Protozoan Pathogens in Clinical Parasitology

Protozoan Pathogens in Clinical Parasitology

Protozoan pathogens are single-celled eukaryotic organisms that can cause various diseases in humans. This resource covers the classification, life cycles, and pathogenic mechanisms of medically important protozoa, including Entamoeba histolytica and Giardia lamblia. It is designed for second-year medical technology students studying clinical parasitology. Key topics include the diagnosis and treatment of protozoan infections, as well as prevention strategies. Understanding these organisms is crucial for effective patient care and disease management.

Key Points

  • Explains the life cycle and pathogenicity of Entamoeba histolytica.
  • Covers diagnostic methods for identifying protozoan infections in clinical settings.
  • Details treatment options for common protozoan diseases like giardiasis.
  • Discusses preventive measures to control the spread of protozoan infections.
,
  • Krissia Gracelle Cernal
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C L I N I C A L P A R A S I T O L O G Y
LEC 10: PROTOZOA
MS. MARTINA DEANNE C. MENDOZA, RMT
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General Characteristics
• The Kingdom Protozoa consists of single-celled eukaryotic
organisms that are spherical to oval or elongated in shape.
• The classification of these organisms is mainly based on the
organ of locomotion utilized.
• Not all protozoa are parasitic. Some are facultative parasites
capable of a free-living state (e.g., Acanthamoeba and Naegleria).
• Facultative parasites normally reside in the soil or water but
can cause severe illness when they gain entrance into the central
nervous system or the eyes.
• Reproduction among protozoa is through binary fission
(flagellates, ciliates, and amebae).
• Sporozoans reproduce through both sexual and asexual means.
• Asexual reproduction is achieved through a process called
merogony or schizogony.
• Due to their small size, protozoan infections are most often
diagnosed through microscopic examination of body fluids,
tissue specimens, or feces.
• Most of the parasitic protozoa infections are diagnosed by
demonstrating the motile, feeding, dividing stage of the parasite
called trophozoite, or the dormant, non-motile form called the
cyst.
• The trophozoite is the motile (with pseudopods or “false
feet”) and feeding form and is the pathogenic stage.
• The cyst is the non-motile form and is the infective stage for
most intestinal protozoan parasites, except for Trichomonas
vaginalis where cyst forms are not found.
EXCYSTATION vs ENCYSTATION
Excystation – the morphologic conversions from the cyst form
into the trophozoite form. It occurs in the ileocecal area of the
intestine
Encystation – the conversion of trophozoites to cysts. It occurs
in the intestine when the environment becomes unacceptable for
continued trophozoite multiplication
Phyla of Medical Importance
Kingdom Protista
Phylum
Sarcomastigaphora
Phylum
Apicomplexa
Phylum
Microspora
Subphylum
Mastigaphora
Superclass
Rhizopoda
Organ of
locomotion
– Flagella (whip-
like) Subphylum
Sarcodina Class
Zoomastigaphora
Organ of
locomotion
– Pseudopodia
(hyaline foot
like)
Has apical
complex at the
anterior end
called,
apicoplast
Example:
Babesia,
Plasmodium,
Coccidian
(Cryptosporidium
parvum,
Cyclospora
cayetanensis,
Toxoplasma
gondii)
Spore-
forming
parasites
Cyst vs Trophozoites
TROPHOZOITES
CYSTS
➢ The form that feed,
multiplies (binary fission), and
possesses pseudopods (motile)
➢ Delicate and fragile
➢ Easily destroyed by the
gastric juices of the stomach
➢ Susceptible to the
environment outside the host
➢ Replication only occurs in
the trophozoite stage
➢ The nonfeeding stage
characterized by a thick
protective cell wall to protect
the parasite from the harsh
outside environment
➢ Infective, environmentally
resistant stage
Conditions that trigger
encystation:
1. Amoeba overpopulation
2. pH change
3. Too much or too little food
supply
4. Too much or too little
available oxygen
Laboratory Diagnosis
• Amebic trophozoites and cysts may be seen in stool samples
Trophozoites are primarily recovered from stools that
are soft, liquid, or loose consistency
Cysts are more likely seen in formed stool specimens
Presence of either or both morphologic forms is
diagnostic
• Proper determination of organism size, using the ocular
micrometer, is essential when identifying the amoebas
• The appearance of key nuclear characteristics, such as the
number of nuclei present and the positioning of the nuclear
structures, is crucial to differentiate the amoebas correctly
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• Cytoplasmic inclusions and motility also aid in the
identification of amoebas
• Standard microscopic procedures
Saline wet preparations – often show motility of the
amebic trophozoites
Iodine wet preparation – internal cytoplasmic and the
nuclear structures may be seen
Permanent stains – to confirm parasite identification
Classifications of Amoebas
Entamoeba Histolytica
• A pseudopod-forming non-flagellated protozoan parasite.
• It is the most invasive of the Entamoeba parasites and the only
member of the family to cause colitis and liver abscess.
• E. histolytica, E. dispar, and E. moshkovskii are morphologically
identical and of the same size.
To differentiate the three species:
PCR-Restriction Fragment Length Polymorphism
(PCR-RFLP)
Typing with monoclonal antibodies
• E. hartmanni, formerly referred to as “small race” of E.
histolytica, is differentiated primarily based on size.
Life Cycle
• Once the infective cyst is ingested, excystation occurs in the
small intestine, where cyst undergoes nuclear followed by
cytoplasmic division to form eight trophozoites.
These motile amoebas settle in the lumen of the large
intestine, where they replicate by binary fission and
feed on living host cells.
