Medical Parasitology deals with the study of animal parasites, which infect and

produce diseases in human beings.

PARASITE
A parasite is a living organism that lives on or in another organism known as a
host.It obtains nourishment and protection while offering no benefit in return.
Consequently, the host suffers from various diseases, infections, and discomforts.
However, in some cases, the host may show no signs at all of infection by the
parasite.parasites may be seen with the naked eye(macroscopically) or with a
microscope(microscopically)
A Parasites may be classified as
1. Ectoparasite: They inhabit the surface of the body of the host without
penetrating into the tissues. They are important vectors transmitting the pathogenic
microbes. The infection by these parasites is called as infestation, e.g . fleas or
ticks
2. Endoparasite: They live within the body of the host (e.g., Leishmania).
Invasion by the endoparasite is called as infection.
The parasites included in medical parasitology are: Protozoa, Helminthes, and
Arthropods.
TYPES OF PARASITES
 An obligatory parasite that is completely dependent on its host during part
or all of its life cycle and can’t survive without it e.g. hookworms.
 A facultative parasite that can change its life style between free-living in
the environment and parasitic according to the surrounding conditions. e.g.
Strongyloides stercoralis.
 An accidental parasite that affects an unusual host e.g. Toxocara canis (a
dog parasite) in man.
 A temporary parasite that visits the host only for feeding and then leaves it.
e.g. Bed bug visiting man for a blood meal.
 A permanent parasite that lives in or on its host without leaving it e.g. Lice.
 An opportunistic parasite that is capable of producing disease in an immune-
deficient host (like AIDS and cancer patients). In the immuno-competent
host, it is either found in a latent form or causes a self-limiting disease e.g.
Toxoplasma gondii.
 A zoonotic parasite that primarily infects animals and is transmittable to
humans. e.g. Fasciola species.
 Aberrant parasite – one which is never transmitted from man to man and
which develops abnormally in man (hydatid, Angiostrongilus, Toxocara)
 Erratic Parasite – one that wanders from its predilection sites into an organ
where it is not usually found, e.g. Entamoeba histolytica in lung or liver of
the host

PATHOGENESIS OF PARASITIC DISEASES

The parasites can cause damage to humans in various way
 Mechanical trauma:
a. Eggs: Trematode eggs being large in size, can be deposited inside
theintestinal mucosa (Schistosoma mansoni), bladder (Schistosoma
haematobium), lungs (Paragonimus), liver (Fasciola hepatica) and can
cause mechanical irritation
b. Larvae: Migration of several helminthic larvae (hookworms,
Strongyloides or Ascaris) in the lungs produce traumatic damage of the
pulmonary capillaries leading to pneumonitis
c. Adult worms: Adult worms of hookworm, Strongyloides, Ascaris or
Taenia get adhere to the intestinal wall and cause mechanical trauma
 Space occupying lesions: Certain parasites produce characteristic cystic
lesion that may compress the surrounding tissues or organs, e.g., hydatid
cysts and neurocysticercosis
 Inflammatory reactions: Most of the para sites induce cellular proliferation
and infiltration at the site of their multiplication, e.g., E. histolytica provokes
inflammation of the large intestine leading to the formation of amoebic
granuloma. Adult worm of W. bancrofti causes mechanical blockage and
chronic inflammation of the lymphatics and lymph vessels. Trematode eggs
can induce inflammatory changes (granuloma formation) surrounding the
area of egg deposition
 Enzyme production and lytic necrosis: Obligate intracellular parasites of
man (Plasmodium, Leishmania and Trypanosoma), produce several
enzymes, which cause digestion and necrosis of host cells. E. histolytica
 produces various enzymes like cysteine proteinases, hydrolytic enzymes and
amoebic pore forming protein that lead to destruction of the target tissue
 Toxins: Some of the parasites produce toxins, which may be responsible for
pathogenesis of the disease, e.g., E. histolytica. However, in contrast to
bacterial toxin, parasitic toxins have minimal role in pathogenesis
 Allergic manifestations: Many metabolic and excretory products of the
parasites get absorbed in the circulation and produce a variety of allergic
manifestations in the sensitized hosts Examples include schistosomes
causing cercarial dermatitis, rupture of hydatid cyst producing anaphylactic
reactions and occult filariasis (tropical pulmonary eosinophilia)
 Neoplasia: Some of the parasitic infections can contribute to the
development of neoplasia (e.g., S. haematobium causes bladder carcinoma,
Clonorchis and Opisthorchis cause cholangiocarcinoma)
 Secondary bacterial infections: Seen in some helminthic diseases
(schistosomiasis and strongyloidiasis)

