parasitology

Introduction to Medical Parasitology

Learning Objectives

At the end of this section, the student is expected to:

  • Define terms related to medical parasitology.

  • Identify features of parasites.

  • Determine sources of infection.

  • Explain modes of transmission.

  • Understand routes of transmission.

  • Explain general life cycles of parasites.

  • Determine general characteristics of parasites.

  • Explain immune responses against parasitic infections.

  • Know the taxonomy and nomenclature of parasites.

Definitions

Parasitology

  • Definition: Parasitology is the science that deals with parasites.

  • Parasite: A living organism that takes nourishment and other needs from a host.

  • Host: An organism that supports the parasite.

  • Medical Parasitology: (from Greek: Para = beside, Sitos = food) The study of the parasites of humans and their medical consequences including:

    • Biological features of human parasites.

    • The relationship between humans and parasites.

    • Prevention and treatment of parasitic diseases.

Scope of Medical Parasitology

  • Broad Definition of Parasitology: Includes viruses, bacteria, fungi, protozoa, and metazoa (multicellular organisms that infect their host species). For historical reasons, viruses, bacteria, and fungi have been traditionally incorporated into the field of microbiology.

Categories of Parasites Mentioned

  • Helminths: Parasitic worms.

  • Protozoa: Single-celled animals, ranging in size from 2-100 µm.

  • Arthropods: Animals that feed on human blood and tissue fluids, categorized as ectoparasites (e.g. lice, mites).

  • Parasitology defines organisms that live on or within another organism (the host).

Human Parasitology

  • Classes of Human Parasites:

    • Medical Helminthology: Includes classes such as Nematoda, Trematoda, Cestoda, Metacanthocephala.

    • Medical Protozoology: Includes Phylum Sarcomastigophora (Amoeba, Flagellates), Apicomplexa, Microsporidia, and Ciliophora.

    • Medical Arthropodology: Includes Class Insecta, Arachnida, Crustacea, and Chilopoda.

Importance of Parasitology

  • Identification: Understanding different kinds of parasites and their associated diseases.

  • Pathogenesis: Knowledge of how parasites interact with and affect their hosts.

  • Vaccine Development: Contributions towards creating new vaccines and drugs against parasites.

  • Diagnosis and Detection: Studying methods for diagnosing and detecting parasitic infections.

  • Treatment and Prevention: Offering insights for treating and preventing parasitic diseases.

  • Control Measures: Understanding how to implement control measures against parasites.

Host–Parasite Relationship

Symbiosis

  • Definition: Any association between two different organisms, in which one partner lives in or on another's body, interacting in some manner.

  • Each organism in symbiosis is termed a symbiont.

  • Types of symbiotic relationships include:

    • Mutualism: Both partners benefit and are metabolically dependent on one another.

    • Commensalism: One partner benefits while the other is neither helped nor harmed (e.g. E. coli in humans).

    • Parasitism: One partner benefits at the expense of the other (e.g. Ascaris lumbricoides and humans).

Types of Parasites

I. Habitat

  1. Endoparasite: Lives inside the body of its host (e.g. tapeworms, flukes, protozoa).

  2. Ectoparasite: Lives on the external surface of its host (e.g. fleas, ticks).

II. Dependency on the Host

  1. Obligate Parasite: Entirely dependent on a host for survival.

  2. Facultative Parasite: Can live independently or parasitically in another organism.

III. Lifecycle Duration

  1. Permanent Parasite: Lives its entire adult life on/in a host.

  2. Temporary Parasite: Only spends a short time on a host.

IV. Pathogenicity

  • Pathogenic Parasite: Has potential to cause disease.

  • Non-Pathogenic: Does not cause disease under normal circumstances.

  • Opportunistic Parasite: Causes disease when the host's immune system is weakened.

V. Lifecycle Complexity

  1. Monoxenous Parasites: Require only one host to complete their life cycle (e.g. Ascaris lumbricoides).

  2. Heteroxenous Parasites: Require two or more hosts to complete their life cycle (e.g. Plasmodium species).

VI. Host Range

  1. Euryxenous Parasites: Broader host range.

  2. Stenoxenous Parasites: Narrow host range.

Other Types of Parasites

Aberrant Parasite

  • Found in unusual locations within the host (e.g. Ascaris larvae migrating to the brain).

Incidental Parasite

  • Occurs in hosts where it typically does not occur (e.g., Fasciola in human liver).

