Comprehensive Study Notes: Parasite Ecology and Introduction to Parasitology

Course Information and Administration

Instructor: Dr. Spencer Mukai.
Course Details: Parasitology SC/BIOL 4360.300 - Winter 2026.
Location/Time: LSB 105; Mondays 7:00-10:00 PM.
Office Locations: 213 LB (Keele) or 344 YH (Glendon).
Student Hours: Mondays 5:00-6:00 PM (In-person) or by appointment via Zoom.
Sessional Dates: * First day of class: Jan. 5, 2026. * Last day of class: April 6, 2026. * Drop deadline: March 9, 2026. * Course withdrawal period: March 10 - April 6, 2026. * Winter Term exam period: April 8–24, 2026.
Required Text: Parasitology: A Conceptual Approach by Eric S. Loker and Bruce V. Hofkin (1st ed 2015 or 2nd ed 2022). Garland Science.

Evaluation Breakdown

Weekly Quizzes: $10\%$ (Due 1 week after posting).
Parasite ID Test 1: $10\%$ (Feb. 9).
Midterm Exam: $25\%$ (Feb. 23) (Delivered in-class).
Graphic Research Summary & Peer-Review: $15\%$ * Submission due: March 20. * Three peer evaluations due: March 27.
Parasite ID Test 2: $10\%$ (March 30).
Final Exam: $30\%$ (April 8–24, 2026).

Evolutionary and Ecological Rationale for Studying Parasitology

Ubiquity of Parasitism: It is estimated that over $60\%$ of species on Earth have parasitic lifestyles.
Impact on Humans: Humans are known hosts for over one hundred species of parasites living internally or externally.
Scientific Discipline: Parasitology is an interdisciplinary and integrative discipline covering life histories (structure, function, systematics, taxonomy, development, behavior, ecology, and control), immunology, cell/molecular biology, and physiology.
Global Health: Parasites are significant pathogens for humans, animals, and plants.
Evolutionary Drivers: Parasites drive host evolution and ecosystem dynamics.

Human Parasite Habitats and Specific Examples

Eye: Onchocerca volvulus, Acanthamoeba, Loa loa.
Face: Demodex folliculorum, Demodex brevis.
Mouth: Trichomonas tenax, Entamoeba gingivalis.
Brain: Toxoplasma gondii, Naegleria fowleri, Echinococcus granulosus.
Tongue: Trichinella spiralis.
Lungs: Paragonimus westermani, Echinococcus granulosus.
Liver: Fasciola hepatica, Leishmania donovani, Toxoplasma gondii, Echinococcus granulosus, Plasmodium sp., Clonorchis sinensis.
Small Intestine: Giardia lamblia, Ascaris lumbricoides, Necator americanus, Ancylostoma duodenale, Taenia solium, Taenia saginata, Diphyllobothrium latum.
Large Intestine/Cecum: Schistosoma mansoni, Trichuris trichiura, Enterobius vermicularis, Entamoeba histolytica.
Urogenital System: Wuchereria bancrofti, Trichomonas vaginalis, Schistosoma haematobium.
Skeletal Muscle: Trichinella spiralis, Echinococcus granulosus.
Circulatory System: Plasmodium sp., Trypanosoma sp., Schistosoma sp..
Skin: Pediculus humanus humanus, Leishmania sp., Dermatobia hominis, Onchocerca volvulus.

Global Burden of Parasitic Infection (Statistical Overview)

Helminths (Parasitic Worms): Total infections approx. $4.46 \times 10^9$ . * Ascaris (Nematoda): $1221 \times 10^6$ infections; $60,000$ deaths/yr. * Hookworms (Nematoda): $740 \times 10^6$ infections; $65,000$ deaths/yr. * Trichuris (Nematoda): $795 \times 10^6$ infections; $10,000$ deaths/yr. * Filarial worms (Nematoda): $657 \times 10^6$ infections; $50,000$ deaths/yr. * Schistosomes (Platyhelminthes): $200 \times 10^6$ infections; $200,000$ deaths/yr.
Protozoa: * Plasmodium: $219 \times 10^6$ infections; $700,000$ deaths/yr. * Entamoeba: $50 \times 10^6$ infections; $40,000$ deaths/yr.

Symbiotic Relationships and Definitions

Symbiosis: [Gk, "living together"] Any organism spending part or all of its life in close association with another living organism of a different species, regardless of benefit or harm.
Commensalism: ["Eating at the same table"] Unidirectional benefit where one partner benefits and the other is neither helped nor harmed. No physiological dependency or trophic relationship exists between partners. * Phoresy: ["To carry"] A mechanical transport relationship where a phoront (smaller) is carried by a host. Can be facultative (cattle egrets) or obligatory (barnacles on whales). * Inquilinism: An organism (inquiline) lives within the nest or burrow of another species. * Metabiosis: One organism depends on another for the indirect preparation of a suitable environment (e.g., oak gall wasps).
Mutualism: Both organisms depend on each other physiologically; both benefit. Usually obligatory (e.g., termites and gut microbes).
Parasitism: A relationship where the parasite is physiologically dependent on the host, causing harm or living at the host's expense. Typically, parasites are smaller than hosts and show host specificity.

