Human Parasitology
(人体寄生虫学)
Liwei Li
Department of Medical Microbiology and
Parasitology
College of Medicine, Zhejiang University
lilw2@zju.edu.cn
Introduction to Parasitology
• F. E. G. Cox. History of Human Parasitology.
Clin Microbiol Rev. 2002 October; 15 (4): 595–
612
http://www.pubmedcentral.nih.gov/articlerender.f
cgi?artid=126866
• Olson & Guselle. Are pig parasite a human
health risk?
http://www.banffpork.ca/proc/2000pdf/Chap13Olson.pdf
You need to know in the course of
human parasitology
•
•
•
•
•
What is parasitology?
What are parasitism, parasite and host ?
What is the life cycle of a parasite?
How is the host-parasite interplay ?
How do we diagnose the infections with
parasites?
• What do we need to understand in the
epidemiology of parasitic infections?
• What are principles of control of parasitic
diseases?
Pathogens
microbes
bacteria
viruses
fungi
parasites
protozoa helminthes arthropods
Mosquito
Pathogens
Infectious or
communicable
diseases!
Definition of Parasitology
Parasitology is a discipline dealing with the biology of
animal parasites, ecology of parasitism with emphasis on
parasite--host and parasite--environmental interactions.
Human parasitology or Medical parasitology is restricted
in studying those parasites that are of importance in
medicine
Protozoology, helminthology and entomology
Parasitology is usually in the scope of preventive
medicine and the foundation of clinical parasitic diseases
Subject outline
In this course we will concentrate on 3 major
groups of parasites:
1. Medical protozoa - flagellates, amoebae,
malarial organisms
2. Medical Helminthes - parasitic worms such as
the flukes, tapeworms, and roundworms
3. Medical Arthropods - insects and arachnids
that are ectoparasites and carriers (vectors) of
diseases
Why do we study parasitology?
1. Parasites provide unique examples of
biological phenomena not found in freeliving organisms
2. _
• Medical importance
• Veterinary importance
• Economic importance
Medical Importance of Parasites
• Humans are hosts to over 100 species of
parasites.
• Many of these parasites are causative agents of
major public health problems of the world.
Medical Importance of Parasites

Ten major tropical diseases (UNDP/World
bank/TDR, 2000)
•
Malaria(疟疾)
•
Shistosomaiasis（血吸虫病）
•
Filariasis (丝虫病，Lymphatic filariasis and Onchocerciasis)
•
Leishmaniasis（利什曼病）
•
Trypanosomiasis (锥虫病，African trypanosomiasis and chagas
disease )
•
Leprosy（麻风病）
•
Tuberculosis （结核病）
•
Dengue fever （登革热）
Major human parasites
• Estimated World Prevalence of the Major
Parasitic Infection of Human:
–
–
–
–
–
–
–
–
–
–
–
Malaria
Schistosomiasis
Lymphatic filariasis
Onchocerciasis
Leishmaniasis
Trypanosoma cruzi (South America)
Ascaris infection
Hookworm infection
Amoebiasis
Trichuriasis
Gardiasis
300-500 million
200 million
120 million
85 million
12 million
18 million
1300 million
1300 million
60 million
900 million
200 million
(WHO,1999)
Examples of Medical Importance
in the World
Medical Importance in China
• Five major parasitic diseases
•
•
•
•
•
Malaria
Schistosomiasis
Kala-azar (黑热病)
Filariasis
Hookworm disease
Medical Importance in China
Medical Importance in China
Parasite infection
Estimated number
of cases(2004)
Malaria (P.v & P.f infection)
60.2 thousand (2006)
Ascariasis (large roundworm infection)
85.9 million
Trichuriasis (whipworm infection)
29.1million
Hookworm disease(A. d and N. a infection) 39.3 million
Clonorchiasis (oriental liver fluke infection) 12.5 million
Fasciolopiasis (intestinal fluke infection)
2 million
Paragonimiasis(lung fluke infection)
2.95 million
Taeniasis (pork and beef tapeworm
infection)
1.5 million
Schistosomiasis (blood fluke infection)
843 thousand (2003)
China’s parasite infection based on
the nationwide parasite survey
Medical Importance in China
 Food-borne parasitic disease is still a big
problem.
