Introduction to Parasitology
What is Parasitology?
A parasite is an organism that live on or within another organism called
the host .
Parasitology is a science of studying parasitism and a discipline dealing
with the biology of parasites (including its morphology, embryology,
physiology, biochemistry and nutrition, etc.), ecology of parasitism with
emphasis on parasite-host and parasite-environment interactions.
• the parasites studied in this discipline involve parasitic protozoa,
parasitic helminthes, certain lesser groups of worms, parasitic
arthropods, and the vectors of parasites, that is, parasitology is
largely an amalgamation of protozoology, helminthology,
entomology and acarology.
• Parasitology has also been subdivided into medical or human
parasitology, veterinary parasitology, fish parasitology and plant
nematology
What is Medical Parasitology?
Medical parasitology or human parasitology is restricted to studying
those parasites that are living in or on the body of humans and with
aspects of the host-parasite relationship having medical significance. In
other words, medical parasitology is the subject which deals with the
parasites that infect man, the diseases caused by them, clinical picture and
the response generated by man against them.
Medical Parasitology
Parasitic diseases
Parasites
Prevention
Transmission
Treatment
Diagnosis
Pathogenesis
Life Cycle
Morphology
Medical parasitology
Medical protozoology
Medical helminthology
Medical arthropodology
i.e. Medical entomology
Medical Parasitology in Medical Education
Public Health &
Preventive Medicine
Clinical
Medicine
Medical Parasitology
Biology,Biochemistry,Anatomy,
Histology,Physiology,Immunology
History of Human Parasitology
Early written records
• 3000 to 400 BC, the first written records of parasitic infections come from Egyptian
medicine, particularly the Ebers papyrus of 1500 BC discovered at Thebes. Intestinal
roundworms and tapeworms.
• 800 to 300 BC, writings of Greek physicians such as Hippocrates. Worms from fishes,
domesticated animals, and humans.
• 3000 to 300 BC, writings of physicians from China.
• 2500 to 200 BC, from India.
• 700 BC to 400 AD, from Rome.
• Latter part of the first millennium, from the Arab Empire.
• Arabic physician Avicenna (AD 980 to 1037) , wrote important medical works that
contain a great deal of information about diseases clearly caused by parasites. He
recognized not only Ascaris, Enterobius, and tapeworms but also the guinea worm,
Dracunculus medinensis.
History of Human Parasitology
Modern times
• Manson in 1879 filariasis
• Laveran in 1880 malaria parasite
• Ross in 1897 malaria life cycle
• Bruce in 1896-1902 and Chagas in 1908 trypanosomiasis
• Leishman and Donovan in 1900-1911 leishmaniasis ……
• 1879 (Ringer) , 1880 (Manson) : Paragonimus westermani
• 1875 (McConnell) : Clonorchis sinensis
• 1905 (Logan) : Schistosoma japonicum in China
Patrick Manson(1844-1922)
“Father of modern tropical
medicine”
In England Manson became Medical
Advisor to the Colonial Office and in this
capacity persuaded the then Secretary
of State for the Colonies, Joseph
Chamberlain, of the need for providing
facilities in Britain for educating doctors
in tropical diseases. One consequence
of his influence was the creation of
School of Tropical Medicine in both
Liverpool and London in 1898
The first President (and
Ronald Ross Vice-President)
of the Society of Tropical
Medicine and Hygiene (1907)
The first to discover (1877–79) that an
insect (mosquito) can be host to a
developing parasite (the worm Filaria
bancrofti) that is the cause of filariasis
1907 Nobel Laureate in Medicine
in recognition of his work on
the role played by protozoa in
causing diseases
Alphonse Laveran
1902 Nobel Laureate in Medicine
for his work on malaria, by which he has shown how
it enters the organism and thereby has laid the
foundation for successful research on this disease and
methods of combating it
Ronald Ross
Conceptions related to medical parasitology
• Symbiosis
• Parasite and type of parasites
• Host and common type of host
• Life cycle and type of life cycle
Symbiosis
The relationship between two living things (animals). Two
living things live together and involve protection or other
advantages to one or both partner.
