Introduction and Classification of Parasites is an very important and basic topic of Parasitology. Parasitology is the branch of Science which mainly deals about all the Parasites and its infectious diseases. Whole Parasitology covers mainly Parasites, Host and its association between them.For having a very clear cut knowledge about Introduction and Classification of Parasites some important terms should be clear and those are:
- A living organism which receives nourishment and shelter from another organism where it lives is called parasites.
- A parasite does not necessarily cause disease.
- Simply parasitism is living in association with the host.
- The parasite derives all benefits from the association and the host may either not be harmed or may suffer the consequences of this association, a parasite disease.
- A parasite is an organism that is entirely dependent on another organism, referred to as its host, for all or part of its life cycle and metabolic requirements.
Host: an organism which harbors the parasite.
Terms for association of Parasites:
This chapter Introduction and Classification of Parasites also describes different terms associated association of parasites.
An association in which both are so dependent upon each other that one can’t live without the help of the other. None of the partners suffers any harm from the association.
The living together of two or more organism.
An association in which the parasite only is deriving benefit without causing injury to its host.
A relationship between members of different species living in proximity (the same cultural environment) in which one organism benefits from the association but the other is not affected.
An association in which the parasite derives benefit and the host gets nothing in return but always suffers some injury, however slight the injury may be. The host, at the same time, offers some resistance to the injury done by the parasite and there may be some adaptation (tolerance) between the parasite and the host. A parasite has lost its power of independent life.
Classes of parasites
Parasite are mainly classified into following two catagories; along with some other types of Parasites;
Ecto-parasite (ectozoa): lives outside on the surface of the body of the host.
Ecto-parasite (ectozoa): lives outside on the surface of the body of the host.
Endo-parasite (entozoa): lives inside the body of the host: in the blood, tissues, body cavities, digestive tract and other organs.
Temporary parasite: visits its host for a short period.
Permanent parasite: leads a parasitic life throughout the whole period of its life.
Facultative parasite: lives a parasitic life when opportunity arises.
Obligatory parasite: cannot exist without the parasitic life.
Occasional or accidental parasite: attacks an unusual host.
Wandering or aberrant parasite: happens to reach a place where it cannot live.
Classes of host:
Introduction and classification also includes;
either harbors the adult stage of the parasite or where the parasite utilizes the sexual method of reproduction.
In the majority of human parasitic infections, man is the definitive host; in malaria and hydatid disease, however, man acts as the intermediate host.
harbours the larval or asexual stages of the parasite.
In some cases larval developments are completed in two different intermediate hosts which are been referred to as first and second intermediate hosts respectively.
Some parasites require two intermediate hosts in which to complete their life cycle.
e.g., W. brancrofti – intermediate host- mosquito (Culex, Aedes, Anopheles)
- definitive host- man (lymphatic sysyem)
Paratenic host (a carrier or transport host):
a host where the parasite remains viable without further development.
Nomenclature of parasites
Each parasite possesses two names i.e. a generic and a specific;
the former begins with an initial capital and the latter with an initial small letter, after which comes the designator’s name, followed by punctuation and finally the year.
The generic and specific names are in italics but not the designator’s name.
for example, the common intestinal roundworm of man is named Ascaris lumbricoides Linnaeus, 1758. This means that it belongs to the Genus Ascaris and the name of the species lumbricoides was given by Linnaeus in the year 1758.
[when the name assigned to the parasite is later transferred, the correct name is written as usual followed by the original name with the year in parenthesis]
Classification of parasites
The parasites which medical men have to deal are divided into three main groups:
Phylum Protozoa- single-celled organism, multiply in human host, Medical Protozoology
Phylum Platyhelminthes and Phylum Nemathelminthes- multicellular worms, do not normally multiply in human host, medical Helminthology.
Phylum Arthropoda – Medical Entomology
While describing animal parasites certain rules of zoological nomenclature are followed and each phylum may be further subdivided as follows:
Like above all parasites can be classified;
The biomedical discipline of Parasitology is concerned with the parasitic protozoa, helminths and arthropods.
Classification of parasitic protozoans
- All are flagellates.