 Prevalence of pet-borne parasitic disease has
increased.
 Prevalence of opportunistic parasitic disease has
increased.
 Material exchange and population migration has
widened the geographical scope of parasitic
disease.
 Medical professionals generally lack the
knowledge of parasitology.
WHAT TYPES OF LIVING ORGANISMS ARE
PARASITIC?
Parasites occur in two of the five kingdoms of living organisms.
What are the 5 kingdoms?
KINGDOM PROTISTA - contains the single-celled protozoans.
KINGDOM ANIMALIA contains 32 phyla.
Parasites of importance are concentrated in 3 phyla.
•
PHYLUM PLATYHELMINTHES – Class Trematoda; Cestoda
•
PHYLUM NEMATODA – Class Nematoda
•
PHYLUM ARTHROPODA – Class Insecta……
What are parasitism, parasite and host?
Evolution of parasitism:
Understanding start with basic concept of symbiosis
• Symbiosis was first coined by the German de
Bary in 1879 - to mean “living together”. It was
originally coined to refer to all cases where
dissimilar organisms or species (e.g.,
heterogenetic associations) live together in an
intimate association
Interactions of Symbionts
In order to facilitate our understanding of symbiosis, 3
subordinate categories of symbiotic relationships are
indicated. They are: commensalism (including
phoresis), mutualism, and parasitism
Symbiosis (cont.)
1. Mutualism(互利共生)
• This occurs when each member of the association
benefits the other
• The mutuals are metabolically dependent on one
another. Sometimes, one cannot survive in the
absence of the other
•
eg. the flagellate cannot
survive outside the termite
Sea anemones and anemonefish
Symbiosis (cont.)
2. Commensalism(偏利共生,共栖)
•
•
Commensalism means “eating at the same table”
and in many commensalistic relationships one
organism (the commensal) is feeding on food
that was not consumed by the host
Commensalism occurs when one member of the
associating pair, usually the smaller, receives all
the benefit and the other member is neither
benefited nor harmed
•
•
To carry -- phoresis
Example: Remora fish associated with sharks
feeds on leftover food
Symbiosis (cont.)
3. Parasitism(寄生)
•
A parasitos (para: beside; sitos: grain or food): Original
meaning from the Greek is a relationship in which "one
eats at another's table or lives at another's expense."
•
Parasitism is a relationship in which one of the
participants, the parasite, either harms its host (the
part that got harmed) or in some sense lives at the
expense of the host.
Debate: The amoeba Entamoeba invadens is harmless in turtles but
causes 100% mortality in snakes.
Is it then a commensal (when it’s in turtles) or is it a parasite (when it’s in
snakes)?
The true nature of parasitism involves an ecological relationship between the
parasite and its host. A parasite is metabolically dependent on its host.
Symbiosis (sum.)
•The categories of symbiosis are man-made constructs introduced
primarily for convenience (they allow us to categorize natural symbiosis
associations). There can in fact be overlap between various categories.
Commensalism
and Phoresis
Mutualism
Parasitism
Overlap between the major categories of symbiosis
Parasite
— In the relationship known as parasitism, the
partner lives in or on another from which it gains
benefit, always smaller, is the parasite which to
some degree injures its partner.
— Parasites (animal parasites) are invertebrates
that can not live independently and should depend
upon others to maintain their lives (live at the
expense of others).
—Parasites may be classified according to different ways:

residing site---endoparasite / ectoparasite

ecology---obligatory/facultative; accidental or
opportunistic

duration of parasitism---permanent/intermittent
Kinds of Parasites
• An organism that does not absolutely depend on
the parasitic way of life, but is capable of
adapting to it if placed in such a relationship is
known as a facultative parasite
• If an organism is completely dependent on the
host during a segment or all of its life cycle the
parasite is known as an obligatory parasite
• Parasites that live within the body of their host
(intestinal tract, liver, etc.) are called
endoparasites
• Parasites that are attached to the outer surfaces
of their hosts are called ectoparasites
Host
— Definition: In the parasitism, the partners
which provide the food and shelter for
parasites, and to some degree are injured by
this association, are scientifically called as
hosts.
larger, more complex and better regulated
bodies
Types of the host
• A definitive host(终宿主) is the host in which the
parasite become sexually mature (where the
adult worm harbor or undergoing sexual
reproduction).