• Commensalism
• Mutualism
• Parasitism
Commensalism
Both partners are able to lead independent lives, but one
may gain advantage from the association when they are
together and least not damage to the other
A female pea crab in the mantle cavity of its
mussel host. The crab does not damage the
mussel and uses its shell purely for protection
Mussel
Mutualism
An association which is beneficial to both living things
A selection of ciliates from the rumen of cattle or sheep.
The rumen contains enormous numbers of ciliates that
break down cellulose in the feed
Parasitism
An association which is beneficial to one partner and harmful to the
other partner. The former that is beneficial to is called parasite, the
latter that is harmful to is called host.
e.g., Human / Hookworm
Anterior end of a hookworm
Other classification
Parasites may be classified according to different ways:
Obligate parasite: a parasite which cannot survive in any other manner,
e.g., filaria
Facultative parasite: an organism which may exist in a free-living state
and which if opportunity presents itself may become parasitic, e.g.,
Strongyloides stercoralis
Endoparasite and ectoparasite
A parasite which lives in or on the body of the host is called
endoparasite (protozoa and helminthes) or ectoparasite
(arthropods)
Host and type of host
Host : an organism that harbors the parasite usually larger
than the parasite
• Intermediate host : the host harboring the larvae or asexual stage of
parasite
• Final host (definitive host) : the host harboring adult or sexual stage
of parasite
Host and type of host
• Reservoir host : animals harboring the same species of parasites as man,
e.g., schistosome/human, buffalo
Potential sources of human infection
• Paratenic host (transport host) : a host which acts as a transporting
agent for the parasite and in which, the parasite, usually a larval stage,
dose not undergo any further development but if they could go into a
final host by accident it would continue to develop till sexual maturity,
e.g., Paragonimus Westermani /human, wild boar
Life cycle and type of life cycle
Life cycle : the whole biological course of growth, development
and reproduction of a parasite
• The direct type : only one host (final host, no intermediate host). e.g.,
Ascaris lumbricoides, Hookworm, and Enterobius vermicularis
• The indirect type : life cycle with more than one host (intermediate host
and final host) . e.g., plasmodium, lymphatic filaria
Classification of parasites
Protozoa
(Unicellular)
Nematodes
Trematodes
Parasites Helminthes
Cestodes
(Multicellular )
Arthropods (Ectoparasite)
(Endoparasite)
Protozoa
Plasmodium
RBC
PROTOZOA
• Amebae (found in stool)
•
•
•
•
•
Entamoeba coli
Entamoeba histolytica
Endolimax nana
Iodamoeba butschlii
Dientamoeba fragilis
• Flagellates (found in stool)
• Giardia lamblia
• Chilomastix mesnili
• Ciliates, Coccidia, Blastocystis
• Balantidium
•
•
•
•
•
•
Cryptosporidium
Isospora belli
Sarcocystis
Cyclospora
Microsporidium
Blastocystis hominis
• Blood-Borne Protozoa
•
•
•
•
•
Babesia
Leishmania
Trypanosoma brucei
T. cruzi
Plasmodium
• Other
• Toxoplasma
• Naegleria fowleri
• Acanthamoeba
Intestinal and luminal protozoa significant to
human health include
• Entamoeba histolytica (Amoebae)
• Balantidium coli (Ciliates)
• Giardia lamblia and Trichomonas vaginalis (Flagellates)
• Cryptosporidium parvum and Isospora belli (Sporozoa)
Protozoa Found in Stool: Amebae
pathogen
Entamoeba histolytica/dispar
• E. histolytica is a pathogen and E. dispar is a nonpathogenic species that can
also occur in the large intestine. Morphologically indistinguishable
• E histolytica
