Medical Parasitology Introduction

Medical Parasitology

• Medical Parasitology is the branch of medical sciences that deals with organisms (parasites) that live temporarily or permanently on or within the human body (host).
• The host-parasite relationship is a competition for supremacy, where the host may have the upper hand and remain healthy or lose the competition, leading to disease development.

Types of Parasites

• Human parasites can be either unicellular (protozoa) or multicellular (helminths and arthropods).
• Parasites can live inside the host (endoparasites) or on the host surface (ectoparasites).
• Endoparasites are classified into intestinal, atrial, or those that inhabit body tissues, causing serious health problems.
• Ectoparasites are arthropods that either cause diseases or act as vectors transmitting other parasites.

Host-Parasite Relationship

• Human evolution and parasitic infections have gone side by side, with most parasitic diseases and methods of transmission discovered thousands of years ago.
• Environmental changes, human behavior, and population movement have a significant impact on transmission, distribution, prevalence, and incidence of parasitic diseases in a community.
• Parasites can invade the human body through various routes, including the oral route, skin, arthropod vectors, and sexual contact.

Host Defense Mechanisms

• Host defense mechanisms consist of innate immunity, which mediates initial protection against infection, and adaptive immunity, which is more effective.
• Once parasites have evaded innate host defenses, adaptive cellular and humoral immune responses are promoted against a wide array of antigenic constituents.

Diagnosis and Control of Parasitic Diseases

• Diagnosis of parasitic diseases depends on several laboratory methods, imaging techniques, and endoscopy, in addition to clinical picture and geographic location.
• Parasitic diseases may be presented by a wide variety of clinical manifestations according to the tissue invaded.
• Direct microscopy is based on detection of the parasite by examination of different specimens (stool, urine, blood, CSF, and tissue biopsies).
• Immunodiagnostic techniques include antigen and antibody-detection assays.
• Molecular-based diagnostic approaches offer great sensitivity and specificity.
• Control and prevention of parasitic diseases depend on the interactions among many factors, including the environment, human behavior, and socio-cultural influences.

Classification of Parasites

• The classification of parasites is controversial, with no universally accepted system.
• Parasites form part of the animal kingdom, which comprises about 800,000 identified species categorized into 33 phyla.
• The most acceptable taxonomic classification of human parasites includes endoparasites and ectoparasites.
• Endoparasites are sub-classified into helminthic parasites (multicellular organisms) and protozoan parasites (unicellular organisms).

Endoparasites

• Most parasites of humans live inside the host (endo- means internal).
• Endoparasites include helminthes (worms of various types), protozoa, or sometimes larval stages of arthropods (insects, mites, etc.).
• Both helminthic and protozoan parasites can infect different tissues and organs of the human body.

Ectoparasites

• Human ectoparasites live on the host (ecto- means outside of).
• Ectoparasites include fleas, lice, mosquitoes, bugs, mites, ticks, etc.
• In general, ectoparasites attach to the skin to feed and do not remain on the host for their entire lives.

Life Cycles of Parasites

• Life cycles of parasites may be simple or complex.
• Parasites that are characterized by a simple or direct life cycle have only one host and are described as monoxenous.
• Parasites with more complex life cycles involving multiple hosts are described as having indirect or heteroxenous life cycles.