Introduction to parasitology

introduction to parasitology
parasitism - an association where one of two organisms (parasite) benefits, lives and gain shelter and nutrition at the expense of the other (the host). the host may suffer from wide range of functional and organic disturbances e.g. worms like ascaris lumbricoides reside in the gastrointestinal tract of man, and feed on of intestinal food causing various illnesses.
parasitology: is the science of parasitism and parasites, its study of host-parasite relationships.
medical parasitology: is the branch of medical sciences dealing with organisms
(parasites) which live temporarily or permanently, on or within the human body (host). medical parasitology consists of: medical protozoology, medical helminthology and medical arachnoentomology.
parasite it is organisms lives upon or within another living organism (host), it obtains its nourishment and other needs from a host the host supports the parasite
types of parasites
1-location
- an ectoparasite (external parasite) a parasitic organism that lives on the outer surface of its host on skin or hair of host e.g. lice, ticks, mites etc.
- an endoparasite (internal parasite) a parasitic organism that lives inside the body of the host may be just under the surface or deep in the body e.g. entamoeba histolytica.
2– required or not
- obligatory parasite this parasite is completely dependent on the host during a segment or all of its life cycle i.e can’t live apart from its hoost e.g. plasmodium spp. can not survive without a host i.e. completely adapted for parasitic existence.
- facultative parasite is an organism that can exist in a free living state or as a parasite and hence does not absolutely depend on the parasitic way of life. its can change its life style between free-living in the environment and parasitic according to the surrounding conditions. e.g. strongyloides stercoralis.

3- amount of time spent
- temporary parasite lives free of its host during part of its life cycle, spends only a short time on a host
- permanent parasite lives in its host from early life until maaturity or death.
- accidental parasite – when a parasite attacks an unnatural host and survives.i.e toxocara canis (a dog parasite) in man


- host - “the organism in or on, which the parasite lives and causes harm”, the host providing food and/or protection. some parasites require more than one host to complete their life cycle or may not require a host during some stages.
types of hosts
- definitive host: a host that harbours a parasite in the adult stage or where the parasite undergoes sexual method of reproduction. e.g. man is dh for schistosoma haematobium, while female anopheles mosquito is dh for plasmodium species (malaria parasites).

- intermediate host: the host in which the larval stage of parasite lives or the asexual
multiplication takes place. it’s required in the life cycle of parasite, the parasite lives during a period of its development only. in some cases, larval development is completed in two different intermediate hosts, referred to as first and second intermediate hosts.
- reservoir host: the host harboring a parasite in nature, serving as a source of infection for other susceptible hosts. reservoir hosts show no sign or symptom of disease. it maintains the life cycle of the parasite in nature such.g. sheep are rh for fasciola hepatica.
- vector: “carrier” of a parasite from one host to another. often an insect. (e.g.an arthropod) that transports a pathogenic organism from an infected to a non-infected host”.
modes of parasitic infection
- filth-borne or contaminative - where personal hygiene and community sanitation lacking. infectious stages remain viable for long periods in contaminated soil.
- soil or water-borne - water or dirt which can contain eggs, etc. larvae can penetrate skin of bare feet.
- food-borne ingestion of contaminated food and water or undercooked meat in which the infective stage has developed.
- arthropod-borne through the bite or faeces of infected vector. it is the most difficult of all to control such as mosquitoes transmitting malaria, etc.
- congenital from mother to foetus.
- inhalation of dust carrying the infective stage of parasite.
source of parasitic infection
1- food -meat ( t.saginata, t. spiralis)
2- vegetables(ascaris, e.histolytica)
3- water - (protozoal, cysts) (e.histolytica, cryptosporidium)
- cercaria (schistosoms)
- cyclops (d.medinensis)
4- soil contaminated with faeces ancylostoma ,strongyloides .
5- association with domestic animals
-dog :(hydatid disease ,toxocariasis ,leishmania)
-cat : (toxoplasma oocyst).
6- arthropods :
- blood sucking (malaria,trypanosome,leishmania).
- mechanical transmission (ova, protozoal cysts)
- myiasis (invasion of tissue by larvae of flies)
7- blood transfusion : (erythrocytic stages of plasmodium).
8-congenital transplantation : (toxoplasma, plasmodium).
9-sexual intercourse : (trichomonas vaginalis).
10-inhalation of dust: (enterobius ova).
life cycle
1) direct (simple): when a parasite requires only one species of host to complete its development .g. histolytic. the parasite generally spends most of its life in or on the host, and may reproduce within the host. transmitted from one host to another through the air, by a fomite, or in contaminated food or water. parasites with a simple or direct life cycle are described as monoxenous.

2) indirect life cycle (complex): when a parasite requires two or more species of hosts to complete its development. parasites with more complex life cycles involving multiple hosts are described as heteroxenous. the primary or definitive host of a heteroxenous species is the one in which adult parasites live and reproduce sexually. the secondary or intermediate host (ih) is the host where immature l ife stages of the parasite live and reproduce asexually. in many cases, the parasite passes through critical developmental stages in the ih. the latter may also aid in transmitting parasites to their final host. e.g. filariasis.
effect of parasite on the host
the damage which pathogenic parasites produce in the tissues of the host may be described in the following two ways
(a) direct effects of the parasite on the host
• mechanical injury - may be inflicted by a parasite by means of pressure as it grows larger, e.g. hydatid cyst causes
blockage of ducts such as blood vessels producing infraction.
• deletingrious effect of toxic substances- in plasmodium falciparum production of toxic substances may cause rigors and other symptoms.
• deprivation of nutrients, fluids and metabolites, parasite may produce disease by competing with the host for nutrients.
(b) indirect effects of the parasite on the host:
tissue damage may be caused by immunological response of the host, e.g. nephritic syndrome following plasmodium infections. excessive proliferation of certain tissues due to invasion by some parasites can also cause tissue damage in man, e.g. fibrosis of liver after deposition of the ova of schistosoma

