Parasitology and Immunology Lecture Notes

Overview of Parasitology and Its Global Importance

• Parasitology is defined as the study of the most common mode of life on Earth. There are more parasitic organisms on the planet than non-parasitic ones.
• The field is centered on animal parasites of humans, domestic animals, and wildlife.
• Parasite Definition: An organism that lives in or on another organism (the host) and either harms the host or lives at the expense of the host.

Factors Increasing the Importance of Parasitology

• Parasite Resistance: Increased resistance to drugs and treatments.
• Insecticide Resistance: Insects often serve as vectors (e.g., mosquitoes).
• Increased Mobility: Popularity in traveling to tropics and subtropics.
• Migration and Geopolitics: Migration of refugees from war-torn areas and military personnel returning from service abroad.
• Environmental Modifications: Changes to the landscape altering ecosystems.
• Climate Change: Disease-causing parasites spreading from tropical regions to more temperate zones.
• Widespread Immunosuppression: Conditions such as AIDS, cancer chemotherapy, and organ transplantation increase vulnerability.
• Environmental Pollutants: The indiscriminate release of toxic chemicals and carcinogens into the environment.

Diverse Examples of Parasitism

• Ophiocordyceps unilateralis: A parasitic fungus that protrudes a stalk from an ant's head (the inspiration for the parasites in the game/show The Last of Us).
• Pocketbook Mussel (Lampsilis ovata and Lampsilis fasciola): Mussels that use mimicry to attract hosts.
• Tongue-eating Isopod (Cymothoa exigua): A parasite that physically replaces the tongue of a fish.
• Common Vampire Bat (Desmodus rotundus): Feeds on the blood of other animals.
• Dodder Vine (Cuscuta sp.): Uses specialized structures called haustoria to obtain nutrients from a host plant.
• Common Cuckoo (Cuculus canorus): Engages in Brood Parasitism, where it is raised by a different species, such as the Reed Warbler (Acrocephalus sp.). They are termed "cuckoo" because their behavior is considered erratic/crazy.
• Hybridization Example: Pelophylax lessonae and Pelophylax ridibundus interacting; male ridibundus with female lessonae producing P. esculentus.
• Candiru or Toothpick Fish (Vandellia cirrhosa): A small parasitic fish known for entering orifices.
• Parasitologist Definition: A quantitative person who seeks truth in strange places; described as a person who sits on one stool while staring at another.

Questions & Discussion

Question: The example of brood parasitism described in lecture involved which of the following organisms?

• A. Common Vampire Bat (Desmodus rotundus)
• B. Candiru or toothpick fish (Vandellia cirrhosa)
• C. Common Cuckoo (Cuculus canorus)
• D. Pocketbook mussel (Lampsilis ovata)
• E. Tongue-eating isopod (Cymothoa exigua)
• Answer: C. Common Cuckoo (Cuculus canorus)

Basic Organismal Interactions and Symbiosis

• Symbiosis ($+\,,\,+$): An interaction among organisms in which one organism lives with, in, or on the body of another.
• Symbionts: The organisms involved in a symbiotic relationship with a host.

