Question

describe the life cycle of a specific helminth ( including terminology)

Answer 1

Cestodes –

In general language they are called tapeworms. They have long flat ribbon-like bodies with a single anterior holdfast organ called scolex and numerous segments. They do not have a gut and all nutrients are taken up through the tegument. They do not have a body cavity and are flattened to facilitate perfusion to all tissues. Segments exhibit slow body flexion produced by longitudinal and transverse muscles. All tapeworms are hermaphroditic and each segment contains both male and female organs.

Life cycle-

Cestodes form three main life-cycle stages: eggs, larvae and adults.

Adult worms infect definitive hosts means those in which sexual development occur.

Larval stages may be free-living or parasitize invertebrate vectors, intermediate hosts.

They produce eggs that embryonate in vitro or outside the host.

The emergent larvae undergo 4 metamorphoses before they mature as adult male or female worms. Cestode eggs released from gravid segments embryonate to produce 6-hooked embryos that are called “hexacanth oncospheres” which are ingested by intermediate hosts. The oncospheres penetrate into host tissues and become metacestodes which is an encysted larvae stage. When eaten by definitive hosts, they excyst and form adult tapeworms.

There are four modes of transmission by which the larvae infect new hosts are faecal-oral, transdermal, vector-borne and predator-prey transmission.

1. Faecal-oral- Eggs or larvae passed in the faeces of one host and ingested with food/water by another host.
2. Tran-dermal- Tran-dermal of infective larvae penetrate the skin and migrate through the tissues to the gut where adults develop and produce eggs that are voided in host faeces
3. Vector born- Transmission of larval stages taken up by blood-sucking arthropods and injected into new human hosts.
4. Predator transmission-Transmission of encysted larvae within prey animals being eaten by predators where adult worms develop and produce eggs.