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Section | III Microbiology 339
Part V
Medical Parasitology
Topic 38
Protozoans
LONG ESSAYS
Q. 1. Describe the morphology and life cycle of Ent- 3. Cystic Stage
amoeba histolytica. Write about laboratory diagnosis of
1. The cyst is rounded and is surrounded by a highly retrac-
intestinal amoebiasis.
tile membrane called cyst wall and varies greatly in size.
Ans. Entamoeba histolytica is an important human patho- 2. The cyst begins as uninucleate body, develops into
gen, causing amoebic dysentery as well as hepatic amoe- binucleate and quadrinucleate body. A mature cyst is a
biasis and other extraintestinal lesions. quadrinucleate spherical body having clear and hyaline
cytoplasm.
This parasite is more commonly found in tropics and
3. In the early stage of development, cytoplasm shows
subtropics, although its distribution is worldwide.
chromatoid bars as retractile oblong bars with rounded
ends and glycogen mass which stains brown with
MORPHOLOGY OF E. HISTOLYTICA iodine.
E. histolytica occurs in three forms which are
1. the trophozoite,
LIFE CYCLE
2. precystic stage and
3. cyst. 1. E. histolytica passes its life cycle in only one host, the
man. The methods of reproduction of this parasite are
1. Trophozoite excystation, encystations and multiplication.
2. The mature quadrinucleate cysts are the infective forms
1. It is the infective form of the parasite.
of the parasite. These cysts are swallowed along with
2. It is irregular in shape, actively motile with single pseu-
contaminated food and drink. The cyst wall is resistant
dopodium. Size ranges from 18 to 40 mm, average size
to the action of trypsin in the intestine.
being 20–30 mm.
3. The excystation occurs when the cyst reaches the
3. The cytoplasm is divisible into two portions, a clear
caecum or the lower part of the ileum. Each cyst lib-
ectoplasm and a granular endoplasm.
erates a single amoeba with four nuclei, a tetranucle-
4. Red blood cells, sometimes leucocytes and tissue debris
ate amoeba which eventually forms eight amoebulae
are usually found in the endoplasm.
by division of nuclei with successive fission of cyto-
5. The nucleus is spherical in shape and contains a central
plasm.
dot-like karyosome. The nucleus is lined with a delicate
4. The trophozoite phase of parasite is responsible for
nuclear membrane.
producing characteristic lesion of amoebiasis.
6. A fine thread-like linen network having a spoke-like
5. After sometime, the parasite finds it difficult to continue
radial arrangement between karyosome and nuclear
the life solely in the trophozoite stage.
membrane are seen.
6. A certain number of these trophozoites are discharged
into the lumen of the bowel and are transformed into the
2. Precystic Stage precystic forms from which the cyst are developed and
1. It is round or slightly oval in shape varying in size from passed in feces.
10 to 20 mm, with a blunt pseudopodium projecting 7. The cysts produced in an infected individual are un-
from the periphery. able to develop in the host in which they are produced
2. The endoplasm is free of red blood cells and other in- and therefore necessitate a transfer to another suscep-
gested food particles. Nucleus will be similar to that of tible host where they can grow and continue their life
trophozoite. cycle.
, Click here to Visit - www.thedentalhub.org.in
340 Quick Review Series: BDS 2nd Year
Pathogenicity EXTRAINTESTINAL LESIONS
1. The infection occurs through ingestion of the cysts. Fae- b. Secondary or Metastatic Lesions
cal contamination of drinking water and food are the (Extraintestinal Amoebiasis)
primary causes. Incubation period varies a great deal but
is generally 4–5 days. i. In amoebic liver abscess: In amoebic liver abscess
2. E. histolytica can cause two types of pathological le-
sions as follows: Trophozoites enter radicals of portal vein from base of ulcer
a. Intestinal amoebiasis or primary lesion
b. Extraintestinal amoebiasis or secondary or meta- Capillary system of liver filters them and traps them
static lesions
Trophozoites multiply and carry on cytolytic action
a. Primary or Intestinal Amoebiasis
Focal necrosis of liver cell
1. The trophozoites liberated after excystation enter
through crypts of Lieberkuhn and penetrate directly Large abscess formed
through the columnar epithelium of the mucous mem-
brane and reach the submucous coat. 1. Amoebae often are transported through the portal
2. Here the amoebae multiply, pass in various directions circulation to the liver. Hepatic invasion is multifo-
destroying the submucous tissue, formation of abscesses cal, right lobe being affected more commonly.
which finally break down leading to development of ul- 2. The amoebae initiate the lytic necrosis and with in-
cers. The ulcers are multiple and confined to the colon. creasing size of the lesion and continuing necrosis,
3. The distribution of ulcers may be generalized or local- there occurs considerable leucocyte infiltration. There
ized at ileocaecal region or sigmoidorectal region. is hepatomegaly which is known as amoebic hepatitis.