• An intermediate host(中间宿主) is host in which
the parasite undergoes larval development but
does not reach sexual maturity, parasites often
can undergo asexual reproduction in this type of
host.
Definitions of Hosts (cont.)
Reservoir host( 保 虫 宿 主 ) is referred to those
animals that harbor an infection that can be
transmitted to humans.
Even if the animal is the normal host of the parasite, it is the
reservoir for the zoonotic infection of people ． Thus ， the
reservoir host shares the same stage of the parasite with humans.
Zoonosis( 人 兽 共 患 病 ): a disease of animals that
may be transmitted to humans under natural
conditions.
Definitions of Hosts (cont.)
A transport/paratenic/transfer host ( 转 续 宿
主 ) : When parasite enters the body of an
abnormal
host
and
not
undergoes
any
development but continues to stay alive and be
infective to the normal host．The host is called
the transport host．
not necessary for the completion of the parasite’s life cycle
Definitions of Hosts (cont.)
• Immune compromised hosts are persons who
are considered to have reduced resistance to
illness include:
infants, hospital patients,
pregnant
women,
frail,
elderly
people,
malnourished individuals, people with controlled
physical or metabolic disorders (e.g., diabetes or
high blood pressure), people with AIDS.
Definitions of Hosts (cont.)
• Opportunistic
parasitic
infection:
Any
infection caused by a parasite that does not
normally cause disease in humans; occurs in
persons with abnormally functioning immune
systems (as AIDS patients or transplant patients
receiving immunosuppressive drugs).
– For example: Toxoplasma gondii, Crypsosporidium
(Pneumocystis jeroveci (carinii) -----
Pneumocystis pneumonia, PCP)
Definitions of Hosts (cont.)
Vector parasite infections may be carried
from one host to another by means of
arthropod vectors. A vector may also be a
host if development of the parasite takes
place with its body.(If the arthropod is simply
an instrument of passive transfer, we refer to
it as a mechanical vector)
What is the Life Cycle
Life cycle described the ontogenesis, development
and reproduction of the parasite, tracking it through
the various phases of its life history which will
encompass both parasitic and non-parasitic stages.
The key to understanding the
transmission of a parasite
species and parasitic disease
is its life-cycle
Generalized stages of a parasite’s life cycle
Stage in human host (linking to pathogenesis)
Stage to discharge (diagnostic stage)
Stage developing outside human host
( in external environment, intermediate host or
insect host)--- (linking to transmission)
Stage infecting men (infective stage)
Parasite Life Cycle—A generalized mode
Invading
Oral
Skin
Vector
contact
Human residing
stage
Infective
Pathogenesis
Site
No.
Stage to discharge
stage
(soil, water, animal host, insect)
Epidemiology
Extra-Human development
Via:
feces
Urine
Sputum
Vector
blood
Diagnosis
The types of life cycle of parasites
Direct type: one host (definitive host)
---geo-helminth
Indirect type: more one hosts(intermediate host(s)
and definitive host)
---bio-helminth
Life cycle (cont.)
• Simple or Direct Life Cycle (monoxenous) is
one in which there is only one host where the
parasite often spends most of its life, usually as
an adult, and where it reproduces.
Life cycle (cont.)
• Many parasites have more complex cycles which
include 2 or more hosts and are classified as
having indirect life cycles.
