2. Definition of Terms
• A parasite is an organism that lives on or in a host organism and gets
its food from or at the expense of its host.
• “Parasitism is a relationship between species, where one organism,
the parasite, lives on or inside another organism, the host, causing it
some harm, and is adapted structurally to this way of life”
3. Classification of Parasites
Location in the host:
• Ectoparasites: such as ticks, lice, fleas, and Sarcopties scabi which live
on the surface of other organism.
• Endoparasites: such as some protozoa and worms which live within
the bodies of other organisms.
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4. Classes of Host
Definitive hosts- they harbor a parasite while it reproduces sexually.
Intermediate hosts- they harbor the parasite during some
developmental
Paratenic hosts- they harbor a parasite without showing any
development of parasite.
Accidental host - a host which is usually not infected with a particular
parasite.
Reservoir host are infected animals that make parasites available for
transmission to other hosts.
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5. Types of Relationship
• Symbiosis- parasites that does no harm to the host but simple act of
living together.
• Commensalism-one organism is benefited and the other remains
unaffected.
• Mutualism is another kind of symbiosis in which both the partners
are benefited
• Parasitism- an organism benefits at the cost of the host.
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6. Taxonomy and Classification
• The parasites of medical importance fall into two kingdoms: Protista
(eukaryote; unicellular) and Animalia (eukaryote; multicellular).
• The microscopic, single-celled, eukaryotic (having a true nuclear
membrane)
• Macroscopic, multicellular worms possessing well differentiated
tissues and complex organ systems.
NOTE: ATTENDANCE DURING LABORATORY PRACTICALS IS MANDOTORY
FOR IDENTIFICATION OF PARASITES AND FURTHER DISCUSSION
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7. Protozoa
• Protozoa are unicellular organisms which are widely distributed in
nature.
• Each consist of a true membrane bound nucleus and cytoplasm. Each
protozoan cell is able to perform all the vital functions of life, and
unlike the metazoan cells, it is capable of an independent existence.
• Most activities such as nutrition, respiration and excretion occurs
through osmosis.
• Locomotion is an important characteristic of protozoa and this takes
place by means of three basic types of organelles: flagella, cilia, and
pseudopodia.
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8. Protozoa
The shape, size, mode of reproduction and type of locomotive
organelle have been used to divided these into four major classes.-
i. Sarcodina- (Rhizopods)/Amoebae;
ii. Ciliophora/Ciliates;
iii. Mastigophora (Flagellates) and
iv. Coccidia (Sporozoa).
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9. Protozoa Reproduction
Reproduction could be by asexual and sexual mode.
Asexual reproduction occurs by:
• Binary fission (transverse as in ciliates or longitudinal as in
flagellates) into two daughter cells.
• Multiple fission (Schizogony) – the nucleus of the parent cell
undergoes multiple divisions, resulting in the formation of
daughter cells (e.g. plasmodium).
• Endodyogeny- In this the cell undergoes a single internal budding
resulting in the formation of two daughter cells (e.g. Toxoplasma).
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10. Protozoa Reproduction (Contd.)
• Sexual Reproduction occurs by either of the following two methods:
• Conjugation- in this process two cells become temporarily attached to
each other during which time there is exchange of nuclear material.
On completion of conjugation the cells separate and there is no
increase in the number of cells. It is seen in ciliates.
• Syngamy (Gametogony) – It is a permanent fusion of male and female
gametes. This is seen in sporozoa. The male and female gametes fuse
to form the zygote.
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11. Encystation
• Protozoa (e.g. Entamoeba histolytica, Giardia intestinalis and
Balantidium coli) may show a resting phase when they enclose
themselves within a resistant membranous wall and are then
designated as cysts.
• Cysts are non-motile structures and resist unfavourable conditions
better than trophozoites (the active or vegetative stage).
• These, hence, serve double purpose of protection and reproduction.
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12. Helminthes
• The helminthes are worms that are elongated, bilaterally symmetrical,
covered with thick cuticle and vary in length.
• The common helminthic parasites of human beings can be placed in
one of the three classes on the basis of body and alimentary tract
configuration, nature of the reproductive system and need for more
than a single host species for the completion of life cycle.
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13. General Features of Helminthes
• They do not possess organs of locomotion, therefore locomotion is
generally by muscular contraction and relaxation.