• Cysts = infectious form
• trophozoites = invasive form
• Contaminated water and poor sanitation
• Colon biopsy shows “flask-shaped” ulcer
• Non-intestinal disease = extraintestinal amebiasis (liver abscess)
• Serology
Entamoeba histolytica/dispar
Cysts <= 10 um
In diameter
Up to 4 nuclei in
the cyst
Clean chromatin
Bulls-eye nucleoli
Amebic abscess
Amebic liver abscess
Entamoeba histolytica
Serology – high %
positive in extraintestinal cases
Flask-shaped ulcer of intestinal amebiasis
Entamoeba histolytica/dispar
Trophozoites &
Trophozoite with
ingested rbcs
Cysts
Entamoeba coli
Trophozoites & Cysts
Entamoeba coli cyst and trophozoite
Trophozoite is the form that
invades intestines
Cyst >=15 mm
Up to 8 nuclei
Shed from host
Lives in environment
Nucleus has a chromatin ring
The cytoplasm appears dirty
Protozoa Found in Stool: Flagellates
Giardia lamblia
• Contaminated water, undercooked
foods
• Mild diarrhea to severe
malabsorption
• Foul, watery diarrhea
• Day-care center outbreaks
• Cysts/trophozoites may be seen in
stool, but can be hard to find;
Fluorescent stains available
• Duodenal aspirations
CYSTS
TROPHOZOITE
“falling leaf” motility
Giardia lamblia trophozoite
Waxing and waning
symptoms
Can be irregularly
Shed in stool material
& can be difficult to find
Russia & Mexico
-Hot beds
Only invades intestine
Flagyl (Metronidazole)
is drug of choice
Giardia lamblia cysts
Giardia lamblia cysts
Giardia lamblia trophozoites
Trichomonas vaginalis
• Urogenital protozoan
• Scant, watery vaginal
discharge
• Four flagella, short
undulating membrane
Protozoa Found in Stool:
Ciliates, Coccidia, Blastocystis
Ciliates
• Balantidium coli
•
•
•
•
•
Mainly in swine
Contact with swine & poor hygiene
Only ciliate that’s pathogenic to humans
Similar disease as amebiasis, but extraintestinal invasion rare
Largest (50-200 um) trophozoite; surface covered with cilia;
macronucleus
• Cyst 40-60 um
• Readily identified in fresh, wet mounts
Only protozoa with cilia
50 microns
In intestine can cause flask-shaped ulcers like those caused by E. histolytica
Blood-Borne Protozoa
Organism
Trypanosoma
brucei
T. cruzi
Leishmania
donovani
Babesia microti
Transmission
Tsetse fly
Reduvid (kissing)
bug
Phlebotomine
sandfly
Ixodes tick
Disease/Sympto
ms
African
trypanosomiasis;
Sleeping sickness
Encephalitis;
cardiac failure
Diagnosis
Treatment
Hemoflagellate in
blood or lymph
node
Blood stage:
Suramin or
petamidine
isethionate
American
trypanosomiasis;
Chagas disease:
megacolon, cardiac
failure.
Hemoflagellate in
blood or tissue.
C- or commashaped
CNS:
melarsoperol
Nifurtimox and
Benzonidazole.
Visceral
leishmaniasis (Kalaazar),
granulomatous skin
lesions
Iraq/Iran/Afghanist
an
Hemolytic
anemia,
Jaundice, fever,
hepatomegaly
Intracellular
(macrophages)
leishmanial bodies
with kinetoplast
Pentosam;
Pentamidine
isethionate.
Maltese cross in rbc
None;
self resolving.
Trypanosomes
• 2 different diseases
• Chagas disease (American trypanosomiasis)
• Trypanosoma cruzi
• Reduviid / Triatome (kissing) bug
• African sleeping sickness (African trypanosomiasis)
• T. brucei (gambiense and rhodesiense)
• Tsetse fly
Trypanosoma brucei  Sleeping sickness
(African trypanosomiasis)
• Vector: Tsetse fly
• The two T. brucei species that cause African trypanosomiasis are
indistinguishable morphologically
• T. b. gambiense
• T. b. rhodesiense
• A typical trypomastigote has:
• A small kinetoplast located at the posterior end
• A centrally located nucleus
• An undulating membrane, and
• A flagellum running along the undulating membrane, leaving the
body at the anterior end
• 14 to 33 µm in length
• Trypomastigotes are the only stage found in patients.
