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A parasite is an organism that obtains food and shelter from another organism and derives all benefits from this association. The parasite is termed obligate when it can live only in a host; it is classified as facultative when it can live both in a host as well as in free form. Parasites that live inside the body are termed endoparasites whereas those that exist on the body surface are called ecto-parasites. Parasites that cause harm to the host are pathogenic parasites while those that benefit from the host without causing it any harm are known as commensals.

The organism that harbors the parasite and suffers a loss caused by the parasite is a host. The host in which the parasite lives its adult and sexual stage is the definitive host whereas the host in which a parasite lives as the larval and asexual stage is the intermediate host. Other hosts that harbor the parasite and thus ensure continuity of the parasite's life cycle and act as additional sources of human infection are known as reservoir hosts. An organism (usually an insect) that is responsible for transmitting the parasitic infection is known as the vector.

axonomic classification of protozoa






Sub kingdom

E. histolytica


Sarcodina-- -



move by pseudopodia

further divided into

G. lamblia


Mastigophora move by flagella

P. ,falciparum , P. vivax P. , malariae P. ovale


B. coli


Apicomplexa no organelle of locomotion

Ciliophora move by cillia

E. bienusi



V. PROTOZOA Unicellular, chemoheterotrophic, eukaryotic organisms of kingdom Protista (3-2000 µ m).

Protozoan means “first animal”. 20,000 species, only a few are pathogens. Most are free-living organisms that inhabit water and soil. Some live in association with other organisms as parasites or symbionts. Reproduce asexually by fission, budding, or schizogony. Some exhibit sexual reproduction (e.g.: Paramecium). Trophozoite: Vegetative stage which feeds upon bacteria and particulate nutrients. Cyst: Some protozoa produce a protective capsule under adverse conditions (toxins, scarce water, food, or oxygen).

V. PROTOZOA (Continued) Nutrition Most are heterotrophic aerobes. Intestinal protozoa can grow anaerobically. Some ingest whole algae, yeast, bacteria, or smaller protozoans. Others live on dead and decaying matter. Parasitic protozoa break down and absorb nutrients from their hosts. Some transport food across the membrane. Others have a protective covering (pellicle) and required specialized structures to take in food. – Ciliates take in food through a cytostome.

Digestion takes place in vacuoles. Waste may be eliminated through plasma membrane or an anal pore.

 Protozoan classification Four major groups of protozoa are recognised and often given the status of phylum* . Note, however, that in the animal kingdom proper (Metazoa* ), phyla are distinguished on their different body plans and that no comparable body plans are found in Protozoa.

Protozoan classification • The groups are: • flagellates (or Mastigophora) • amoebae (or Sarcodina) • sporozoans (or Sporozoa, Apicomplexa) and • ciliates (or Ciliophora).

• Kingdom Animalia • Subkingdom Protozoa • Phylum Sarcomastigophora • Phylum Apicomplexa • Phylum Ciliophora • Phylum Microspora • Subkingdom Metazoa • Phylum Nematoda • Phylum Platyhelminthes

Protozoa as Human Parasites Taxonomy:

Kingdom: Protista





Mastigophora (mastigo = whip = flagellates)

Trypanosoma, Leishmania, Giardia, Trichomonas

Sarcodina (amoebae)

Entamoeba, Naegleria, Acanthamoeba


Plasmodium, Toxoplasma, Cryptosporidium, Isospora



• • • • • • • • • • • • •

INTESTINE Entamoeba histolytica Giardia Isospora Cryptosporidium Cyclospora Strongyloides Ascaris Trichuris hookworm pinworm tapeworms intestinal flukes

SYSTEMIC • Toxoplasma • malaria • filaria • Toxocara • hydatid cysts • cysticercosis • Schistosoma • liver flukes • lung flukes • Protozoa • Nematodes • Cestodes • Trematodes

Protozoa .“eukaryote”..has genetic material encased in a nuclear membrane (unlike bacteria and viruses) • ..classified traditionally by morphology (eg. organelles of locomotion), life cycle and mechanisms of reproduction etc.

• Mastigophora: movement with flagella -

e.g. Trichomonas, Giardia • Sarcodina: pseudopodia, e.g. Entamoeba histolytica • Apicomplexa: apical complex, no locomotor apparatus; • sexual reproduction, e.g. cryptosporidium, malaria, toxoplasma • Ciliophora: movement with cilia, e.g. Balantidium.

INTESTINAL PROTOZOA • • • • • • • • • •

Pathogenic Entamoeba histolytica Balantidium coli Giardia lamblia Dientamoeba fragilis Cryptosporidium parvum Enterocytozoon bieneusi Septata intestinalis Cyclospora cayetanensis Isospora belli

• • • • • • • • •

Commensal Entamoeba hartmani Entamoeba dispar Entamoeba coli Endolimax nana Iodamoeba bütschlii Chilomastix mesnili Trichomonas hominis Blastocystis hominis

• Sarcodina:

(**=pathogenic) Entamoeba histolytica**             

Entamoeba dispar Iodomoeba butschlii Dientamoeba fragilis** Endolimax nana   Entamoeba coli Entamoeba hartmani

Entamoeba histolytica ((amoebiasis • Phylum Sarcomastigophora. • Subphylum - Sarcodina: pseudopodia,

• Life Cycle                         

Biology • Two morphological stages occur; • Trophozoite - metabolically active

invasive stage, moves with pseudopodia, ingests RBC, lives in colon and is found in fresh diarrheal stool; • divides by binary fission. • Trophozoite 10-60 µm • cogwheel distribution of nuclear chromatin • Hematophagous • unidirectional movement with pseudopodia

• Cyst - "vegetative" inactive form resistant to unfavourable environmental conditions outside human host; • 4 nuclei • This is the infective form resistant to stomach acid if swallowed • survives up to 30 days; • excyst to trophozoite on passing through stomach • cyst 10-20 µm • chromotoidal body        

Adaptations • Cyst: a dormant form characterized by a hardened external covering in which metabolic activity has ceased.

– Form in response to nutrient deficiency, drought, and decreased oxygen concentration, or pH or temperature changes. – Emerge when conditions improve

Contractile vacuole • An organelle that expels fluid from the

cell. • Freshwater organisms are usually hypertonic relative to their environment so water continually diffuses into them. • To maintain homeostasis, it must use contractile vacuole to rid cell of excess water.


• Digests (liquifies) human host cells

(colon wall, neutrophils, liver cells) • Disease states: - asymptomatic carrier- symptomatic infection - amoebic dysentery - mucoid bloody    - amoebic - liver or lung abscess

:Diagnosis • - stool examination - for trophozoites and cysts - amoebic serology - abscess aspirate - Entamoeba dispar  a non-pathogen is indistinguishable by microscopy and is a much more common intestinal protozoan than Entamoeba histolytica. Antigen capture and PCR tests can distinguish E. dispar from E. histolytica in heavier infections.            

• Treatment:

Invasive states (Dysentery, Liver abscess): metronidazole

• Carrier states: diiodoquine, diloxanide furoate, or paromomycin

Other Sarcodina


404414 intestinal and luminal protozoa  
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