- They have one or more whip like flagella for locomotion at some stage of their life cycle. In some cases, there is the presence of undulating membrane (Eg. Trypanosoma).
- The mastigophore includes the intestinal and genitourinary flagellates and the blood and tissue flagellates.
- The intestinal and genitourinary flagellates are Giardia, Trichomonas, Dientamoeba, Chilomastik, etc.
- The blood and tissue flagellates are Trypanosoma, Leishmania, etc.
- They reproduce asexually by binary fission.
- They are all typically amoeboid and include Entamoeba, Endolimax, Iodamoeba, Naegleria, Acanthamoeba, etc. amoebae consist of a shapeless mass of moving cytoplasm which is divided in to granular endoplasm and clear ectoplasm.
- They move by pushing out the ectoplasm to form pseudopodia (false feet) into which the endoplasm then low.
- Amoebae reproduce asexually by simply dividing into two (binary fission)
The members of this super-class undergoes complex life cycle with alternating sexual and asexual reproductive phases involving two different hosts.
Coccidia are intacellular parasites that reproduce asexually by a process called schizogony (merogony) and sexually by sporogony.
Class Coccidia includes Isospora and Toxoplasma and class Haematozoa includes the malarial parasites-Plasmodium species.
These are the complex protozoa bearing cilia (short hairs) distributed in rows or patches by which they move.
They have two kinds of nuclei (macronucleus and micronucleus) and a large contractile vacoule.
Balantidium coli, a giant intestinal ciliate of humans and pigs, is the only human parasite representative of this group.
Classification of Parasitic Helminths:
Classification of Parasites includes; parasitic worms, or helminths, of human beings belong to two phyla- Platyhelminthes (flatworms) and Nemathehelminthes (Round worms).
Nematoda( Round Worms)
- These are tape-like, segmented and hermaphrodite organism. They have suckers in their head and in some species also hooks that attach he tapewor to its host.. It consists of a head (scolex) and many proglottids.
- Alimentary canal and body cavity are absent. Examples are Diphyllobothrium, Taenia, Echinococcus, Hymenolepsis, etc.
- They are leaf-like unsegmented organism. Sexes are not separate except Schistosomes which are diecious. They don’t have hooks and suckers in their head. Alimentary canal is present but is not complete (anus absent).
- The body cavity is absent.
- Examples are Schistosoma, Gastrodiscoides, Fasciolopsis, Fasciola, Clonorchis, Heterophyes, etc.
- Their body is elongated, cylindrical and unsegmented. Sexes are separate (diecious). They also lack hooks and suckers.
- They possess the complete alimentary canal and body cavity. Examples are:
i. Small intestine only: Ascaris lumbricoides (Common round worm), Ancyclostoma duodenale (The old world hook worm), Necator americanus (American hookworm)
ii. Caecum and vermiform appendix: Enterobius vermicularis (Threadworm or pin worm), Trichuris trichuria (Whipworm).
2. Somatic (inside the tissues and organs)
i. Lymphatic system: Wuchereria bancrofti, Brugia malayi
ii. Subcutaneous tissue: Onchocerca volvulus, Dracunculus medinensis
iii. Lungs: Strongyloides stercoralis
iv. Conjunctiva: Loa loa
Scheme followed in parasitological studies
The Introduction and Classification of Parasites along with the study of animal parasites infecting man and producing clinical manifestations should include:
- History of discovery of the parasite
- Geographical distribution
- Habitat inside the human host
- Morphology and life cycle
- Modes of infection: Reservoir host, source of infection, portal of entry, vehicle of transmission
- Effects of the parasites: pathogenic lesions, clinical manifestations
- Immunological responses
- Methods for specific diagnosis
- Approved therapy for eradication of parasitic infection
- Prophylactic measures for the prevention of parasitic infection of the individual as well as of the community.
Protozoal parasite consists of a single “cell-like unit” which is morphologically and functionally complete.
Differences between protozoa and metazoan
A single “cell like unit”
Physiology ----- A single cell performs all the functions: reproduction, digestion, respiration, excretion
A number of cells, making up a complex individual
Physiology ----- Each special cell performs a particular function.
