Introduction

Man and other living things on earth live in an entangling relationship with each other.

They don’t exist in an isolated fashion. They are interdependent; each forms a strand in

the web of life. Medical parasitology is the science that deals with organisms living in

the human body (the host) and the medical significance of this host-parasite

relationship.

ASSOCIATION BETWEEN PARASITE AND HOST

A parasite is a living organism, which takes its nourishment and other needs from a

host; the host is an organism which supports the parasite. The parasites included in

medical parasitology are protozoa, helminthes, and some arthropods. (See box 1 for

broader classification of parasites). The hosts vary depending on whether they harbor

the various stages in parasitic development. (See box 2)

2

BOX 1. DIFFERENT KINDS OF PARASITES

• Ectoparasite – a parasitic organism that lives on the outer surface of its host, e.g.

lice, ticks, mites etc.

• Endoparasites – parasites that live inside the body of their host, e.g. Entamoeba

histolytica.

• Obligate Parasite - This parasite is completely dependent on the host during a

segment or all of its life cycle, e.g. Plasmodium spp.

• Facultative parasite – an organism that exhibits both parasitic and non-parasitic

modes of living and hence does not absolutely depend on the parasitic way of

life, but is capable of adapting to it if placed on a host. E.g. Naegleria fowleri

• Accidental parasite – when a parasite attacks an unnatural host and survives. E.g.

Hymenolepis diminuta (rat tapeworm).

• Erratic parasite - is one that wanders in to an organ in which it is not usually found.

E.g. Entamoeba histolytica in the liver or lung of humans.

Most of the parasites which live in/on the body of the host do not cause disease

(non-pathogenic parasites). In Medical parasitology we will focus on most of the disease

causing (pathogenic) parasites. However, understanding parasites which do not

ordinarily produce disease in healthy (immunocompetent) individuals but do cause

illness in individuals with impaired defense mechanism (opportunistic parasites) is

becoming of paramount importance because of the increasing prevalence of HIV/AIDS

in our country.

3

BOX 2. DIFFERENT KINDS OF HOSTS

• Definitive host – a host that harbors a parasite in the adult stage or where the parasite

undergoes a sexual method of reproduction.

• Intermediate host - harbors the larval stages of the parasite or an asexual cycle of

development takes place. In some cases, larval development is completed in two

different intermediate hosts, referred to as first and second intermediate hosts.

• Paratenic host – a host that serves as a temporary refuge and vehicle for reaching an

obligatory host, usually the definitive host, i.e. it is not necessary for the completion of

the parasites life cycle.

• Reservoir host – a host that makes the parasite available for the transmission to

another host and is usually not affected by the infection.

• Natural host – a host that is naturally infected with certain species of parasite.

• Accidental host – a host that is under normal circumstances not infected with the

parasite.

There is a dynamic equilibrium which exists in the interaction of organisms. Any

organism that spends a portion or all of its life cycle intimately associated with another

organism of a different species is considered as Symbiont (symbiote) and this

relationship is called symbiosis (symbiotic relationships).

The following are the three common symbiotic relationships between two organisms:

Mutualism - an association in which both partners are metabolically dependent upon

each other and one cannot live without the help of the other; however, none of the

partners suffers any harm from the association. One classic example is the relationship

between certain species of flagellated protozoa living in the gut of termites. The

protozoa, which depend entirely on a carbohydrate diet, acquire their nutrients from

termites. In return they are capable of synthesizing and secreting cellulases; the

cellulose digesting enzymes, which are utilized by termites in their digestion.

4

Commensalism - an association in which the commensal takes the benefit without

causing injury to the host. E.g. Most of the normal floras of the humans’ body can be

considered as commensals.

Parasitism - an association where one of the partners is harmed and the other lives at

the expense of the other. E.g. Worms like Ascaris lumbricoides reside in the

gastrointestinal tract of man, and feed on important items of intestinal food causing

various illnesses.

Once we are clear about the different types of associations between hosts and

parasites, we can see the effect the parasite brings to the host and the reactions which

develop in the host’s body due to parasitic invasion.