Trypanosoma brucei gambiense in a blood film
Filamentous structures found in blood
TRYPANOSOMA GAMBIENSE
Leishmania
• Obligate intracellular parasite
• Vector: female sand fly bite
• Visceral leishmaniasis (kala azar)
• L. donovani
• Cutaneous leishmaniasis
• L. tropica
• L. braziliensis
Leishmania
• Leishmania amastigotes
• Macrophages filled with amastigotes (arrows),
several of which have a clearly visible nucleus
and kinetoplast
• Amastigotes are being freed from a rupturing
macrophage
Leishmania – Clinical Disease
• Cutaneous
• Single or few chronic, ulcerating lesions; many
species
• Latin America, southern Europe, Middle east,
southern Asia, Africa
• Mucocutaneous in Latin America
• Visceral
• primarily L. donovani complex (Asia), L.
infantum/chagasi (Africa and Latin America), others
• Hepatosplenomegaly, anemia, cytopenias, systemic
symptoms
• India, Bangladesh, Nepal, Sudan, and Brazil
• Important OI in HIV infection
Leishmania donovani
Skin lesion
Sand Fly
Kinetoplast next to nucleus
MALARIA
• Protozoan
• Transmitted by the anopheles mosquito
• Endemic to tropical areas
Malaria Symptoms
• Fever and chills
• Splenomegaly
• Headache
• Abdominal pain
• Diarrhea
• Myalgia
• Blackwater fever (hemolysis, hemoglobinuria, renal failure) –
P falciparum only
Malaria
• Distinction is between P. falciparum and non-falciparum
• P. falciparum = rapidly progressive and LETHAL (malignant tertian fever),
often chloroquine-resistant
• Non-falciparum = rarely cause severe manifestations, often chloroquine
sensitive
• Relapsing malaria
• Dormant hepatic phase
• Hypnozoites of P. vivax and P. ovale
MALARIA
Life Cycle of Plasmodium
Species
Malarial Preparations
Thick smear
Thin smear
Drop of blood on slide •
Water rinse to eliminate •
rbc’s
Stain with Giemsa stain (not •
Wright-Giemsa) with proper
pH
Concentrated to spot malaria •
parasites
• Feather edge smear
• For optimal morphology,
stain with Giemsa (not
Wright-Giemsa) stain with
proper pH
• Speciation of malaria
• Parasitemia (%)
Malaria Diagnosis
• Microscopy is most often used
• Antigen detection – EIA available
• Molecular methods
“Rosette” schizont
Other Protozoa
• Toxoplasma
Organism
Toxoplasma gondii
Transmission
Oral from cat fecal
material
or meat
Disease/Symptoms
Adult: flu like;
congenital: abortion,
neonatal blindness and
neuropathies
Diagnosis
Intracellular (in
macrophages)
tachyzoites
Treatment
Sulphonamides,
pyemethamine,
possibly
spiramycin (nonFDA)
Toxoplasma gondii
•
•
•
•
Coccidian protozoan
House cat = definitive host
Ingestion of infective oocysts from contaminated cat feces
Ingestion of improperly cooked meat from animals that serve as
intermediate hosts (rodents)
• Symptoms
• Predilection for lung, heart, lymphoid organs, CNS/eye
• Infectious mono-like; lymphadenitis, hepatitis, rash, encephalomyelitis,
myocarditis, chorioretinitis
• Transplacental infection
• 1st trimester  spontaneous abortion, stillbirth or severe disease
• 2nd/3rd trimester  CNS infections (epilepsy, encephalitis, intracranial calcifications,
MR, chorioretinitis, blindness, hearing loss), jaundice, rash
• AIDS - Encephalitis; mass lesions in brain
Toxoplasma gondii
• Diagnosis
• Serology EIA
• Anti-toxo IgM – congenital and acute infection; may persist for months
• Anti-toxo IgG – common; if positive, gestations safe from intrauterine toxoplasmosis
infection
• PCR
Toxoplasma gondii
Can be diagnosed by serology
Toxoplasma gondii
• Toxoplasma gondii cyst in brain tissue stained with hematoxylin and
eosin
HELMINTHS
• Nematodes (roundworms)
• Trematodes (flukes)
• Cestodes (tapeworms)
Nematodes
• Enterobius
• Ascaris
• Trichuris
• Necator and Ancylostoma (Hookworm)
• Microfilaria – Wucheria, Brugia, Loa loa, Mansonella, and
