Parasitology Quiz Basics

Parasitology

• Parasitology is the study of parasite-host relationships
• A parasite is an organism that lives in or on another organism (host)
• Parasites typically benefit from the interaction while harming the host
• Parasites live within the host's body and obtain nourishment from it
• Parasites can cause damage to the host
• Many infections, infestations, and diseases are caused by parasites
• Parasites present difficulties in developing effective treatments

Disease Terminology

• Epidemiology: The study of how diseases occur, spread, and are distributed in populations
• Endemic: A disease native to a certain region
• Epidemic: A disease spreading rapidly to many individuals at the same time
• Pandemic: A widespread epidemic affecting large geographic areas, possibly global
• Chronic: A disease that develops slowly and lasts a long time (potentially lifelong)
• Etiology: The factors involved in causing a disease
• Pathogenesis: The development pattern of a disease
• Idiopathic: A disease with unknown causes
• Communicable: A disease that can be transmitted from person to person
• Incubation: The latent period of an infection before symptoms appear

Important Questions in Parasitology

• What effects do parasites have on the host's internal systems?
• What effects do parasites have on host tissues?
• How do parasites alter organ structure and function?
• How does the host's immune system respond to a parasite?

Challenges to the Control of Parasite Infections

• Resistance to antiparasitics and antibiotics
• Emigration
• Vacation travel
• Construction in new habitats
• Climate change
• Diseases predisposing to infection
• Treatments predisposing to infection

Parasite Prevalence

• Many parasites cause major public health problems globally, especially in tropical regions with poor sanitation.
• Recent estimates of global parasite prevalence:
  • Ascaris: 1.5 billion
  • Hookworms: 1.3 billion
  • Whipworms: 1 billion
  • Filarial worms: 657 million
  • Malaria: 270 million
  • Schistosomes: 500 million
  • Amebiasis: 50 million
  • Taeniad tapeworms: 50 million
  • Clonorchis: 20 million
  • Chagas' Disease: 15 million

Types of Parasites

• Obligatory Parasites: Unable to survive outside of their hosts
• Facultative/Opportunistic Parasites: Can exist independently of hosts
• Endoparasites: Live inside the body (e.g., hookworms)
• Ectoparasites: Live on or in the skin or surface of the body (e.g., some mites)

Types of Parasitic Hosts

• Accidental/Incidental Host: A host that isn't the normal host for a parasite
• Definitive Host: Hosts where the adult and/or sexual stage of a parasite occurs
• Intermediate Host: Hosts where the larval or asexual stage of a parasite occurs
• Reservoir Host: Hosts harboring parasites that can also infect humans
• Carrier: A host without symptoms but capable of transmitting the parasite

Modes of Parasite Transmission

• Arthropod Vectors: Via bites (primary mode); blood meals
• Consumption of Contaminated Food/ Water & Hand-to-Mouth Transmission
• Direct Transmission: Via parasite drilling through skin, for example through bare feet
• Sexual Transmission: Through body fluids
• Mouth-to-Mouth Contact: Via kissing
• Eye Contact: With contaminated swimming water

Factors that Contribute to Parasite Infestations

• Poor hygiene (includes increased population density, for example prisons)
• Poor sanitation/lack of indoor plumbing (especially in developing countries)
• Compromised/marginal water sources
• Poor public health practices
• Environmental factors/changes affecting parasite life cycles and breeding
• Global travel & immigration & tourism
• Immunocompromised individuals, for example, children in daycare, AIDS patients

Parasite-Host Relationships

• Symbiosis: The general association of organisms of different species
  • Commensalism: Beneficial to one organism, neutral to the other
  • Mutualism: Beneficial to both organisms
  • Parasitism: Beneficial to one organism, detrimental to the other
• Some commensals may be inaccurately classified as parasites (example Entamoeba coli)

Adaptations to Parasitism

• Result of preadaptation changes allowing survival in harsh environments.
• Loss of locomotor structures in obligate parasites
• Loss of metabolic activities making the parasite dependent on the host for products.