HOST
Host is defined as an organism, which harbors the parasite and provides
nourishment and shelter.
Hosts may be of the following types:
 Definitive host: The host in which the adult parasites replicate sexually
(e.g., anopheles species), is called as definitive host. The definitive hosts
may be human or nonhuman living things
 Intermediate host: The host in which the parasite under goes asexual
multiplication is called as intermediate host. (e.g., in malaria parasite life
cycle, humans are the intermediate hosts). Intermediate hosts are essential
for the completion of the life cycle for some parasites. Some parasites
require two intermediate hosts to complete their different larval stages.
These are known as the first and second intermediate hosts respectively
(e.g., Amphibian snails are the first intermediate host and aquatic plants are
the second inter mediate host for Fasciola hepatica
Hosts can also be:
 Reservoir host: It is a host, which harbours the parasites and serves as an
important source of infection to other susceptible hosts. (e.g., dog is the
reservoir host for cystic echinococcosis,)
 Paratenic or transport host in whom the parasite does not undergo any
development but remains alive and infective to another host. Paratenic hosts
 bridge gap between the intermediate and definitive hosts. For example, dogs
and pigs may carry hookworm eggs from one place to another, but the eggs
do not hatch or pass through any development in these animals.
 Vector is an arthropod that transmits parasites from one host to another, e.g.
female sand fly transmits Leishmania parasites
HOST-PARASITE RELATIONSHIP
The relationship between the parasite and the host, may be divided into the
following types:
1. Symbiosis: It is the close association between the host and the parasite.
Both are interdependent upon each other that one cannot live without the
help of the other. None of them suffer any harm from each other
2. Commensalism: It is an association in which the parasite only derives the
benefit without causing any injury to the host. A commensal is capable of
living an independent life
3. Parasitism: It is an association in which the parasite derives benefit from
the host and always causes some injury to the host. The host gets no benefit
in return.

In medical parasitology, each of the medically important parasites are

discussed under the standard subheadings of morphology, geographical
distribution, means of infection, life cycle, host/parasite relationship,
pathology and clinical manifestations of infection, laboratory diagnosis,
treatment and preventive/control measures of parasites.
1. Morphology - includes size, shape, color and position of different
organelles in different parasites at various stages of their development. This
is especially important in laboratory diagnosis which helps to identify the
different stages of development and differentiate between pathogenic and
commensal organisms. For example, Entamoeba histolytica and Entamoeba
coli
2.Geographical distribution –there are certain diseases that occur or are
prevalent than others in a certain region. Even though revolutionary
advances in transportation has made geographical isolation no longer a
protection against many of the parasitic diseases, many of them are still
found in abundance in the tropics.
Distribution of parasites depends upon:
 a. The presence and food habits of a suitable host: • Host specificity, for
example, Ancylostoma duodenale requires man as a host where
Ancylostoma caninum requires a dog. • Food habits, e.g. consumption of
raw or undercooked meat or vegetables predisposes to Taeniasis
b. Easy escape of the parasite from the host- the different developmental
stages of a parasite which are released from the body along with faeces and
urine are widely distributed in many parts of the world as compared to those
parasites which require a vector or direct body fluid contact for
transmission.
c. Environmental conditions favoring survival outside the body of the host,
i.e. temperature, the presence of water, humidity etc.
d. The presence of an appropriate vector or intermediate host – parasites
that do not require an intermediate host (vector) for transmission are more
widely distributed than those that do require vectors.
3.life cycle
A parasite life cycle consists of two common phases;
1) The route a parasite follows inside the host body and
2) the route a parasite follows outside of the body.
This provides crucial information pertinent to epidemiology, prevention and
control.
Once the infecting organism is introduced into the body of the host, it reacts
in different ways and this could result in:
A) Carrier state – a perfect host parasite relationship where tissue
destruction or damage by a parasite is balanced with the host’s
tissue/damage repair. At this point the parasite and the host live
harmoniously, i.e. they are at equilibrium, as in reservoir host.
B) Pathology and clinical manifestations of infection:
1) Disease state -this is due to an imperfect host parasite relationship where
the parasite dominates the upper hand. This can result either from lower
resistance of the host or a higher pathogenecity of the parasite.
2) Parasite destruction –occurs when the host takes the upper hand.