host-parasite interactions
Adaptations to parasitism
 Profound morphological adaptation to their way of life
 Organs not necessary to a parasitic existence are
frequently lost or degenerated
 Reproductive system is very highly developed in
association with increased reproductive capacity
 Specialized attachment organs in the form of suckers
and hooks have been developed
 Physiological and biochemical adaptations
 Immune evasion
host-parasite interactions
The harmful effects on the host
• Depriving for nutrition
• Mechanical damage
• Toxic effects
• Immune-pathological consequences
Ascaris in small intestine
Hookworm and anemia
Complete blockage of
intestine caused by Ascaris
Allergy caused by mosquito bites
schistosomiasis
host-parasite interactions
Effects of the host to the parasite
• Genetic constitution of the host may profoundly
influence
the
host-parasite
relationship
(racial
variations in resistance to certain strains of
Plasmodium vivax; sickle cell trait increased resistance
to infection with P. falciparum )
host-parasite interactions
Effects of the host to the parasite
•Anti-parasitic immune responses
Natural immunity– mucocutaneous barrier, blood brain
barrier, phagocyte, complement, defensins…
Acquired immunity
Sterilizing immunity (cutaneous leishmaniasis)
Non-sterilizing immunity– premunition ( 带 虫 免 疫 ),
concomitant immunity (伴随免疫)
host-parasite interactions
• Colonized (“infected”), asymptomatic
– Differences in host susceptibility
• Many people are asymptomatically infected with
Toxoplasma gondii.
– Capable of spreading microbe
• Amoeba carrier
• Colonized, infected, symptomatic
• Infected, host death
Characteristics of parasitic
disease (infection)
• Chronic infection, carrier
• Suppressive infection(隐性感染)
• Polyparasitism
• Eosinophilia（嗜酸性粒细胞增多）
• IgE ↑
• Larva migrans(幼虫移行症)
• Ectopic parasitism(异位寄生)
Diagnosis
Diagnostic techniques
• Etiological techniques
• Immunodiagnostic techniques
• Other molecular diagnostics
Epidemiology & principles of
control
• Basic and essential links for parasitic
diseases:
– Source of infection: patients, carriers,
reservoir hosts
– Route of infection
– Susceptible population
Epidemiology & principles of
control(cont.)
• Endemic factors
– Natural factors
• One won’t get parasitic diseases in the polar
area
– Social factors
• Eating habits, hygienic habit…
• Sexual transmitted diseases…
Epidemiology & principles of
control(cont.)
• Principal of disease control
– Control the source of infection
– Cut off the route of transmission
– Massive protection, esp. for the
susceptible population
• no successful vaccine
Epidemiology & principles of
control(cont.)
• Health Education
Round Worms
Introduction
• Among the commonest of all parasites
and responsible for diseases of major
importance in humans
• Non-segmented roundworms belonging
to the Phylum Nemathelminthes, Class
Nematoda
• The
sexes
are
usually
separate
(Dioecious), the male which is smaller
than the female commonly has a curved
posterior end
Morphology
•
•
•
•
•
Cylindrical and slender
Bilaterally symmetrical
Sex-differentiated
Celomic cavity: protocoele
The supporting body wall consists of
cuticle layer, syncytial layer (subcutical
layer) and longitudinal muscular layer
• The alimentary tract is a simple tube
extending from the mouth to the anus
Morphology
• No circulatory system
• The reproductive system is in
tubular form with dioecious (sexdifferentiation):
– Male: testis, vas deferens, seminal
vesicle, and ejaculatory duct, also
cloaca and spicule
– Female: ovary, oviduct, seminal
receptacle, uterus, ovejector and
vagina
Digestive system
male reproductive system
female reproductive system
Physiology
• The methods of obtaining food may be classified
as
–
–
–
–
Sucking with ingestion of blood (Ancylostoma)
Ingestion of lysed tissues and blood (Trichuris)
Feeding on the intestinal contents (Ascaris)
Ingestion of nourishment from the body fluids (filarial
worms)
• Metabolism: mainly aerobic metabolism, and
most species need free-living periods for larvae,
which are capable of withstanding a wide range
of environmental condition
• During larval development, nematodes pass
through several molts or ecdysis, both inside and
outside the host
Life cycle
• Geo-helminths
– Only one host: the larvae pass from host to
host directly or after a free-living existence
– Transmission to a new host depends upon
• the ingestion of the mature infectious
eggs with larvae (Ascaris, Pinworm)
• the penetration of the skin or mucous