Characteristics of Parasites

• Can adapt morphologically to their host environment
• Can protect themselves from the host's immune system (masking antigenicity).

Adaptations to Parasitism (Continued)

• Increased reproductive capacity, especially during infective (often larval) stages
• Host entry mechanisms (e.g., enzymatic digestion, specialized burrowing structures)
• Survival mechanisms within the host (e.g., encystation to evade the immune system).

Effects of the Parasite on the Host

• Host enzyme systems may be compromised by parasite secretions.
• Tissue damage from parasite overpopulation
• Obstruction of intestines or blood vessels from parasite overpopulation
• Host tissue damage/perforation by parasite structures (e.g., suckers, hooks).
• Deprivation of essential nutrients by the parasite

Effects of the Host on the Parasite

• Host genetic background can influence parasite infection (e.g., sickle cell trait linked to resistance to malaria).
• Host nutritional status can affect parasite survival (e.g., high protein diets unfavorable for some intestinal parasites, while high carbohydrate diets are favorable to some tapeworms).

Acquired Immunity

• Can occur (e.g., in Leishmania infections) but less common than for bacterial or viral infections
• No human vaccines exist yet
• However, premunition (resistance to re-infection) protects against hyperinfection (especially in endemic areas).

Parasites and the Compromised Host

• Nutrition influences infection
• Surgery, transfusions, intubation, and prolonged hospitalizations may increase infection risk
• Benefits and harms of specific treatments need to be weighed (e.g., aggressive leukemia treatment linked to toxoplasma; corticosteroid use potentially linked to amebic colitis)

Important Groups of Animal Parasites

• Single-celled protozoa (e.g., flagellates, amebae, malarial organisms)
• Multicellular worms (e.g., flukes, tapeworms, roundworms)
• Arthropods (e.g., insects, arachnids, can be ectoparasites or disease carriers)

Main Targets of Parasitic Infestations

• Gastrointestinal and urogenital tracts
• Blood and tissue
• Liver, lung, and other major organs
• Other locations (e.g, eyes)

Symptoms of Parasitic Infestations

• Diarrhea
• Abdominal pain and cramping
• Fever
• Chills
• Elephantiasis
• Anemia
• Bowel obstruction
• Skin lesions
• Blindness
• Enlargement of major organs

Prevention of Parasitic Infestations

• Interrupt the transmission cycle
  • Education
  • Insecticides
  • Protective clothing
  • Water treatment
  • Personal hygiene
  • Sanitation practices
  • Safe food handling
  • Safe sex practices

Stages of the Parasitic Life Cycle

• Mode of transmission
• Infective stage
• Diagnostic stage

Generic Parasitic Life Cycle

• Parasites emerging from water, food, soil
• Parasites ingested (potentially directly or through an intermediate)
• Parasites establishing residence
• Parasite multiplication and competition for resources
• Parasite emerging from the host (diagnostic stages).
  • Visual Representation includes arrows depicting the parasite movement.

Typical Fecal-Oral Life Cycle

• Cyst stage
• Passed in feces
• Resistant wall
• (Excystation)
• Trophozoite stage
• Feeding
• Replication (in the large intestine)
• (Encystment)
• Diagram shows the two stages and their transition between each other, including the relevant parasite movement in the cycle.

Techniques Used in the Identification of Parasites

• Histological: Acid fast stain, Trichome stain, H and E stain, Skin tests
• Molecular: Polymerase chain reaction (PCR), Deoxyribonucleic acid (DNA) hybridization, Antigen-antibody detection (e.g., enzyme immunoassay [EIA])

Summary

• Parasitic infections widely prevalent, particularly in tropical and developing countries
• Treating parasitic infections is becoming increasingly difficult
• Many different organisms can play parasitic roles at some point in their life cycles