The below mentioned article provides study notes on Class Rhizopoda:- 1. Medical Parasitology 2. Reproduction of Protozoa 3. Intestinal Protozoa 4. Clinical Features 5. Mastigophora (Intestinal Protozoa) 6. Sporozoa.
Contents:
- Medical Parasitology of Class Rhizopoda
- Reproduction of Protozoa
- Intestinal Protozoa
- Clinical Features of Class Rhizopoda
- Mastigophora (Intestinal Protozoa)
- Sporozoa
1. Medical Parasitology of Class Rhizopoda:
It is a branch of Medical Sciences which deals with Parasites and their relationship with the host and consists of:
(1) Protozoology (study of protozoa),
(2) Helminthology (Study of helminths),
(3) Entomology (Study of insects).
A parasite is an organism which adapts itself to live in or on another organisms on which it is depended for its nutrition or metabolism.
Human parasites belong to animal kingdom protozoa, helminths and arthropods:
1. Ectoparasite is the one which lives on the surface of the body, e.g., the human louse.
2. Endoparasite is normally found inside the human body, e.g., the roundworm.
3. Commensal lives in or on a host for its own benefit but does not produce disease, e.g., Entamoeba coli.
4. Pathogens are parasites which are pathogenic to man, e.g., E. histolytica.
5. Symbionts are two organisms which live in close association, so that each derives benefit from the presence of other.
6. Infection occurs when a parasite establishes itself within a host; and Infestation when a parasite lives superficially on the host.
A host is any animal that harbours a parasite.
A host may be:
(1) Definite,
(2) Intermediate,
(3) A reservoir.
A definite host is one that harbours the vegetative, the adult, or the sexual stage of the parasite. Man is a definite host for Taenia saginata. An intermediate host is the one which harbours the cystic, larval, immature, or sexual stage. Cattle are intermediate host for T. saginata. A reservoir is an animal which replaces man in the life cycle—Antelope is reservoir host of African trypanosomiasis.
Scheme of study of parasites:
1. History of the parasite,
2. Geographical distribution,
3. Habitat,
4. Morphology,
5. Life- cycle;
6. Modes of infection,
7. Pathogenicity and clinical features,
8. Laboratory diagnosis,
9. Treatment and prophylaxis.
Protozoa are microscopic single celled animals. Helminths (Worms) are metazoa and are provided with tissues and organs derived from three embryonic layers.
2. Reproduction of Protozoa:
May be by
(1) Asexual multiplication,
(2) Sexual reproduction.
1. Asexual multiplication:
(a) By binary fission (e.g. E. histolytica) or
(b) By multiple fission or Schizogony (e.g. Plasmodia in RBCs).
2. Sexual reproduction:
(i) By conjugation in which there is interchange of nuclear material due to a temporary union (e.g. Ciliata) or
(ii) By syngamy or sporogony in which male and female mate to produce zygote which finally divides into sporozoites.
African trypanosomes reproduces asexually in the blood by repeated longitudinal fission. American trypanosomes roll into round shapes in the tissue, then elongate to trypanosome form and reenter the blood-stream.
3. Intestinal Protozoa:
I. RHIZOPODA – Entamoeba histolytica, E. coli, Acanthamoeba castellani, Naegleria fowleri.
II. MASTIGOPHORA – Giardia intestinalis, Trichomonas hominis
III. SPOROZOA – Isospora belli, Cryptosporidium, Toxoplasma gondii
IV. CILIATA – Balantidium coli.
E. histolytica is cosmopolitan, occurs in three forms:
Trophozoite (Fig. 104.1) Precystic (Fig. 104.2) and Cystic (Fig. 104.3, 4, 5) forms and passes its life cycle in one host only, i.e., man.
When the fully developed cysts containing four nuclei are swallowed by man, they pass down the intestine where the excystation of oocytes takes place at alkaline pH, hence the trophozoites (Vegetative amoebae) are formed and invade the intestinal mucosa, multiply, cause flask shaped ulceration of the intestine, few enter the blood-stream, are carried to other organs (i.e. liver, lung, brain) where they give rise to amoebic abscess, some are discharged into the lumen of the gut and are transformed into small precystic forms from which the adult cysts are developed and passed in the faeces.
Thus the life cycle is repeated.
