Introduction to Parasitology & Host-Parasite Interactions

Parasite

An organism that feeds or lives off a host, typically harming the host.

Protozoa

Single-celled eukaryotic microorganisms.

Helminths

Parasitic worms.

Arthropods in Parasitology

Invertebrates that are usually ectoparasites or vectors for other pathogens.

Binomial Nomenclature Formatting

Genus (capitalized) + specific epithet (lowercase); formatted in italics or underlined.

Reasons to Study Parasitology

Provides unique biological phenomena; essential for medical, veterinary, and economic fields.

Common Parasitic Diseases in the US

Pinworms, giardiasis (beaver fever), cutaneous larval migrans, and tick-borne diseases.

Prevalence of Parasitism

100% of organisms can be hosts; 50% to 80% of all organisms are parasites.

Symbiosis

A close relationship where two different species live together.

Parasite vs. Predator: Size & Population Growth

Parasites are smaller than hosts and grow faster; predators are larger than prey.

Micropredator (Intermittent Parasite)

An organism (e.g., mosquito, tick) that feeds on host tissue intermittently.

Parasitoid

An organism whose immature stage feeds inside a host, usually killing it.

Ectoparasite

A parasite that lives on the outer body surface of its host.

Endoparasite

A parasite that lives inside the body of its host.

Obligate Parasite

A parasite that completely depends on a host to survive.

Facultative Parasite

A normally free-living organism that can become parasitic, often causing fatal infection.

Accidental (Incidental) Parasite

A parasite residing in an unnatural host species, usually failing to survive.

Permanent Parasite

A parasite that lives its entire adult life on or within a host.

Temporary Parasite

A parasite that visits hosts only briefly during a specific life cycle stage.

Zoonotic Parasite

A parasite that normally infects animals but can also infect humans.

Definitive Host (D-Host)

The host in which a parasite reaches sexual maturity.

Intermediate Host (I-Host)

A required host in which parasite development or asexual reproduction occurs.

Vector

An organism that actively transmits a parasite from one host to another.

Paratenic Host

A host where the parasite survives without undergoing further development.

Reservoir Host

An animal host harboring an infection that can be transmitted to humans.

Monoxenous (Direct) Life Cycle

A life cycle completed within a single host species.

Same infective in - same infective out

Heteroxenous (Indirect) Life Cycle

A life cycle requiring two or more different host species to complete.

One stage in - second stage out

Enterobius vermicularis Life Cycle

Monoxenous life cycle where the infective stage is an egg.

Fasciola hepatica Hosts

Heteroxenous: D-host is livestock, 1st I-host is snail, 2nd I-host is water plants.

Coelozoic Parasite

A parasite residing in body cavities or organ lumens.

Histozoic Parasite

A parasite residing within tissues, such as muscle or organ walls.

Host Microhabitats

Distinct physiological regions within a host organ defined by pH, enzymes, and surface structures.

Niche Partitioning in Parasites

Spatial separation of parasite species inside a host due to current or past competition.

Infrapopulation

All individuals of one parasite species in a single host individual.

Intensity

The number of parasites of a given species residing in a single infected host.

Metapopulation

All infrapopulations of a parasite species within a single host population.

Mean Intensity Formula

Total number of parasites in a host population divided by the number of infected hosts.

Suprapopulation

All parasites of a species across all its host species in an ecosystem.

Trophic Transmission

Parasite transmission occurring when a predator ingests an infected prey host.

Red Queen Hypothesis

Species must constantly evolve to survive alongside co-evolving opposing species.

Virulence

The degree or severity of disease caused by a pathogen or parasite.

Virulence Evolution Hypothesis 1

Parasites evolve toward lower virulence because host death reduces parasite fitness.

Virulence Evolution Hypothesis 2

Parasites evolve an optimal virulence that balances host survival with maximal transmission.

Subclinical Infection

An infection without visible signs or symptoms.

Factors Determining Infection vs. Disease

Parasite burden, host immune response, host genetics, and parasite pathogenicity.

Primary Cause of Parasitic Disease Pathology

Host defense mechanisms, particularly the immunoinflammatory response.

Pathology vs. Symptoms

Pathology is physical/cellular damage; symptoms are reported or observable clinical signs.

Malaria: Pathology vs. Symptoms

Pathology: anemia, blocked capillaries, hypoxia. Symptoms: cyclical severe chills, high fever, fatigue.

Dog Heartworm: Pathology vs. Symptoms

Pathology: heart disruption, vessel inflammation, enlarged heart. Symptoms: dry cough, lethargy, dyspnea.

Epidemiology

The study of ecological aspects of disease, including distribution, transmission, prevalence, and incidence.

Prevalence Formula

(Number of infected hosts / Total number of hosts sampled) × 100.

Incidence

The number of new parasitic infections per unit of time.

Abundance Formula

Total number of parasites in a host population divided by total hosts sampled.

Overdispersed Parasite Distribution

Distribution pattern where most hosts harbor few/no parasites, while a few hosts harbor most (variance/mean > 1).

Three Categories of Parasite Control

Reducing transmission, anti-parasitic drugs, and vaccines.

Prophylactic vs. Therapeutic Anti-parasitic Drugs

Prophylactic drugs prevent infection; therapeutic drugs treat existing infection.

One Health Concept

Interdisciplinary approach recognizing that human, animal, and environmental health are interconnected.

Protozoans

Single-celled eukaryotic microorganisms without formal taxonomic status as a single group.