Hosts and Types of Hosts

  • Definition of Host: An organism that harbors and provides sustenance for another organism (the parasite).

Types of Hosts

  1. Definitive Host: Harbors the adult or sexual stage of the parasite.

  2. Intermediate Host: Harbors the larval or asexual stage (e.g., Fasciola's immature stages in snails).

  3. Accidental/Incidental Host: Harbors parasites that do not usually infect them, potentially causing harm (e.g., parasite in the 'wrong' host).

  4. Reservoir Host: Carries a parasite that can infect humans.

  5. Carrier Host: Carries parasites and can transmit them without necessarily showing symptoms (e.g. E. histolytica).

Vector and Types of Vectors

Definition of Vector

  • An organism (usually an arthropod) that transfers infective forms of a parasite from one host to another.

Classification of Vectors

  1. Biological Vectors: Development or multiplication of the parasite occurs within the vector (e.g., Plasmodium in Anopheles mosquitoes).

    • Types include:

      • Propagative: Multiplication without developmental change.

      • Cyclodevelopmental: Developmental change without multiplication.

      • Cyclopropagative: Both multiplication and developmental change.

  2. Mechanical Vectors: Transmits parasite without being essential to its life cycle; no development occurs (e.g., housefly).

Other Terminologies

Infective Stage

  • Stage of the parasite that can invade the human body and sustain life (e.g. embryonated egg of Ascaris).

Infective Route

  • Specific entry point the parasite uses to invade (e.g., skin penetration by hookworms).

Infective Mode

  • The means by which the parasite enters (e.g. cercariae penetrating skin).

Geohelminthes

  • Helminths that complete their life cycle without requiring development in intermediate hosts.

Biohelminthes

  • Helminths that must develop in intermediate hosts to complete their life cycle.

Heterogenetic Parasites

  • Parasites that alternate between sexual and asexual reproduction during their life cycles (e.g., coccidial parasites).

Trophozoite

  • The active, motile feeding stage in the life cycle of apicomplexan parasites (e.g., Giardia spp.).

Cyst

  • A protective stage in the life cycle of some protozoan parasites; usually the infective stage.

Infestation

  • Establishment of arthropods upon or within a host.

Zoonosis

  • Diseases transmittable from animals to humans.

Biological Incubation (Prepatent) Period

  • Time between initial infection and detection of the parasite.

Clinical Incubation Period

  • Interval between exposure and first symptoms.

Autoinfection

  • An infected individual acts as a source of hyperinfection to themselves.

Superinfection (Hyperinfection)

  • Reinfection with the same parasite.

Retroinfection

  • Retrograde infection caused by newly hatched larvae reaching prior infection sites.

Epidemiology of Parasites

  • Distribution: Parasites are globally distributed; most are found in tropical regions.

  • Factors influencing prevalence include:

    • Environmental conditions.

    • Socioeconomic factors (poverty, sanitation, hygiene).

    • Availability of suitable hosts for parasites.

    • Parasite escape mechanisms from hosts.

    • Economic and social conditions that support parasite proliferation.

Transmission of Parasites

Key Factors

  • Route of transmission.

  • Sources of infection.

Sources of Infection

  1. Contaminated soil: Exposure often due to geohelminth infections.

  2. Contaminated water: May contain various infective stages (e.g., blood flukes, cysts of amoebas).

  3. Raw or insufficiently cooked meat: Can transmit multiple parasites (e.g. Trichinella spiralis).

  4. Blood-sucking arthropods: (e.g., female Anopheles mosquito for malaria).

  5. Animals: Wild/domestic animals that carry parasites.

  6. Humans: Clothes, bedding, and environments contaminated by infected individuals.

Modes of Transmission

Direct Transmission

Horizontal Transmission:

  • Through methods such as:

    • Feco-oral route (common for intestinal parasites).

    • Sexual intercourse.

    • Blood transfusion.

    • Direct skin penetration.

Vertical Transmission:

  • Transmission from mother to child via:

    • Congenital (transplacental).

    • Transmammary (via breast milk).

Indirect Transmission

  • Involves a complex life cycle requiring biological vectors or intermediate hosts.

Routes of Transmission

  • Accidental Ingestion of infective eggs/larvae from contaminated sources (e.g., food, water, or unwashed hands).

  • Skin Penetration by larvae (e.g. hookworms).

  • Insect Bites: Passes infectious material (e.g. Filarial worms).