4. Clinical Features of Class Rhizopoda:
The incubation period is 4-5 days. E. histolytica causes amoebic dysentery or amoebiasis which is characterised by the passage of blood and mucus in the stool. The macroscopic and microscopic differences between the stools of amoebic and bacillary dysentery and described next:
In intestinal amoebiasis a definite diagnosis can be done by:
(1) Stool examination in which the cysts (in formed stool), and the trophozoites (in diarrhoeal stool) are identified;
(2) Sigmoidoscopy, in which scrapings from any lesions in the rectum are examined for trophozoites.
In extra-intestinal amoebiasis, the following procedures are followed:
(1) Clinical diagnosis of amoebic liver abscess (ALA) by (a) hepatomegaly, pain, fever and sweating, which is later confirmed by the presence of cysts and trophozoites in the tissues, (b) X-ray or ultrasonography of the abdomen, (c) Immunodiagnostic tests.
(2) The haemagglutination, complement fixation, gel diffusion; recent cellulose acetate precipitin (CAP), Dot immunobinding assay (DIB) and Sandwich Enzyme linked Immunosorbent Assay (ELISA) tests.
Treatment:
Amicline plus is a complete current amoebicide used for eradication of extra intestinal and intestinal amoebiasis,
Prophylaxis:
Personal prophylaxis consists of:
(1) Use of boiled drinking water,
(2) Protection of food and drink from flies, cockroaches and rats,
(3) Avoiding eating unwashed raw vegetables and fruits, and
(4) Personal cleanliness while taking food.
Community prophylaxis comprises of:
(1) Effective sanitary disposal of faeces;
(2) Protection of water supplies from faecal pollution;
(3) Avoidance of the use of human excreta as fertiliser and
(4) Detection and isolation of carriers.
E. coli (Figs. 104.6, 7, 8, 9) are worldwide and live in the large intestine of man. They are similar morphologically and in their life cycle to E. histolytica and are commensal. Recently E. dispar is accepted as non-pathogenic.
Naegleria fowleri (Free living amoeba), Acanthamoeba castellani, elongated, 10-40 microns, are rapidly motile. Their cytoplasm contains vacuoles and mitochondria (absent in Entamoeba) no red blood cells. They develop flagella after 2-4 hours in distilled water. They can be stained with Giemsa stain.
Though they survive transit through the gastrointestinal tract and passed in stool, they are known to cause primary amoebic meningoencephalitis (PAM).
In children, swimming in warm soil-contaminated pools, these amoebae enter via the nose and cribriform plate, into the brain tissue where they cause extensive haemorrhage and damage in the cerebrum and cerebellum; even they may enter from skin ulcers.
There is report of corneal ulceration or keratitis from contaminated saline used with contact lenses. Balamuthia mandrillaris; a recent free living amoeba, is pathogenic to man.
Microscopic examination of cerebrospinal fluid (CSF) will show the trophozoites and red blood cells but no bacteria.
Treatment with amphotericin B has been successful. Sulphonamide and B-hydroxystibamide have been recommended for Acanthamoebiasis.
Prophylaxis by avoiding swimming in warm soil contaminated water, contaminated saline should not be used with the contact lenses.
5. Mastigophora (Intestinal Protozoa):
Giardia Intestinais:
Causal agent of giardiasis has both trophozoite and cyst stages. Human infection is brought about by ingestion of food or drink contaminated with cysts. Within 30 minutes of ingestion, the cyst hatches out two trophozoites which then multiply in enormous numbers and colonies in the duodenum. When conditions in the duodenum become unfavorable encystment occurs and the cysts are passed out along with faeces.
Their trophozoites attach, themselves on the epithelial cells of the intestine and disturb the intestinal function leading to malabsorption of fat. There is mild steatorrhoea (passage of stools with excess fat), loose motions, chronic enteritis and cholecystopathy. Recently, 1994, there is a report of giardiasis of stomach.
Diagnosis is by:
(1) Microscopic examination of trophozoites in diarrhoea stools and cysts in formed cysts; trophozoites in the duodenal aspirates;
(2) Fluoroscopy which may demonstrate the hyper-motility of the jejunum and
(3) X-ray which may reveal mucosal defects. CIEP can detect G. intestinalis in faeces by using faecal antigen.