• The trophozoite encyst producing uninucleate cysts, which then
undergo two successive nuclear divisions to form the
characteristic quadrinucleate cysts.
Encystation occurs in the intestinal lumen.
Modes of Transmission
Hand-to-mouth contamination
Food or water contamination
Sexual intercourse (unprotected sex)
Flies and cockroaches may also serve as vectors
Pathogenesis
• The range of symptoms varies and depends on two major
factors:
1. The location(s) of the parasite in the host
2. The extent of tissue invasion
• Asymptomatic Carrier State
Three factors (acting separately or in combination):
1. The parasite is a low-virulence strain
2. The inoculation into the host is low
3. The patient’s immune system is intact
In these cases, amoebas may reproduce but the
infected patient shows no clinical symptoms.
• Symptomatic Intestinal Amebiasis
Patients infected with E. histolytica who exhibit
symptoms often suffer from amebic colitis. It is an
intestinal infection caused by the presence of amoebas
exhibiting symptoms.
Clinically presents as gradual onset of abdominal pain
and diarrhea with or without blood and mucus in the
stools.
The onset of amebic colitis may be sudden after an
incubation period of 8 – 10 days, or after a long period
of asymptomatic cyst carrier state.
The most serious complication of amebic colitis is
perforation and secondary bacterial peritonitis.
Some patients may transit from amoebic colitis into a
condition characterized by blood and mucus in the
stool known as amebic dysentery.
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Ameboma – occurs in less than 1% of intestinal
infections
It clinically presents as a mass-like lesion with
abdominal pain and history of dysentery.
It can be mistaken for carcinoma.
• Symptomatic Extraintestinal Amebiasis
Amebic liver abscess (ALA) – the most common
extraintestinal form of amoebiasis
Cardinal manifestation: fever and right upper
quadrant (RUQ) pain
Pain is either localized in or referred to the right
shoulder.
Hepatomegaly
Rupture into the pericardium is the most serious
complication of ALA.
Second most common is intraperitoneal rupture.
Venereal Amebiasis
▪ Men become infected with penile amebiasis after
experiencing unprotected sex with a woman who has
vaginal amebiasis.
▪ The disease may also be transferred during anal
intercourse.
▪ In the examination of the genital area, trophozoite
form of E. histolytica is most encountered.
• Acute amebic colitis should be differentiated from bacillary
dysentery of the following etiology: Shigella, Salmonella,
Campylobacter, Yersinia and enteroinvasive Escherichia coli.
• The differential diagnoses of ALA include pyogenic liver abscess,
tuberculosis of the lIver, and hepatic carcinoma.
Laboratory Diagnosis
Standard Method
Microscopic detection of the trophozoites and cysts in stool
specimens.
Ideally, a minimum of 3 stool specimens collected
on different days should be examined.
For detection of trophozoites, fresh stool specimens
should be examined within 30 minutes from
defecation.
DFS with saline: can observe trophozoite motility
(unidirectional movement)
Saline and methylene blue: Entamoeba species will
stain blue, thus, differentiating them from WBC.
Saline and iodine: the nucleus and karyosome can be
observed to differentiate E. histolytica from the non-
pathogenic amebae (E. hartmanni, E. coli, Endolimax
nana)
FECT and MIFC – are more sensitive than the DFS for
detection of cysts
Noting the following morphological structures can
differentiate E. histolytica from non-pathogenic species, E.
hartmanni, E. coli, E. nana, and Iodameba butschlii.
Size of the cyst
Number of nuclei
Location and appearance of the karyosome
Characteristic appearance of chromatoid bodies
Presence of cytoplasmic structure – glycogen vacuole
• Stool Culture – not routinely available
More sensitive than stool microscopy
Medium: Robinson’s and Inoki
• Differentiation between E. histolytica and E. dispar can only be
done by:
PCR
Enzyme-linked immunosorbent assay (ELISA)
Isoenzyme analysis
Treatment
• Two objectives:
To cure invasive disease at both intestinal and
extraintestinal sites
To eliminate the passage of cysts from the intestinal lumen
• Metronidazole is the drug of choice for the treatment of
invasive amebiasis.
• Diloxanide furoate is the drug of choice for asymptomatic cyst
passers.
Prevention and Control
Boiling water or treating with iodine crystals
Properly washing of food products
Avoiding the use of human feces as fertilizer
Good personal hygiene and sanitation practices
Protection of food from flies and cockroaches
Avoid unprotected sexual practices
Commensal Amebae
• The presence of commensal amebae in the stools of an
individual is significant for two reasons:
1. The amebae may be mistaken for the pathogenic E.
histolytica
2. It is an indication of fecal contamination of food or
water
• Genus Entamoeba
Has a spherical nucleus with a distinct nuclear
membrane lined with chromatin granules
Karyosome is found near the center of the nucleus
Trophozoites usually have only one nucleus
• Genus Endolimax
Has a vesicular nucleus
Large, irregularly shaped karyosome anchored to the
nucleus by achromatic fibrils
• Genus Iodamoeba
Characterized by a large chromatin-rich karyosome
surrounded by a layer of achromatic granules and
anchored to the nuclear membrane by achromatic
fibrils.
• All species have the following stages (except for Entamoeba
gingivalis, which has no cyst stage and does not inhabit the
intestine):
Trophozoite
Precyst
Cyst
Metacystic trophozoite
• Mode of transmission: Ingestion of viable cysts in food or water
• Excystation occurs in the alkaline environment of the lower
small intestines.
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End of Document
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FAQs of Protozoan Pathogens in Clinical Parasitology