Phyla of Parasitic Protozoa

Euglenozoa, Metamonada, Apicomplexa, and the supergroup Amoebozoa.

Hemoflagellates

Flagellated protozoans that reside in the blood during at least one stage of their life cycle.

Trypanosomatidae Morphological Features

Kinetosome (basal body), kinetoplast (mitochondrial DNA), pellicular tubules, flagellar pocket, and undulating membrane.

Promastigote Morphology

Elongated form with a anterior flagellum, central nucleus, and no undulating membrane.

Epimastigote Morphology

Flagellum emerges mid-body, creating a partial undulating membrane; kinetoplast is near the nucleus.

Trypomastigote Morphology

Posterior kinetoplast with full-length undulating membrane ending in a free anterior flagellum.

Amastigote Morphology

Spherical, non-motile intracellular form with an internal flagellum.

Trypanosoma brucei brucei

Subspecies infecting livestock (causing Nagana) but non-pathogenic to humans.

Trypanosoma brucei gambiense

Causes chronic African Sleeping Sickness in humans in West and Central Africa.

Trypanosoma brucei rhodesiense

Causes acute African Sleeping Sickness in humans in East Africa.

Vector for African Trypanosomiasis

Tsetse flies (Glossina species).

Stumpy Trypomastigote

Non-replicating bloodstream stage of T. brucei infective to the tsetse fly vector.

Winterbottom's Sign

Swollen lymph nodes in the posterior cervical triangle, characteristic of African Sleeping Sickness.

Antigenic Variation in Trypanosomes

Periodic switching of Variant-Specific Glycoproteins (VSGs) on the cell surface to evade host antibodies.

Trypanosoma cruzi

Etiological agent of American Trypanosomiasis (Chagas disease).

Trypanosoma cruzi Hosts

Definitive: humans; Vector: kissing bugs (Triatominae); Reservoirs: armadillos, opossums, dogs.

Trypanosoma cruzi Transmission Mode

Stercorarian transmission: infectious trypomastigotes in bug feces are scratched into the bite wound.

Romana's Sign

Unilateral palpebral edema associated with acute Chagas disease infection at the conjunctiva.

Chronic Chagas Disease Pathology

Severe cardiomyopathy (heart enlargement) and megaviscera (megaesophagus and megacolon).

Xenodiagnosis

Diagnostic method where uninfected vector bugs feed on a patient and their feces are later examined for parasites.

Chagas Disease Control Measures

Vector control using residual insecticides, improved housing (tin roofs, sealed walls), and blood donor screening.

Chagas Disease in Dogs

High prevalence in working/outdoor dogs; causes fever, swollen lymph nodes, and potential heart failure.

Leishmania Vector

Female sand flies (Phlebotomus and Lutzomyia spp.).

Leishmania Intracellular Stage

Amastigotes multiplying inside human macrophage cells.

Cutaneous Leishmaniasis

Localized skin ulcers caused by amastigotes multiplying in reticuloendothelial cells of the skin.

Mucocutaneous Leishmaniasis (Espundia)

Caused by L. braziliensis; secondary mucosal destruction of nasal and oral cavities.

Visceral Leishmaniasis (Kala-azar)

Severe systemic infection targeting visceral organs (liver, spleen, bone marrow), causing hepatosplenomegaly.

Post-Kala-azar Dermal Leishmanoid

Dermal macular or nodular lesions appearing after incomplete treatment of visceral leishmaniasis.

Phylum Metamonada General Characteristics

Flagellated protozoans lacking mitochondria and dictyosomes (Golgi bodies), possessing multiple flagella.

Giardia duodenalis Synonyms

Giardia lamblia and Giardia intestinalis.

Giardia duodenalis Microhabitat

Small intestine of humans and other mammals.

Giardia duodenalis Transmission Mode

Fecal-oral route via ingestion of infective cysts in contaminated water or food.

Giardia duodenalis Trophozoite Morphology

13 µm × 7 µm, binucleate (2 nuclei), 8 flagella, central endosomes, ventral adhesive disk, and 2 median bodies.

Giardia duodenalis Cyst Morphology

8-12 µm long, oval, containing 2 to 4 nuclei, median bodies, and a distinct cyst wall clearing.

Giardia duodenalis Excystation & Colonization

Alkaline duodenum stimulates excystation; quadrinucleate cyst yields 2 binucleate trophozoites that adhere via ventral disk.

Giardiasis Pathogenesis & Symptoms

Villi inflammation/atrophy impairs absorption; causes foul flatulence, fatty stools (steatorrhea), abdominal cramps, and diarrhea.

Giardia duodenalis Diagnosis Methods

Cysts in formed stool, trophozoites in diarrheal stool, fluorescent antibody staining, ELISA (antigen detection), or PCR.

Giardia Cyst Water Treatment & Control

Filtration below 7 µm, boiling (>55°C), or iodine tablets; routine chlorination is often insufficient.

Trichomonas vaginalis Microhabitat & Life Stage

Human urogenital tract; exists solely as a trophozoite (no cyst stage produced).

Trichomonas vaginalis Trophozoite Structure

Single nucleus, axostyle (microtubular rod), undulating membrane, multiple flagella, and no mitochondria.

Trichomonas vaginalis Transmission

Direct transmission via sexual intercourse; most common non-viral sexually transmitted infection.

Trichomoniasis Clinical Manifestations

Men usually asymptomatic; women experience vaginal itching, burning, inflammation, and excessive frothy mucus discharge.