The life cycle of the parasite may be direct (simple) or indirect (complex).
 Direct/simple life cycle: When a parasite requires only one host to
complete its development, it is referred as direct/simple life cycle
 Indirect/complex life cycle: When a parasite requires two hosts
(one definitive host and another intermediate host) to complete its
 development, it is referred as indirect/complex life cycle. Some of
the helminths require three hosts (one
definitive host and two intermediate hosts)
4.laboratory diagnosis
Depending on the nature of the parasitic infections, the following specimens are
selected for laboratory diagnosis:
•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.
•Stools - examination of the stool forms an important part in the diagnosis of
intestinal parasitic infections.
 trophozoites, cystic forms and parasite eggs may be detected. Some adult
worms and their larvae may also be found in the stools.
•Urine –when the parasite localizes in the urinary tract, examination of the urine
will be of help in establishing the parasitological diagnosis
.Sputum –examination of the sputum is useful in the following:
•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.
•Biopsy material –varies with different parasitic infections.
–e.g. spleen punctures in cases of kala-azar,
–muscle biopsy in cases of Cysticercosis, Trichinelliasis, and Chagas’
disease,
–Skin snip for Onchocerciasis.
 Urethral or vaginal discharge
–for Trichomonas vaginalis
 Indirect evidences •changes indicative of intestinal parasitic infections are:
-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.
Serological tests –are carried out only in laboratories where special antigens are
available.
5.Modes of Transmission
The infective stages of various parasites may be transmitted from one host to
another in the following ways:
 Oral or feco-oral route: It is the most common mode of transmission of the
parasites. Infection is transmitted orally by ingestion of food, water or
vegetables contaminated with feces containing the infective stages of the
parasite. (e.g., cysts of E. histolytica, and ova of Ascaris lumbricoides)
 Penetration of the skin and mucous membranes: Infection is transmitted by
the penetration of the larval forms of the parasite through unbroken skin
(e.g., filariform larva of Strongyloides stercoralis and hookworm can
penetrate through the skin of an individual walking barefooted over fecally
contaminated soil), or by introduction of the parasites through bloodsucking
insect vectors. (e.g., Plasmodium species, Leishmania species and
Wuchereria bancrofti)
 Sexual contact: Trichomonas vaginalis is the most frequent parasite to be
transmitted by sexual contact. However, Entamoeba, Giardia and
Enterobius are also transmitted rarely by sexual contact among
homosexuals
 Bite of vectors: Many parasitic diseases are transmitted by insect bite such
as: malaria (female anopheles mosquito), filariasis (Culex), leishmaniasis
(sandfly), Chagas’ disease (reduviid bug) and African sleeping sickness
(tsetse fly)
 Vertical transmission: Mother to fetus transmission is important for few
parasitic infections like Toxoplasma gondii, Plasmodium spp. and
Trypanosoma cruzi.
 Blood transfusion: Certain parasites like Plasmodium species, Babesia
species, Toxoplasma species, Leishmania species and Trypanosoma species
can be transmitted through transfusion of blood or blood products
  Autoinfection: Few intestinal parasites may be transmitted to the same
person by contaminated hand (external autoinfection) or by reverse
peristalsis.
6.preventive measures
They include:
 Reduction of the source of infection–the parasite is attacked within the
host, thereby preventing the dissemination of the infecting agent.
 –Sanitary control of drinking water and food
 Proper waste disposal –through establishing safe sewage systems, use of
screened latrines, and treatment of night soil.
 The use of insecticides and other chemicals used to control the vector
population.
 Protective clothing that would prevent vectors from resting on the surface
of the body and inoculate pathogens during their blood meal.
 Good personal hygiene.
7.Treatment of parasitic diseases
Treatment of parasitic disease is primarily based on chemotherapy and in
some cases by surgery.
 Antiparasitic Drugs Various chemotherapeutic agents are used for the
treatment and prophylaxis of parasitic infections
 Surgical Management; For management of parasitic diseases like
cystic echinococcosis and neurocysticercosis surgery is indicated.
TAXONOMY OF PARASITES
According to the binomial nomenclature as suggested by Linnaeus, each parasite
has two names: a genus and a species name. These names are either derived from:
names of their discoverers, Greek or Latin words of the geographical area where
they are found, habitat of the parasite, or hosts in which parasites are found and its
size and shape. All parasites are classified under the following taxonomic units—
the kingdom, subkingdom, phylum, subphylum, super class, class, subclass, order,
suborder, super family, family, genus and species.
The generic name of the parasite always begins with an initial capital letter and
species name with an initial small letter, e.g., Entamoeba histolytica
CLASSIFICATION OF MEDICAL PARASITOLOGY
Parasites of medical importance come under the kingdom called protista and
animalia. Protista includes the microscopic single-celled eukaroytes known as
protozoa. In contrast, helminthes are macroscopic, multicellular worms possessing
well differentiated tissues and complex organs belonging to the kingdom animalia.
Medical Parasitology is generally classified into:
1• Medical Protozoology - Deals with the study of medically important protozoa.
2• Medical Helminthology - Deals with the study of helminthes (worms) that
affect man.
3• Medical Entomology - Deals with the study of arthropods which cause or
transmit disease to man.
PHYLUM PROTOZOA
These are unicellular organisms in which the various activities of metabolism,
locomotion, etc, are carried out by organelles of the cell. Protozoa of medical
importance are grouped in the following classes:
Class - Sarcodina (Amoebae):
a) Genus, Entameba: e.g. Entamoeba histolytica
b) Genus Endolimax e.g. Endolimax nana
c) Genus Iodameba e.g. Iodameba butchlii
d) Genus Dientmeba e.g. Dientameba fragilis