Classification of Parasites by Type and Lifestyle

Obligate Parasites: Cannot survive if isolated from the host.
Facultative Parasites: Free-living organisms capable of becoming parasitic in favorable conditions (e.g., Naegleria fowleri, Halocephalobus gingivalis). * Naegleria fowleri: Causes primary amoebic meningoencephalitis (PAM); fatality rate >95\%.
Microparasites: Microscopic (bacteria, viruses, protozoa).
Macroparasites: Large enough for the naked eye (worms, arthropods).
Endoparasites: Live within the host (intestines, vessels, liver).
Ectoparasites: Attached to the outer surface (lice, mites, ticks).
Parasitic Castrators: Divert host resources to parasite growth, stopping host reproduction; can cause "gigantism" (e.g., trematodes in snails).
"Body Snatchers": Parasites that alter host behavior for trophic transmission (e.g., Toxoplasma gondii in mice, Leucochloridium paradoxum in snails).
Hyperparasitism: A parasite of another parasite.
Brood Parasitism: Laying eggs in the nest of another bird (e.g., cuckoo and reed warbler).
Cleptoparasitism: Stealing resources gathered by another animal.

Epidemiological Terms and Disease Measurement

Infective Stage: Life cycle stage capable of entering a host and continuing development.
Pathogenicity: Genetic ability of an organism to cause damage/disease.
Virulence: Degree of pathology caused; often correlated with multiplication rates.
Morbidity: Condition of being diseased/sick; incidence of sickness in a population.
Mortality: Incidence of death.
Control: Reduction to a locally acceptable level; requires continued effort.
Elimination: Reduction to zero in a specific geographical area.
Eradication: Permanent worldwide reduction to zero (e.g., no further intervention needed).
Disability-Adjusted Life Years (DALY): A measure of disease burden calculated as: $DALY = \text{Years of Life Lost (YLL)} + \text{Years Lived with Disability (YLD)}$ * One DALY represents the loss of one year of healthy life.
Economic Impact Examples: * Animal Trypanosomiasis ("Nagana") in Africa: >1.3 \times 10^9\,USD annual losses. * Lymphatic Filariasis in Southern India: $811 \times 10^6\,USD$ annual productivity loss.

Life Cycle Strategies and Direct/Indirect Transmission

Direct Transmission (Monoxenous): Requires no other organism to move between hosts (e.g., Ascaris lumbricoides, Giardia).
Indirect Transmission (Heteroxenous): Requires intermediate hosts or vectors (e.g., Schistosoma sp., Trypanosoma cruzi). * Diheteroxenous: Two hosts (Final host and one Intermediate host). * Triheteroxenous: Three hosts (Final, First Intermediate, and Second Intermediate).
Host Roles: * Definitive (Final) Host: Where the parasite reaches sexual maturity and undergoes sexual reproduction. * Intermediate Host: Required for development but sexual maturity is not reached. * Paratenic (Transfer) Host: Not required for completion of life cycle but serves as a temporary refuge/trophic bridge. * Reservoir Host: A non-human host in which the parasite normally dwells and remains a source of infection (can be Sylvatic/wild or Domestic).

Vectors: Mechanical vs. Biological

Vector: Actively transports a parasite to a vertebrate host.
Mechanical Vector: Transports the parasite with no development or multiplication within the vector (e.g., house fly Musca domestica carrying Entamoeba cysts).
Biological Vector: Parasite undergoes development or multiplication. * Cyclodevelopmental Transmission: Development occurs, but no increase in numbers (e.g., Wuchereia bancrofti in mosquitoes). * Propagative Transmission: Multiplication occurs, but no developmental progression (e.g., Dengue virus). * Cyclopropagative Transmission: Both development and multiplication occur (e.g., Plasmodium).

Evolutionary Adaptations to Parasitism

Morphological: * Sacculinization: Reduction/loss of sensory and locomotory organs in intestinal forms. * High SA:VOL Ratio: Dorsoventrally flattened helminths for nutrient absorption. * Attachment Organs: Suckers (acetabula), hooks, pincers, adhesive discs (e.g., Giardia).
Biochemical: * Metabolism: Shift to anaerobic respiration (facultative or obligatory) in low-oxygen environments. * Unique Organelles: * Hydrogenosomes: Release hydrogen as a byproduct of anaerobic ATP production. * Mitosomes: Involved in Fe-S protein metabolism; do not produce ATP. * Glycosomes: Contain glycolytic enzymes. * Metabolic Gaps: Schistosoma cannot synthesize sterols; many Platyhelminthes lack enzymes for purine synthesis. * Detoxification: Specialized heme detoxification (hemozoin production) in hematophagous parasites.
Immunological: * Antigenic Variation: Changing surface proteins (Trypanosoma brucei). * Immunological Privilege: Residing in macrophages (Leishmania) or erythrocytes (Plasmodium). * Immunomodulation: Suppressing host response via cytokines (IL-10, TGF- $\beta$ ).
Life Cycle/Reproductive: * Polyembryony: Development of a single zygote into multiple offspring (e.g., Schistosoma sporocysts). * Hypobiosis: Developmental arrest in response to environment (e.g., Ancylostoma duodenale L3).