• The outer covering, known as cuticle or integument is tough and
may be armed with spines or hooks. It is resistant to intestinal
digestion.
• Digestive system is absent or rudimentary.
• Nervous system is primitive
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14. General Features of Helminthes (contd.)
• Reproductive system is very well developed
• These may be monoecious (hermaphroditic) or diecious (separate
sex)
• Both self fertilization and cross-fertilisation may take place.
• Eggs are produced in enormous numbers, but only a few of them
survive and manage to infect a suitable host.
• Helminthes, by and large, do not multiply in the human body, so
that single infection does not lead to disease.
• The adult worm or their eggs/larvae can be widely distributed in
various organs and tissues of the body.
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15. Helminths
i. Platyhelminthes (Flat worms) –
a. Trematoda (Flukes)
b. Cestoda (Tape worms)
ii. Nemathelminthes (Round worms) –
c. Nematodes.
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16. Platyhelminthes
• The body is flattened dorsoventrally, they have no body cavity, the
alimentary canal is either totally absent or rudimentary and they are
mostly hermaphroditic (both sexes present in same organism).
Suckers are present.
Further division to:
• Trematodea
• Cestodea
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17. Trematodea (Flukes)
• These are ecto- or endoparasites without epidermis or external cilia;
they are leaf-like worms with no body cavity and incomplete
alimentary canal consisting of a mouth, pharynx and a bifurcated
intestine.
• They are covered with a cuticle and are provided with one or more
suckers. The body is not segmented.
• They are hermaphrodites with paired testes, but a single ovary,
except the Schistosomes in which the testes vary in number from four
to eight.
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18. Cestodea (Tapeworms)
• These are tape-like, segmented worms often with hooks and suckers.
• They have no body cavity (coelom);
• the internal organs are embedded in a loosely connected
parenchyma.
• They have no circulatory or respiratory systems, no alimentary canal,
no exoskeleton and no definitive anus and are hermaphrodite.
• The excretory system is comprised of flame cells and ducts
(protonephridia).
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19. Nemathelminthes
• Nematodea (roundworms)- the only class of nemathelminthes.
• They are cylindrical, unsegmented and have body cavity with
complete alimentary canal.
• They have no hooks or suckers but have a well developed buccal
capsule.
• They are sexually differentiated.
• Nematodes could be found in the intestine or tissue.
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20. Method of Acquisition of Parasites
Ingestions – through contaminated food & drinks e.g. Ascaris,
Entamoeba histolytica, Taenia spp., Diphylobothrum latum,
Dracunculus medinensis.
Skin penetration e.g. Hookworm, Strongyloides stercoralis,
Schistosoma spp. (Schistosoma haematobum)
Insect bite – Wuchereria bancrofti and Brugia (mosquito); Loa loa
(chrysops fly);Onchocerca volvulus (Simulium damnosum);
Plasmodium spp. (Anopheles mosquito); T. b. gambiense (Glossina
spp.); T. b. rhodesiense (Glossina spp.); Leshmania spp.
(Phlebotomus spp. (sand flies).
Sexual contact e.g. Trichomonas vaginalis.
Rubbing infected insect faeces into the site of the insect bite e.g.
Bug & Trypanosoma cruzi.
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21. Mechanism of Disease Production By Parasites
1. By mechanical injury – e.g. rupture of red blood cell by malarial parasite,
intestinal obstruction by Ascaris lumbricoides, lysis of cells by Entamoeba
histolytica; pressure following the growth e.g. Hydatid cysts; Blockage of
ducts such as blood vessels or lymphatics e.g. Strongyloides stercoralis;
Filarial worms (Oedema and elephantiasis e.t.c).
2. 2. By the deleterious action of toxic substances e.g. –malarial parasite
pigments (rigors, chills e.t.c), Histolytic enzymes of Entamoeba histolytica.
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22. Disease Mechanism (Contd.)
• (3) Deprivation of nutrients, fluids and metabolites, by competing
with the host for food e.g.
• -Diphyllobothrium latum (fish tapeworm) → Vit.B12 deficiency
(Megaloblastic Anaemia).
• -Blood sucking activities of hookworm - Necator americanus &
Ancylostoma duodenale → Iron deficiency Anaemia
• - Competing for other micronutrients = Protein energy malnutrition as
in Ascaris, Strongyloides etc.
