Introduction To Parasitology
Introduction to Parasitology
Parasitology to a lay man is the study of parasites. A parasite is an organism that lives in or on a host causing harm to the host. Also, parasites are metabolically dependent on their hosts. Good knowledge on the biology of a parasite, transmission cycles, and human behaviors that promote transmission is necessary to eliminate or reduce infection.
The course, therefore, introduces students to the biology of common parasites in the helminth category in tropical countries and some other countries of the world. To facilitate learning, diagrams on their morphologies and life cycles are included in the guide. For coherence, epidemiology and control are treated simultaneously.
The contents of this course, therefore, will serve as the basis for studying parasites.
Information on the diagnosis of parasites will equip those willing to practice clinical parasitology with practical knowledge.
The Course
This course Guide tells you briefly what to expect from reading this material which bothers on the study of parasite “Parasitology.”
The association ‘Parasitism’ is a harmful association in favor of the parasite which likely evolved from a non-harmful association called commensalism. The location
and the behavior of parasites has enabled us to place them into different groups.
The parasite can not survive without a host organism. Therefore the study of the host in parasite transmission is equally important.
Parasites can be grouped into those with zoological, veterinary, and medical importance. Those invading livestock (veterinary parasites) and humans (human parasites) are touched in this Course Guide. Generally, parasitic helminths are
grouped into trematodes, cestodes, and nematodes. The digeneans are the only groups with medical importance in the class Trematoda, while nematodes form the majority of the helminthic infection. The entire course focuses on the following; lifecycle,
morphology, diagnosis, epidemiology, and control of the parasite.
Life cycle:
This involves the routes of transmission and parasite development from the immature to the adult stage. Some helminths have direct lifecycles involving no intermediate host. Examples in this category are Ascaris lumbricoides, hookworms, and Enterobius vermicularis. On the other hand, some show long lifecycles, which
could either be homogenous or heterogeneous. Homogenous indirect life cycle utilizes a single intermediate host. e.g., Schistosoma spp that make use of specific
snail intermediate hosts for intramolluscan development. In heterogenous long lifecycle, two or more intermediate hosts are utilized in the transmission cycle. e.g.
Diphyllobothrium latum that makes use of cyclops and fish as the first and second intermediate hosts respectively.
Morphology:
This is gross anatomy. Each helminth group has generalized features. The morphologies of the adults are often different from the immature stages. The adult trematodes are often leaf-like with two suckers (the oral and ventral suckers).
The reproductive system is often hermaphroditic. The cestode body is divided into head, neck, and strobila. The integuments are metabolic in function. The distinctive
morphological features of either the adult or the larval form are often used in the diagnosis of helminth infection.
Diagnosis:
The methods used in the detection of the parasite in the host is dependent on various factors. The site of infection of the parasite, morphological indifferences in some closely related species, the cost-effectiveness of different diagnostic methods and
introduction of new technologies and proficiency of laboratory technologists to manage these new technologies.
Parasites that inhabit the intestine will best be diagnosed through stool examination .e.g. Ascaris lumbricoides. In infection due to Paragonimus spp whose adult flukes reside in the lungs, diagnosis is made through sputum examination. Usually, the determination of parasitic helminths is made by looking out for the striking feature(s) of the eggs laid by the adult parasites. However, when the
eggs of two or more closely related species are morphologically indistinguishable, such as those of the two Hookworm species (Necator americanus and Ancylostoma duodenal) and the hexacanth eggs of Taenia spp., the adult parasites could be used.
The conventional method diagnosis (the use of a microscope) can sometimes show limitation, especially in the detection of parasites with very low intensity. Therefore,
more sensitive methods like Enzyme-Linked Immunosorbent Assay (ELISA) and parasite gene amplification using Polymerase Chain Reaction (PCR) are some of the
modern technologies currently available. Nevertheless, these methods are underused in Nigeria due to the cost and lack of technological know-how.
Epidemiology and control
– Epidemiology deals with the study of the cause of diseases and factors favoring transmission. In epidemiology, a good understanding of the transmission cycle is necessary. This will enable one to target control at stage(s) which will break the transmission cycle of the parasite.
The killing of the snail intermediate hosts of Schistosomiasis will lead to a significant reduction in
disease occurrence. In this case, the intramolluscan development, which gives rise to the infective stage ‘cercaria’ will not occur. Epidemiology also offers knowledge of
the behaviors that predispose people to infection.
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