membranes by the larvae (Hookworm)
• Bio-helminths
– Have an intermediate host
– Transmission to a new definitive host is
intermediated by the arthropod--- (filarial
worm)
Common medical nematodes
species
• Intestinal lumen residing nematodes:
– Ascaris, Hookworm, Whip worm, Pinworm---the adult parasite inhabit in human
intestinal tract
• Blood and tissue residing nematodes:
– Filaria, Thichinella----the location of the
adult parasite is blood or tissue
Ascaris lumbricoides
(似蚓蛔线虫,蛔虫)
General Introduction
•
Common saying “round worm of man”
•
The largest of the intestinal nematodes
parasitizing humans
•
The most common worm found in human
•
It is worldwide in distribution and most
prevalent through out the tropics, sub-tropics
and more prevalent in the countryside than in
the city
Morphology
• Adult:
– Cylindrical in shape
– Creamy-white or pinkish in color
– The female averages 20-35cm in length, the
largest 49cm
– The male is smaller, averaging 15-31cm in
length, has a typical curled tail with a pair sickle
like copulatory spines
– On the tip of the head there are three lips,
arranged as a Chinese word “ 品 ”
– Male has a single reproductive tubule
– The female has two reproductive tubules and
the vulva is ventrally located at the posterior
part of the anterior 1/3 of the body
Adult worm of A. lumbricoides
The lips of A. lumbricoides
The three lips are
seen at the anterior
end. The margin of
each lip is lined with
minute teeth which are
not visible at this
magnification
Morphology
Egg
• There are three kinds of the eggs
– fertilized eggs
– unfertilized eggs
– decorticated eggs
• We usually describe an egg in 5 aspects
– size, color, shape, shell and content
Morphology
• Fertilized egg:
–
–
–
–
–
an average size 60×45µm
broad oval in shape
brown in color
The shell is thick
Albuminous coat is thick and stained brown by
bile
– The content is a fertilized ovum
– There is a new-moon(crescent) shaped clear
space at each end inside the shell
Morphology
• Unfertilized egg
Longer and slender than fertilized egg
– The shell and albuminous coat are
thinner
than those of the fertilized egg
– The content is made of many
refractable
granules various in size
–
• Decorticated egg:
– Both fertilized and unfertilized eggs
sometimes
may
lack
their
outer
albuminous coats and are colorless
Life Cycle
Life Cycle
•
•
•
•
Site of inhabitation: small intestine
Infective stage: embryonated eggs
Route of infection: by mouth
Blood-lung
migration:
intestine--blood stream --- right side of the heart -- lung --- respiratory tree --- coughed up
and swallowed --- small intestine
Life Cycle
• No intermediate and reservoir hosts
• The
time
from
the
ingestion
of
embryonated eggs to oviposition by the
females is about 60-75 days
• Life span of the adult: about 1 year
• Female
may
produce
approximately
240,000 eggs per day, which are passed
in feces
Pathogenesis
1. The blood-lung migration phase of
the larvae: During the migration
through the lungs, the larvae may
cause a pneumonia (temporary).
– The symptoms of the pneumonia are
low fever, cough, blood-tinged sputum,
asthma
– The clinical manifestation is also called
Loeffler’s syndrome
Pathogenesis
2. The intestinal phase of the adults
• No symptoms to vague abdominal pains or
intermittent colic, especially in children
• A heavy worm burden can result in
malnutrition
• Wandering adults may block the appendical
lumen or the common bile duct and even
perforate the intestinal wall which cause
complications of ascariasis:
•
•
•
•
•
intestinal obstruction
Appendicitis
biliary ascariasis (the most common one)
perforation of the intestine
cholecystitis, pancreatitis and peritonitis
Diagnosis
• The symptoms and signs are for
reference only
• Intestinal ascariasis: feces are
examined for the ascaris eggs
– Direct fecal film: it is simple and effective
and is the first choice
– brine-floatation method:
– recovery of adult worms: when adults or
adolescents are found in feces or vomit
and tissues
• Ascaris pneumonitis: examination
of sputum for Ascaris larvae is
sometimes
Epidemiology
• Worldwide distribution, very common
in China, especially in the countryside.
Infection rate:
rural >urban;
children > adults
• Factors favoring the spread of the
transmission:
– Simple life cycle
– Enormous egg production ( 240,000 eggs/ day/ female )
– Eggs are highly resistant to ordinary disinfectants
( due to the ascroside) which may remain viable for
several years
– Social customs and living habits.
