Article 3: Introduction to Clinical Parasitology

Question 1

Symbiosis vs. Commensalism vs. Parasitism

Answer 1

Symbiosis (Mutualism): both partners benefit

Commensalism: one partner benefits; the other is unaffected

Parasitism: one partner benefits at the EXPENSE of the other. All infectious agents causing illness belong to this category.

Question 2

Ectoparasite

Answer 2

Live ON SURFACE of host. Usually arthorpods (e.g. ticks and mites)

Question 3

Endoparasite

Answer 3

Live WITHIN the body of host. Mostly protozoa and helminths

Question 4

Obligate Parasites (Majority)

Answer 4

Must spend at least part of their life cycle in association with a host (e.g. Schistosoma (trematode) and Filaria (nematodes))

Question 5

Facultate Parasites

Answer 5

Capable of leading both a free and parasitic existence. (e.g. Naegleria (ameba) and Strongyloides (nematode))

Question 6

Vector

Answer 6

insect that transmits infectious agent from one host to the next

Question 7

Protozoa

Answer 7

unicellular eukaryotic microbe

Question 8

Helminth

Answer 8

worm

Question 9

Nematode

Answer 9

roundworm

Question 10

Cestode

Answer 10

tapeworm

Question 11

Trematode

Answer 11

fluke (complex flatworms)

Question 12

Parasitic infections currently account for _____ compared to other infectious agents, worldwide

Answer 12

greater morbidity and mortality

***Uncommon in the US, however, those who are immunosuppressed or immunocompromised are at increased risk

Question 13

Definitive Host

Answer 13

Species in which the parasite reproduces SEXUALLY and reaches sexually maturity (adulthood) (in parasites that have a life cycle involving more than one host)

Question 14

Intermediate Host

Answer 14

Species in which the parasite reproduces ASEXUALLY or larval stages of development occur (in parasites that have a life cycle involving more than one host)

Question 15

In general, the more complicated an organism’s life cycle, the ___ likely change of survival it has

Answer 15

Less

Question 16

How do parasitic disease occur in non-endemic countries?

Answer 16

Travel to endemic countries, immigration (e.g. malaria)

Question 17

Reservoir

Answer 17

Animal (definitive host) that serves to maintain the parasite’s life cycle in the environment

Question 18

What three types of parasites does medical parasitology concern itself with?

Answer 18

Protozoa, Helminths, and (to a lesser extent) Arthropods

Question 19

Trophozoite

Answer 19

Metabolically active, motile, “feeding stage” of a Protozoa

Question 20

Protozoa

Answer 20

Single celled eukaryotes with a nucleus and cell organelles

• Many can form a cyst (protects against environment)
• Those that don’t form cysts will be transmitted via an insect vector

Question 21

Protozoa Classes

Answer 21

1. Ameba (locomotion: pseudopodia)
2. Flagellates (locomotion: flagella)
3. Sporozoa (locomotion: gliding)
4. Ciliates (locomotion: cilia)

Question 22

Schizogony

Answer 22

A type of ASEXUAL reproduction performed by Sporozoans and some Amoebas

-Multiple intracellular nuclear divisions (mitosis) that precede cytoplasmic division

Question 23

Sporogony

Answer 23

A type of SEXUAL reproduction that entails multiple nuclear divisions (meiosis) followed by cytokinesis after zygote formation

Question 24

Helminths

Answer 24

Multicellular organisms with an elongated appearance

• Can vary from a few mm to over 40 feet
• Thick membranous coat known as a CUTICLE
• Have hooks or suckers to aid in attachment
• Two classes: Roundworms (nematodes) and Flatworms (cestodes and trematodes)

Question 25

Transmission of Parasitic Diseases involves what three factors?

Answer 25

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

Question 26

Name the 5 mechanisms of transmission:

Answer 26

1. Ingestion: consumption of food/water with parasitic eggs (helminths) or cyst (protozoa); usually involves fecal contamination
2. Penetration/Inoculation: larval forms of some helminths have develops the ability to directly penetrate the skin (hookworms or schistosomes); also can occur via insect bite (e.g. malaria or trypanosomes)
3. Direct transmission: Trichomonas vaginalis, causative agent of a common STD, is transmitted through SEXUAL CONTACT
4. Congenital transmission: mother to infant (e.g. toxoplasmosis and malaria)
5. Transfusion and Transplantation: potentially any blood or tissue dwelling parasite. Most frequently observed with malaria and American trypanosomes (Trypanosoma cruzi)

Question 27

What are the two mechanisms by which parasites produce pathology?