4. The typical amoebic ulcer is flask-shaped in cross-sec- 3. The lesions may develop into amoebic abscesses,
tion, multiple ulcers may coalesce to form large necrotic which may vary in size from a few millimetres to
lesions with ragged or undermined edges and covered several centimetres. The centre of the abscess con-
with brownish slough. tains thick chocolate brown pus similar to anchovy
5. The ulcers do not generally extend deeper than the sub- sauce which is liquefied necrotic liver tissue. It does
mucous layer. Occasionally the ulcer may involve the not contain any bacteria or amoebae. The amoebae
deeper tissues and may cause perforation and peritonitis. are located at the periphery.
6. Erosion of a blood vessel may lead to haemorrhage. 4. The liver abscesses may be solitary or may be mul-
7. The superficial lesions generally heal without scarring tiple. It may lead to jaundice.
but the deep ulcers form the scars which may lead to 5. If untreated, some abscesses tend to rupture into ad-
strictures, partial obstruction and thickening of the gut jacent tissues and organs.
wall, occasionally granulomatous growth may develop ii. Lungs: In pulmonary amoebiasis
in the intestinal wall from a chronic ulcer (amoeboma).
Trophozoites
INTESTINAL LESION
Portal circulation
Metacystic trophozoite liberated after excystation
Pulmonary circulation
Penetrate mucous membrane (by amoeboid Pulmonary abscess
activity and proteolytic enzymes)
1. It usually occurs following rupture of hepatic abscess
Reach submucosa and multiply rapidly through diaphragm by direct extension.
2. Rarely, it may occur by direct haematogenous spread
from the colon.
Destroy considerable area of submucosa and
mucous membrane iii. Brain
1. A rare variety of secondary amoebiasis presenting as
amoebic brain abscess, arising as a complication of
Coagulative necrosis begins and abscess forms
either hepatic or lung abscess or both.
2. Generally a small single abscess most commonly
Flask-shaped ulcer developed located in cerebral hemisphere.
Section | III Microbiology 339
Part V
Medical Parasitology
Topic 38
Protozoans
LONG ESSAYS
Q. 1. Describe the morphology and life cycle of Ent- 3. Cystic Stage
amoeba histolytica. Write about laboratory diagnosis of
1. The cyst is rounded and is surrounded by a highly retrac-
intestinal amoebiasis.
tile membrane called cyst wall and varies greatly in size.
Ans. Entamoeba histolytica is an important human patho- 2. The cyst begins as uninucleate body, develops into
gen, causing amoebic dysentery as well as hepatic amoe- binucleate and quadrinucleate body. A mature cyst is a
biasis and other extraintestinal lesions. quadrinucleate spherical body having clear and hyaline
cytoplasm.
This parasite is more commonly found in tropics and
3. In the early stage of development, cytoplasm shows
subtropics, although its distribution is worldwide.
chromatoid bars as retractile oblong bars with rounded
ends and glycogen mass which stains brown with
MORPHOLOGY OF E. HISTOLYTICA iodine.
E. histolytica occurs in three forms which are
1. the trophozoite,
LIFE CYCLE
2. precystic stage and
3. cyst. 1. E. histolytica passes its life cycle in only one host, the
man. The methods of reproduction of this parasite are
1. Trophozoite excystation, encystations and multiplication.
2. The mature quadrinucleate cysts are the infective forms
1. It is the infective form of the parasite.
of the parasite. These cysts are swallowed along with
2. It is irregular in shape, actively motile with single pseu-
contaminated food and drink. The cyst wall is resistant
dopodium. Size ranges from 18 to 40 mm, average size
to the action of trypsin in the intestine.
being 20–30 mm.
3. The excystation occurs when the cyst reaches the
3. The cytoplasm is divisible into two portions, a clear
caecum or the lower part of the ileum. Each cyst lib-
ectoplasm and a granular endoplasm.
erates a single amoeba with four nuclei, a tetranucle-
4. Red blood cells, sometimes leucocytes and tissue debris
ate amoeba which eventually forms eight amoebulae
are usually found in the endoplasm.
by division of nuclei with successive fission of cyto-
5. The nucleus is spherical in shape and contains a central
plasm.
dot-like karyosome. The nucleus is lined with a delicate
4. The trophozoite phase of parasite is responsible for
nuclear membrane.
producing characteristic lesion of amoebiasis.
