07 - Parasitology Review

Question 1

Describe the clinical and epidemiological significance of human parasitic infections commonly found in the US and worldwide.

Answer 1

Top 15 worldwide parasitic diseases are dominated by helminths (worms). Thus, parasitic infections resultant of poor conditions in which humans inhabit. Estimated 70% of world’s population infected with parasitic helminth; in North America, becoming rare.
Only four of top 15 diseases are protozoan-based. Protozoal diseases are common in tropical and subtropical regions. Transmission generally by ingestion of an infective parasite life-stage or insect vector. Areas where sanitation and control of vectors poor. No longer confined due to travel and immigration.

Question 2

Describe and discuss the key characteristics that identify protozoa

Answer 2

1. Unicellular, eukaryotic. Since eukaryotic, related to human cells. Antiprotozoal drugs are also toxic to human host. Less easily treated than bacterial infections.
2. Pathogenic to humans by one of two ways. Intestinal and urogenital tracts or blood cells and tissues.
3. Most clinically important parasites of humans. Onlya few are pathgenic out of tens of thousands. Can be intracellular or extracellular parasites.
4. Most protozoa have a dormant immotile cyst stage (allows survival of hostile environment). Also have motile, actively feeding and reproducing stage.

Question 3

Clinically relevant protozoa (and motility)

Answer 3

Amoebas: Extend cytoplasmic projections (pseudopodia - “fake feet”); some have flagella.
Flagellates: Rotating whip-like flagella; liquid environment.
Ciliates: Synchronus beating of hair-like cilia. “Row boat” motion, cilia arranged methodically like oars.
Sporozoa: Non-motile.

Question 4

Describe and discuss the key characteristics that identify helminthes

Answer 4

1. Multicellular, eukaryotic worms.
2. Have preferred sites of infestation, often the intestines with little damage. Can disseminate to vital organs (brain, lungs, liver) with severe damage.
3. Infection starts with ingestion of: larvae in raw or undercooked pork, beef, fish; eggs in feces; transmission by insect bite; direct skin penetration.

Question 5

Description of clinically relevant helminthes: Cestodes (tapeworms)

Answer 5

Ribbon-like, segmented worms primarily intestinal parasites. Absorb soluble nutrients from host. Can grow to 15m. Reasons for clinical injury include sequestering of host’s nutrients, excreting toxic waste, mechanical blockage of intestine.

Question 6

Description of clinically relevant helminthes: Trematodes (flukes)

Answer 6

Small, approx 1cm, leaf-like worms. Infest various organs of human host. All parasitic trematodes use freshwater snails as an intermediate host. Categorized into two groups. Hermaphroditic flukes (reproduce asexually in snail before maturing in secondary host) and sexual flukes aka schistosomes (mating occurs in human liver).

Question 7

Description of clinically relevant helminthes: Nematodes (roundworms)

Answer 7

Elongated, nonsegmented worm tapered at both ends. Have own digestive system including mouth, intestine, anus. Can invade almost any human tissue type. Categorized by infecting the intestine or not and whether infectious at egg-stage or larval-stage. Most common nematode infection in US is Enterobiasis (pinworm).

Question 8

Describe life cycle and human acquisition of Ascarias

Answer 8

Lifecycle: Eggs expelled in fecal matter. Mature in soil for 2-3 weeks before infections. Hatch in small intestine. Worms migrate to bloodstream where they travel to lungs, up trachea and are re-swallowed.
Acquisition: Oral ingestion of eggs.

Question 9

Describe life cycle and human acquisition of Hookworm

Answer 9

Lifecycle: Larvae excreted in feces. Penetrate skin, travel via circulatory system to lungs. Crawl up trachea and re-swallowed.
Acquisition: Skin penetration.

Question 10

Describe life cycle and human acquisition of Pinworm

Answer 10

Lifecycle: Eggs deposited perianally. Ingestion of eggs. Matures in colon.
Acquisition: Oral ingestion of eggs.

Question 11

Describe life cycle and human acquisition of Trichinosis

Answer 11

Lifecycle: Ingestion of meat containing cysts. Pigs contract from eating scraps of meat of other infected animals.
Acquisition: Ingestion of contaminated pork.

Question 12

Describe life cycle and human acquisition of Filariasis (elephantiasis)

Answer 12

Lifecycle: Mosquito bite introduces larvae. Larvae mature in lymphatics. Can migrate to blood where they are taken up by another mosquito. Develop further larvae in mosquito and reinfect human.
Acquisition: Mosquito bite.

Question 13

Describe life cycle and human acquisition of River Blindness

Answer 13

Lifecycle: Larvae develop into adults. Re-ingested by blackfly. Produce larvae.
Acquisition: Bite of blackfly.

Question 14

Describe life cycle and human acquisition of Malaria (remainder are protozoa)

Answer 14

Lifecycle: Mosquito bite introduces. Ultimately invades RBCs. Reproduce and rupture RBCs, mosquitos reingest.
Acquisition: Mosquito bite.

Question 15

Describe life cycle and human acquisition of Giardia

Answer 15

Lifecycle: Ingestion of cysts. Multiplication within colon wall. Expelled in fecal matter.
Acquisition: Contaminated water.

Question 16

Describe life cycle and human acquisition of Leishmania

Answer 16

Life cycle: Bite of sandfly. Invade macrophages, reproduce, kill host cell. Reingested by sandfly.
Acquisition: Sandfly bite.