3. • Parasitology
‘the study of parasitism’
• a multidisciplinary subject covering many topics including:
morphology, taxonomy, biology, behaviour, life-cycles, pathogenesis,
epidemiology, ecology, physiology, biochemistry, genetics and
molecular biology, as well as the diagnosis, immunology and
treatment of infections.
4. Parasite:
• an organism which lives in or on another organism (its host) and
benefits by deriving nutrients at the other's expense.
Parasitism:
• Parasitism is a non-mutual relationship between species, where one
species, the parasite, benefits at the expense of the other, the host.
5.
6. Classes of parasites:
• Ectoparasites: parasites such as lice and flies that live on the body’s outer surface.
• Endoparasites: parasites the infect the internals parts of the body, such as trypanosomes
or Ascaris worms.
• Facultative parasite: is an organisms that may survive and dwell in the absence of a host
but that occasionally infects a host organism.
• Temporary parasite: Visits its host for a short period.
• Permanent parasite: leads a parasitic life throughout the whole period of its life.
7. • Obligatory parasite: Cannot exist without a parasitic life.
• Occasional or Accidental Parasite: Attacks an unusual host.
• Wandering or Aberrant parasite: Happens to reach a place where it
cannot live.
• Free living: The term free living describes the non parasitic stages of
existence which are lived independently of a host. E.g. hookworm
have active free living stages in the soil.
8. Classes of Hosts
• Definitive Host: Either harbours the adult stage of the parasite
In majority of human parasitic infections, man is the definitive host.
• Intermediate host: It harbours the larval stages of the parasite.
• In some cases larval developments are completed in two different
intermediate hosts .These are referred as first and second
intermediate hosts respectively.
9. • Paratenic Host-( A carrier and transport host):
• A host where the parasite remains viable without further
development.
• Reservoir host: It is a host which harbour the parasite and serves as
an important source of infection to other susceptible hosts.
• Epidemiologically, reservoir hosts are important in the control of
parasitic disease.
10. • Vector: A vector is an agent, usually an insect, that transmits an
infection from one human host to another.
• Mechanical vector: This is a vector which assists in the transfer of
parasitic forms between hosts but is not essential in the life cycle of
the parasite.
• e.g. a housefly that transfers amoebic cysts from infected faeces to
food that is eaten by humans.
11. Effects of parasites on their host:
• Many parasites cause harmful effects to their host, but in most cases
these effects are not of such importance that the host is being killed.
• Wasting
African trypanosomiasis and leishmaniasis may lead to severe loss of
weight in both animals and man.
• Superinfections
In the case of (muco)cutaneous leishmaniasis ulcerations may lead to
superinfections with bacteria
12. • Production of toxic compounds
It is thought that the African trypanosome, when in the central
nervous system, produces aromatic amino-acid analogues that may
influence brain function.
• Immunosuppression
Malaria, shistosomiasis, etc., lead to a certain degree of immune
suppression which renders the infected host more susceptible to
other diseases.
13. • Allergic reactions
In the case of Onchocerciasis (river blindness) the presence of the
filarial worms under the skin may lead to depigmentation due to
allergic reactions.
• Anaphylactic shock may be induced by the sudden release of large
amounts of parasite internal antigens into the bloodstream.
• In malaria this occurs when the merozoites are released in waves from
infected erythrocytes.
• In African trypanosomiasis or sleeping sickness this occurs when the immune
response leads to the massive killing and lysis of the circulating parasites.
• Also drug treatment leading to a massive killing of the parasites may result in
anaphylactic shock.
14. • Mechanical damage
• In the case of malaria the lysis of erythrocytes does lead to haemolysis and
anaemia.
• In the case of ascaris infection the presence of the worms in the small
intestine may lead to intestinal occlusions
• Irritative reflexes (intestinal contractions: ascaris)
• Irritation of skin and tissues by ecto- and endoparasites
15. Terminologies
• Infection: contamination with parasites present inside of the host
organism, such as by malaria parasites or by schistosomes.
• Infestation: contamination with parasites present on the outside of
the host organisms, such as by ectoparasites or the contamination of
a habitat with mosquitos.
• Congenital transmission: transfer of pathogens from mother to
foetus via the placenta. In this case the child will be born infected.
16. • Haemolysis: lysis of red blood cells due to the damaging action of intra
erythrocytic parasites such as Plasmodium or Babesia or by immunological
reactions due to the presence of trypanosomes in the circulation.
• Haematophagous: bloodsucking, used for insects that need blood either as the
major nutrient, or for producing fertilized eggs (female mosquitoes or sand flies).
• Host: an organism that gives food and shelter to an other organism (often a
parasite).