Structure of Protozoa:
The structure of a protozoal cell is composed of;
1. Cytoplasmic body
It may be divisible in to two portions:
- Ectoplasm: the external hyaline portion; its function is protective, locomotive and sensory.
- Endoplasm: the internal granular portion; its function is nutritive and reproductive
Structures developed from ectoplasm
Organelles of locomotion
- Pseudopodia: prolongation of temporary ectoplasmic process, seen in Rhizopodea (E. histolytica)
- Flagella: long delicate thread-like filaments, seen in Zoomastigophorea. (Giardia intestinalis)
- Cilia: fine needle-like filaments covering the entire surface of the body, seen in Ciliatea. (B. coli)
- Contractile vacuoles: situated inside the endoplasm; excretory function.
- Rudimentary digestive organ, such as cytostome (cell mouth) and cytopharynx, seen in Balantidium coli.
- Cyst wall: a thickened resistant wall, seen in the cystic stage
It is the most important structure, as it controls the various functions and regulates reproduction.
It is situated inside the endoplasm and its structure is often of great help in the differentiation of genera and species.
Its structure comprises of the following:
bound externally by a well-defined nuclear membrane
chromatin granules, lining the inner side of the nuclear membrane
karyosome (plastin) situated inside the nucleus either centrally or peripherally
The protozoal parasite possesses the property of being transformed from an active (trophozoite) to an inactive stage, losing its power of motility and enclosing itself within a tough wall.
The protoplasmic body thus formed is known as a cyst. At this stage, the parasite loses its power of growth and multiplication.
The cyst is the resistant stage of the parasite and is also infective to its human host.
In order to reach a new host, it must be transferred mechanically, either by a carrier or by some intermediaries (insect-house-flies), to food and drink which become contaminated with the cysts of protozoa.
The protozoal parasites may exist in two stages: trophozoite and cyst, as in intestinal flagellates and amoebae.
In such cases the parasite multiplies only in the trophic stage.
The methods of reproduction or multiplication among the parasitic protozoa are of the following types:
1. Asexual multiplications:
In this process, the individual parasite divides either longitudinally or transversely into two more or less equal parts.
Before division all the structures are duplicated.
ii. by multiple fission or schizogony:
The nucleus of the parent cell at first undergoes repeated divisions which are then surrounded by the cytoplasm.
When the multiplication is completed, the parasitic body or the schizont ruptures and liberates these daughter individuals which in their repeat their life cycle.
i. By conjugation:
In this process, a temporary union of two individuals occurs during which time interchange of nuclear material takes place.
Later on, the two individuals separate, each being rejuvenated by the process, as in Ciliatea.
ii. By syngamy:
In this process, sexually differentiated cells, called gametes, unite permanently and a complete fusion of the nuclear material takes place.
The resulting product is then known as a zygote, as in Plasmodia.
- A protozoal parasite may multiply vigorously by asexual method for a long time, and later by a change of process it either has recourse to sexual method of reproduction or undergoes encystment for a change of its host.
- The sexual method of reproduction often occurs in a different host other than the one utilized for asexual multiplication; the process is known as alternation of generation accompanied by alternation of host, as seen in Plasmodia.
- A protozoal parasite may pass its life cycle in one or two hosts.
Second host not required:
examples are Rhizopodea, intestinal flagellates and Ciliates.
The parasite in this group adapts themselves for passive transfer from one host to another by encystment.
The species multiplies asexually in its trophic stage and in circumstances unfavorable for its existence, secretes a resistant cyst-wall, transforming itself into a cyst.
When the condition becomes favorable again, the organism leaves the cyst (excystment) and continues its life in the trophozoite stage.
Thus encystment is not a reproductive process, but a means of protection of the species from extinction.
In amoeba, however, multiplication may take place within the cyst where the nucleus divides into 2, 4 or 8 daughter nuclei. Later, when the organism leaves the cyst, complete division of the cell occurs and a number of individual organisms are liberated.
Both the processes, encystment and excystment, occur in one and the same host, but after encystment a transference to another new host is required for further development.
Second host required:
examples are Trypanosoma, Leishmania and Plasmodia.
