Parasites and life cycles

Question 1

Outline the life cycle of Taenia saginata.

(6)

Answer 1

1. Eggs or gravid proglottids in faeces are passed into the environment.
2. Cattle become infected by ingesting vegetation contaminated by eggs.
3. Oncospheres hatch, penetrate intestinal wall and circulate to musculature.
4. Oncospheres develop into cysticerci in muscles.
5. Humans infected by ingesting raw/undercooked meat.
6. Adults live in the small intestine and reproduce.

Question 2

Outline the life cycle of Taenia multiceps

9

Answer 2

1. Eggs or gravid proglottids in faeces and passed into environment.
2. Embryonated eggs are ingested by the intermediate host - sheep.
3. Oncospheres hatch, penetrate the intestinal wall and circulate to tissue.
4. Oncospheres develop into coenuri containing many protocolises within the tissue of the intermediate hosts- these being the head (brain and eyes).
5. Dogs, the definitive host become infected by ingesting the meat from the intermediate host.
6. Adults in the small intestine reproduce.
7. Humans are infected from the embryonated eggs that are ingested from dog faeces.
8. Oncospheres hatch, penetrate the intestinal wall and circulate to tissue.
9. Coenuri develop in the brain, eyes and subcutaneous tissues of people.

Question 3

Outline the life cycle of Taenia ovis.

6

Answer 3

1. Eggs or gravid proglottids in faeces are passed into the enviroment.
2. Sheep become infected by ingesting vegetation contaminated by eggs.
3. Oncospheres hatch, penetrate intestial wall and circulate to musculature.
4. Oncospheres develop into cysticerci in muscles.
5. Humans infected by ingesting raw/undercooked meat.
6. Adults live in the small intestine and reproduce.

Question 4

Outline the life cycle of Echinococcus granulosus.

9

Answer 4

1. Embryonated eggs in the environment are consumed by sheep the intermediate host.
2. Oncospheres hatch and penetrate the intestinal wall.
3. They migrate to various organs (commonly the liver and lung) and tissues and form hydatic cysts.
4. Dogs, the definitive host eat infected organs.
5. Protoscolex develop from the ingested cysts, mature into scolex and attach to the intestine.
6. Adults mature in the small intestine and reproduce.
7. Humans, and sheep, consume embryonated eggs from the faeces.
8. Causing oncospheres to hatch in the intestine of humans, causing hydatic cysts in people.

Question 5

Outline three differentials for a white mass in the liver of a sheep.

Answer 5

Echinococcus granulossus
Echinococcus multilocularis (same as the above but with foxes instead of dogs and rats instead of sheep)
Abscesses

Question 6

Outline the life cycle for Muellaris capillaris

5

Answer 6

1. An infected snail (intermediate host) in ingested by a sheep (DH).
2. The ingested larvae migrate to the lungs by the lymphatic system.
3. In the alveoli, the larvae mature to the adult stage and produce eggs.
4. The eggs hatch and larva are coughed up and swallowed.
5. Larvae are released into the environment where they infect snails.

Question 7

Outline the lifecycle for Taenia solium.

8

Answer 7

1. Eggs or gravid proglottids in faeces are passed into the environment.
2. Embryonated eggs are ingested by swines.
3. Oncospheres hatch, penetrate intestinal wall and circulate to musculature.
4. Oncospheres develop into cysticercoid.
5. Humans will become contaminated via consumption of infected meat, causing taeniasis, where mature tapeworms will develop and live in the small intestine. This can be treated with praziquantel.
6. However, humans may consume embryonated eggs.
7. Oncospheres hatch, penetrate the intestinal wall and circulate to the musculature.
8. From this, cysticerci develop in any organ, notably the brain and eyes.

Question 8

Outline the life cycle for Trichinella spiralis

6

Answer 8

1. Pigs consume infected meat scraps/animals (like rats).
2. The HCl in the stomach activates the encysted larva which hatch.
3. There larva are released into the small intestine which develop into mature adults.
4. These adults reproduce, producing more larva which enter the circulation.
5. The larva become encysted in striated muscles.
6. Humans consume the infected (undercooked) pork and the cycle happens (but inside the human).

Question 9

Outline the life cycle of Ascaris suum

5

Answer 9

1. Ascarids eggs in soil mature.
2. Eggs with L3 ingested by a pig.
3. L3 hatches and penetrates the intestinal mucosa and travels to the liver.
4. Migration to lungs and from lungs to bronchial tree, coughed, swallowed and back into the intestine.
5. Adults in intestine mate and produce highly resistant eggs.

Question 10

Outline the life cycle of Fasiola spp.

8

Answer 10

1. Unembroynated eggs are passed into faeces.
2. Eggs become embryonated in water.
3. Miracidia hatch from eggs and seek out the snail, the intermediate host.
4. Miracidia develop into cercaria which leaves the snail and becomes free-swimming.
5. Cercaria encyst on moist vegetation, becoming metacercaria.
6. Ruminants eat metacercaria on vegetation.
7. Immature flukes develop in the duodenum, penetrate the intestinal wall and migrate through liver parenchyma to the biliary ducts.
8. Ruminants are the definitive host

Question 11

Outline the life cycle of Taenia hydatigena.

9

Answer 11

1. Eggs or gravid proglottids in faeces and passed into enviroment.
2. Embryonated eggs are injected by the intermediate host - sheep.
3. Oncospheres hatch, penetrate the intestinal wall and circulate to the liver.
4. Oncospheres develop into cysticerci containing many protoscolices in the liver
5. Dogs, the difinitive host become infected by injesting the meat from the intermediate host.
6. Adults in the small intestine reproduce.
7. Humans are infected from the embryonated eggs that are injested from dog faeces.
8. Onchospheres hatch, penetrare the inestinal wall and circulate to tissue.
9. Cysticerci develop in the brain, eyes and subcutaneous tissues of people.

Question 12

What is the lifecycle for diphyllobothrium (10)?

Answer 12

1. Unembryonated eggs passed in faeces of definitive host (humans)
2. Eggs embryonate in water
3. Corocidia hatch from eggs and ingested by fish
4. Procercoid larval develop in the body cavity of crustaceans
5. Infected crustacean ingested by secondary intermediate host
6. Procercoid released from crustacean and develops into plerocoid larvae
7. Predator eats secondary intermediate fish
8. Plerocoid invades tissue of predator
9. Definitive host ingest infected fish
10. Adults develop in small intestine

White cysts in fish intestine -> killed by normal cooking and freezing

Question 13

What is the lifecycle for anisakidae (7)?

Answer 13

1. Definitive host (dolphins and seals) excrete Unembryonated eggs in faeces.
2. Eggs become embryonated in water and L3 develop in eggs
3. L3 larvae hatch and become free swimming
4. Ingested by crustaceans
5. L3 migrates to mesentary and muscle disuse
6. Paratenic host (fish) are infective to definitive host once larvae enters the tissue
7. Diagnosis through gastroscopic examination

Causes gastrointestinal irritation in humans and is killed by freezing and heat treatment (marinating and cold smoking will not kill larvae)