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Liwei LiDepartment of Medical Microbiology and
Parasitology
College of Medicine, Zhejiang University
Human Parasitology
(人体寄生虫学)
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.fcgi?artid=126866
• Olson & Guselle. Are pig parasite a human health risk?http://www.banffpork.ca/proc/2000pdf/Chap13-Olson.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
Infectious or
communicable
diseases!
Mosquito
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
Definition of Parasitology
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 free-living 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.
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)
Medical Importance of Parasites
Major human parasites• Estimated World Prevalence of the Major
Parasitic Infection of Human:– Malaria 300-500 million
– Schistosomiasis 200 million
– Lymphatic filariasis 120 million
– Onchocerciasis 85 million
– Leishmaniasis 12 million
– Trypanosoma cruzi (South America) 18 million
– Ascaris infection 1300 million
– Hookworm infection 1300 million
– Amoebiasis 60 million
– Trichuriasis 900 million
– Gardiasis 200 million
(WHO,1999)
Medical Importance in China
• Five major parasitic diseases
• Malaria
• Schistosomiasis
• Kala-azar (黑热病)
• Filariasis
• Hookworm disease
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 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……
KINGDOM PROTISTA - contains the single-celled protozoans.
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.)
Overlap between the major categories of symbiosis
Parasitism
Commensalism
and Phoresis
Mutualism
•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.
Parasite
— In the relationship known as parasitism, the
partner lives in or on another from which it gainsbenefit, always smaller, is the parasite which tosome degree injures its partner.
— Parasites (animal parasites) are invertebratesthat can not live independently and should dependupon others to maintain their lives (live at theexpense 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.
Reservoir host( 保虫宿主 ) is referred to thoseanimals that harbor an infection that can betransmitted 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.
Definitions of Hosts (cont.)
Zoonosis(人兽共患病 ): a disease of animals thatmay be transmitted to humans under naturalconditions.
A transport/paratenic/transfer host (转续宿主 ) : When parasite enters the body of anabnormal host and not undergoes anydevelopment but continues to stay alive and beinfective to the normal host.The host is calledthe transport host.not necessary for the completion of the parasite’s life cycle
Definitions of Hosts (cont.)
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)
Vector parasite infections may be carriedfrom one host to another by means ofarthropod vectors. A vector may also be ahost if development of the parasite takesplace with its body.(If the arthropod is simplyan instrument of passive transfer, we refer toit as a mechanical vector)
Definitions of Hosts (cont.)
What is the Life CycleLife 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
Human residing
stage
Invading
OralSkinVectorcontact
Infective
stage
EpidemiologyExtra-Human development
(soil, water, animal host, insect)
Diagnosis
Stage to discharge
Via:fecesUrineSputumVectorblood
Pathogenesis
Site
No.
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.
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
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
host-parasite interactions
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