Class - Mastigophora (Flagellates):

Have one or more flagella
a) Genus Giardia e.g. Giardia lamblia
b) Genus Trichomonas e.g. Trichomonas vaginalis
c) Genus Trypanosoma e.g. Trypanosoma brucci
d) Genus Leishmania e.g. Leishmania donovani

Class Sporozoa:
No organs of locomotion (except in gamete stages).
1) Genus Plasmodium e.g. Plasmodium falciparum
2) Genus Toxoplasma e.g. Toxoplasma Gondi
3) Genus Cryptosporidium e.g. Cryptosporidium Parvum
4) Genus Isospora e.g. I. Beli

Class Ciliata (Ciliates)

Have cilia all over the clell body, e.g. Balantidium coli

PHYLUM PLATHYHELMINTHES
Dorso-ventrally flattened organisms which are usually hermaphroditic (except
Schistosomes).
Respiratory and blood vascular systems are absent.
Two classes are important in medical parasitology:
a.Class – Trematoda (Flukes):
Leaf-shaped species that have an alimentary canal
Genus Schistosoma e.g. S. mansoni
Genus Fasciola e.g. F. hepatica
b.Class – Cestoda (Tape worms):
Species with segmented body and with no alimentary canal
Mainly hermaphroditic

(a) Diphylobotrium e.g. D. latum

(b) Genus Taenia e.g. T. saginata and T. solium
(c) Genus Echinococcus e.g. E. granulosus
(d) Genus Hymenolepsis e.g. H. nana
Phylum nemathyhelminthes
a.Class Nematoda (Round worms):
Cylindrical worms, both ends tapered.
Have alimentary canal
Are dioecious have- have male and female in different organisms
Can be divided into:
Intestinal Nematodes e.g. A. Lumbricoides
Somatic Nematodes e.g. W. bancrofti
Phylum arthropoda
Most arthropods are of medical importance in that they either cause pathological
conditions or transmit pathogenic organisms to man.
a.Class - Arachnida e.g. ticks, spiders, mites, scorpions, etc. 4 pairs of limbs.
Mostly 2 body parts – cephalothorax and abdomen.
b.Class - Insecta – includes all the insects, e.g. Mosquitoes, Tsetse flies, Sand flies,
House flies, etc. Body divided into 3 parts:
Head, with one pair of antennae;
Thorax, consisting of 3 segments bearing 3 pairs of legs and, typically, two pairs of
wings (not all);
Abdomen, consisting of a variable number of segments but commonly lacking
appendages.
c.Class - Crustacia e.g. lobsters, crabs, shrimps, etc. Most species are aquatic and
breathe by means of gills. Have 2 pairs of antennae on the head and several pairs of
limbs on thorax and abdomen
d.Class- diploda e.g. millipede
e.Class - Chilopoda e.g. Centipedes
f.Class - Pentastomida e.g. tongue worms
The first three classes – Arachnida, Insecta and Crustacia are the most common
classes of arthropods of medical significance.
MEDICAL PROTOZOLOGY
Protozoa are microscopic unicellular, or single-celled, organisms. However, they
do exhibit an incredibly large range of sizes, but most of the organisms discussed
in this course will be 3-50 µm. This small size necessitates the use of a microscope
to detect protozoa. An electron microscope is needed for detailed morphological
studies.
Protozoa are found in moist environments virtually everywhere. As a group, the
protozoa are extremely adaptable. Individual species, though, generally have very
specific niches. Like all other organisms, protozoa must be able to acquire and
metabolize nutrients from their environment.
Protozoa exhibit a wide variety of morphologies. Shapes range from the