In Trypanosoma and Leishmania the second host is required for the development of a special cycle which is essential for the continuation of the species from one host and the sexual method of reproduction in another host.
The helminthic parasites are multicellular, bilaterally symmetrical animals having three germ layer (triploblastic metazoa).
Difference between cestode, trematode and nematode
Head space-----------Suckers, often with hooks
Sexes---------Not separate(monoecious),Except Schistosomes which are diecious
Head space--------Suckers, no hooks
Alimentary canal--------Present but incomplete; no anus
Shape---------Elongated, cylindrical; unsegmented
Head space---------No suckers, no hooks. Well developed buccal capsule in some species
Alimentary canal---------Present and complete; anus present
General characters of Cestodes
1. The majority of cestodes are long, segmented and tape-like, hence called tape-worms. They are flattened dorsoventrally.
2. Sizes vary from a few millimeters to several meters.
3. Adults worms are found in the intestinal canal of man and animal.
4. Head is provided with suckers (cup like) and sometimes with hooks which serves as organs of attachment.
5. There are three regions in an adult worm: i) a head (scolex), ii) a neck and iii) a strobila (a body or trunk) consisting of a series of segments (proglottides).
6. Sexes are not separate, i.e., each individual worm is a hermaphrodite. Newly formed poglottids are small an immature. Mature proglottids contain fuly developed reproductive organs (several testes, bilobed ovary, and a uterus which may be coiled or consist of a central stem with side branches). Proglottidis that contain eggs are called gravid segments.
7. Body cavity is absent.
8. There is no mouth or digestive system. Alimentary canal is entirely absent.
9. Excretory and nervous systems are present.
10. Reproductive system is highly developed and complete in each segment.
11.Fertilization takes place between the segments; it may be a self fertilization or a cross-fertilization.
According to maturity of the reproductive organs three types of segments of the strobila can be recognized from the front backwards:
immature: mae and female organs not differentiated
mature: male and female organs have become differeniated
gravid: uteri are filled with eggs
Terms used in the Cestode
Strobila: body and trunk of the adult worm
Scolex: commonly called head that carries the organs of attachment or suckers
Neck: the region of growth behind the head
Proglottid: an individual segment comprising the complete unit of a tapeworm.
Rostellum: a beak-like projection on the head which carries hooklets in the armed species. It may remain invaginated between the suckers.
Oncosphere: a six-hooked (hexacanth) embryo inside the egg.
Embryophore: the enveloping membrane surrounding the onchosphere.
Egg-shell: the outer membrane enclosing the onchosphere with the embryophore and containing the yolk materials.
Cysticercus: it is the resting stage of the larva in the intermediate host where it develops into a bladder-worm. Itconsists f a hollow vesicle with the invaginated scolex on its wall and a central cavity containing a little fluid.
Cysticercoid: small bladder containing the invaginated head proximally and a solid, elongated portion as a caudal appendage.
Coenurus: a larval stage in the form of bladder-worm containing many invaginated scolices
Hydatid cyst: the larval stage of the genus Echinococcus
Coracidium: a ciliated onchosphere of Diphyllobothrium
Procercoid: the first larval stage of Diphyllobothrium found in cyclops.
Plerocercoid: the second larval stage of Diphyllobothrium.
In Pseudophyllidean cestodes the egg is operculated and has a single covering.
In cyclophyllidean cestodes the egg is not operculated and has two covering
outer covering: egg shell
inner covering: emryophore
General characters of Trematodes (Digenetic)
- These are leaf-shaped, unsegmented flat worms, called flukes.
- Size varies from 1 mm to several cm in length.
- The organs of attachment are two strong muscular cup-shaped depressions, called suckers. The one surrounding the mouth is called the oral sucker and the other, on the ventral surface of the body, is called the ventral sucker (acetabulum).
- Sexes are not separate, i.e., each individual worm is hermaphrodite (monoecious) except the Schistosomes.
- Body cavity is absent.
- The alimentary canal is present but incomplete. The anus is absent. The oesophagus bifurgates in front of the ventral sucker in to a pair of blind intestinal caeca or crura which may be simple (as in C. sinensis) or branched (as in F. hepatica) or may reunite to form a single caecum (as in Schistosomes).