Types of Interactions

1. Neutralism ($0\,,\,0$): A lack of benefit or detriment experienced by either member. It describes interactions where the fitness of one species has absolutely no effect on the other. It is theoretical and difficult to prove; noted as Dr. Sharp's least favorite interaction.
2. Competition ($-\,,\,-$): Overlap in ecological niches where populations depend on the same food, shelter, or resources, negatively affecting each other's survival.
   • Examples: Carnivorous animals (like Cape Buffalo/Lion) competing for prey; plants competing for sunlight, water, and pollinators; the bladderwort plant (Utricularia) competing with tiny fish for small crustaceans/insects.
3. Phoresis: From the Greek "to carry." One organism (the phoront) is mechanically carried by a host without physiological dependence.
   • Symbols: No symbols given ($0\,,\,0$ usually implied, but transcript says "NO SYMBOLS").
   • Examples: Bacteria on the legs of a fly; fungal spores on beetle feet; aquatic snail eggs on bird feet; Dermatobia hominis (human botfly) female depositing eggs on a mosquito's surface.
4. Protocooperation ($+\,,\,+$): A mutually beneficial symbiosis that is not physiologically necessary for survival.
   • Examples: Egyptian plover eating residuals from crocodile teeth; cattle egrets removing ectoparasites from bovines; hermit crabs using shells covered by sea anemones; ants and aphids (can sometimes shift toward parasitism) where aphids provide honeydew.
5. Mutualism ($+\,,\,+$): A type of symbiosis where both benefit and the interaction is usually obligatory and physiologically dependent.
   • Examples: Termites and their intestinal protozoan fauna; blood-sucking leeches and their intestinal bacteria; Cleaner Wrasse (Labroides dimidiatus) at reef cleaning stations (without cleaners, the reef dies); Lichen (microscopic green algae/cyanobacteria combined with filamentous fungi).
6. Commensalism ($+\,,\,0$): Meaning "eating at the same table"; the symbiont (commensal) benefits while the host is unaffected.
   • Examples: Pilot fish and remoras; barnacles on a whale; epiphytes (ferns, bromeliads) on a tree; remoras on whale sharks using a transformed dorsal fin (sucking disk) for a free ride.
7. Amensalism ($-\,,\,0$): An asymmetrical interaction where one organism negatively affects another but is not affected itself.
   • Examples: Bread mold Penicillium producing penicillin to kill bacteria; Black Walnut tree (Juglans nigra) producing juglone, which is toxic to other plants.
8. Predation ($+\,,\,-$): An interaction where a predator kills its prey outright and does not subsist on it while it is alive (e.g., lion and impala).
9. Parasitism ($+\,,\,-$): A symbiosis where the symbiont benefits at the expense of the host (e.g., human infected with Guinea worm, Dracunculus medinensis).

Comparing Predation and Parasitism

Describing and Categorizing Parasites

Macroparasites versus Microparasites

• Macroparasites: Large parasites that do not multiply in the host of interest (e.g., helminths, arthropods). They reproduce, but the population inside the host does not increase through those offspring. They have stable populations and cause endemic diseases.
• Microparasites: Small parasites that multiply within the host (e.g., fungi, protists, bacteria, viruses). They induce lasting immunity, have unstable populations, and cause epidemic diseases.
• Terminology:
  • Endemic: "In or within people"; disease native to a particular area.
  • Epidemic: "Above or on people"; rapid spreading of diseases.

Specific Classifications

• Ectoparasite: Lives on the outer surface (e.g., lamprey).
• Endoparasite: Lives inside the host (e.g., nematodes).
• Obligate Parasite: Cannot complete its life cycle without a parasitic phase (most parasites).
• Facultative Parasite: Normally free-living but can become parasitic accidentally (e.g., Naegleria fowleri, Micronema deletrix).
• Accidental (Incidental) Parasite: Found in a host other than its normal host; neither is well-adapted to the other (e.g., Toxocara canis in humans causing visceral larval migrans).
• Permanent Parasite: Lives its entire adult life on/in a host (e.g., Trichinella spiralis).
• Temporary (Intermittent) Parasite / Micropredator: Contacts the host only to feed and then leaves (e.g., mosquitoes, bed bugs, fleas).
• Parasitoid: A parasite early in development that ultimately kills the host at completion (e.g., Pseudacteon phorid flies; accounts for approximately $10\%$ of described insect species).
• Hyperparasitism: An organism that is a parasite of another parasite (e.g., Plasmodium spp. inside an Anopheles mosquito; tapeworm juvenile in a flea).
• Colozoic: Parasite living in the lumen of a hollow organ (e.g., intestine).
• Histozoic: Parasite living within the tissues (e.g., liver).
• Vector: Any agent (water, wind, or insect host) that transmits a disease organism.

Host Categories and Life Cycles

Types of Hosts

• Definitive Host: Where the parasite achieves sexual maturity. If no sexual reproduction exists, the host most important to humans or the human itself is defined as definitive.
• Intermediate Host: Where the parasite develops but does not reach sexual maturity.
• Paratenic (Transport) Host: The parasite survives here without undergoing further development.
• Reservoir Host: A host (often wildlife like wolves or coyotes) that sustains the parasite when not infecting humans.
• Accidental Host: Infected by chance; not the normal host.

Life Cycles

• Urban (Domestic) Cycle: Occurs within human environments and involves domesticated animals.
• Sylvatic Cycle: Exists normally in the wild and involves wildlife.
• Example: Dioctophyme renale (Giant Kidney Worm):
  • Eggs embryonate in water.
  • Intermediate host: Earthworms.
  • Paratenic host: Fish/Frogs.
  • Definitive host: Canids (wolves/coyotes serve as reservoirs).

