RA3: Introduction to Clinical Parasitology

Question 1

Ecto vs endoparasite

Answer 1

• Ecto: live on surface of host – usually arthropod (tick and mite)
• Endo: live within body of host – usually protozoa and helminth

Question 2

Lifestyles of parasites

Answer 2

1. ) Obligate: must spend part of life cycle in association with host
2. ) Facultative: capable of leading free and parasitic existence

Question 3

What is a nematode vs helminth vs cestode vs trematode? Describe reproduction

Answer 3

1. ) Helminth = multicellular worm
   - Two classes based on body shape
   - A.) roundworms (nematodes): separate sexes, with smaller male typically
   - B.) flat/tapeworms (cestodes and trematodes/aka fluke): cestodes w/o alimentary canal and are segmented, while flukes have alimentary canal and are non-segmented with leaf-shape. Hermaphroditic.

Question 4

Hosts of nematode

Answer 4

• One host, larva/eggs pass from host to host directly or after a free-living existence

Question 5

Hosts of cestods/trematodes

Answer 5

• Complex life cycle involving one or more intermediate host

Question 6

Structure of helminth

Answer 6

• Cuticle: thick membranous coating that protect it from environment
• Hooks/suckers: aid in attachment to host, not in all

Question 7

Clinical / medical definition of parasite?

Answer 7

• Infections caused by helminth, protozoa or arthropod

Question 8

Manner for endemic parasitic infections to spread to non-endemic countries

Answer 8

• Travel and immigration

Question 9

Types of parasitic hosts?

Answer 9

• Definitive: host in which parasite reaches sexual maturity/adulthood
• Intermediate: host that harbors larval or asexual stages of the parasite
• Reservoir: animal that serves to maintain the parasite’s life cycle in the environment

Question 10

What are protozoa? What are trophozoites? 4 classes of protists?

Answer 10

• Unicellular eukaryotic microbes. Have a feeding stage where they are known as trophozoites. Protists are classified into 4 main classes based on their means of locomotion: 1.) ameba (pseudopodia), 2.) flagellates (flagella), 3.) sporozoa (gliding), 4.) ciliates (cilia)*
• First three classes cause infections in man, *only single ciliate is known to infect man – Balantidium coli cause symptoms similar to amebic dystentery

Question 11

How do protozoa reproduce?

Answer 11

• Asexually via process known as binary fission
• Some produce via process known as schizogony (not sporogony): described as multiple intracellular nuclear divisions that precede cytoplasmic division

Question 12

Describe function of protozoan cysts

Answer 12

• Many protozoans encyst during their life cycle. Formation of cysts protects the organism from environment during passage from host to host – this is typically shed by infected host.

Question 13

Transmission of protozoa can take place in two forms

Answer 13

1. ) Cyst

2. ) Vector

Question 14

Three factors involving transmission of parasitic disease

Answer 14

1. ) source of infection
2. ) mode of transmission
3. ) presence of susceptible host

Question 15

Methods of transmission of parasites

Answer 15

1. ) ingestion
2. ) penetration/inoculation
3. ) direct transmission
4. ) congenital transmission
5. ) transfusion/transplantation

Question 16

Mechanisms for pathology of parasitic infections

Answer 16

1. ) Mechanical damage: obstruction, blockage of lymphatics, damage caused by migration, lysis
2. ) Damage d/t parasitic products: hydrolytic enzymes such as proteases, phospholipases, collagenases and hyaluronidases
3. ) Immunopathology: response of host to parasite can cause pathology – granuloma (blocks blood flow in cases of liver parasites), autoimmune response to structurally similar antigens, immune complexes leading to vasculitis and nephritis, hypersensitivity leading to anaphylaxis

Question 17

Immune response to fight off parasitic infections

Answer 17

1. ) neutralizing antibody
2. ) antibody + complement
3. ) opsonization
4. ) activated macrophages – against intracellular protozoa
5. ) CD8+ T cell activation – against parasite infected host cells
6. ) IgE and eosinophil mediated helminth destruction

Question 18

Why do helminthic infections pose a challenge for immune system?

Answer 18

1. ) too big for phagocytosis

2. ) covered by protective cuticle or sheath

Question 19

What is eosinophilia typically the hallmark of?

Answer 19

• Helminthic infection

Question 20

How parasites evade the immune defenses?

Answer 20

1. ) Size: helminthes
2. ) Anatomical location: eg. In lumen of gut
3. ) Intracellular sequestration: reside for a period inside host cells
4. ) Formation of cyst: impenetrable to immune system
5. ) Avoidance of phago-lysosomal destruction
6. ) Antigenic variance
7. ) Antigenic masking/coating
8. ) Immunosuppresion

Question 21

Obstacle in treating parasitic infections

Answer 21

• Parasites in clinical/medical sense are helminthes and protozoa, ie. eukaryotes – drugs exhibit toxicity to host as well. Differential toxicity is commonly achieved by preferential uptake of drug by parasite, metabolic alteration of drug by parasite or differences in susceptibility of functionally equivalent sites in parasite and host
• Anti-protozoals: target rapidly proliferating metabolically active cells
• Anti-helminthics: target non-proliferating adult helminthes, most affect neuromuscular system, CHO metabolism or egg production of adult worms