EFFECT OF PARASITES ON THE HOST

The damage which pathogenic parasites produce in the tissues of the host may be

described in the following two ways;

(a) Direct effects of the parasite on the host

• Mechanical injury - may be inflicted by a parasite by means of pressure as it grows

larger, e.g. Hydatid cyst causes blockage of ducts such as blood vessels

producing infraction.

• Deleterious effect of toxic substances- in Plasmodium falciparum production of

toxic substances may cause rigors and other symptoms.

• Deprivation of nutrients, fluids and metabolites -parasite may produce disease by

competing with the host for nutrients.

(b) Indirect effects of the parasite on the host:

Immunological reaction: Tissue damage may be caused by immunological

response of the host, e.g. nephritic syndrome following Plasmodium infections.

Excessive proliferation of certain tissues due to invasion by some parasites can

also cause tissue damage in man, e.g. fibrosis of liver after deposition of the ova of

Schistosoma.

5

BASIC CONCEPTS IN MEDICAL PARASITOLOGY

In medical parasitology, each of the medically important parasites are discussed under

the standard subheadings of morphology, geographical distribution, means of infection,

life cycle, host/parasite relationship, pathology and clinical manifestations of infection,

laboratory diagnosis, treatment and preventive/control measures of parasites. In the

subsequent section some of these criteria are briefly presented.

Morphology - includes size, shape, color and position of different organelles in different

parasites at various stages of their development. This is especially important in

laboratory diagnosis which helps to identify the different stages of development and

differentiate between pathogenic and commensal organisms. For example,

Entamoeba histolytica and Entamoeba coli.

Geographical distribution - Even though revolutionary advances in transportation has

made geographical isolation no longer a protection against many of the parasitic

diseases, many of them are still found in abundance in the tropics. Distribution of

parasites depends upon:

a. The presence and food habits of a suitable host:

• Host specificity, for example, Ancylostoma duodenale requires man as a host

where Ancylostoma caninum requires a dog.

• Food habits, e.g. consumption of raw or undercooked meat or vegetables

predisposes to Taeniasis

b. Easy escape of the parasite from the host- the different developmental stages of a

parasite which are released from the body along with faeces and urine are widely

distributed in many parts of the world as compared to those parasites which

require a vector or direct body fluid contact for transmission.

c. Environmental conditions favoring survival outside the body of the host, i.e.

temperature, the presence of water, humidity etc.

d. The presence of an appropriate vector or intermediate host – parasites that do not

require an intermediate host (vector) for transmission are more widely distributed

than those that do require vectors.

6

Once we are clear about the geographical distribution and conditions favoring survival in

relation to different parasites, effective preventive and control measures can more easily

be devised and implemented.

Life cycle of parasites - the route followed by a parasite from the time of entry to the

host to exit, including the extracorporeal (outside the host) life. It can either be simple,

when only one host is involved, or complex, involving one or more intermediate hosts. A

parasite’s life cycle consists of two common phases one phase involves the route a

parasite follows inside the body. This information provides an understanding of the

symptomatology and pathology of the parasite. In addition the method of diagnosis and

selection of appropriate medication may also be determined. The other phase, the route

a parasite follows outside of the body, provides crucial information pertinent to

epidemiology, prevention, and control.

Host parasite relationship - infection is the result of entry and development within the

body of any injurious organism regardless of its size. Once the infecting organism is

introduced into the body of the host, it reacts in different ways and this could result in:

a. Carrier state - a perfect host parasite relationship where tissue destruction by a

parasite is balanced with the host’s tissue repair. At this point the parasite and

the host live harmoniously, i.e. they are at equilibrium.

b. Disease state - this is due to an imperfect host parasite relationship where the

parasite dominates the upper hand. It can result either from lower resistance of

the host or a higher pathogenecity of the parasite.

c. Parasite destruction – occurs when the host takes the upper hand.