6. A fine thread-like linen network having a spoke-like
5. After sometime, the parasite finds it difficult to continue
radial arrangement between karyosome and nuclear
the life solely in the trophozoite stage.
membrane are seen.
6. A certain number of these trophozoites are discharged
into the lumen of the bowel and are transformed into the
2. Precystic Stage precystic forms from which the cyst are developed and
1. It is round or slightly oval in shape varying in size from passed in feces.
10 to 20 mm, with a blunt pseudopodium projecting 7. The cysts produced in an infected individual are un-
from the periphery. able to develop in the host in which they are produced
2. The endoplasm is free of red blood cells and other in- and therefore necessitate a transfer to another suscep-
gested food particles. Nucleus will be similar to that of tible host where they can grow and continue their life
trophozoite. cycle.
, Click here to Visit - www.thedentalhub.org.in
340 Quick Review Series: BDS 2nd Year
Pathogenicity EXTRAINTESTINAL LESIONS
1. The infection occurs through ingestion of the cysts. Fae- b. Secondary or Metastatic Lesions
cal contamination of drinking water and food are the (Extraintestinal Amoebiasis)
primary causes. Incubation period varies a great deal but
is generally 4–5 days. i. In amoebic liver abscess: In amoebic liver abscess
2. E. histolytica can cause two types of pathological le-
sions as follows: Trophozoites enter radicals of portal vein from base of ulcer
a. Intestinal amoebiasis or primary lesion
b. Extraintestinal amoebiasis or secondary or meta- Capillary system of liver filters them and traps them
static lesions
Trophozoites multiply and carry on cytolytic action
a. Primary or Intestinal Amoebiasis
Focal necrosis of liver cell
1. The trophozoites liberated after excystation enter
through crypts of Lieberkuhn and penetrate directly Large abscess formed
through the columnar epithelium of the mucous mem-
brane and reach the submucous coat. 1. Amoebae often are transported through the portal
2. Here the amoebae multiply, pass in various directions circulation to the liver. Hepatic invasion is multifo-
destroying the submucous tissue, formation of abscesses cal, right lobe being affected more commonly.
which finally break down leading to development of ul- 2. The amoebae initiate the lytic necrosis and with in-
cers. The ulcers are multiple and confined to the colon. creasing size of the lesion and continuing necrosis,
3. The distribution of ulcers may be generalized or local- there occurs considerable leucocyte infiltration. There
ized at ileocaecal region or sigmoidorectal region. is hepatomegaly which is known as amoebic hepatitis.
4. The typical amoebic ulcer is flask-shaped in cross-sec- 3. The lesions may develop into amoebic abscesses,
tion, multiple ulcers may coalesce to form large necrotic which may vary in size from a few millimetres to
lesions with ragged or undermined edges and covered several centimetres. The centre of the abscess con-
with brownish slough. tains thick chocolate brown pus similar to anchovy
5. The ulcers do not generally extend deeper than the sub- sauce which is liquefied necrotic liver tissue. It does
mucous layer. Occasionally the ulcer may involve the not contain any bacteria or amoebae. The amoebae
deeper tissues and may cause perforation and peritonitis. are located at the periphery.
6. Erosion of a blood vessel may lead to haemorrhage. 4. The liver abscesses may be solitary or may be mul-
7. The superficial lesions generally heal without scarring tiple. It may lead to jaundice.
but the deep ulcers form the scars which may lead to 5. If untreated, some abscesses tend to rupture into ad-
strictures, partial obstruction and thickening of the gut jacent tissues and organs.
wall, occasionally granulomatous growth may develop ii. Lungs: In pulmonary amoebiasis
in the intestinal wall from a chronic ulcer (amoeboma).
Trophozoites
INTESTINAL LESION
Portal circulation
Metacystic trophozoite liberated after excystation
Pulmonary circulation
Penetrate mucous membrane (by amoeboid Pulmonary abscess
activity and proteolytic enzymes)
1. It usually occurs following rupture of hepatic abscess
Reach submucosa and multiply rapidly through diaphragm by direct extension.
2. Rarely, it may occur by direct haematogenous spread
from the colon.
Destroy considerable area of submucosa and
mucous membrane iii. Brain
1. A rare variety of secondary amoebiasis presenting as
amoebic brain abscess, arising as a complication of
Coagulative necrosis begins and abscess forms
either hepatic or lung abscess or both.
2. Generally a small single abscess most commonly
Flask-shaped ulcer developed located in cerebral hemisphere.
