MEDICAL PARASITOLOGY AND ENTOMOLOGY
STUDY NOTES
LECTURE 1: INTRODUCTION TO MEDICAL PARASITISM
Objectives
Upon completion of this lecture you will learn various terms and phrases associated with medical parasitology and different types of parasites affecting man, and, where possible, the vectors of those parasites. You will also be in a position to identify and describe the life cycles of the parasites and their vectors. In addition, you will be able to design methods of surveillance and control of the parasites and their vectors. Detailed topics, subtopics and objectives for each lecture are listed in the course outline on pages 3 and 4 above.
1.1 Parasite – an organism which lives at the expense of another larger organism, its host. The parasite is physiologically dependent on the host, has a higher reproductive capacity than the host and may kill it in heavy infections. Medical parasitism can be defined as a state in which an organism is metabolically dependent to a greater or lesser extent on another organism, the host, in which the parasite causes some harmful effect.
1.2 ASSOCIATION BETWEEN PARASITE AND HOST
•A parasite is a living organism, which takes its nourishment and other needs from a host; and may cause harm to the host
•The host is an organism which supports the parasite.
•The parasites included in medical parasitology are:
Protozoa, Helminthes, and some Arthropods.
•The hosts vary depending on how they harbor the various stages in parasitic development.
1.3 COMMON RELATIONSHIPS
•Symbiont - Any organism that spends a portion or all of its life cycle intimately associated with another organism of a different species is considered a symbiont (symbiote) and this relationship is called symbiosis (symbiotic relationships).
•Mutualism - is an association in which both partners are metabolically dependent upon each other where none of the partners suffers any harm from the association.
•Commensalism - an association in which the commensal takes benefit without causing any detrimental or beneficial effects on the host, (e.g. most of the normal flora in the human body).
•Parasitism - an association where one of the partners (the parasite) lives at the expense of the other (the host) causing a harmful effect, e.g. hookworm.
1.4 DIFFERENT KINDS OF PARASITES
•Accidental parasite – one that attacks an unnatural host and survives, e.g. Hymenolepis diminuta, a rat tape worm, in man.
•Erratic Parasite – one that wanders into an organ where it is not usually found, e.g. Entamoeba histolytica in lung or liver of the host.
•Ectoparasite –a parasitic organism that lives on the outer surface or in the superficial tissues of its host, e.g. lice, ticks, mites, jiggers etc.
•Endoparasites –parasites that live inside the body of their host, e.g. Schistosomes.
•Obligate Parasite -This parasite is completely dependent on the host during part or all of its life cycle, e.g. Plasmodium spp.
•Facultative parasite – an organism which may employ either a free-living or a parasitic mode of life, e.g. Strongyloides.
•Aberrant parasite – one which is never transmitted from man to man and which develops abnormally in man (multilocular hydatid, Angiostrongilus, Toxocara)
•Zoonotic parasite (Zoonosis) Parasites or diseases that are transmitted between man and other vertabrates, e.g. Echinococcosis (hydatid disease).
1.5 DIFFERENT KINDS OF HOSTS
•Definitive host – a host that harbors a parasite in the adult stage or where the parasite goes through sexual method of reproduction.
•Intermediate host – a host that harbors the larval stage 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 parasite life cycle (crabs and prawns in Angiostrongylus cantonensis).
•Reservoir host – a host that harbors the parasite and makes the parasite available for 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.
1.6 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 on the host
•Mechanical injury - may be inflicted by a parasite by means of pressure as it grows larger, e.g. Hydatid cyst which causes blockage of ducts such as blood vessels, producing infarction.
•Deleterious effect of toxic substances - in Plasmodium falciparum production of toxic substances may cause rigors and other symptoms.
•Deprivation of nutrients, fluids and metabolites - parasites 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 Schistosomes.
2. BASIC CONCEPTS IN MEDICAL PARASITOLOGY
2.1 In medical parasitology, each of the medically important parasites is discussed under the standard subheadings of:
A) Morphology - e.g. size, shape, color and position of different organelles in different parasites at various stages of their development. This is especially important in laboratory diagnosis.
B) Geographical distribution – areas where certain diseases occur or are prevalent. 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:
•The presence and food habits of a suitable host;
•Host specificity, for example, Ancylostoma duodenale requires man as a host whereas Ancylostoma caninum requires a dog.
•Easy escape of part of the parasite life cycle from the host for continuation of the life cycle – the different developmental stages of a parasite which are released from the body along with faeces and urine