Laboratory diagnosis – depending on the nature of the parasitic infections, the

following specimens are selected for laboratory diagnosis:

a) Blood – in those parasitic infections where the parasite itself in any stage of its

development circulates in the blood stream, examination of blood film forms one

of the main procedures for specific diagnosis. For example, in malaria the

parasites are found inside the red blood cells. In Bancroftian and Malayan

filariasis, microfilariae are found in the blood plasma.

7

b) Stool – examination of the stool forms an important part in the diagnosis of

intestinal parasitic infections and also for those helminthic parasites that localize

in the biliary tract and discharge their eggs into the intestine.

In protozoan infections, either trophozoites or cystic forms may be detected; the

former during the active phase and the latter during the chronic phase. Example,

Amoebiasis, Giardiasis, etc.

In the case of helmithic infections, the adult worms, their eggs, or larvae are

found in the stool.

c) Urine – when the parasite localizes in the urinary tract, examination of the urine

will be of help in establishing the parasitological diagnosis. For example in

urinary Schistosomiasis, eggs of Schistosoma haematobium are found in the

urine. In cases of chyluria caused by Wuchereria bancrofti, microfilariae are

found in the urine.

d) Sputum – examination of the sputum is useful in the following:

• In cases where the habitat of the parasite is in the respiratory tract, as in

Paragonimiasis, the eggs of Paragonimus westermani are found.

• In amoebic abscess of lung or in the case of amoebic liver abscess bursting

into the lungs, the trophozoites of E. histolytica are detected in the sputum.

e) Biopsy material - varies with different parasitic infections. For example spleen

punctures in cases of kala-azar, muscle biopsy in cases of Cysticercosis,

Trichinelliasis, and Chagas’ disease, Skin snip for Onchocerciasis.

f) Urethral or vaginal discharge – for Trichomonas vaginalis

Indirect evidences – changes indicative of intestinal parasitic infections are:

a. Cytological changes in the blood – eosiniphilia often gives an indication

of tissue invasion by helminthes, a reduction in white blood cell count is

an indication of kala-azar, and anemia is a feature of hookworm

infestation and malaria.

b. Serological tests – are carried out only in laboratories where special

antigens are available.

8

Treatment – many parasitic infections can be cured by specific chemotherapy. The

greatest advances have been made in the treatment of protozoal diseases.

For the treatment of intestinal helminthiasis, drugs are given orally for direct action on

the helminthes. To obtain maximum parasiticidal effect, it is desirable that the drugs

administered should not be absorbed and the drugs should also have minimum toxic

effect on the host.

Prevention and control - measures may be taken against every parasite infectiving

humans. Preventive measures designed to break the transmission cycle are crucial to

successful parasitic eradication. Such measures include:

􀂃 Reduction of the source of infection- the parasite is attacked within the host, thereby

preventing the dissemination of the infecting agent. Therefore, a prompt diagnosis

and treatment of parasitic diseases is an important component in the prevention of

dissemination.

􀂃 Sanitary control of drinking water and food.

􀂃 Proper waste disposal – through establishing safe sewage systems, use of screened

latrines, and treatment of night soil.

􀂃 The use of insecticides and other chemicals used to control the vector population.

􀂃 Protective clothing that would prevent vectors from resting in the surface of the body

and inoculate pathogens during their blood meal.

􀂃 Good personal hygiene.

􀂃 Avoidance of unprotected sexual practices.

CLASSIFICATION OF MEDICAL PARASITOLOGY

Parasites of medical importance come under the kingdom called protista and animalia.

Protista includes the microscopic single-celled eukaroytes known as protozoa. In

contrast, helminthes are macroscopic, multicellular worms possessing welldifferentiated

tissues and complex organs belonging to the kingdom animalia. Medical

Parasitology is generally classified into:

• Medical Protozoology - Deals with the study of medically important protozoa.

9

• Medical Helminthology - Deals with the study of helminthes (worms) that affect

man.

• Medical Entomology - Deals with the study of arthropods which cause or transmit

disease to man.