• (4) Allergic manifestation (Visceral larval migrans, roundworm
infections)
• (5) Inflammatory process (trichinellosis, Leishmaniasis)
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23. Life Cycle of Parasites
• No intermediate host (Direct life cycle) e.g. Ascaris Lumbricoides,
Enterobius vermiculaus; Balantidium coli, Entamoeba histolytica.
E.t.c
• One intermediate host: - e.g Taeniasis ( pig or cow), Echinococcus
granulosus (dog); Trypanosoma cruzi (bug); Plasmodium spp ( man)
e.t.c
• Two intermediate hosts e.g Paragonimus westermani ( snail and
crustacean); Diphyllobothrium latum (cyclop and fish); Clonorchis
sinensis ( snail and fish)
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25. Malaria
• caused by P.falciparum, P. malariae, P. ovale, and P. vivax.
• Insect vector is the female anopheles mosquito.
• Over a billion people in the world live in malarious area
• 300-500million clinical cases are reported each year (90% in Africa)
• 0.5-3million deaths occur yearly (majority in children < 5yrs)
• Plasmodium falciparum (malignant tertian) has developed
resistance to most categories of antimalarial drugs currently in use.
However, ACT still remains effective for malaria treatment in
endemic regions.
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26. • Resurgence has been reported in areas where malaria has
initially being eradicated.
• Very few new therapies have been developed in the last
century.
• The people at risk of severe malaria are:-non-immune
travelers/migrant workers, children< 5yrs, and pregnant
women.
27. Economic importance of malaria
•Loss of man hour
•Cost of treatment and investigations
•In pregnant women - premature
delivery, still birth/ abortion.
•Attendant problem of acute and
complicated cases including death.
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28. Amoebiasis
Entamoeba histolytica.
Acquired feaco-orally
On a global scale it infects about 480million people, and some
48million suffer from invasive amoebiasis.
It affects about 50% of inhabitants of some developing
countries, 10% of the world population and even 1% of the
population of America are infected with E. histolytica.
Invasive Amoebiasis produces an estimated 40million disabling
infections and 40,000 deaths annually.
Amoebiasis has been ranked among parasitic causes of death.
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29. Amoebiasis (contd.)
• Entamoeba histolytica is most prevalent in tropical and
subtropical areas, incidence varies widely e.g.
• higher among persons in mental hospitals and orphanages.
• Age -children under 5years lower rate than other age groups.
Reasons for higher incidence in the tropics
• lower standards of sanitation and
• favourable environment (cysts live long)
30. Amoebiasis (contd.)
Manner of disposal of human wastes - most important factor in its epidemiology.
• Transmission depends heavily on contaminated food and water.
• Filth flies, and cockroaches also are important mechanical vectors of cysts.
• Polluted water supplies, such as wells, ditches, and springs are common sources of infection.
• Carriers (cyst passers) handling food can infect the rest of their family group or hundreds of
people if the carrier works in a restaurant.
• The use of human feaces as fertilizer contributes heavily to transmission.
• While humans are the most important reservoir of this disease, dogs, pigs, and monkeys are also
important.
31. Trypanosomiasis
American Trypanosomiasis
• In Latin America, Tyrpanosoma cruzi infects an estimated10million
individual annually,
• Triatoma spp are the vectors of T. cruzi
• The infection is acquired when the faeces of infected bug is rubbed on
the conjunctiva or skin abrasions.
• The acute phase begins with facial swelling and pronounced oedema
of the eyelids of one eye (Romana’s sign) (this happens in 50% of the
cases).
• If parasite enters through abrasion on the skin it gives rise to
circumscribed area of erythema and swelling, which is called
chagoma.
• Chagoma are most commonly seen on the face or skin
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32. African Trypanosomiasis
• In Africa, Trypanosoma brucei rhodensiense or T. b. gambiense causes
sleeping sickness (one of the most lethal human infections).
• They are found in the heart of Africa, from 15º N to 15º S latitude. T.
b. gambiense is found in west central and central Africa, while T. b.
rhodesiense occurs in Central and East Central Africa
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33. African Trypanosomiasis (contd.)
• In African trypanosomiasis, infective trypanosomes of T. b. gambiense
and T. b. rhodesiense are introduced through the bite of the tsetse fly
• They multiply at the site of inoculation to cause variable induration
and swelling (the primary lesion), which may progress to form a
trypanosomal chancre.