amorphous and ever-changing forms of ameba to relatively rigid forms dictated in
part by highly ordered cytoskeletons or secreted walls or shells. Several protozoan
species express photosynthetic or other pigments and thus are colored. Many
protozoan species exhibit complex life cycles with multiple stages
FEEDING OF PROTOZOA
Many protozoa simply absorb solutes (i.e., osmotrophy) from their media, while
some are scavengers that ingest solid material (i.e., phagotrophy). Predatory
protozoa either actively hunt down or passively ambush other organisms (typically
bacteria or other protozoa).
Some protozoa are photosynthetic and can capture the energy of the sun and
convert it to usable chemical energy (i.e., autotrophic or phototrophic). Many
protozoa are not restricted to a single feeding mechanism and can utilize
combinations of the above (i.e., heterotrophic, mixotrophic)
MOVEMENT OF PROTOZOA
Cilia and flagella are subcellular structures which propel protozoa through a fluid
medium. Flagella are long whip-like structures which propel the organism as a
result of wave-like beat which is propagated through their length. Flagellated
protozoa typically have one or a few flagella per organism. In contrast, ciliated
protozoa are usually covered with rows of numerous cilia. The beats of these cilia
are coordinated and function like oars to propel the organism. Cilia and flagella
can also assist in the procurement of food, reproduction and other functions. Cilia
and flagella are made up of the same protein components and are actually
equivalent structures. Both are membrane bound filamentous projections from the
cell.
In contrast to the swimming exhibited by flagellates and ciliates, ameba are
protozoa that crawl along a solid substratum in a fashion known as 'ameboid
movement'. The ameba projects out a pseudopodium, or false foot, from the cell
body. The pseudopodium then attaches to the substratum and then pulls the rest of
the cell body forward. The force involved in this movement is generated by
another cytoskeletal system, which is comprised of actin and myosin. Actin forms
long filaments, also known as microfilaments, and myosin is a motor protein which
moves along the microfilaments in an ATP dependent manner
Apicomplexa (sporozoa) also crawl along a substratum, but by a different
mechanism than the ameba. The mechanism of this so-called 'gliding motility' is
just beginning to be understood and probably involves both microfilament and
microtubule based cytoskeletal systems. Apicomplexa also exhibit intracellular
forms and invasion of the host cell also involves this gliding motility. Cellular
motility involves force generation through either the microtubule-based
cytoskeletal elements or the microfilament-based cytoskeletal elements. This is
true for protozoa and other eukaryotes
REPRODUCTION OF PROTOZOA
Protozoa, like all other organisms, reproduce. The most common form of
reproduction in protozoa is asexual binary fission. In other words, a single
organism will divide into two equal organisms. A slight modification of this binary
fission, called budding, is when one of the newly formed cells is smaller than the
other. Typically the larger cell is called the mother and the smaller is the daughter.
Some protozoa will form an intracellular bud and essentially give birth. Another