What are the main characteristics of protozoan pathogens?
Protozoan pathogens are primarily single-celled eukaryotic organisms that can vary in shape from spherical to elongated. They are classified based on their mode of locomotion, including flagella, cilia, and pseudopodia. Some protozoa are obligate parasites, while others can exist in a free-living state. Their ability to reproduce through binary fission or sexual means contributes to their survival and pathogenicity.
How is Entamoeba histolytica transmitted and what diseases does it cause?
Entamoeba histolytica is primarily transmitted through the fecal-oral route, often via contaminated food or water. This protozoan can cause intestinal diseases such as amebic dysentery and can lead to extraintestinal complications like amebic liver abscess. Symptoms often include abdominal pain, diarrhea, and in severe cases, perforation of the intestinal wall. Understanding its transmission is essential for implementing effective public health measures.
What diagnostic methods are used for protozoan infections?
Diagnostic methods for protozoan infections typically involve microscopic examination of stool samples to identify trophozoites and cysts. Techniques such as saline wet preparations and iodine staining help visualize the organisms. Additionally, molecular methods like PCR can provide accurate identification of specific species. It is crucial to collect multiple stool samples over several days due to the intermittent shedding of cysts.
What treatment options are available for giardiasis?
Giardiasis, caused by Giardia lamblia, is commonly treated with medications such as metronidazole and tinidazole. These drugs are effective in eliminating the protozoan from the gastrointestinal tract. Patients are also advised to maintain hydration and may require dietary adjustments during recovery. Preventive measures, including proper sanitation and safe drinking water, are essential to avoid reinfection.
What are the implications of protozoan infections in immunocompromised patients?
Immunocompromised patients are at a higher risk for severe protozoan infections due to their weakened immune systems. Infections such as those caused by Acanthamoeba can lead to serious complications like granulomatous amebic encephalitis. Early diagnosis and prompt treatment are critical in these populations to prevent life-threatening outcomes. Understanding the unique challenges faced by immunocompromised individuals is vital for healthcare providers.

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