• They spread to lymph nodes, bloodstream and, in terminal stages, to
the central nervous system where they produce the typical sleeping
sickness syndrome: Lassitude, inability to eat, tissue wasting,
unconsciousness, and death in severe cases.
34. Leishmaniasis
• Epidemiology: parts of Europe, Asia and Latin America and also
prevalent in India.
• SPECIES-
• Leishmania donovani – causes visceral leishmaniasis (Kala-azar)
• L. tropica – cause of cutaneous leishmaniasis or oriental sore;
Baghdad boil or other local names.
• L. braziliensis – cause of American leishmaniasis which
predominantly affects mucous membranes.
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35. Transmission of Leishmaniasis
• The transmission of visceral leishmaniasis is related to the activities
of humans and the biology of sandflies, Phlebotomus spp. exist
mainly at attitudes below 2000 feet, most commonly in flat plains
areas.
• For humans to become infected, they must be in sandfly areas
• Age affects the course and outcome
• Males >females
36. Filariasis
• Onchocerca volvulus – river blindness
• Wuchereria bancrofti – elephantiasis (Lymphatic filariasis)
• Loa loa (eye worm) – calabar swelling.
Other filarial are:
• Brugia malayi – causes elephantiasis
• Brugia timori
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37. Onchocerciasis
• River blindness is a disease caused by this large filarid worm in areas
of Africa (>30 million are infected). O. volvulus occur along the
courses of fast running rivers
• It is endemic from Senegal in the West to Uganda and Ethiopia in the
east and as far south as Zambia.
• Small endemic areas occur in the Yemen Arab Republic, Saudi Arabia
and in Central America (Mexico & Guatemala).
• O. volvulus is also found in South America especially in Ecuador with
smaller foci in Brazil, Venezuela and Columbia.
38. Onchocerciasis
• Transmitted by Simulium damnosum, live their larval stages only in clear,
fast-running streams.
• The adult flies survive only where there is high humidity and plenty of
streamside vegetation.
• Anyone who intrudes into such an area is viciously attacked by these
insects.
• The resulting nodules are called onchocercomas.
• ‘Sowda’- allergic rxn resulting in skin darkening.
• Hanging skin (elephant skin)- from loss of elasticity
• Leopard skin- depigmentation in chronic cases.
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39. Ocular Onchocerciasis
• The most serious complication of onchocerciasis - microfilariae in the
skin of the face migrate into the eye.
• microfilariae can be found in the cornea and in the anterior chamber,
causing iridocylitis, choroiditis in early cases. Characterized by redness
and irritation of the eye.
• Progressive changes caused by inflammatory reaction around
damaged and dead microfilariae can cause sclerosing keratitis which
can lead to blindness. Cataract and glaucoma can also result.
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40. Lymphatic Filariasis (Elephantiasis)
• Wuchereria bancrofti and Brugia malayi infections are endemic in
Asia and Africa.
• They are also found in America, Japan, Srilanka, Australia and China.
• It is however hyper endemic in Asia, India and China.
41. Lymphatic Filariasis (Elephantiasis) 2
• Many mosquito vectors of Wuchereria have a preference for human
blood and often breed near human habitation.
• A wide range of mosquitoes can transmit the parasite, depending on
the geographic area. In Africa, the most common vector is Anopheles
and in the Americas, it is Culex quinquefasciatus. Aedes and
Mansonia can transmit the infection in the Pacific and in Asia.
• The species of mosquito serving as vector in a particular area seems
to follow the periodicity of the worms (Nocturnal)
42. Schistosomiasis
• The five principal species that infect man are:
• Schistosoma haematobium– Terminal spine
• Schistosoma mansoni - Lateral spine
• Schistosoma japonicum- Minutely spined
• Schistosoma intercalatun- Terminal spine
• Schistosoma mekongi - Minutely spined.
• The intermediate hosts are Bulinus spp for S. haematobium,
Omphalaria spp for S. mansoni and for S. japonicum is Oncomelania.
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43. Schistosomiasis (Contd.)
Epidemiological factor for transmission
- Human waste in water containing intermediate host of the Schistosoma
- availability of suitable species of snail host
- vulnerable to infection are farmer who wade in their irrigation water, fishermen
who wade in their lakes and streams, children who play in any contaminated
body of water, and women who wash clothes in streams.