variation of fission is a multiple fission or segmentation. In this situation, several
rounds of nuclear replication occur. This multinucleated cell will then form
multiple progeny simultaneously.
Many protozoa exhibit sexual reproduction in addition to the asexual forms of
reproduction. This sexual reproduction can involve the production and fusion of
gametes in processes similar to higher organisms. The Ciliophora undergo a
conjugation in which opposite mating types will pair and directly exchange
genetic material (i.e., DNA). Sometimes sexual reproduction is an obligatory step
in the life cycle, whereas in other cases the organism can reproduce asexually with
an occasional round of sexual reproduction
1.AMOEBA
Amoeba is a single celled protozoa that constantly changes its shape. The word
“amoeba” is derived from the Greek word “amoibe” meaning “change”. They
constantly change their shape due to presence of an organ of locomotion called as “
pseudopodium
Amoebae are classified as intestinal amoebae and free living amoebae.
 Intestinal amoebae: They inhabit at in the large intestine of humans and
animals. Entamoeba histolytica is the only pathogenic species. Others are
nonpathogenic such as E. dispar, E. moshkovskii, E. coli, E. polecki, E.
hartmanni, E. gingivalis, Endolimax nana, and Iodamoeba butschlii
 Free-living amoebae: They are small free living and opportunistic
pathogens. Examples are Acanthamoeba species, Naegleria fowleri,
Balamuthia mandrillaris and Sappinia diploidea
All human intestinal amoebae have:
1) a trophozoite from which is motile organism, feed, and reproduce,
2) Precystic form, intermediate between cyst and triphozoite stage.oval with
blunt pseudopodia
3) a cystic form which is the non-feeding, non-motile, dormant stage of
protozoa.
 Among amoeba, E. gingivalis has only a trophozoite form.
 Amoeba reproduce asexually by simply dividing into two (binnary fission).
Entamoeba histolytica
E.histolytica is an enteric protozoan parasite with worldwide distribution. It is
responsible for amoebic dysentery (bloody diarrhea) and invasive extraintestinal
amebiasis (such as liver abscess, peritonitis, pleuropulmonary abscess). E.
histolytica is worldwide in distribution but more common in tropical and
subtropical countries.
EPIDEMIOLOGY
Amoebiasis is a major health problem worldwide. The largest burden of the
disease occurs in tropics of China, Central and South America, and Indian
subcontinents aff ecting 10% of the world’s population. (500 million)
It is the third most common parasitic cause of death in the world (after malaria and
schistosomiasis). Approximately 50 million cases and 110, 000 deaths are reported
annually by WHO (World health Oranization
In industrialized countries, risk groups include men who have sex with men,
travelers, recent immigrants, immunocompromised persons, and institutionalized
populations.
MORPHOLOGY
E. histolytica has three stages(1) trophozoite, (2) precyst and (3) cyst (immature
and mature)
.Trophozoite
● Average size is about 20*25micrometer
● Shows active amoeboid movement (directional) in fresh warm specimen.
● In dysenteric specimens, the amoebae contain ingested red cells. This is
diagnostic of E. histolytica.
● Single nucleus is present which has a central karyosome (not always visible)

