Liwei LiDepartment of Medical Microbiology and

Parasitology

College of Medicine, Zhejiang University

lilw2@zju.edu.cn

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?

microbes parasites

Pathogens

bacteria viruses fungi protozoa helminthes arthropods

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)

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 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

Ascaris in small intestine

Hookworm and anemia

Complete blockage of intestine caused by Ascaris

Allergy caused by mosquito bites

schistosomiasis

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

Epidemiology & principles of control(cont.)

• Health Education