Host-Parasite Coevolution

Key Evolutionary Terms

• Population: Group of individuals of a single species in a given area.
• Mutation: Permanent change in DNA (sequence, position, loss/duplication, insertion). Mutation provides the raw material for selection.
• Selective (Adaptive) Value: Relative reproductive success (fitness) of an allele or genotype.
• Preadaptation: A trait that predisposes an organism for survival in a new environment before it is encountered (e.g., dinosaur feathers for insulation, fish larynx for gulping air).
• Adaptation: Evolutionary process leading to better suitability to a habitat; can be anatomical, physiological, or behavioral.
• Selection: Forces causing differential survival and reproduction (Natural Selection). A reciprocal selection pressure exists where hosts select for better parasites and parasites select for better hosts.
• Evolution: Genetic changes in populations over time leading to heritable differences; occurs only when gene frequency changes.

Interests of Evolution

• Parasite Interests: Adaptations to encounter the host and survive within the host's immune system.
• Host Interests: Adaptations to avoid encounters and eliminate the parasite if infected.
• Fitness: Measured by reproductive outcome, not necessarily the amount of damage caused.

Red Queen Hypothesis

• Proposed by Leigh Van Valen ($1973$).
• Metaphor from Through the Looking Glass: "It takes all the running you can do, to keep in the same place."
• Species must evolve constantly to survive (stay extant) because the competition/environment is also evolving.
• Arms Race: As a parasite acquires a new antigen, the host develops a new antibody. Parasites often have the upper hand due to shorter generation times.

Adaptations for Parasite Transmission

A. Reproductive

• High reproductive potential (producing more eggs/sperm than free-living relatives).
• Asexual reproduction.
• Hermaphroditism and self-fertilization.

B. Behavioral

• Adaptive advantages in movement (e.g., Schistosoma mansoni cercariae).
• Altering host behavior (e.g., Dicrocoelium species involving snail, ant, and cow).

C. Morphological

• Size increases (larger than free-living relatives).
• Attachment organs: Suckers, hooks, spines, penetration organs, and cysts.
• Loss (regression) of structures: Reduction in sense organs, locomotion, and digestion.

D. Biochemical

• Reduced or modified catabolic pathways.
• Elaborate transport mechanisms for nutrient uptake (monomers like glucose/amino acids).
• Reduced synthetic capacities.

E. Immunological

1. Absorption of Host Antigen: Schistosoma spp.
2. Antigenic Variation: Trypanosoma brucei (VATS), Giardia.
3. Occupying Immunologically Privileged Sites: Leishmania spp., Trypanosoma cruzi, Plasmodium spp.
4. Disruption of Host Immune Response: Entamoeba histolytica.
5. Molecular Mimicry: Trypanosoma cruzi.
6. Loss or Masking of Surface Antigens: Trypanosoma brucei, Entamoeba histolytica, Ancylostoma caninum.

F. Life Cycle Adaptations

• Reduction of the free-living phase to avoid environmental variability.
• Responding to chemical stimuli from hosts.
• Changing the behavior of intermediate hosts to facilitate ingestion by the final host.
• Infection of multiple hosts (Secondary/Tertiary):
  • Increases reproductive potential through asexual stages (e.g., Digeneans).
  • Increases range in space and time.
  • Provides survival during periods of host scarcity.
  • Channels the parasite toward the definitive host via the food chain.
• Example: Life cycle of Prosthogonimus macrorchis (big testes):
  • Definitive host: Chicken oviduct.
  • 1st Intermediate host: Snail (miracidium enters eggs, which hatch in the snail).
  • 2nd Intermediate host: Dragonfly (parasite enters through the anus; metacercaria is the infectious stage).

Taxonomy and Systematics

• Descriptive Parasitology: Non-hypothesis driven study focusing on the discovery of new species, morphology, life cycles, and population surveys.
• Identification: There are over $3500$ species of mosquitoes, but only about $40$ regularly infect humans.
• Binomial Nomenclature: System by Carolus Linnaeus ($1707-1778$). Includes Genus (capitalized) and species epithet (uncapitalized), both italicized.
• Hierarchical Classification: Taxonomic categories move from inclusive to exclusive (Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species).

Disciplines

• Taxonomy: Study of scientific classification, ordering, and naming.
• Systematics: Study of classification and diversity within an evolutionary context; seeks to understand the origin of diversity.
• Systematists Tasks: Describe in detail, name species, preserve collections, create identification keys, investigate evolutionary histories, and consider environmental adaptations.