Precyst
It is the intermediate stage between trophozoite and cyst.
  It is smaller to trophozoite but larger to cyst (10–20 µm)
 It is oval with a blunt pseudopodia. Food vacuoles and RBCs
disappear. Nuclear

Cyst
It is the infective form as well as the diagnostic form of the parasite found in the
feces of carriers as well as patients with active disease structures are same as that
of trophozoite
 It measures 10–20 µm (average 12–15 µm) in diameter
 Nuclear structures are same as in trophozoites.
 First, the cyst is uninucleated; later the nucleus divides to form binucleated
and finally becomes quadrinucleated cyst
 Cytoplasm of uninucleated cyst contains 1–4 numbers refractile bars with
rounded ends called as chromatoid bodies (aggregation of ribosome) and a
large glycogen mass (stains brown with iodine)
 Both chromatoid body and glycogen mass gradually disappear, and they are
not found in mature quadrinucleated cyst
 Cysts are present only in the gut lumen; they never invade the intestinal
wall.
Note: Trophozoites and immature cysts can be passed in stool of amoebic
patients, but they can’t serve as infective form as they are disintegrated in
the environment or by gastric juice when ingested
 Mode of transmission
 Feco-oral route (most common)
By ingestion of contaminated food or water with mature quadrinucleated
cysts
 Sexual contact: Rare, either by anogenital or orogenital contact. (especially
in developed countries among homosexual males)
 Vector: Very rarely, flies and cockroaches may mechanically transmit the
cysts from feces, and contaminate food and water

LIFE CYCLE
Life cycle of E.histolytica has two stages: motile trophozoite and non-motile
cyst. Trophozoites are found in intestinal lesions, extra-intestinal lesions and
diarrheal stools whereas cyst predominate in non-diarrheal stools
When they cyst of E. histolytica reaches caecum or lower part of ileum
excystation occurs and an amoeba with four nuclei emerges and that divides
by binary fission to form eight trophozoites.
Trophozoites migrate to the large intestine and lodge in to the submucosal
tissue.
Trophozoites grow and multiply by binary fission in large
intestine (Trophozoite phase of life cycle is responsible for producing
characteristics lesion of amoebiasis).
Certain number of trophozoites are discharged in to the lumen of the bowel
and are transformed into cystic forms.
The cysts thus formed are unable to develop in the same host and therefore
necessitate a transference to another susceptible host. The cysts are passed in
the feces.

Note: Because of the protection conferred by their walls, the cysts can
survive days to weeks in the external environment. Cysts are not highly
resistant and are readily killed by boiling. But they are resistant to
chlorination or can be removed by filtration. Trophozoites can also be
passed in diarrheal stools, but are rapidly destroyed once outside the body.

