Nematodes

PARASITES

Introduction

Soybean Cyst Nematode and Egg, Scanning Electron Micrograph - Wikimedia Commons

Mononchidae eating a Mononchidae - Wikimedia Commons

Nematodes are one of the most prolific life-forms on this planet. Most are free-living, with species adapted to every imaginable habitat. Some are parasitic on plants, and others on animals. The animal parasitic nematodes form a large and diverse array of organisms.

The Nematodes

Recognition Features

• A unique feature of the nematodes is that the fluid in the body cavity is maintained under high pressure (this is why nematodes often burst after collection)
• They therefore have a tough elastic cuticle which often forms elaborate structures useful for identification
• A muscular pharynx is needed to pump food into the intestine
• The shape of the pharynx is characteristic in some groups
• There is a nerve ring around the pharynx and four longitudinal nerves with ganglia that co-ordinate movement (many anthelmintics act by disrupting neuromuscular co-ordination)
• The sexes are separate:
  • the female tail generally ends in a blunt point
  • males usually have two chitinous rods that can be protruded through the cloaca to hold the female - these are called spicules and, being chitinous, are easily seen under the microscope. As these differ in shape and size between species, they are very useful in identification
• The bursate nematodes are characterised by a large expansion of the cuticle of the male tail to form a clasping organ (the bursa)
• Other modifications of the cuticle on both male and female nematodes useful for identification include:
  • alae (wing-like protrusions)
  • vesicles (swellings around the whole circumference of the body)
• The heads of some nematodes have structures such as:
  • leaf-like lips around the mouth (the leaf-crown)
  • a buccal cavity
  • teeth or cutting plates

Feeding Habits

• Many intestinal nematodes are closely applied to the mucosal surface
• Some swallow ingesta and/or host secretions.
• Others suck a plug of mucosa into the buccal cavity (plug feeders), leaving a circular ulcer
• Yet others bury their heads deep into the mucosa and suck blood

General Life-Cycle

• The basic life-cycle is very simple:
  • egg → L1 → L2 → L3 → L4 → adult
• With a moult in between each larval stage
• But there are many variations on this theme

Nematode Eggs

• These vary greatly in appearance and are very useful for diagnosis as they can often be detected and counted in faecal samples
• NOTE: we use the typical strongyle egg to provide us with a yard-stick for comparing sizes. A "large egg", for example, means larger than a strongyle egg. A typical strongyle egg is approximately 80µm long

Nematode Superfamilies

BURSATE

• Trichostrongyloidea

• Strongyloidea

• Ancylostomatoidea (Hook worms)

• Metastrongyloidae

NOTE: the Trichostrongyloidea, Strongyloidea and hookworms are very closely related, and are often collectively known as "the strongyles". Typically, they produce almost identical eggs, called "typical strongyle eggs".

NON-BURSATE

• Ascaridoidea

• Oxyuroidea

• Rhabditoidea

• Trichinelloidea

• Spiruroidea

• Filarioidea

From the veterinary view-point, the nematodes are most conveniently grouped into superfamilies because their members often have similar life-cycles and epidemiological features. Superfamily names end in -oidea (the anglicised form is -oid, as in trichostrongyloid).

Nematodes of Different Species

Cattle

Small Ruminants

Horses

Pigs

Poultry

Small Animals

Occlusion of pancreatic ducts sometimes happens Pancreas Parasitic - Pathology