Treatment:
Metronidazole or quinacrine can eliminate the infection. Only by personal hygiene, its spread can be prevented.
Trichomonas hominis found in ileocaecal region, may invade the vagina; but they are nonpathogenic (Fig. 106.11).
6. Sporozoa:
Isospora belli are parasites of the small intestine of man and form oocysts and sporocysts. After ingestion by man, the mature oocysts liberate sporocysts which enter into the cells of the villi of small intestine where they multiply by schizogony (Trophozoite, schizont and merozoites) and ultimately the oocysts develop by sporogony and passed in the faeces.
The lesions produced by I. belli is unknown in man; but in calf; there is mucous diarrhoea. In man, the symptoms are anorexia, nausea, abdominal pain and diarrhoea. The infection in man is usually self- limited. In immuno-compromised (AIDS) patients, I. belli causes severe complications (diarrhoea).
Oocysts of I. belli can be demonstrated microscopically in the faeces.
There is no specific treatment for human infection. Rest with bland diet is effective.
Prophylaxis can be done by adopting the methods recommended for amoebiasis and giardiasis.
Cryptosporidium are coccidia related to Isospora, having long been known as parasites of fowls, rodents, cattle and have probably been an un-recognised cause of self-limited, mild gastroenteritis and diarrhoea in man. It is distributed in many countries including India. The children are more susceptible than adults.
Those parasites are minute (2-5 microns) intracellular sphere found in great number just under the mucosal epithelium of the stomach or intestine. The mature trophozoite (Schizont) divide into arc shaped merozoites which are released from the infected mucosal cell to begin a new life-cycle.
Oocysts (4-5 microns) containing 4 sporozoites are passed into faeces and become infective agents. If stained by modified Ziehl-Neelsen staining, oocysts in the stool appear faintly blue with reddish or purple corpuscles. Reverse Passive Haemagglutination (RPHA) is a promising technique for the detection of Cryptosporidium antigen in human faeces.
Patients infected with Cryptosporidium have watery diarrhoea, cramps, upper abdominal pain (exacerbated by food ingestion), weight loss and flatulence, nausea, vomiting, anorexia, myalgia, malaise, dehydration, fever not common. The disease is self-limited. They cause severe intractable diarrhoea in the immuno-compromised persons (e.g. those with AIDS) and immuno-competent individuals.
Only supportive therapy is available for those with AIDS or congenital immunodeficiency. Spiromycin (Rovamycin) may temporarily be effective.
Prophylaxis:
Though initially recognised in animals, cryptosporidium infections is increasingly recognised as important cause of diarrhoea in both immuno-compromised and immuno-competent individuals. Previously, it was in animals but the transmission from human to human is possible.
Toxoplasma gondii is distributed all over the world and is an obligatory, intracellular, coccidial parasite.
It may occur in two forms:
Extracellular and intracellular forms.
The life cycle of T. gondii may be (a) enteric and (b) exo-enteric.
(a) Enteric Cycle:
After eating mouse brain containing cysts of T. gondii, cycles of schizogony and sporogony develop inside the intestinal mucosa of the cat ultimately liberating oocysts in the infected cat’s faeces.
(b) Exo-Enteric Cycle:
The oocysts, after ingestion, liberate sporozoites which, in man, penetrate the mucosa of the intestine and are carried by blood and lymph stream to distant organs (brain, eyes, liver, lymph nodes, heart, skeletal muscles and placenta of the pregnant uterus) where they form pseudocysts which multiply causing damage to the central nervous system and musculature.
Toxoplasmosis may be congenital or acquired. In congenital toxoplasmosis, there is encephalitis, chorioretinitis, hydrocephalus, mental retardation and convulsion. Polymerase Chain Reaction (PCR) test on amniotic fluid is rapid, safe and accurate for prenatal diagnosis.
Acquired toxoplasmosis is very rare in adults, often fatal. In laboratory diagnosis, complement fixation test of Warren and Sabin; skin test of Sabin and Feldman are often used; ELISA (recent) test is used to assist the diagnosis. Indirect haemagglutination (IHA) test can be used.
Radiological diagnosis is done to show cerebral calcification.
Treatment is unsatisfactory. Sulphonamides and pyrimethamine are currently used.