NOTE;Trophozoite is responsible for disease conditions;

The trophozoites invade the colonic epithelium and secrete enzymes that
cause localized necrosis. Little inflammation occurs at the site.
As the lesion reaches the muscularis layer, a typical “flask shaped” ulcer
forms, that can undermine and destroy large areas of intestinal epithelium.
Progression into submucosa leads to invasion of the portal circulation by the
trophozoites
DISEASE
a.Amoebic dysentery
Amoebic dysentery occurs when E. histolytica trophozoites invade the wall
of the large intestine and multiply in the submucosa, forming large flask-
shaped ulcers. The amoebae ingest red cells from damaged capillaries.
Compared with bacillary dysentery, the onset of amoebic dysentery is less
acute, lasts longer, and there is usually no significant fever. Without
treatment dysenteric attacks may recur for several years.
b.Amoebic liver abscess
Occasionally E. histolytica amoebae are carried to the liver in the portal
circulation and form abscesses, usually in the right lobe. Amoebic liver
abscesses are more common in adults than in children with a higher
frequency in men (3 to 1 rate). There is pain and tenderness over the liver,
wasting, and fever with chills and night sweats. Patients with large or
multiple abscesses may become jaundiced and anaemic. There is usually a
raised white cell count with neutrophilia and a significantly raised ESR. The
centre of the abscess contains a viscous pink-brown or grey-yellow fluid
consisting of digested liver tissue. It is referred to as ‘pus’ but contains very
few pus cells. As part of treatment (not diagnosis) the fluid may be aspirated
and sent to the laboratory to examine for trophozoites. Amoebic liver
abscess can be diagnosed serologically
 Clinical Findings
Acute intestinal amebiasis
 dysentery (i.e. bloody, mucus containing diarrhea)
 lower abdominal discomfort,
 flatulence
Chronic amebiasis:
 Low grade symptoms such as occasional diarrhea, weight loss and fatigue
also occurs.
 Roughly 90% of infected individuals have asymptomatic infection.
 Ameboma, a granulomatous lesion may form in the cecal or rectosigmoid
areas of the colon in some patients. These lesions resemble an
adenocarcinoma of the colon and must be distinguished from them
Diagnosis
It rests on finding either trophozoites in diarrheal stools or cysts in formed
stools. Diarrheal stools should be examined within one hour of collection to
see the ameboid motility of the trophozoite. The
trophozoite characteristically contain ingested red blood cells.
Characteristics of Stool
Macroscopic appearance of stool: Offensive dark brown semisolid stool, and
mixed with blood, mucus and much fecal matter.
General microscopic examination:
Presence of charcot-Leyden crystals.
E. histolytica infection is distinguished from bacillary dysentery by the lack
of high fever and absence PMN leukocytosis
Laboratory diagnosis methods:
A. Microscopy:
E. histolytica can be distinguished from other amoebas by two major criteria
1.Nature of the nucleus and the trophozoite.
The E. histolytica nucleus has a small central nucleolus and fine chromatin
granules along the border of the nuclear membrane. The nuclei of other
amebas are quite different.
Note: The trophozoites of Entamobea dispar, a nonpathogenic species of
Entamoeba, are morphologically indistinguishable from those of E.
histolytica
Cyst size and number of its nuclei.
Mature cysts of E. histolytica are smaller than those of Entamoeba coli and
contain four nuclei, whereas E. coli cysts have eight nuclei.
B. Antigen detection: detection of E. histolytica antigen in the stool
C. Serologic testing is useful for the diagnosis of invasive amebiasis.
C. Detection of nucleic acid of this protozoan parasite by PCR based assay.

Prevention and control of E. histolytica infection

● Preventing faecal contamination of the environment by using latrines and
protecting water supplies from faecal contamination.
● Handwashing after defaecation and before eating.
● Covering food and water to prevent contamination from flies which can
act as cyst carriers.
● Not eating green salads or other uncooked foods which may contain cysts,
usually as a result of fertilization with untreated human faeces.
● Boiling drinking water (E. histolytica cysts are killed at 55 °C).
● Health education, particularly of food handlers, and also in schools and
community health centres