, two distinct daughter amoebae are produced from a single parent amoeba.

Binary fission in ciliates takes place in a transverse plane at right angles to the long axis and develops across the narrow part of the protozoan body. This type of fission is sometimes referred to as homothetogenic fission. Ciliates usually contain two types of nuclei, a macronucleus and a micronucleus. These nuclei divide in different manners.

While the micronucleus divides by regular mitosis, the macronucleus divides amitotically. The macronucleus elongates and pinches off in two daughter fragments. Each daughter ciliate receives a copy of the micronucleus and the macronucleus.

Binary fission also takes place in flagellated protozoa, but the fission is generally along the long axis of the body (longitudinal binary fission). The flagella are regenerated from the basal bodies which divide before the nuclear division initiates.

2. Multiple fission:

Some protozoa asexually reproduce by multiple fission or schizogamy. In this process, the nucleus divides mitotically to produce a large number of nuclei before the body divides.

Each nucleus with the surrounding cytoplasm, forms a daughter protozoan. The daughter protozoans then separate. Multiple fission is best studied in Plasmodium, the malarial parasite, though it has also been observed a certain amoebae such as foraminiferans and radiolaria.

3. Budding:

Many protozoa accomplish asexual reproduction by budding. In this process, daughter nuclei formed as a result of mitotic division migrate into a cytoplasmic protrusion (bud), which is ultimately separated from the parent protozoan by fission.

Sexual Reproduction:

Only some protozoans carry out sexual reproduction. The most common method of sexual reproduction is conjugation in which there is an exchange of gametes between paired protozoa of complementary mating types (conjugants).

Conjugation is most prevalent among ciliates like Tetrahymena and Paramecium. In Tetrahymena (Fig. 10.4), firstly, the two conjugant ciliates unite fusing their pellicles at the contact point. The macronucleus in each degrades.

The individual micronuclei divide twice by meiosis to form four haploid pronuclei, three of which disintegrate. The remaining pronuclear divides again mitotically to form two gametic nuclei, a stationary one and a migratory one, the migratory nuclei pass into the respective conjugates. Then the conjugates separate, the gametic nuclei fuse, and the resulting diploid zygote nucleus under goes three rounds of mitosis.

The eight resulting nuclei in each conjugant have different fates: one of them is retained as a micronucleus, three others get degraded, and the four remaining ones fuse and reform a macronucleus. Each separated conjugant now undergoes binary fission to result in progeny protozoans.

Essay # 7. Significance of Protozoa:

Protozoa, the unicellular microscopic organisms, occur almost everywhere. They are present in water, in moist soil, in air, or even inside the bodies of humans and many animals and plants. It appears at the first glance that these microorganisms may not be of any significance, but the fact is that protozoa exert for more influence in worldly affairs. They are useful as well as harmful both.

They aspects of the significance of protozoa are the following:

Useful Protozoa:

1. Protozoa play an important role in the economy of nature. The constitute considerably a large part of planktonic life consisting of small, free floating microorganisms; the planktons are an important link in the many food chains (food chain is a series of organisms, each feeding on the preceding one) and food webs (food web is a complex interlocking series of food chains) of aquatic environments.

For example, in marine waters, the protozoa (zooplankton) that feed on the photosynthetic phytoplankton are eaten by insect larvae, small crustaceans, worms, etc. which in turn become food for large marine organisms.

2. Numerous holozoic protozoa feed on putrefying bacteria in various water bodies and thus help indirectly in the purification of water. These protozoa play an important role in the sanitary betterment and improvement of the modern civilized world in keeping water safe for drinking purpose.

3. Although bacteria play prominent role in the process of degradation of sewage, the role of protozoa is also quite appreciable. Biological sewage treatment involves both anaerobic digestion and/or aeration. Anaerobic protozoa (e.g., Epalxis, Metopus, Saprodinium) play active role in anaerobic steps, while aerobic protozoa (e.g., Aspidisca, Bodo, Paramecium, Vorticella) contribute in aeration steps of sewage treatment stages.

4. Nitrates and phosphates accumulate in the waste water of settling tanks during industrial waste treatment. The settling tanks are illuminated and, as a result, the growth of microalgae and protozoa is promoted in them. These microorganisms remove the inorganic material from the waste water and improve the quality of it. The autotrophs that grow on the surface of improved waste water are skimmed, dried, and used as bio-fertilizer.

5. Many protozoa are used as research organisms for biochemical and molecular biological studies because many biochemical pathways used by protozoa occur in all eukaryotic cells. This use of protozoa is mostly because many of them are easily cultured and maintained in the laboratory, and they reproduce asexually resulting in ‘clones’ that possess genetic makeup identical to their parents.

For convenience, species of protozoa such as Euplotes, Paramecium, and Tetrahymena have been used to study the cell-cycles and nucleic acid biosynthesis during cell division. Studies of mating types and killer particles in Paramecium have shown a relationship between genotype and the maintenance of cytoplasmic inclusions and endosymbionts.

Harmful Protozoa:

1. Though fewer than 20 genera of protozoa cause diseases in humans, their impact is formidable. Some of the human diseases caused by protozoa are very important. There are over 150 million cases of malaria reported worldwide every year, and the disease itself is responsible for the deaths of more than a million children under the age of 15 per year in tropical Africa alone.

Estimates reveal that there are at least 8 million cases of trypanosomiasis, 12 million cases of leishmaniasis, and over 500 million cases of amebiasis throughout the world every year. However, some important protozoan diseases caused in humans are given in Table 10.2.

2. Protozoan pathogens cause a number of important diseases in animals. Some important animal diseases caused by protozoa are given in Table 10.3.

3. Certain protozoa have been reported as causal agents of plant diseases. To distinguish them from protozoa pathogenic to humans and animals, plant parasitic protozoa have been placed in a separate genus named Phytomonas. Some of the plant diseases caused by protozoa are given in Table 10.4, though test of pathogenicity (Koch’s postulates) have not been provided so far against any of these diseases.