– Disposal of feces is unsuitable
Prevention
• Treatment to ascariasis:
Mebendazole(甲苯咪唑)
Albendazole(阿苯达唑)
• Sanitary disposal of feces.
• Hygienic habits such as cleaning
of hands before meals.
• Health education.
Hookworms (钩虫)
Two major species of hookworms
can infect human
Necator americanus
(美洲板口线虫)
Ancylostoma duodenale
(十二指肠钩口线虫)
General Introduction
•
•
•
•
•
•
Human intestinal nematode of smaller size,
inhabits the small intestine
World-wide distribution, about 900 million
infections in the world
A. duodenale is prevalent in Southern Europe,
North Africa, Northern Asia (North China),
and the more pathogenic one
N. americanus is the predominant species in
the Western hemisphere and equatorial Africa
(South China)
Many areas are endemic for both species
Heavy infection may evoke anemia known as
"Yellow Laziness"
Morphology: Adults
Ancylostoma duodenale
• Female is 10-13 mm in
length by 0.6 mm in
diameter
• Males are 8-11 mm by 0.4
mm
• Posterior end has an
umbrella-shaped bursa
with riblike rays
• Two pairs of curved teeth
on the ventral wall of its
buccal capsule
Morphology: Adults
Necator americanus
• Females are 9-11 mm in
length by 0.4 mm in
diameter
• Males are 7-9 mm by 0.3
mm
• Smaller than A. duodenale
• A pair of semilunar cutting
plates on the ventral wall
of the buccal capsule
Buccal capsule
Ancylostoma duodenale
Necator americanus
Morphology: Egg
oval or can-shaped
with a thin, hyaline
shell, measured 6075 by 36-40 µm.
Life cycle
Life cycle
•
•
•
•
No intermediate host is necessary
Larva takes a free living mode
The filariform larva is the infective stage
Infection routes: skin penetration(mainly); orally
swallow; maternal-child
• Residing in human upper small intestine: duodenum,
jejunum
• Blood-Lung migration: skin --- lymphatic system --right side of the heart --- lung --- respiratory tree --coughed up and swallowed --- small intestine
• Persisting migrans( 迁 延 移 行 ): Ancylostoma
duodenale
Pathogenesis
1. Hookworm larvae dermatitis:
Penetration of the skin by the filariform
larvae
may
be
asymptomatic
in
previously uninfected individuals.
However, those experiencing repeated
infections develop itching,
known as
"ground itch" or "dew itch".
Pathogenesis
2. Migration of pre-adult cause
temporary pulmonary inflammation:
In heavily infected individuals
(i.e., 500-1000 worms), there can
be symptoms of pneumonia during
the
migratory
phase
in
the
developmental cycle of these worms
Pathogenesis
3. Abdominal pains, diarrhea, loss of
appetite…
4. Anemia
– Especially in young children
– Hypoproteinemic because of some loss of
serum proteins
– Iron-deficiency
– Why the small worms can cause anemia?
• The pump-like action when worms suck blood
• The worms secrete an anticoagulant, which
facilitates bleeding
• The worms usually change the sites when suck
blood
5. Allotriophagy (Geophagy): due to the irondeficiency
6. Ancylostomiasis in infant
Allotriophagy
Laboratory diagnosis
• Brine floatation is the method
of first choice
• Hookworm larvae cultivation
is used for species identification
• Hookworm larvae in sputum
Epidemiology
•
•
Most prevalent in the tropical and
subtropical zones
In China, mostly mixed infected,
while
A.duodenale
is
somewhat
northward
distributed
and
N.
americanus in southward
Epidemiology
• Moist, shady, sandy, or loamy soil
favors persistence of these worms
• Larvae can survive for up to 6 weeks
• Do not live long in clay, dry, hard
packed soils, or where temperatures are
freezing, or are higher than 45C
Principles of Control
1. Chemotherapy: Albendazole; Mebendazole
2. Sanitary disposal of human feces is the
most effective control measure in
preventing the spread of infection with
the hookworms
3. Protection of the susceptible population