Answer 27

1. Physical presence of parasite or its products

2. Immunopathology caused by the host’s response to infection

Question 28

Mechanical Damage

Answer 28

Caused by physical presence of parasite or parasite movement within the host. Examples of this type of pathology include obstruction of the intestine by high worm burdens (Ascaris), blockage of the lymphatics by filarial worms, damaged caused by the migration of helminth larvae through tissues, and lysis of RBC by malaria parasites.

Question 29

Damage caused by Parasite Products

Answer 29

Numerous parasites, particularly helminths and intestinal protozoa, produce products that aid in the establishment and persistence of infections. Often these products cause severe damage to host tissues and organs, and include hydrolytic enzymes such as proteases, phospolipases, collagenases and hyaluronidases.

Question 30

Immunopathology

Answer 30

The response of the host to parasitic infection can often result in or contribute to pathology

(e.g. Schistosome eggs (granuloma formation), Trypanosoma cruzi (Chagas disease–>myocarditis and neuropathy caused by autimmune response)

Question 31

Immune Responses to Protozoal Infection

Answer 31

1. ) neutralizing antibody. e.g. block entry of parasite into host cells
2. ) antibody + complement. e.g. lysis of blood dwelling parasites
3. ) antibody/complement opsonization (plus neutrophils or macrophages) e.g. phagocytosis and clearance of pathogens
4. ) Activated macrophages. e.g. destruction of intracellular protozoa such as Leishmania, Toxoplasma, and Trypanosoma cruzi
5. ) CD8+ cytotoxic T cells. e.g. lysis of parasite infected host cells

Question 32

Immune Responses to Helminthic Infections

Answer 32

Appears to be mediated by IgE and eosinophils (Hallmark of helminthic infections)

IgE Abs bind to the surface of the worm, are recognized, and are bound by Fc receptors on eosinophils, which then dump their cytotoxic granules onto the parasite

Question 33

Two problems for the immune system posed by Helminthic Infections

Answer 33

1. Too big to be phagocytosed

2. They are often covered with a protective cuticle or sheath that protects them from complement-mediated lysis

Question 34

What are the eight ways in which parasites avoid immune destruction?

Answer 34

1. Size (large size makes phagocytosis unlikely)
2. Anatomical location (i.e. in the gut; makes it difficult for immune system to reach them)
3. Intracellular sequestration (within the host cell)
4. Formation of cysts
5. Avoidance of phago-lysosomal destruction (survive and replicate within macrophages)
6. Antigenic variation
   - Two classic examples (1. Malaria parasites have man different life cycle stages, each with a unique antigen; 2. African trypanosomes present successive “waves of infection” so that the immune system is constantly trying to “catch-up”)
7. Antigenic masking (Helminths coat themselves with host cellular or serum proteins, “masking” their own antigens”)
8. Immuno-suppression (thought to involve the production of immunosuppressive cytokines by macrophages and T-cells in response to parasitic antigens or infections; host is susceptible to SECONDARY INFECTIONS)

Question 35

What is the main challenge in treating parasitic infections?

Answer 35

They are EUKARYOTIC cells, so treating these infections often results in a fair amount of TOXICITY TO THE HOST

Question 36

How is differential toxicity commonly achieved?

Answer 36

By 1) preferential uptake of drug by the parasite, 2) metabolic alteration of the drug by the parasite, or 3) differences in the susceptibility of functionally equivalent sites in the parasite and the host.

Question 37

Anti-protozoals

Answer 37

Generally, target RAPIDLY PROLIFERATING, METABOLICALLY ACTIVE cells

Mnemonic: “Anti-PROtozoals treat rapidly PROliferating cells”

Question 38

Anti-helminthics

Answer 38

Generally, targeted at non-proliferating adult helminthes. Most affect the neuromuscular system, carbohydrate metabolism, or egg (larvae) production of adult worms.