Taxonomy and Species Concepts in Parasitology

Biological Species Concept (BSC): Focuses on interbreeding and reproductive isolation. Limitation: many parasites are asexual (e.g., Entamoeba) or involve cryptic gamete exchange (Trypanosoma).
Morphological Species Concept (MSC): Focuses on structural features. Limitation: Phenotypic plasticity and cryptic species.
Phylogenetic Species Concept (PSC): Smallest diagnosable group sharing common ancestry. Uses synapomorphies (shared derived traits) and molecular tools.
Horizontal (Lateral) Gene Transfer (HGT/LGT): Acquisition of genes from different species. (e.g., nematodes acquiring cellulase from plant microbes). Wolbachia bacteria provide vital growth genes to filarial nematodes.

Evolutionary Histories of Human Parasites

Lice Transfers: * Pediculus humanus (Head/Body louse): An "heirloom parasite" inherited from human-chimp common ancestors approx. 5–6 million years ago (mya). * Pthirus pubis (Pubic louse): A "souvenir parasite" acquired via host switch from gorillas to humans approx. 3–4 mya.
Schistosoma mansoni Biogeography: Native to Africa; introduced to the New World (South America) in the 1600s via the trans-Atlantic slave trade. Hybridization of S. haematobium x S. bovis in Corsica (Cavu River) allowed the parasite to infect local Bulinus snails previously resistant, showing higher pathogenicity.

Pathological Categories of Infection

Damage to Cells/Organs: Mechanical obstruction (e.g., Ascaris blocking intestines), tissue penetration, or cell death.
Alteration of Cell Growth Patterns: * Hypertrophy: Increase in cell size (e.g., T. cruzi in cardiomyocytes). * Hyperplasia: Increase in cell number (e.g., Fasciola hepatica causing bile duct proliferation). * Metaplasia: Conversion of cell type; usually reversible (e.g., S. haematobium in bladder epithelium). * Neoplasia: Abnormal proliferation/tumors (e.g., Opisthorchis viverrini and cholangiocarcinoma).
Nutrient Interference: Diverting vitamins (B12 deficiency from Diphyllobothrium latum) or fat malabsorption (Giardia).
Immunopathology: Harm caused by the immune response (e.g., granulomas around Schistosoma eggs causing liver fibrosis; cerebral malaria from cytokine storms and vessel occlusion).

Intestinal Physiology and Diarrhea Types

Osmotic Diarrhea: Malabsorption of solutes (sodium, lactose) or damage to villus tips leads to water retention in the lumen.
Inflammatory Diarrhea: Cytokine release stimulates ion secretion or alters paracellular permeability via Tight Junction (TJ) disruption.
Secretory Diarrhea: Overstimulation of crypt cells (Cl- secretion) via toxins or cytokines.
Exudative Diarrhea: Loss of blood and pus due to large-scale epithelial death/sloughing.
Motility Diarrhea: Increased peristalsis moving feces too quickly for absorption.
Intestinal Barrier Layers: 1. Mucosa: Inner layer containing enterocytes and mucins (intrinsic/extrinsic barrier). 2. Submucosa: Connective tissue with blood/lymph vessels. 3. Muscularis Externa: Smooth muscle (circular and longitudinal). 4. Serosa: Outer lubricant layer.

Detailed Parasite Study: Entamoeba histolytica

Lineage: Amoebozoa. Anaerobic; possesses mitosomes rather than mitochondria.
Life Cycle: Fecal-oral ("5 F's"). Trophozoite (feeding/replicating) and Cyst (resistant, chitinous wall, 4 nuclei).
Pathogenesis Factors: * Adherence: Mediated by Gal/GalNAc lectin binding to host galactose residues. Disruption of TJs follows. * Cytolytic Activity: Production of amoebapores that perforate host membranes. * Trogocytosis: "Cell nibbling" where the amoeba ingests host cell fragments. It may acquire host MHC I proteins to mask itself from the immune system. * Cysteine Proteases: CP5 is highly expressed in Eh compared to non-pathogenic E. dispar.
Clinical Outcomes: * Amoebic Colitis: Flask-shaped ulcers in the large intestine. * Hepatic Amoebiasis: Most common extraintestinal site; results in "anchovy sauce" liver abscesses. $80\%$ of cases occur in males (testosterone-related cytokine inhibition). * Pulmonary/Brain Abscesses: Rare, secondary to liver abscess rupture or hematogenous spread.