17. • Quartan fever: fever caused by malaria parasites with a periodicity of
72 hours
• Tertian fever: fever caused by malaria parasites with a periodicity of
48 hours
• Relapses: spontaneous return of the parasitaemia and the disease
symptoms after a period of apparent cure.
• Sylvatic: from the forest or present in the forest
18. • Thrombocytopenia: condition where there is an abnormally small
number of thrombocytes or blood platelets in the circulating blood.
• Therapeutic window: difference between the ED50 (half-maximal
effective dose) and LD50 (half-maximal toxic dose), indicating the
dose range in which the drug is active.
19. • Zoonosis: a parasitic disease mainly infecting animals and
occasionally humans. The animal host serves as the major parasite
reservoir.
• Acaricide: chemical substance having a cidal or lethal effect on the
Acarida, a subclass of the arthropods to which ticks and mites belong.
20. • Anaemia: reduced number of erythrocytes or red blood cells often
resulting by haemolysis due to the damaging action of intra erythrocytic
parasites such as Plasmodium or Babesia or by immunological reactions
due to the presence of trypanosomes in the circulation.
• BCG: Bacille Calmette Guerin of Mycobacterium bovis.
• The glycoproteins of the cell wall of this bacterium are a powerful adjuvant
used as a constituent of a vaccine in combination with vaccinating
molecules or cell preparations to stimulate the immune response.
21. • Adjuvant: an additive to a vaccine in order to stimulate or potentiate
the immune response.
• In experimental animals Freund's adjuvant is often used.
• In humans this is not allowed and as adjuvant BCG is often used
22. Sources of infection
• Contaminated soil and water
• Freshwater fishes: Diphyllobothrium latum and Clonorchis sinensis.
• Crab and crayfishes: Paragonimus wertermani.
• Raw or undercooked pork: Trichinella spiralis, T.solium.
• Raw or undercooked beef- T.saginata, Toxoplasma gondii.
• Watercress- Fasciola hepatica
• Blood sucking insects
• Housefly-Mechanical carrier- E. histolytica
23. • Dog: Echinococcus granulosus and Toxocara canis.
• Cat: T. gondii.
• Man: E.histolytica, Enterobius vermicularis and H. nana.
• Autoinfection-May occur with E. vermicularis and S. stercoralis
leading to hyperinfection.
24. • Portal of entry into the body
• Mouth: Commonest portal of entry of the parasites is oral through
contaminated food, water, soiled fingers or fomites.
• Skin-Entry through skin is another important portal of entry of
parasites. Infection with A. duodenale, N. americanus and S.
stercoralis is acquired when filariform larvae of these nematodes
penetrate the unbroken skin of an individual walking over faecal
contaminated soil.
25. • Schistosomiasis caused by S. haematobium, S. mansoni and S.
japonicum is acquired when the cercarial larvae, in water, penetrate
the skin.
• Sexual contact: Trichomonas vaginalis is transmitted by sexual
contact.
• Congenital-Infection: with T. gondii and Plasmodium spp. May be
transmitted from mother to fetus (transplacentally).
26. • Inhalation-
• Iatrogenic infection: Malaria parasites may be transmitted by transfusion of
blood from the donor with malaria containing asexual forms of erythrocytic
schizogony.
• This is known as trophozoite induced malaria or transfusion malaria.
• Malaria parasites may also be transmitted by the use of contaminated
syringes and needles. This may occur in drug addicts.
27. MCQs
• Which of the following disease causing agent penetrate through skin?
a) Shistosoma species
b) Trichomonas vaginalis
c) Plasmodium malariae
d) Trichuris trichiura
28. MCQs
• Which of the following disease causing agent penetrate through skin?
a) Shistosoma species
b) Trichomonas vaginalis
c) Plasmodium malariae
d) Trichuris trichiura
29. MCQs
• A parasite that feeds on another parasite:
a) Epiparasite
b) Hyperparasite
c) Social parasite
d) Both a and b
e) All of the above
30. MCQs
• A parasite that feeds on another parasite:
a) Epiparasite
b) Hyperparasite
c) Social parasite
d) Both a and b
e) All of the above
31. MCQs
• Onchocerciasis is also called
a) River blindness
b) Night blindness
c) Black water blindness
d) None of the above
32. MCQs
• Onchocerciasis is also called
a) River blindness
b) Night blindness
c) Black water blindness
d) None of the above
33. MCQs
• Which of the following parasite cause intestinal infection:
a) Plasmodium
b) Giardia lamblia
c) Leishmania donovani
d) Toxoplasma gondii
34. MCQs
• Which of the following parasite cause intestinal infection:
a) Plasmodium
b) Giardia lamblia
c) Leishmania donovani
d) Toxoplasma gondii