Describing animal parasites follow certain rules of zoological nomenclature and each

phylum may be further subdivided as follows:

Super class Super family

Phylum Subphylum Class Order Family Genus Species

Subclass Subfamily

10

FIGURE 1. CLASSIFICATION OF MEDICALLY IMPORTANT PARASITES

PROTOZOA METAZOA (HELIMINTHS)

Sarcodina (Amoebae): Platyhelminthes:

(a) Genus, Entameba: Trematodea:

E.g. Entameba histolytica (a) Genus Schistosoma

(b) Genus Endolimax E.g. S. mansoni

E.g. Endolimax nana (b) Genus Fasciola

(c) Genus Iodameba E.g. F. hepatica

E.g. Iodameba butchlii Cestoda:

(d) Genus Dientmeba (a) Genus Diphylobotrium

E.g. Dientameba fragilis E.g. D. latum

Mastigophora (Flagellates): (b) Genus Taenia

(a) Genus Giardia E.g. T. saginata

E.g. G. lamblia (c) Genus Echinococcus

(b) Genus Trichomonas E.g. E. granulosus

E.g. T. vaginalis (d) Genus Hymenolepsis

(c) Genus Trypanosoma E.g. H. nana

E.g. T. brucci Nemathelminthes:

(d) Genus Leishmania (a) Intestinal Nematodes

E.g. L. donovani E.g. A. lumbricoides

Sporozoa (b) Somatic Nematodes

(1) Genus Plasmodium E.g. W. bancrofti

E.g. P. falciparum

(2) Genus Toxoplasma

E.g. T. gondi

(3) Genus Cryptosporidum

E.g. C. parvum

(4) Genus Isospora E.g. I. beli

Ciliates

E.g. Balantidium coli

Parasitology

11

GENERAL CHARACTERISTICS OF MEDICALLY IMPORTANT PARASITES

Medically important protozoa, helminthes, and arthropods, which are identified as

causes and propagators of disease have the following general features. These

features also differ among parasites in a specific category.

(1) PROTOZOA

Protozoan parasites consist of a single "cell-like unit" which is morphologically and

functionally complete and can perform all functions of life. They are made up of a

mass of protoplasm differentiated into cytoplasm and nucleoplasm. The cytoplasm

consists of an outer layer of hyaline ectoplasm and an inner voluminous granular

endoplasm. The ectoplasm functions in protection, locomotion, and ingestion of food,

excretion, and respiration. In the cytoplasm there are different vacuoles responsible

for storage of food, digestion and excretion of waste products. The nucleus also

functions in reproduction and maintaining life.

The protozoal parasite possesses the property of being transformed from an active

(trophozoite) to an inactive stage, losing its power of motility and enclosing itself within

a tough wall. The protoplasmic body thus formed is known as a cyst. At this stage the

parasite loses its power to grow and multiply. The cyst is the resistant stage of the

parasite and is also infective to the human host.

Reproduction – the methods of reproduction or multiplication among the parasitic

protozoa are of the following types:

1. Asexual multiplication:

(a) Simple binary fission – in this process, after division of all the structures, the

individual parasite divides either longitudinally or transversely into two more

or less equal parts.

(b) Multiple fission or schizogony – in this process more than two individuals

are produced, e.g. asexual reproduction in Plasmodia.

12

2. Sexual reproduction:

(a) Conjugation – in this process, a temporary union of two individuals occurs

during which time interchange of nuclear material takes place. Later on, the

two individuals separate.

(b) Syngamy – in this process, sexually differentiated cells, called gametes,

unite permanently and a complete fusion of the nuclear material takes

place. The resulting product is then known as a zygote.

Protozoa are divided into four types classified based on their organs of locomotion.

These classifications are: amoebas, ciliates, flagellates, and sporozoans.

TABLE 1. CLASSIFICATION OF THE PATHOGENIC PROTOZOA:

PROTOZOA ORGAN OF IMPORTANT HUMAN

LOCOMOTION PATHOGENS

1. Rhizopoda Pseudopodia Entamoeba histolytica

(Amoeba)

2. Mastigophora Flagella Trypanosomes

(Flagellates) Leishmania

Trichomonas

Giardia

3. Sporozoa None, exhibit a slight Plasmodium.Spp

Amoeboid movement

4. Ciliates Cilia Balantidium coli

13

(2) HELIMINTHS:

The heliminthic parasites are multicellular, bilaterally symmetrical animals having three

germ layers. The helminthes of importance to human beings are divided into three

main groups with the peculiarities of the different categories described in table 2.