- Excretory and Nervous systems are present.
- Excretory system consists of “flame cells” and collecting tubules which opens posteriorly, in to excretory pore.
- Reproductive system is highly developed and complete in each individual. The genital organ lie between the two branches of the intestine.
- The worm is oviparous, since eggs are liberated.
- Eggs are all operculated (with lid) (except those of Schistosomes) and can developed only in water. Trematode eggs do not float in saturated solution of common salt
Life cycle passes in two different hosts
Definitive host: generally man
Intermediate host: a fresh-water snail or mollusc for larval development.
A second intermediate host (fish or crab) is required for encystment in some trematodes.
The eggs liberated by the definitive host gain access into the water. A free swimming ciliated embryo, miracidium develops and hatches out of the eggs. The miracidium gains access to its proper intermediate host (snail or mollusc) and localises in the liver or lymph spaces for further development.
Larval development in the snail
i.The miracidium is transformed into sac like structure called sporocyst.
ii.The sporocyst changes into a redia, having the following structural characters: an oral sucker, a pharynx, sac-like intestine and a birth-pore through which the next generation escapes.
iii.The cerceria represents the final stage of larval development in the snail and is infective to man. The cercaria is provided with suckers and has an intestine like that of the adult worm. It possesses a tail by means of which it propels itself in water.
when mature, the cercariae escape from the snail into the water and ma remain free in water or encyst (metacercaria) either in water-plants or in another intermediate host, a fresh water fish or a crab.
From a single miracidium a large number of cecariae are developed.Man is infected either by drinking ontaminated water or by ingesting encysted cercariae in the water-plant, fish, or crab.Free cerceria, in some cases, can enter directly through the skin.On entering the definitive hst, the young proceed to their sites of loalisation to grow into adult worm, become sexually mature and repeat the cycle.
General characters of Nematodes
- The Nematodes are unsegmented worms with out any appendage. They are elongated and cylindrical in appearance; both ends often pointed.
- The size show a great variation (5 mm- up to 1 m) D. medinensis measuring 1 meter or more.
- The body is covered with a tough cuticle.
- The worm possesses a body cavity in which the various organs, such as digestive and genital systems float.
- Excretory and nervous system are rudimentary.
- The alimentary canal is complete, consisting of an oral aperature, mouth cavity, oesophagus, intestine and a subterminal anus.
- The nematodes of man are all diecious helminths. The male is generally smaller than the female and its posterior end is curved or coiled Ventrally.
- Females are either viviparous (produce larvae) or oviparous (lay eggs)
- The discharged eggs may hatch directly into infective larvae or they may require special conditions in which to hatch and up to three developmental stages before becoming infective larvae. Each stage involves a shedding of the old cuticle (moulting).
Sources of infection and portal of entry
- by contamination of food or drink
- by contamination of the skin or mucous membrane
- by the agency of insect host
- by contamination of food or drink- gain entrance in to the digestive tract. Example Cysts of E. histolytica, eggs of A. lumbricoides after contaminating with food or drink the infective forms may remains in the flesh of some intermediate hosts
- beef containing the larval stages of T. saginata
- pork containing the larval forms of T. solium
- crab or crayfish containing metacercerial forms of P. westermani
- sometimes the intermediate host harbouring the infective form may be taken up as a whole eg: Cyclops infected with a larval forms of Dracunculus medinensi are ingested with water
- the infective forms may come out its intermediate host and encyst in aquatic plants, eaten as food by man. Eg: metacercarial forms of F. hepatica
- by contamination of the skin or mucous membrane
- the filariform larvae of A. duodenale, N. americanus which abound in damp soil, may penetrate the unbroken skin of an individual walking over such places bare-footed.
- The cercarial forms of S. haematobium and S. mansoni and S. japonicum in infected water, may penetrate the skin of a person coming in contact with such water.
- by the agency of insect host
- an infected blood sucking arthropod may introduce the organism directly in to the blood or in to the skin or in to the skin layers at the time of obtaining a blood-meal.
