Introduction of parasitology

Terms

• Parasitology [Gr. Parasitos parasite-logy] is the science of parasitism and parasites.

• Medical Parasitology is the science or study of parasites of humans. Medical

Parasitology consists of: Medical Protozoology, Medical Helminthology and Medical

Arachnoentomology.

• Medical Protozoology is the study of human parasites of Protozoa.

• Medical Helminthology is the study of human parasitic worms of Cestodes,

Nematodes and Trematodes.

• Medical Arachnoentomology is the study of parasites of Arthropoda.

• Symbiosis is the living together or close association of two dissimilar

organisms. There are three forms of the symbiosis: mutualism, commensalism and

parasitism.

• Mutualism is symbiosis in which both parties benefit.

• Commensalism is symbiosis in which one party (commensalis) is benefited and the

other party (host) receives neither benefit nor harm.

• Parasitism [Gr.parasitios eating with another] is symbiosis in which one party

(parasite) benefits at the expense of the other (host).

• Parasite lives upon or within another living organism (host) at whose expense it

obtains some advantage.

• External parasite (ectoparasite) lives on skin or hair of host.

• Internal parasite (endoparasite) lives in body organs, body tissues, body cells, body

cavities of host.

• Facultative parasite is an organism which may be parasitic upon another but which is

capable of independent existence.

• Obligatory parasite can’t live apart from its host.

• Temporary parasite lives free of its host during part of its life cycle.

• Permanent parasite lives in its host from early life until maturity or death.

• Host is an organism that harbours or nourishes another organism (parasite). The hosts

divide into: definitive host, intermediate host and reservoir.

• Definitive host (final h.) is a host in which a parasite attains sexual maturity; harbours

the adult or sexually mature parasite.

• Intermediate host harbours the immature or asexual stages of the parasite.

• Reservoir host an animal that harbours the same species of parasites as man and

constitute a source of infection to him.

• Vector is an arthropod that carriers a parasite to its host.

• Invasive diseases are caused by animals.

• Protozoan diseases are caused by Protozoa.

• Anthroponotic diseases are characteristic for humans.

• Zoonotic diseases are characteristic for animals.

• Anthropozoonotic diseases are characteristic for humans and animals.

• There are four ways of agent transmission of invasious diseases:

• 1) contagion (by skin contact, sexual contact);

• 2) alimentary or faecal-oral transmission (ingestion of raw or undercooked food or

use of drinking water containing the infective stage of the parasite);

• 3) blood (by bite of vector containing the infective stage, blood transfusion);

• 4) congenital (through the placenta).

The Protozoa

• Kingdom Animalia

• Subkingdom Protozoa

• Phylum 1. Sarcomastigophora

• Subphylum Sarcodina. Class Lobozea. Type species: Entamoeba histolytica,

Entamoeba coli, Entamoeba gingivalis.

• Subphylum Mastigophora (or Flagellates).

• Class Zoomastigophorea. Type species: Trypanosoma brucei gambriense,

Trypanosoma brucei rhodesiense, Trypanosoma cruzi, Leishmania donovani,

Leishmania tropica, Giardia lamblia , Trichomonas vaginalis, Trichomonas

intestinalis, Trichomonas buccalis.

• Phylum 2. Apicomplexa. Class Sporozoa. Type species: Plasmodium vivax,

Plasmodium malariae, Plasmodium falciparum, Plasmodium ovale, Toxoplasma

gondii.

• Phylum 3. Ciliophora. Class Ciliata. Type species: Balantidium coli.

Morphology and Ultrastructure of Protozoa

1) Protozoa are unicellular animal organisms.

2) Each protozoon performs all functions of life.

3) Sizes is from 1 micro;m until 150 micro;m.

4) The protozoa have cytoplasm and nucleus.

5) The cytoplasm is differentiated into ectoplasm (the outer layer) and endoplasm

(the inner layer).

6) The ectoplasm functions in: protection, locomotion, ingestion of food, excretion

and respiration.

7) Locomotion either by pseudopodia, cilia and flagella.

8) The endoplasm encloses: organelles, contractile vacuoles for osmoregulation, food

vacuoles containing food during digestion.

9) 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.

10) Reproduction may be asexual or sexual.

• Class Lobozea:

• 1) Motion is by pseudopodia.

• 2) Reproduction is by binary fission.

• 3) The production of a cyst is one of the stages in the life cycle.

• 4) The pathogenic species for man is Entamoeba histolytica, the non-pathogenic

(commensal) species are E. gingivalis, E. coli.

• Parasite: Entamoeba histolytica

• Disease: Amoebiasis, or amoebic disentery

• Geographical distribution: Cosmopolitan

• Morphology: Three forms: 1) forma magna;

2) forma minuta;

3) cyst.

• Host: Homo sapiens

• Transmission: faecal-oral (alimentary)

• Infective stage: mature cyst

• Localisation: large intestine

• Pathogenicity:

1) Intestinal amoebiasis: formation of ulcerus of the wall of the intestine, acute or

chronic diarrhoea, stool containing blood and mucus; may be asymptomatic infection.

2) Extra- intestinal amoebiasis: abscess of liver, lung, brain, skin.

• Laboratory diagnosis: Fresh stools are examined under the microscope. E. histolytica

(forma magna and cysts with 4 nuclei) can be demonstrated in the stools.

• Prophylaxys: Treatment of patients and asymptomatic cyst carriers; protection of

foodstuffs and water from flies and contamination with faeces, the staff of catering

establishments must be examined for cysts carriage, health education of the

population.