  • Inhalation: Contaminated air containing infective stages (e.g., E. vermicularis).

General Life Cycles of Parasites

  • Describe developmental cycles of parasites, which may include various stages of host and environmental interactions.

  • Phases of Life Cycle:

    • Growth and maturation.

    • Reproduction (both sexual and asexual).

    • Transmission.

  • All phases are crucial for the parasite's survival.

Simple or Direct Life Cycle

  • Also termed monoxenous; requires a single host. Parasite remains mostly within this host, reproducing successfully.

Indirect Life Cycle

  • Known as heteroxenous; requires two or more hosts, often involving multiple developmental stages.

Infections & Diseases

  • Not all parasitic infections lead to clinically significant diseases. Infection levels correlate directly to the quantity of infective stages encountered.

  • Disease causation involves both parasite and host factors.

Parasitic Infections & Disease

Parasite Factors:

  • Strain type, host adaptation, load, occupied sites, and metabolic byproducts.

Host Factors:

  • Include genetics (e.g. resistance to certain parasites), age, sex, immunity, nutrition, infection intensity, and co-existing conditions.

Pathogenesis and Pathology

  • Parasitic infections often asymptomatic. Pathology emerges from parasite size, activity, metabolism, immune responses, and inflammatory responses.

Pathogenesis Mechanisms

  • Nutrient Competition: e.g. D. latum consumes vitamin B-12.

  • Tissue Destruction: Injuries upon entry or after establishment (e.g. intestinal lesions from some worms).

  • Tissue Changes: Including hyperplasia, hypertrophy, and metaplasia influenced by the parasite (e.g. liver fluke enlargement of bile duct).

Host Immunity & Immune Evasion

Host Responses

  • Complexity stems from:

    • Structure, metabolism, life cycle of the parasite.

    • Evasion strategies employed by parasites.

  • Types of Immunity:

    • Innate (non-specific) Immunity.

    • Acquired (Specific) Immunity.

Innate Immunity

  1. Skin: Protective barrier against penetration.

  2. Body Secretions: Intestinal secretions can wash away luminal parasites.

  3. Serum Factors: High-density lipoproteins may kill certain parasites.

  4. Erythrocyte Factors: Hemoglobin S and Duffy antigen influence susceptibility to malaria.

Immune Recognition

  • First detection of parasites by immune cells (macrophages, dendritic cells, neutrophils) using pattern-recognition receptors.

  • Activation of an immune response follows.

Phagocytosis

  • Macrophages engulf and kill parasites via phagocytosis, employing reactive oxygen species and enzymes.

Activated Immune Cell Signaling

  • Cytokine release by activated immune cells enhances other immune responses and promotes inflammation.

Acquired Immunity

  • Initiated by antigen presenting cells upon parasite entry into the body.

T-Cell Activation

  1. CD4+ Helper T Cells: Activate macrophages and B cells, coordinating the immune response.

  2. CD8+ Cytotoxic T Cells: Identify and kill infected host cells expressing parasite antigens.

Role of Immune Cells

  • NK Cells: Attack infected cells directly or through antibody interaction, releasing cytolytic agents.

  • B Cells: Differentiate into plasma cells that produce antibodies, directly kill parasites, activate complement, and improve phagocytosis.

  • Eosinophils: Essential against helminthic infections; levels of eosinophils vary with the invasiveness of helminths.

Immune Evasion by Parasites

  • Helminths survive host defenses by mechanisms such as:

    • Large size and motility.

    • Acquisition of host molecules to disguise themselves.

    • Changes in membrane properties which enhance resistance to immune attacks.

  • Release of substances to depress immune function or digest antibodies.

Taxonomy and Nomenclature of Parasites

  • Parasites fall under the animal kingdom and eukaryotes, classified into:

    • Protozoa: Unicellular organisms.

    • Metazoa: Multicellular organisms (including helminths and arthropods).

  • Protozoan Classification: Based on morphology and locomotion, consisting of around 45,000 species, with most human-affecting species belonging to the phylums Sarcomastigophora and Apicomplexa.

  • Metazoan Classification: Includes worms (helminths) and arthropods.

Nomenclature of Parasites

  • Common vs. Scientific Name: Parasites are named using binomial nomenclature, comprising:

    • Genus (capitalized).

    • Species (not capitalized).

  • Example: Ascaris lumbricoides (scientific name is either underlined or italicized).