Chapter 1

Introduction to Parasitology
How should you study this course?

Start with learning objectives and try to keep these in mind as you go through the course. Learn your glossary.
You should research any issues that you are unsure about. Look in your textbooks, access the on-line resources indicated at the end of the chapters and discuss with your peers and instructor.
Questions and puzzles from book will be assigned homework. Some questions will be from instructor.


Learning Outcomes
After completing this course, you should be able to:
Discuss how important parasites can be classified
State the meaning of commonly-used terms
Describe how parasitic infections affect communities and knowledge of their life cycles.
Discuss the epidemiology, basic life cycle, clinical presentation, management and control of some important parasitic infections

Key definitions: What is ….?
Medical parasitology: “the study and medical implications of parasites that infect humans”
A parasite: “a living organism that acquires some of its basic nutritional requirements through its intimate contact with another living organism”. Parasites may be unicellular or complex multicellular.
Eukaryote: a cell with a well-defined chromosome in a membrane-bound nucleus. All parasitic organisms are eukaryotes
Protozoa: unicellular organisms, e.g. Plasmodium
Metazoa: multicellular organisms, e.g. helminths (worms) and arthropods (ticks, lice)
An endoparasite: “a parasite that lives within another living organism” – e.g. malaria, Giardia
An ectoparasite: “a parasite that lives on the external surface of another living organism” – e.g. lice, ticks

Key definitions: What is ….?
Host: “the organism in, or on, which the parasite lives and causes harm”
Definitive host: “the organism in which the adult or sexually mature stage of the parasite lives”
Intermediate host: “the organism in which the parasite lives during a period of its development only”
Zoonosis: “a parasitic disease in which an animal is normally the host - but which also infects man”
Vector: “a living carrier (e.g.an arthropod) that transports a pathogenic organism from an infected to a non-infected host”.

Key definitions Parasitology
Parasitology covers many aspects of parasites of humans and their hosts.
morphology
biochemistry
physiology and life cycles of parasites
immunological, pathological and clinical responses of the host to the presence of parasites,
treatment and control

Parasitology
Parasites of human parasites and hosts but also, domestic animals (livestock and pet animals) as well as wildlife animals are considered.
Parasitology studies genesis and development of parasitoses in the host. Parasitology also studies taxonomy and systematics of parasites, and living needs of parasites in environment and in animal host.
Diagnosis and prevention of parasitoses are designed using observations and methods

Parasitology
Protozoology focuses on important protozoans
Helminthology focuses on important helminth parasites
Entomology focuses on important Arachnids and Insects

Important features of the host-parasite relationship. 
Parasitism always involves two species, the parasite and the host.
Many of these parasitic associations produce pathological changes in hosts that may result in disease.
Successful treatment and control of parasitic diseases requires not only comprehensive information  about the parasite itself but also a good understanding of the nature of parasites' interactions with their hosts.
The parasite is always the beneficiary and the host is always the provider in any host-parasite relationship.

Classification
The classification of parasites is controversial - there is no universally accepted system
Parasites form part of the animal kingdom which comprises some 800,000 identified species categorised into 33 phyla (but it is estimated that there may be ~10m species in total)
The parasitic organisms that are of importance for human health are eukaryotes - they have a well defined chromosome in a nuclear membrane (as opposed to prokaryotes which have no nuclear membrane, e.g. bacteria)

Sub-Kingdoms
Parasites are classified into 2 sub-kingdoms: protozoa (unicellular) and metazoa (multicellular)
Protozoan (unicellular) parasites are classified according to morphology and means of locomotion. Most species that cause human disease belong to the phylums sarcomastigophora and apicomplexa
Metazoa (multicellular) include the worms (helminths) and arthropoda (posses an external skeleton) e.g. ticks, lice
Note that the genus starts with a capital letter and the species is always written in italics, e.g. Plasmodium falciparum, Giardia lamblia

Taxonomic classification of protozoa
Taxonomic classification of helminths
Parasitism
The protozoa, arthropods and helminthes are traditionally defined as parasites.
However, there are members of the scientific community who designate all infectious agents of animals as parasites including viruses, bacteria and fungi.
This broader definition of parasites includes viruses, bacteria and fungi as well as the arthropods, helminthes and protozoa.
micro parasites and macro parasites.

Parasites are an extremely varied group. They range from:
Round Worms (Nematodes)
Tapeworms (Cestodes)
Flukes (Trematodes/Digena)
Flies, mosquitoes, fleas, lice and ticks
Protozoa such as Giardia.
The table below illustrates some of these parasites. 

Characteristics
Unique life cycles with a set of well-defined and recognizable stages unique to each group.
The multicellular nematodes, for example, have life cycles which include adult males and females. They reproduce sexually and eggs are laid by the female. Larvae develop, in these eggs, hatch and progress through a series of developing larval stages until they reach adulthood as sexually mature males and females and the cycle begins again.

The Importance of Life Cycles 
Life cycles of parasites may provide two important pieces of information.
Information that has predictive value with respect to the pathogenicity of each particular parasite. Information of epidemiological significance that assists to develop effective control programs.

Importance of Life Cycles
How does a host acquire an infection? does the host become infected by direct ingestion of an infective stage? by ingestion of an intermediate host or transport host containing  infective stages? by skin penetration of an infective stage?
by direct contact with an infectehost ?via the bite of a vector?


Importance of Life Cycles
What is the predilection site of the parasite is in its definitive host.
The predilection site is the place in the host where adult males and females are found.
In those parasites without recognizable male and female forms, it's the site(s) where sexual and/or asexual reproduction occurs. 
A related third question asks how the parasite reaches the predilections site and migration through the host body.

Importance of Life Cycles
How a parasite leaves its definitive host to return either to the environment or to its intermediate host.  
What are the identity of the two life cycle stages; the (infective) stage entering the host and the stage leaving the host? the latter is often called the diagnostic stage if it is valuable in diagnosing parasitic infections?

Epidemiology
Although parasitic infections occur globally, the majority occur in tropical regions, where there is poverty, poor sanitation and personal hygiene
Often entire communities may be infected with multiple, different organisms which remain untreated because treatment is neither accessible nor affordable
Effective prevention and control requires "mass intervention strategies” and intense community education. Examples include:

General improved sanitation: pit latrines, fresh water wells, piped water
Vector control: insecticide impregnated bed nets, spraying of houses with residual insecticides, drainage, landfill
Mass screening and drug administration programs which may need to be repeated at regular intervals

Diagnostic methods
A number various methods are used to identify and to diagnose parasites in animals:
Coprological examination
Flotation
Sedimentation
McMaster method
Baermann method
Hematology
Bloodsmear and Giemsa-stain
Diff-Quick
QBC
Skin scraping

Diagnostic methods
Immunological methods
Indirect immunofluorescence
ELISA
Immunoblotting (Western blot)
Complement fixation test
Molecular biological methods
PCR