- In some Moslem countries, the religious requirement of ablution, that is, washing
the anal or urethral orifices after urination or defecation is an important factor in
transmission.
• The role of reservoir hosts and of strain of the parasite have some importance as
epidemiology factors, depending on the species
44. Taeniasis
• Infections are tapeworms
• Two species are common- Taenia solium (pork tapeworm) and Taenia
saginata (beef tapeworm).
• Intermediate hosts are pig or cow.
• Transmission is due to ingestion of raw, poorly or undercooked meat.
• Taenia solium tapeworm infections can lead to cysticercosis, which is
a disease that can cause seizures disorders.
45. Dracunculus Medinensis (Guinea Worm)
• It is caused by Dracunculus medinensis and acquired by drinking water that
contains infected Cyclops (a micro-crustacean).
• The female worms cause severe pain and allergic reactions including
Urticaria, fever, nausea and vomiting. Damage to the worms in the skin
can produce severe inflammation.
• Usually the legs and feet are infected.
• Three conditions necessary for parasites life cycle
• the skin of an infected individual must come in contact with water,
• the water must contain the appropriate species of microcrustacean, and
• the water must be used for drinking.
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46. Intestinal Helminthes
• it is estimated that there are more worms than the people, especially
in the developing countries.
• These worms are acquired faeco-orally e.g. Ascaris lumbricoides
(affects 1billion), Trichuris trichiura, Enterobious vermicularis, or
through skin penetration: - hookworm (Ancylostoma duoderale,
Necator americanus) affects about 900million individuals and
Strongyloides stercoralis.
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47. Intestinal Roundworm (Contd.)
• Nematodes are roundworms, and can parasitize either intermediate
or final hosts.
• In intermediate hosts, worms in the juvenile, larval, or developmental
stages are found, whereas in final (or definitive) hosts, worms occur in
the adult or sexually reproductive stage.
• In some instances the same host serves in both capacities, as with the
many mammalian hosts of Trichinella, the agent of trichinosis (Human
are dead end hosts for Trichinella spiralis).
48. Parasitic Infections in Compromised Host
• Parasitic opportunistic infection occur with protozoan such as;
Toxoplasma gondii (CNS involvement), Cryptosporidium parvum,
Microsporidium and Isospora belli (Enteritis>1month duration), and
helminth e.g. Strongyloides stercoralis (disseminated infection).
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49. Methods of Diagnosis of Parasitic Infections
• Direct demonstration of adult parasites (Ascaris)/segments (Taenia).
• Microscopic examination of body fluids and tissues for early stages e.g. ova,
trophozoites, larva and cyst.
• Cultural methods
• Xenodiagnosis
• Animal inoculation
• Histopathology
• Immunodiagnosis-serology
-skin reaction
Radiological & scanning techniques.
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50. Control and Prevention of Parasitic Infections
• Environmental sanitation (mosquitoes, helminths),
• Reduction of man’s contact with the vectors
• Use of prophylactic drugs and mass treatment of susceptible
population in endemic zones.
• Active treatment of infected individuals.
• Control of the different stages of the parasites.
• Vector control (for those parasites with intermediate hosts e.g.
mosquitoes, Cyclops, e.t.c.
• Provision of safe drinking water e.g. Dracunculus medinensis.
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51. For further reading:
• Medical Parasitology Ichpujani RL and Bhatia R. 2nd Edition. 1998. Jaypee, New Delhi.
• Medical Parasitology Muller R and Baker JR 1990 Gower, London.
• Falade C.O, Salako L.A., et al 1997). Comparative efficacy of halofantrine, chloroquine and sulfadoxine –
pyrimethamine for treatment of acute uncomplicated falciparum malaria in Nigerian children – Transaction
of the Royal Society of Tropical Medicine and Hygiene 91, 58-62
• Kremsner P.G., Luty A.FJ.F. and Graninger W. (1997). Combination chemotherapy for Plasmodium falciparum
malaria. Parasitology today 13:(5) 167-168.
• Dr Dada-Adegbola. Ecology of Major Parasites of Public Health Importance. WACP Update. 2010.
• Muller R: Guinea worm disease: Epidemiology, control and treatment. Bull WHO 57; 683 1979.
• Hawkurg F: The distribution of human filariasis throughout the World. Part III, Africa Trop. Dis Bull. 74: 650,
1977.
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