TABLE 2. DIFFERENTIATING FEATURES OF HELMINTHES

CESTODE TREMATODE NEMATODE

Shape Tape like, segmented Leaf like, Unsegmented Elongated,

Cylindrical

Sexes Not separate Not separate Separate.

(monoecious) (monoecious) (diecious)

Except blood flukes

which are dioecious

"Head" End Suckers: with hooks Suckers: no hooks No suckers, and

hooks

Alimentary Absent Present Present and

canal but incomplete complete

Body cavity Absent Absent Present

14

(3) ARTHROPODS

Arthropods, which form the largest group of species in the animal kingdom, are

characterized by having a bilaterally symmetrical and segmented body with jointed

appendages. They have a hard exoskeleton, which helps enclose and protect the

muscles and other organs. An open circulatory system, with or without a dorsally

situated heart pumps the blood (hemolymph) via arteries to the various organs and

body tissues. Blood is returned to the heart through body spaces known as

hemocoeles. In addition, respiratory, excretory, and nervous systems are present.

Arthropods affect the health of humans by being either direct agents for disease or

agents for disease transmission.

The arthropods of medical importance are found in Classes Insecta, Arachnida, and

Crustacia which have their own distinguishing features. In Class insecta the body is

divided into head, thorax, and abdomen, with one pair of antennae. Diseases like

malaria, yellow fever, onchocerciasis, and trypanasomiasis are primarily transmitted

by insects.

FIGURE 2. CLASSIFICATION OF ARTHROPODS

Kingdom Animalia

Phylum Arthropoda

Class Class Class Class Class

Crustacia Arachnida Insecta Chilopoda Pentastomida

e.g. Scorpion e.g. Ticks e.g. Mosquito e.g. Centipedes e.g. tongue worms

N.B. Crustacia, Arachnida, and Insecta are the three most common classes of

arthropods of medical significance, which need closer attention

15

SUMMARY

A parasite is an organism which lives in/on the body of a host. A host is that which

harbors the parasite. There is usually some association such as mutualism,

commensalisms, or parasitism between the parasite and the host. This association

may produce a variety of effects and the host usually tends to react to it.

Understanding the various structural and behavioral components of parasites assists

classification. In general, the protozoa, helminthes and arthropods are the most

commonly studied and the most important parasites in medical parasitology. They are

further sub classified considering many parameters.

16

REVIEW QUESTIONS

1. Explain briefly the various types of parasites and hosts.

2. Explain the three types of symbiotic relationships and give examples.

3. Discuss the mechanisms by which parasites impose their effect on the host.

4. Give examples of reactions that occur in the body of the host following parasitic

invasion.

REFERENCES:

1. Robert F. Boyd, Basic medical microbiology, third edition, 1986

2. K.D. Chaterjee, protozoology and helminthology, twelfth edition, 1980

3. Brown, H.W. and Neva. F.A. Basic clinical Parasitology (5th edn) New York:

1982.

4. Zaman, V. scanning electron microscopy of medically important parasites.

Littleton, MA: Johnwright PSG, 1983.

5. Belding, D., Text book of clinical parasitology, 2nd edition, New York, 1952.

17

UNIT TWO

MEDICAL PROTOZOLOGY

LEARNING OBJECTIVES:

At the end of the lesson, the student should be able to:

• Discuss the classification of medically important protozoa.

• Discuss the pathogenesis and clinical aspects of infections.

• Describe the general epidemiological aspects and transmission patterns of diseases

caused by protozoa.

• Identify the methods and procedures of laboratory diagnosis of pathogenic protozoa in

clinical specimens.

• Discuss treatment options for protozoan infections.

• Implement the preventive and control measures of protozoan infection.