5. Commensalism
• One species (usually smaller, called
commensal) derives food and shelter from the
other (usually larger, called host)
• Host is neither harmed or benefited
6. Parasitism
• One species (parasite) derives food and
shelter from another (host)
• But host is harmed
7. Groups of human parasites
• Protozoa
• Helminths
• Arthropods
All belong to eukaryota
8. Types of parasite
Endoparasite
- live inside the body of host
- they infect
- e.g. protozoa and helminths
Ectoparasite
-live on the surface of the body of host
- they infest
- e.g. arthropods
15. Flagella
1. Long thread like filaments
2. Seen in
i. Giardia lamblia,
ii. Trichomonas,
iii. blood flagellates like Leishmania and
Trypanosomes
16. Cilia
1. Short needle like filaments
2. Cover entire surface of the body
3. Seen in Balantidium coli
17. Stages of life cycle in protozoa
1. Trophozoite:
• It is an active form
• It is usually motile
• It can grow
• It can multiply
2. Cyst:
• (the protozoa is enclosed
within a tough wall)
• It is dormant form
• It is non-motile
• It cannot grow
• It cannot multiply
18. • The helminths are divided into 2 main groups:
1. Phylum Platyhelminthes
2. Phylum Nemathelminthes
19. Phylum Platyhelminthes
(Greek: platy=flat, helminth=worm)
• mostly hermaphrodite (monoecious) except
Schistosomes
• alimentary canal is incomplete or entirely lacking
• body cavity is absent
• when they are flattened, leaf like and
unsegmented, they belong to Class Trematoda
(trematodes)
• when they are flattened, tape like and
segmented, they belong to Class Cestoidea
(cestodes)
21. Phylum Nemathelminthes
(Greek: nematos=thread, helminth=worm)
• sexes are separate
(diecious)
• alimentary canal is
complete
• body cavity is present
• Phylum
Nemathelminthes has
only one Class
Nematoda (nematodes)
22. Stages of life cycle in helminths
1. Eggs (also called ova)
• Produced by adult forms
2. Larvae:
• Hatch out from eggs, either inside the host or in
the environment
• Are immature forms mature into adult forms
3. Adult forms:
• Mature from larval forms
• Produce eggs (oviparous) or larvae (viviparous)
23. Reproduction in parasites
Asexual multiplication:
-binary fission: one divides into two
-multiple fission (or schizogony): one divides
into many
Sexual multiplication: union of two cells
-conjugation: temporary union during which
genetic material is exchanged
-syngamy: sexually differentiated cells called
‘gametes’permanent unionform ‘zygote’
25. Type of host
Definitive host:
-for protozoa: in which sexual multiplication
takes place
-for helminths: in which adult form is present
Intermediate host:
-for protozoa: in which asexual multiplication
takes place
-for helminths: in which larval form is present
26. Types of life cycle
• Simple: in which there is only one host
• Complex: in which there is more than one
host
27. Exit forms
• When immune system gains upper hand
parasite transforms into exit forms reach
environment, vector or another host
28. Types of vectors
• Biological: in which the parasites multiply
• Mechanical: in which the parasites do not
multiply
36. Trophozoites
• They are pear shaped
• They are bilaterally symmetrical
• They measure 12-15 microns in length
• They are broad at anterior and taper at posterior end
• They measure 7-9 microns at their widest
• The dorsal surface is convex
• On the ventral surface, a disc is present (ventral disc)
37. Ventral disc
• The disc is bilobed and rigid
• It occupies most of the anterior part of the ventral
surface
• It is useful for attachment of the parasite to the
intestinal mucosa
39. Axostyles
• The trophozoite is supported by axostyles
• They are two in number
• These axostyles run between the two nuclei
longitudinally
• They run upto the posterior end
40. Median bodies
• There are two rod like structures present on
the axostyles posterior to the nucleus
• These are slightly curved
• They are called median or parabasal bodies
41. Flagella
• One pair of specialized ventral flagella for
attachment to the intestinal wall
• Three pairs of flagella which are dorsal
• Thus the organism has four pairs of flagella
• The trophozoites of G. intestinalis exhibit
motility resembling ‘falling leaf’
42.
43. Why G. intestinalis parasitizes the small
intestine?
• G. intestinalis requires phospho-lipids and
sterols for it’s growth
• But it is unable to synthesize them
• These substances are present in abundance in
small intestine
44.
45. Cysts
• The cysts are ellipsoid
• They measure 12 microns x 7 microns
• They have a well defined wall
• The wall is made up of chitin
• The mature cysts contains four nuclei
• It contains the axostyle and parabasal bodies as in
trophozoites
• It however contains only the remnants of flagella
• It does not have the disc
46. Life cycle
• It is simple i.e. it completes it’s life cycle in a single
host
• Mature cysts are infective forms of G. intestinalis
• Infective dose is 10 to 100 cysts
• Transmission is by fecal oral route
(Note: life cycle is very similar to E. histolytica)
47. Life cycle..2
• Excystation occurs in the proximal small intestine
• It is triggered by the action of pancreatic enzymes
• One cyst liberates two trophozoites
• The trophozoites attach to the intestinal mucosa by ventral
disc
• The feed by pinocytosis on the phospholipids and sterols
present in the small intestine
• They reproduce by binary fission
• When survival is threatened, encystations occurs
• Cysts can remain viable in soil and water for several weeks
• Trophozoites may be present in the feces, but they are not
infective
48.
