Helminths

• Nematoda (roundworms) are a helminth living in a stomach (abomasum) worms e.g. Haemonchus spp. They can also live in the small intestine worms e.g. Nematodirus spp. or lungworm such as Dictyocaulus spp. or CNS worms e.g. Elaphostrongylus cervi.
• Nematodes are commonly called roundworms due to their appearance. They can be parasitic or free-living, and the majority are dioecious.
• Nematoda are cylindrical, unsegmented, and tapered at either end with a cuticle, a colourless, translucent layer covering the body. They have two lateral excretory canals with dorsal and ventral cords carrying their nerves. Their muscle cells stretch longitudinally with their internal organs being suspended in fluid-filled cavity.
• Nematode Life Cycle:

  A) Definitive

  B) Migration

  C) Direct

  D) Paratenic

  E) Indirect

  
• Trematoda are a platyhelminth with two main sub-classes:

  1. Monogenea - Direct life cycle. External parasites of fish and amphibians. Importance in aquaculture
  2. Digenea - Indirect life cycle. Exclusively in vertebrates. Considerable veterinary importance. Adults = Flukes.
• Digenea, or flukes, have 6,000 species overall with an average of 10 – 30mm in length. They occur primarily in bile ducts, alimentary tracts and vascular system. There's a wide variation in lifecycles across the species.
• Digenea are flat and leaf-like with two muscular suckers for attachment. The anterior oral sucker is surrounding mouth and the ventral surface sucker is named the acetabulum. It has an absorptive tegument with spines or scales and the muscle cells and parenchyma below the tegument. There is no body cavity and these worms are generally monoecious.
• Cestoda are tapeworms which are monoecious. Their length ranges from few millimetres to metres.
• Cestoda have a globular head (scolex) with attachment organs. There are four suckers, hooks, and a rostellum. The body is a chain of segments called strobila and each segment is proglottid. The worm absorbs nutrients through a tegument and has no body cavity.
• Routes of helminth infection:

  • Ingestion of eggs e.g. from faeces. Larval stage can form cysts. Cysts development = harmful.
  • Ingestion of cysts e.g. from undercooked meat. Growth of worm in intestinal tract. Adult worms cause minor harm. Danger if cysts get into other organs.
• Helminth parasite lifecycles are varied and as development of some stages may rely on development within the environment or within an intermediate host, there can be considerable variation year to year depending on environmental conditions and/or abundance of intermediate hosts available.
• Variation in parasite development may influence control strategies in regions with differing climatic conditions.
• Preventing access to the intermediate host, if possible, may play an important part in control of parasites with an indirect lifecycle.
• Cestodes typically have a metacestode lifestage within an intermediate host which can vary from small insects to large mammals depending on the species. Ingestion of the metacestode stage in the intermediate host is required for infection of the final host, appropriate assessment of meat and cooking of meat are important control measures to ensure the final host is not infected.