Chapter 6: The helminths

Question 1

Describe platyhelminths and provide two examples

Answer 1

Flatworms: flattened bodies with muscular suckers and/or hooks for attachment to the host

ex:
1) tapeworms (Cestoda-Taenia solium)
2) flukes (Trematoda) (Schistosoma sp. blood fluke)

Question 2

Describe the Nematodes; what is their common name?

Answer 2

Phylum Nematoda (roundworms) have long cylindrical bodies and generally lack specialized attachment organs

Question 3

What are the transmission routes for helminths?

Answer 3

1. via intermediate host: accidental ingestion of larvae in tissue of another host
2. fecal-oral route: accidental ingestion of eggs or larvae originating from feces of infected host
3. active skin penetration: larval stages invade through skin
4. injection by blood-sucking insect: larval stages develop to infectivity in insect intermediate host

Question 4

How do flukes and nematodes feed?

Answer 4

Actively feed on host tissues or on the intestinal content

Question 5

How do tapeworms feed?

Answer 5

Tapeworms have no digestive system and absorb predigested nutrients (directly by diffusion through body wall)

Question 6

Describe indirect life cycles.

Answer 6

Indirect cycles are those where reproductive stages must undergo further development in an intermediate host (tapeworms) or vector (filarial worms) before sexual maturity can be achieved in the final host.

Question 7

Describe direct life cycles.

Answer 7

Direct life cycles, reproductive stages produced by sexually mature adults in one host are released from the body and can develop directly to adult stages after infection of another host via the faecal–oral route (Ascaris) or by direct penetration (hookworm).

Question 8

Summarize the life cycle of Schistosoma sp.

Answer 8

The life cycle of Schistosoma sp. includes several species of water snails, which serve as secondary hosts. Human become infected by larvae (contaminated water) penetrating the skin and takes up residence in the veins of the digestive system. Eggs escape the host in the urine or feces and infect a snail to complete the life cycle.

Question 9

Describe the sex cycle of platyhelminths.

Answer 9

Tapeworms and most flukes are hermaphrodites, except the schistosomes, which have separate sexes (Remember latter have male and female parts that are found together).
The eggs of both flukes and tapeworms develop into larvae that must pass through one or more intermediate hosts and develop into other larval stages before the parasite is again infective to humans.

Question 10

Describe the sex cycle of nematodes

Answer 10

In nematodes, the sexes are separate. Most species liberate fertilized eggs, but some release early-stage larvae directly into the host’s body. Development from egg or larva to adult can be direct and occur in a single host, or may be indirect, requiring development in the body of an intermediate host (Filarial worms).

Question 11

Which two categories of human-specific nematodes were discussed in lecture?

Answer 11

• those that mature within the gastrointestinal tract, some of which may migrate through the body during development (e.g. Ascaris, hookworms, Trichinella, Strongyloides, Trichuris)
• those that mature in deeper tissues (e.g. the filarial nematodes).

Question 12

How do helminths survive the immune response?

Answer 12

Schistosomes disguise themselves from immune system by acquiring host molecules on their outer surface. Others actively suppress the host’s immune responses by releasing factors that interfere with, or divert, protective responses

Question 13

Which helminths actively feed on host tissues or on intestinal content and have a digestive system?

Answer 13

flukes and nematodes

Question 14

Which helminths do not have a digestive system and absorb predigested nutrients?

Answer 14

tapeworms