Phylogenetic Systematics (Cladistics)

• Method to infer evolutionary history (Phylogeny), pioneered by Willi Hennig ($1913-1976$).
• Assumptions:
  1. Characteristics change in lineages over time.
  2. Groups are related by descent from a common ancestor (supported by DNA/glycolysis).
  3. Bifurcating/branching pattern of cladogenesis (splits into exactly two groups).

Cladistic Terminology

• Clade: An ancestor and all its descendants.
• Ingroup: Taxon of interest.
• Outgroup: Related taxon chosen for comparison (more basal).
• Node: Internal branch point/speciation event.
• Sistergroup: Two descendants splitting from the same node.
• Homologous Character: Similar due to shared ancestry.
• Analogous Character: Similar function but different origin.
• Homoplasy: Similarity NOT due to homology (convergence/reversal).
• Polesiomorphic: Ancestral characters (found in both ingroup and outgroup; uninformative).
• Autapomorphic: Derived character found in only one taxon (uninformative novelty).
• Synapomorphy: Shared derived character that sets a taxon apart (informative; e.g., feathers are an apomorphy for birds).

Group Types

• Monophyletic: Includes hypothetical ancestor and ALL descendants (e.g., Mammals).
• Paraphyletic: Includes ancestor but NOT all descendants (e.g., Reptilia excluding birds).
• Polyphyletic: Group that does not share a most recent common ancestor; defined by homoplasies (e.g., wings of birds, bats, and butterflies).

Immunology: The Body's Defense System

• Immunity: Resistance arising from tissues capable of recognizing nonself invaders.
• Innate Immunity: Non-specific; born with it; no prior exposure needed.
• Adaptive (Acquired) Immunity: Specific; requires time to develop; faster on second exposure.
• Passive Immunity: Receiving antibodies (e.g., from mother to baby).

The First Line of Defense: Skin and Mucous Membranes (Nonspecific)

• Skin: Oil and sweat glands ($pH\,4-5.5$); sweat contains lysozyme; epidermis is $10-30$ cells thick; dermis is $15-40$ times thicker.
• Digestive Tract: Saliva (lysozyme); Stomach ($pH\,1.5-3.5$); intestine enzymes; expelling via vomiting/diarrhea.
• Respiratory Tract: Mucus in bronchi; ciliary sweeping; coughing/sneezing.
• Urogenital Tract: Vaginal secretions ($pH\,3.8-4.5$); acidic urine washes out pathogens.

The Second Line of Defense: Phagocytes and Chemical Signals (Nonspecific)

• Key Players: Neutrophils, monocytes, and macrophages. They recognize PAMPs (Pathogen-Associated Molecular Patterns).
• Macrophages: Monocytes mature into macrophages at infection sites; professional antigen-presenting cells (APC) for helper T cells ($T_H$); utilize a respiratory burst; express Fc receptors for IgG.
• Neutrophils: Most abundant circulating leukocytes ($50-70\%$); first to show at damage; phagocytose; express Fc receptors for IgE.
• Eosinophils: ($2-5\%$ of leukocytes); critical for eliminating parasites; degranulation releases enzymes/toxins near parasites; high count associated with parasitic infection/allergies; express Fc receptors for IgE.
• Basophils: Least numerous ($0.5\%$); release histamine for inflammation; express Fc receptors for IgE.
• Natural Killer (NK) Cells: Induce apoptosis (cell suicide) via perforins (pores) and granzymes (activate caspase enzymes); lack T-cell receptors.
• Mast Cells: Located under mucosal surfaces; release histamine; crucial for allergic and inflammatory responses; express Fc receptors for IgE.

Inflammation and Hypersensitivity

• Acute Inflammation: Immediate response.
  • Histamine: Dilates local blood vessels (red/warm).
  • Bradykinins: Increases capillary permeability (edema/swelling).
  • Prostaglandins: Pressure on nerves (pain).
  • Cytokines (IL-1, TNF): Act on hypothalamus to cause fever.
• Delayed Type Hypersensitivity (DTH): Cell-mediated ($T_H1$ cells); takes $24$ or more hours (e.g., poison ivy, tuberculin skin test).
• Immediate Hypersensitivity: Antibody-mediated (IgE); rapid degranulation of mast cells/basophils; basis of allergies.
• Anaphylaxis: Systemic immediate hypersensitivity; widespread inflammatory response; fatal drop in blood pressure and airway blockage.
• Chronic Inflammation: Long-term; includes Fibrosis (scarring/collagen), Granulomas (nodules around persistent antigens), Abscess (pus/pressure), and Ulcers (open areas of inflammation).