diagnosis of parasitic infections
1-clinical
2-laboratory
purpose of laboratory diagnosis:
- confirmation of clinical suspicion
- identification of unsuspected infection
- depending on the nature of the parasitic infections, the following specimens are selected for laboratory diagnosis:
1- blood: in those parasitic infections where the parasite itself in any stage of its development circulates in the blood stream, examination of blood film forms one of the main procedures for specific diagnosis. for example, in malaria the parasites are found inside the red blood cells.
2- stool: examination of the stool forms an important part in the diagnosis of intestinal parasitic infections. in protozoan infections, either trophozoites or cystic forms may be detected the former during the active phase and the latter during the chronic phase. example, amoebiasis, giardiasis, etc. in the case of helmithic infections, the adult worms, their eggs, or larvae are found in the stool.
3- urine – when the parasite localizes in the urinary tract, examination of the urine will be of help in establishing the parasitological diagnosis. for example in urinary schistosomiasis, eggs of schistosoma haematobium are found in the urine.

4- sputum – examination of the sputum is useful in the following:
- in cases where the habitat of the parasite is in the respiratory tract, as in paragonimiasis, the eggs of paragonimus westermani are found.
- in amoebic abscess of lung or in the case of amoebic liver abscess bursting into the lungs, the trophozoites of e. histolytica are detected in the sputum
5) biopsy material - varies with different parasitic infections. for example spleen punctures in cases of kala-azar, muscle biopsy in cases of cysticercosis and chagas’ disease, skin snip for onchocerciasis.
6) urethral or vaginal discharge – for trichomonas vaginalis

indirect evidences – changes indicative of intestinal parasitic infections are:
a. cytological changes in the blood – eosiniphilia often gives an indication of tissue invasion by helminthes, a reduction in white blood cell count is an indication of kala-azar, and anemia is a feature of hookworm infestation and malaria.
b. serological tests – are carried out only in laboratories where special antigens are available.
classification of parasites
parasites are divided into two phyla:
- a. protozoa : these are unicellular parasites
- b. metazoa : these are multi cellular parasites

- a. phylum protozoa: these parasites are unicellular eukaryotic organisms and the single cell carries out all the functions of the parasite like reproduction,digestion, respiration and excretion.
- they usually measure from 1-150 ?m.
- the protozoa have cytoplasm and nucleus. - the cytoplasm is differentiated into ectoplasm (the outer layer) and endoplasm (the inner layer). - the ectoplasm functions in: protection, locomotion, ingestion of food, excretion and respiration. - locomotion either by pseudopodia, cilia and flagella. - the endoplasm encloses: organelles, contractile vacuoles for osmoregulation, food vacuoles containing food during digestion. - the nutrition of all protozoa is holozoic. absorption of liquid food through the body surface, or ingestion of solid particles by the help of pseudopodia or through the cytostome. - examples of protozoa are: entamoeba histolytica, giardia lamblia, plasmodia, leishmania and trypanosoma
- metazoa: multicellular organisms, e.g. helminths (worms) and arthropods (ticks, lice)
- helminths: the common morphological features of helminths are as given under
- no organs of locomotion
- have tough cuticle
- gastro intestinal tract (git) absent or rudimentary or developed nervous system primitive
- very well developed reproductive system
- hermaphrodites or separate sexes
- enormous number of eggs produced
- do not multiply in humans (generally)

protozoa helminthes
unicellular
single cell for all function multicellular
specialized cells
amoebae:
move by pseudopodia.
flagellates:
move by flagella.
ciliates :
move by cilia
apicomplexa
(sporozoa) tissue parasites round worms
(nematodes)
cylindrical, unsegmented
flat worms
1-trematodes:
leaf-like, unsegmented.
2-cestodes:
tape-like, segmented






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classification of medically important parasites


protozoa metazoa (heliminths)
sarcodina (amoebae): platyhelminthes:
(a) genus, entameba: trematodea:
e.g. entameba histolytica (a) genus schistosoma
(b) genus endolimax e.g. s. mansoni
e.g. endolimax nana (b) genus fasciola
(c) genus iodameba e.g. f. hepatica
e.g. iodameba butchlii cestoda:
(d) genus dientmeba (a) genus diphylobotrium
e.g. dientameba fragilis e.g. d. latum
mastigophora (flagellates): (b) genus taenia
(a) genus giardia e.g. t. saginata
e.g. g. lamblia (c) genus echinococcus
(b) genus trichomonas e.g. e. granulosus
e.g. t. vaginalis (d) genus hymenolepsis
(c) genus trypanosoma e.g. h. nana
e.g. t. brucci nemathelminthes:
(d) genus leishmania (a) intestinal nematodes
e.g. l. donovani e.g. a. lumbricoides
sporozoa (b) somatic nematodes
(1) genus plasmodium e.g. w. bancrofti
e.g. p. falciparum
(2) genus toxoplasma
e.g. t. gondi
(3) genus cryptosporidum
e.g. c. parvum
(4) genus isospora e.g. i. beli
ciliates
e.g. balantidium coli