Liwei Li Department of Medical Microbiology and Parasitology College of Medicine, Zhejiang University [email protected] Human Parasitology ( 人体寄生虫学 )

Liwei LiDepartment of Medical Microbiology and

Parasitology College of Medicine, Zhejiang University

[email protected]

Human ParasitologyHuman Parasitology

(( 人体寄生虫学人体寄生虫学 ))

Introduction to Introduction to ParasitologyParasitology

• 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 You need to know in the course of human parasitologyhuman 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!

MosquitoMosquito

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 Definition of ParasitologyParasitology

Subject outlineSubject outlineIn 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?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 ParasitesMedical 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 ParasitesMedical Importance of Parasites

Major human parasitesMajor 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)(WHO,1999)

Examples of Medical Importance Examples of Medical Importance in the Worldin the World

Medical Importance in China 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

Medical Importance in ChinaMedical Importance in ChinaParasite 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 ChinaMedical Importance in ChinaFood-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 WHAT TYPES OF LIVING ORGANISMS ARE PARASITIC?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?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.)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 Sea anemones and anemonefishanemonefish

Symbiosis (cont.)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

SymbiosisSymbiosis (cont.)(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.)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 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 ParasitesKinds 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

HostHost

— 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 hostTypes 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 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.

Definitions of Hosts (cont.)Definitions of Hosts (cont.)

Zoonosis( 人兽共患病 ): a disease of animals that may be transmitted to humans under natural conditions.

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



• 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.


• 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 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 CycleWhat 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

Epidemiology

Extra-Human development

(soil, water, animal host, insect)

Diagnosis

Stage to discharge

Via:fecesUrineSputumVectorblood

Pathogenesis

SiteNo.

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.)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.)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 interactionshost-parasite interactions

The harmful effects on the hostThe harmful effects on the host• Depriving for nutrition• Mechanical damage• Toxic effects• Immune-pathological consequences


Ascaris in small intestineAscaris in small intestine

Hookworm and anemiaHookworm and anemia

Complete blockage of Complete blockage of intestine caused by Ascarisintestine caused by Ascaris

Allergy caused by mosquito bitesAllergy caused by mosquito bites

schistosomiasisschistosomiasis

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 )


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 )


• 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 Characteristics of parasitic disease (infection)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 )

DiagnosisDiagnosisDiagnostic techniques • Etiological techniques• Immunodiagnostic techniques • Other molecular diagnostics

Epidemiology & principles of Epidemiology & principles of controlcontrol

• Basic and essential links for parasitic diseases:– Source of infection: patients, carriers,

reservoir hosts– Route of infection– Susceptible population

Epidemiology & principles of Epidemiology & principles of control(cont.)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…


• 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


• Health Education

Round WormsRound Worms

IntroductionIntroduction

• 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

MorphologyMorphology• 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

MorphologyMorphology

• No circulatory system • The reproductive system is in

tubular form with dioecious (sex-differentiation): – Male: testis, vas deferens, seminal

vesicle, and ejaculatory duct, also cloaca and spicule

– Female: ovary, oviduct, seminal receptacle, uterus, ovejector and vagina

Digestive system

female reproductive system

male reproductive system

PhysiologyPhysiology • 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 cycleLife 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 Common medical nematodes speciesspecies

• 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 IntroductionGeneral 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

• 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 The lips of A. lumbricoidesA. 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

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


• 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


• 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 CycleLife 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


• 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


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

PathogenesisPathogenesis

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


• 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

DiagnosisDiagnosis

• Worldwide distribution, very common in China, especially in the countryside.

Infection rate: rural >urban; children > adults

EpidemiologyEpidemiology

• 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

• 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.

PreventionPrevention

HookwormsHookworms ( ( 钩虫钩虫 ))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 IntroductionGeneral 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:Morphology: Adults 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:Morphology: Adults 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:Morphology: Egg Egg

oval or can-shaped with a thin, hyaline shell, measured 60-75 by 36-40 µm.

Life cycleLife cycle

Life cycleLife 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


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 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


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 iron-deficiency

6. Ancylostomiasis in infant

AllotriophagyAllotriophagy

Laboratory diagnosisLaboratory diagnosis

• Brine floatation is the method of first choice

• Hookworm larvae cultivation is used for species identification

• Hookworm larvae in sputum

EpidemiologyEpidemiology• 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

EpidemiologyEpidemiology

• 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 45C

Principles of ControlPrinciples 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

Documents

Liwei Li Department of Medical Microbiology and Parasitology College of Medicine, Zhejiang University [email protected] Human Parasitology ( 人体寄生虫学 )