- Eg: Leishmania by Phlebotomous (sandfly)
- Wuchereria by Culicin mosquitoes
- Plasmodia by Anopheline mosquitoes
- In this group, the parasites undergo a biological development for a certain period before becoming infective to man.
Parasitic infections diseases
African trypanosomiasis : transmitted by the tsetse fly. Symptoms are fever, headaches and joint pains.
Amebiasis : due to ingestion of water or food contaminated with amoebic cysts.
Ascariasis : due to ingestion of food contaminated with fecal matter containing Ascaris eggs.
Chagas disease : transmitted to humans by blood-sucking bugs, caused by Trypanosoma cruzi.
Clonorchiasis : caused by the Chinese liver fluke (Clonorchis sinensis). Man acquire the infection by eating raw, inadequately cooked, dried, salted or pickled fresh water fish, harbouring the metacercariae of C. sinensis.
Cryptosporidiosis : caused by contaminated material like earth, water, uncooked or cross-contaminated food in contact with the feces of an infected person or animal.
Cysticercosis : caused by larvae of tapeworm, generally found in pork, transmitted by ingestion of undercooked meat of the pork.
Dracunculiasis : caused by drinking water containing infected copepods, caused by Dracunculus medinensis
Echinococcosis : fatal disease due to infection by tapeworm larvae. The eggs are discharged with the feces of the definitive host (dogs) and these are swallowed by the intermediate host hosts, sheep and other domestic animals while grazing in the field, and also by man (particularly children) due to intimate handling of infected dogs.
Enterobiasis : infection due to pinworms leading to poor appetite, restless sleep and itching of the anus. Children are the usual victims and familial infection is common. Transmission is effected from one person to another by the ingestion of eggs. The first infection is either contagious from close association or due to contaminated food or drink. Person handling the night clothes and bed linens of infected patients often contract the infection. (Auto infection and retroinfection)
Fascioliasis : caused by Fasciola hepatica and Fasciola gigantica. Portal of entry is alimentary canal.
;;;;;;;;;;caused when aquatic plants are eaten raw and water drunk unheated.
Filariasis : transmitted by mosquitoes .
Giardiasis : starts in the small intestine, transmitted by faeco oral route, infection of man is brought by ingestion of cysts of G. intestinalis.
Hymenolepiasis : caused by Hymenolepis nana or Hymenolepis diminuta. The first infection occurs through ingestion of food contaminated with eggs liberated along with the faeces of an infected man or rodents.
Isosporiasis : symptoms are diarrhea and weight loss. Sometimes linked with AIDS. Faeco oral route
Leishmaniasis : transmitted by certain varieties of sand fly.
Malaria : transmitted by female Anopheles mosquitoes.
Onchocerciasis : the second leading infectious cause of blindness. Caused by Onchocerca volvulus. The infection is transmitted by the bite of an infected female Simulium. The infective larvae remain localized in the skin and grow into adult worms. The gravid females release actively motile microfilariae which migrate in the skin, subcutaneous tissue and eyes until they die.
Schistosomiasis : due to infection of the blood with schistosome leading to weakness, liver and intestinal damage.An individual bathing in an infected pool or coming in contact with contaminated water is liable to be infected.The cercaria stick to the surface of the skin of the swimmer or bather, by means of their ventral suckers and penetrate the skin.
Taeniasis : infection involving pork and beef tapeworms.The infection is common amongst those eating raw or insufficiently cooked measly pork and beef.
Toxoplasmosis : due to ingestion of raw or partly cooked meat, contaminated cat feces, drinking water containing Toxoplasma, transplacental infection in utero or infected organ transplant or blood transfusion.
Trichinellosis : caused by eating raw or undercooked pork infected with the larvae of a species of
Trichuriasis : infection of the large intestine by a parasite whipworm. Man is infected when the embryonated eggs are swallowed with food or water.
Trichomoniasis : sexually transmitted disease that infects the genitourinary tract.
Trypanosomiasis : caused by parasitic protozoan trypanosomes of the genus Trypanosoma.
So with above description of different disease in this related Introduction and Classification of Parasites topic concludes it.