Parasitology

Question 1

Parasite

Answer 1

An organism that lives and feeds on or in an organism of a different species and causes harm to its host

Question 2

Eukaryotic endo-parasite groups are classified as…

Answer 2

Protozoa and Helminths

Question 3

All parasitic helminths belong to which group? Why?

Answer 3

Opisthokonta

- because they are multicellular and animals

Question 4

Parasitic protozoa are found in…?

Answer 4

3 of the 6 major lineages as all are unicellular organisms

Question 5

What are the 6 major endo-parasite groups?

Answer 5

1. Excavata
2. Opisthokonta (all parasitic helminths belong to this group)
3. Amoebozoa
4. Archaeplastida
5. Rhizaria
6. Chromalveolata

Question 6

Typical characteristics of a parasites?

Answer 6

1. Eukaryotic organisms that have co-evolved with hosts
2. Have complex life cycles, including developmental stages
3. Long lived so cryptic and chronic infections
4. Collectively properties manifest into a wide range of pathologies from a single species

Question 7

Direct life cycle

Answer 7

Single definitive host

Question 8

Indirect life cycle

Answer 8

Involves both definitive and intermediate hosts

Question 9

Disease and damage due to Parasites?

Answer 9

1. May cause acute disease but parasites (unlike other pathogens) also have the capability to produce chronic diseases.
2. Disease may be caused by direct damage due to parasite, or indirect damage caused by immuno-pathological response to infection.

Question 10

3rd most common nematode infection worldwide - estimated 1 billion people carrying infection. Direct life cycle/ oral-faecal transmission

Answer 10

Whip worm/ Trichuris Trichuria (nematode)

Question 11

How may whip worm/Trichuris trichuria be treated?

Answer 11

With Helminth microtubule inhibitor = Albendazole

Question 12

• Most common worm infection in the UK. This worm crawls out of the rectum to peri-anal region to lay eggs at night. Eggs are then spread by scratching and touching bed clothes/furniture

Answer 12

Pin worm/ Enterobius Vermicularis

Question 13

How does Ascaris Lumbricoides (giant round worm) cause disease?

Answer 13

So large that it causes disease by simple mechanical blockage of the intestine
- Can take from 2 weeks to 1 month to develop

Question 14

Toxocara Canis

Answer 14

Helminth parasite with dog as definitive host, causes human toxocariasis when eggs ingested. Larval worm do not develop, but migrate to eyes and brain resulting in blindness and epilepsy, respectively.

Question 15

Tanenia Solium/ Pork tapeworm

Answer 15

• Human as definitive host and pigs as common intermediate host
• Humans invest larval stage cystercerci from infected meat –> cystercircus may develop into a hermaphroditic adult worm intestinal infection causing taeniasis.
• Tapeworm is several metres long but generally asymptomatic - can cause severe cases of anaemia and emaciation
• Adult worm sheds proglottids (containing 50,000 embryolated eggs) –> excreted by humans, and ingested by intermediate host. Eggs hatch + migrate to muscle and brain to settle as cystercerci.
• Severe disease in intermediate host when eggs are ingested and cystercerci grow into cysticercosis…

Question 16

What are cystercerci?

Answer 16

Large fluid filled bladders with worm proteins, may cause a fatal inflammatory response upon accidental rupture.

Question 17

Ivermectin

Answer 17

Neurotransmission inhibitor

• Acts on the glutamate gated Cl- channels
• Anti-helminth drug against trichuriasis/ enterobiasis/ filariasis/ ascariasis/ strongyloidiasis

Question 18

Albendazole/ Mebendazole

Answer 18

Microtubule assembly inhibitor

• Colchicine-sensitive site of tubulin
• Trichuriasis/ Cysticercosis/ Hydatid disease/ Filariasis

Question 19

Praziquantal

Answer 19

Permeability of membranes to calcium ions

- Unknown target but works against Schistosomiasis/ tapeworm and other fluke infections

Question 20

Development of Schistosomiasis? (S. Mansoni)

Answer 20

S Mansoni cercaria in water comes into contact with swimmers –> penetration into skin + loss of tail; larval schistosomula develop into adult worm.
Adult male + female migrate - attach in mesenteric venules living 5-10 years –> deposit 300 to 3000 eggs every day depending on species.
- Eggs hatch in water releasing miracidia –> invade snail host. Sporocysts in snail release motile cercaria into water…

Question 21

Morbidity in schistosomiasis is largely due to…

Answer 21

Egg deposition

• Most eggs transit gut/bladder wall + shed.
• A fraction will reflux with blood back into the liver using granuloma formation/fibrosis/calcification

Question 22

Immune response pathway in Schistosomiasis…

Answer 22

1. Initial Th1 inflammatory response to cercaria penetration + larval worm migration
2. Egg deposition initiates switch to Th2 response, as egg antigens picked by dendritic cells + processed + presented.
3. Basophils activated by antigen reinforce Th2 cytokine profile. Alternatively activated macrophages + eosinophils support would healing mechanisms (collagen deposition)
4. B cells produce ice and igG antibodies –> aid destruction of eggs in granulomas
5. Egg antigens participate in Treg formation, with alternatively activated macrophages, produce IL10 to limit granuloma size + liver tissue wound healing response.