VPRP 0906: Cestodes and Nematodes

• VPRP 0906: Introduction to Microbiology - Parasites: Cestodes

Nematodes

• Nematodes have elongate/cylindrical shapes.
• They possess an alimentary canal.
• Sexes are usually separate in nematodes.
• Nematodes have direct or indirect life cycles.
• Nematode life cycle progresses from L1 (mff) to L2 to L3 to L4 to L5 (immature adult), then to adult, and finally to egg.
• Nematodes have various survival methods.

Parasites of Veterinary Importance

• Phylum Nematoda includes roundworms.
• Phylum Platyhelminthes includes flatworms.
• Class Cestoda refers to tapeworms.
• Class Trematoda refers to flukes.

Cestodes Learning Objectives

• Understand the key characteristics of cestodes.
• Identify the final host (FH) and intermediate host (IH).
• Identify the location of adults in the FH.
• Acknowledge the clinical relevance in each host.
• Know the purpose of adults and larvae.
• Understand the hosts of Taenia solium.
• Know the significance of the wrong life stage in the wrong host.

Cestodes (Tapeworms) Description

• They are flat-bodied without a body cavity.
• They have a strobila (body) with a head, neck, and proglottids (segments).
• Their head (scolex) has 4 suckers or bothria.
• Cestodes proglottids are hermaphroditic.
• They lack an alimentary canal.
• They have an indirect life cycle.
• They possess several types of larval stages.

Scolex

• The scolex can have suckers, a most rostellum, and hooks that are armed or unarmed.

Cyclophyllidea

• They have one intermediate host (IH).
• They have organs of attachment associated with the scolex and suckers.
• Their egg contains an oncosphere.

Pseudophyllidea

• They have two intermediate hosts (IHs). The first is a crustacean.
• They have organs of attachment associated with the scolex and bothria.
• The egg contains an oncosphere with cilia for motility of the organism.

Strobila (Body)

• The strobila has proglottids (segments).
• Proglottids are mature posterior and furthest from the scolex.

Proglottid

• Proglottids contain male and female reproductive organs, vitelline glands, and a genital pore.

Cyclophyllidea Cestodes

• Sucker shape, size, and hooks are defining characteristics.
• Rostellum presence or absence and hooks are defining characteristics.

Proglottid Facts

• Proglottids have shape related to width, length, and fringe.
• Some Cestodes have a few or a lot of proglottids. They also have one or two genital pores, and the position of glands varies.

Cyclophyllidea Cestodes Life Cycle

• Adult present in final host.
• Segments (proglottids) are present in feces.
• Eggs are released from proglottids.
• Eggs are consumed by intermediate host.
• Larvae develop in IH.
• The IH is consumed by final host.
• Adults are rarely a problem unless there is a heavy infection or in young animals.
• The larval stages, can cause damage, particularly in humans.

Segment Release

• Segments release both an oncosphere and a hexacanth embryo.

Oncosphere/Hexacanth Embryo Facts

• It is consumed by an IH
• It forms a larva/larvae which makes a scolex.
• Several types of larvae can exist. However, only one type occurs for each tapeworm species.

Taenia solium

• It is flat and hermaphroditic.
• The final host has the adult tapeworm in the intestine and clinical signs are rare in the final host.
• Proglottids can be found in the feces of the final host.
• Features an oncosphere, which is a hexacanth embryo.
• The intermediate host can have clinical signs.
• The final host eats the IH.
• The final host is humans and the intermediate host is pigs.
• Humans consume the larva by eating undercooked pork.
• Adult T. solium lives in the small intestine of humans.
• Adult T. solium is not normally pathogenic, but can cause Taeniasis, which causes an itchy end.
• Pigs eat the hexacanth embryo in human feces. From the pig, a larva (cysticercus) forms in the muscle or tongue.
• A person eating human feces develops cysticercus, which can occur in the brain: NCC neurocysticercosis.
• Humans are an aberrant and dead-end host when they possess the larval stage of T. solium.