49. Clinical features
• Most of the infections are asymptomatic
• In symptomatic cases, the incubation period is 1-3 weeks
• Symptoms include diarrhea, abdominal pain, bloating,
belching, flatus, nausea and vomiting
• It usually subsides within 10-14 days
• In chronic cases, multiple relapses may occur
• The patients have malabsorption, steatorrohea, and weight
loss
• It can impair growth in children
• It is one of the causes of traveler's diarrhea
50. Laboratory diagnosis
• The gold standard is the microscopic
examination of stool
• It may reveal trophozoites, cysts or both
• Trophozoites exhibit a typical ‘falling leaf’
motility
• The characteristic shape and two nuclei of
trophozoites are seen after staining the thin
fecal smear with Field’s stain
(Note: very similar to E. histolytica)
51. Laboratory diagnosis..2
• Cysts are often shed in feces in ‘showers’
• They may be present on one day and absent on the other
• Hence, it is advisable to examine atleast 6 stool samples
before ruling out giardiasis (In E. histolytica, at least 3 stool
samples are needed)
• The stool should be examined preferably after concentration
techniques
(Note: very similar to E. histolytica)
55. Prevention and treatment
• Not eating or drinking contaminated food and
water
• Adequate cooking
• Treatment is with Metronidazole
• Quinacrine is equally effective
56. Dientamoeba fragilis
• It does not have a flagella
• It uses pseudopodia for motility
• However, electron microscope has revealed
that it has characteristics of flagellates
• Hence, it is considered as a flagellate
57. Organism characteristics
• It exists only as a trophozoite (important!)
• Trophozoites have an average size of 8-12 microns
• They have a pseudopodium
• They have 2 nuclei (important!)
• Each nucleus have 4-8 chromatin granules (important!)
• The granules are centrally placed
• Very rarely, they may show ingested RBCs
62. Life cycle, pathogenicity and clinical
features
• Trophozoites are the infective forms (as they are the only
forms)
• However, it is not known how they survive in the external
environment
• The prevalence of D. fragilis is more in persons suffering from
pinworm (E. vermicularis) infections
• It is possible that eggs or larvae of pinworm facilitate the
transmission of D. fragilis
• The trophozoites live in the mucosal crypts of large intestine
• They do not invade the colonic mucosa
• Multiplication is by binary fission
• Most of the infections are asymptomatic
• When symptoms are present, they include intermittent
diarrhea, abdominal pain and anorexia
63. Laboratory diagnosis
• Stool microscopy reveals characteristic
trophozoites
• Fecal excretion rates vary
• Therefore several samples have to be
obtained
• The samples should be preferably obtained on
alternate days
64. Treatment
• Iodoquinol 650 mg tid for 20 days
• Paromycin 25-30 mg/kg/day in three doses for
7 days
• Paromycin is an aminoglycoside antibiotic
• Tetracycline 500 mg qid for 10 days
67. Organism characteristics
• It exists only as a trophozoite (like D. fragilis)
• Cysts are not formed
• Trophozoite is pear shaped
• It is 10-30 microns x 5-10 microns
• It has a single nucleus (important!)
• It has five flagella
• All arise at the anterior end
• Four flagella are free and one extends along the outer margin
forming an undulating membrane (fin-like extension)
• The undulating membrane reaches upto the middle of the
body
• It is anchored to a flexible rod
• This rod is called the costa
68. Organism characteristics..2
• It has an axostyle
• The axostyle supports the trophozoite
• It originates at the anterior end
• It then curves around the nucleus
• It extends posteriorly
• It extends beyond the body
69. Organism characteristics..3
• In fresh specimens, the trophozoites exhibit a
jerky motility
• They are resistant to changes in environment
• They survive in urine and damp towel for
several hours
• They can survive in water upto 40 minutes
70. Life cycle
• The trophozoite is the infective form (as it is the only
form)
• It lives in vagina and Bartholin glands in females
• In males, it lives in urethra, prostate, seminal vesicles
and edididymis (therefore the term vaginalis can be
misleading)
• The trophozoites divide by binary fission
• It is transmitted by sexual contact and fomites like
shared towels and underclothing
71. Pathogenicity and clinical features
• T. vaginalis produces trichomoniasis
• In females, it presents as vaginitis
• Incubation period is 4-28 days
• Manifestations are vaginal discharge
• Vaginal discharge is yellow and malodorous
• Vulval erythema and itching may be present
• It may also cause cervicitis
• In that case, examination with speculum may reveal
‘strawberry cervix’ (important!)
• In males, the infection is usually asymptomatic
• Some patients develop urethritis
• Rarely prostitis and epididymitis may result
73. Diagnosis
• Direct microscopy of the specimens reveal
motile trophozoites
• Specimens include vaginal or prostatic
secretions
• They may be accidently detected in urine also