Onchocerca
Enterobius vermicularis (pinworm)
• Humans considered only host
• Females 8-13mm, males 2-5 mm
• Dwell in the cecum
• ¼-1/2 inch in thickness, white, lloks like string in stool
• Lay up to 15,000 eggs
• Oval with a flattened side: 50-60um by 20-30um
• Diagnosis- Scotch tape test or anal swab
• Most common helminth in US
Enterobius vermicularis (pinworm) eggs
Asymmetrical eggs
Pinworm larvae
Ascaris lumbricoides (roundworm)
• 1-1.2 billion people infected
• More common in children
• 20,000 death
• Largest helminth to affect humans
• Females 20-35cm long, males 15-30cm with a curved tale
• Can cause intestinal obstruction
Ascaris lumbricoides
Ascaris eggs
Unfertilized eggs-large & oval, mammillated
layer is pronounced
Fertilized eggs- smaller, rounder,
mammillated layer is less obvious
Trichuris Trichiura (whipworm)
• Soil transmitted
• Can be similar to amebiasis
• PVA preserved samples inferior to formalin
• Adults attach to large intestine and are rarely recovered
• Thinnest part- head
• Males are smaller than females
Trichuris trichiura
Necator americanus, Anclyostoma duodenale
(Hookworms)
• Soil transmitted
• 2nd most common helminth infection
• Enter via exposed skin
Necator or Ancylostoma – Hookworm egg
Trichinella spiralis
-Tissue nematode
-All stages occur in single
host
-usually an incidental finding
in muscle
Loa loa (eye worm)
Trematodes (Flatworms)
• Intestinal and Liver flukes
• Fasciolopsis buski
• Fasciola hepatica
• Liver flukes
• Clonorchis sinensis (Chinese liver fluke)
• Paragonimus westermani – oriental lung fluke
• Schistosomes
• S mansoni – intestinal bilharziasis
• S haematobium - urinary
• S japonicum – blood fluke, found in intestines
Trematodes ( Flukes )
Schistosomes
Schistosoma japonicum, mansoni
Fasciolopsis buski ( Giant intestinal fluke)
Distinct nose
Fasciola hepatica
Fasciolopsis buski
Fasciola hepatica
Fasciolopsis buski
Fasciolopsis buski
Cestodes (Tapeworms)
Flattened dorsoventrally, segmented
Head with armed or unarmed scolex
Proglottids immature, mature (sex organs)
Gravid (with eggs)
• Examples
Internal structure of proglottids
Hermaphroditic-ovary, testes, vitellaria,
uterus, genital pore and ducts
• Taenia solium
Lateral excretory and nervous system
No gut-tegument absorbs nutrients
Muscles-longitidinal and horizontal
• Diphyllobothrium latum
• Taenia saginata
• Hymenolepis nana
• Hymenolepis diminuta
• Echinococcus granulosis
Diphyllobothrium latum
• Poorly-cooked fresh-water fish(salmon)
• Scandinavian, Russia, Canada, N. USA, Alaska
• Broad fish tapeworm
• Longitudinal sucker
• Eggs have non-shouldered operculum and knob
• They are not embryonated
• Causes Vit B12 deficiency
Diphyllobothrium latum
Sucking plate
Diphyllobothrium latum
Taenia Species – two species Outstanding
characteristics
Taenia saginata
Taenia Solium
 Beef tapeworm
 4 suckers on scolex
 >13 uterine branches in
proglottids
Ingestion of cysticerci in beef
Intestinal infestation
Ingestion of eggs ->
Non-human pathogen
Pig tapeworm
Ring of thorns/crown on
scolex
<13 uterine branches in
proglottids
Ingestion of cysticerci in pork
Intestinal infestation
Ingestion of eggs ->
Cysticercosis
Taenia eggs
Identical eggs for the two species
Taenia saginata
Proglottid > 12 uterine
branches
Taenia solium
Scolex - Ring of thorns
Proglottis – fewer uterine branches
(<=12 uterine branches)
Medical arthropods
Fly
Sandfly
Mosquito
Soft tick
Louse
Flea
Hard tick
Maggots
Bot fly larvae
Extrudes from the skin
Ticks of
importance
Hard Ticks
Soft tick Expands with blood
engorgement
Hour glass
On tummy
Black Widow spider
Body louse
Body Louse
Hair nit
Flea
Crab louse
Mite
Scabies
Tiny eggs under skin
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