• Life cycle

Life cycle of E. histolytica

Human

Intestinal Protozoa - The Amoebae

Entamoeba hartmanni

Epidemiology - similar to E. histolytica

Formerly called the “small race” of Entamoeba histolytica.

Technologists must be able to differentiate this organism from E.

histolytica because E. hartmanni is non-pathogenic.

Intestinal

Protozoa - The

Amoebae

Entamoeba coli

Significance - this is a harmless commensal; must be differentiated from

pathogens.

Morphology - trophozoites range from 10 to 35 microns in diameter; cysts

range from 10 to 30 microns in diameter and contain 8 to 16 nuclei when

mature; the nucleus exhibits an eccentric karyosome with irregular,

coarse chromatin. The cytoplasm is heavily vacuolated, containing yeast,

bacteria, and debris.

Intestinal Protozoa - The Amoebae

Entamoeba gingivalis

Infective site - the mouth; the organism thrives in diseased gums, but is

not considered a causal agent. It is destroyed in stomach if swallowed.

Transmission - contact with fomites (drinking glasses, eating utensils,

etc.); kissing.

Morphology - resembles E. histolytica, but has no cyst stage. It is the only

species which ingests leucocytes.

Intestinal Protozoa - The Amoebae

Endolimax nana

Occurrence - occurs in about 14% of the US population; 21% worldwide.

Pathogenicity - none.

Morphology - trophozoites range from 5 to 10 microns in diameter. The

nucleus contains a large, blot-like karyosome; there is little or no

peripheral chromatin. Cysts are usually sub-oval, measuring 4 to 6 by 6 to

10 microns.

Iodamoeba butschlii i

Pathogenicity - none.

Morphology - the cyst is often called the “iodine cyst” due to the presence

of a large glycogen vacuole which stains dark brown with iodine.

Tissue Dwelling Amoebae

Naegleria fowleri

Classification - an ameboflagellate; a free-living organism alternating

between amoeboid and flagellated forms; only the amoeboid form is

found in tissues.

Life cycle - the amoeba gains entry via the nasal mucosa, usually during a

swimming event; it moves along the olfactory nerve, gaining access to the

brain via the cribriform plate. Cases are invariably fatal. Infections do

not spread from person-to-person.

Tissue Dwelling Amoebae

Naegleria fowleri

Symptoms - Dramatic and rapidly progressive. Headache, fever, nausea

& vomiting occur within 1 to 2 days. Meningoencephalitis, irrational

behavior, coma & death usually occur within 9 days of exposure.

Diagnosis - Usually made at autopsy. CSF contain motile amoebae. Early

diagnosis is critical. Amoebae in CSF specimens can be cultured on non-

nutrient agar containing bacteria.

Tissue Dwelling Amoebae

Acanthamoeba spp.

Life cycle - also a free-living amoeba. The amoeba reaches the brain

hematogenously after entering a wound or lesion on the skin. More

commonly, the organism is associated with getting into eyes via

contaminated or homemade cleaning solutions.

Symptoms - slow onset (10 or more days). Presents as chronic,

granulomatous lesions in brain. In eye lesions, the infection resembles a

herpes virus infection.

Acanthamoeba keratitis - associated with users of extended-wear contact

lenses

Entamoeba histolytica must be differentiated from other intestinal protozoa

including: E. coli, E. hartmanni, E. dispare,……

Differentiation is possible, but not always easy, based on morphologic

characteristics of the cysts and trophozoites.

The nonpathogenic Entamoeba dispar, however, is morphologically identical

to E. histolytica, and differentiation must be based on isoenzymatic or

immunologic analysis.

Molecular methods are also useful in distinguishing between E. histolytica

and E. dispar and can also be used to identify E. polecki.

Microscopic identification

This can be accomplished using:

Fresh stool: wet mounts and permanently stained preparations (e.g.,

trichrome).

Concentrates from fresh stool: wet mounts, with or without iodine stain, and

permanently stained preparations (e.g., trichrome).

Immunodiagnosis

(Antibody Detection)

1- Antibody detection

2- Antigen detection may be useful as an adjunct to microscopic diagnosis

The indirect hemagglutination (IHA)

The EIA test detects antibody specific for E. histolytica in approximately 95%

of patients with extraintestinal amebiasis, 70% of patients with active

intestinal infection, and 10% of asymptomatic persons who are passing cysts

of E. his

Treatment

Intestinal Amebiasis:

*Asymptomatic amebiasis(cyst passer): Diloxanide furoate ( furamide)

500 mg 3 times daily / 10 days

*Symptomatic amebiasis ( troph. & cyst): - Iodoquinol , 650 mg 3 times daily/

20 days or Metronidazole (Flagyl) , 750 mg 3 times daily/ 10 days

*Amebic colitis: Chloroquine, 250 mg 2 times daily

* Acute amebic dysentery: Emetine hydrochloride, 1mg/kg daily IM

Extraintestinal Amebiasis:

Amebic liver abscess, ameboma:

Metronidazole, as above plus dehydroemetine / 10 days or Metronidazole or

dehydroemetine as above plus Chloroquine , 500 mg 2 times daily / 2 days,…..

Symptoms

Depends on host’s previous exposure to parasite

Chronic, low-level exposure can result in host being asymptomatic.

Less frequent exposure result in severe symptoms.

May also depend on nutritional status of host

Amebic dysentery

Ranges from minor cramping and diarrhea to severe cramping

and 15 to 20 bloody stools a day

Race has no effect on severity

Amebiasis – Symptoms depend on what organ the parasite invades.

Abdominal cavity

Peritonitis, abdominal pain, cramping, and anemia

Liver

Symptoms are similar to hepatitis

Lungs, brains, or heart

May cause the death of the host.