Nematodes: Characteristics and Features

• Nematodes are abundant and diverse metazoan life-forms.
• Majority of nematodes are free-living, in aquatic or terrestrial ecological niches.
• A small minority are parasitic, exploiting plants or animals for part or all of their life cycle.
• Closely related nematodes tend to have similar life cycles, epidemiology, pathogenesis, and drug susceptibilities.
• The superfamily is the best taxonomic level for consideration.

Key Concepts

• Nematodes have high-pressure fluid in their body cavity, unique in the animal world.
• Movement is governed by muscle bundles working against hydrostatic tension and the cuticle's elasticity.
• Large muscular pharynx is needed to pump food into the intestine due to internal forces.
• A nerve ring around the pharynx synchronizes functions.
• Many anthelmintic drugs disrupt neuromuscular coordination.

Recognition Features

• Many nematodes are easily identified under the microscope.
• Recognizing basic morphological features is important.
• The cuticle is a complex structure with microscopic landmarks such as sensory papillae.
• Alae are wing-like protrusions or cuticular swellings that encompass the body.
• The position of a diagnostic feature is indicated by descriptive words like cephalic (head-end), cervical (behind the head), and caudal (tail-end).

Accessory Sexual Structures

• Nematodes have separate sexes.
• The female worm tapers to a blunt point, which is rarely diagnostic.
• The male tail has accessory sexual structures useful for identification.
• Spicules are rod-like structures protruded from the cloaca to assist sperm transfer during mating.
• Spicules are made of chitin and show up well under the microscope.
• Spicules are usually paired, and their shape and size is often diagnostic.
• The bursa is a distinctive holding organ at the posterior end of male worms in specific superfamilies.
• The presence or absence of a male bursa distinguishes bursate and nonbursate nematodes, which is the first step in worm identification.
• The bursa is large enough to be seen with the naked eye in some species.
• Bursal rays form bursa and have a sensory function.
• The bursa is used to hold the female during mating.
• Some species excrete a cementing substance during mating.

Head, Mouth and Associated Structures

• Many nematode species have a simple head and others have useful distinguishing feature
• Leaf crown is rows of leaf-shaped structures arranged around the mouth.
• Buccal cavity is a large space behind the mouth.
• Teeth and cutting plates may be around the mouth and/or at the base of the buccal cavity.
• The mouth leads to the muscular pharynx directly or via the buccal cavity.
• The pharynx is a prominent internal feature.
• The rhabditiform esophagus, has two thickenings along its length separated by a distinct constriction and is seen in free-living and plant parasitic nematodes
• Rhabditiform esophagus also occurs in preparasitic life-cycle stages of many animal nematodes.
• Parasitic stages of animal nematodes need specialized feeding techniques so the shape of the pharynx differs, assisting in the identification process.
• Free-living nematodes, can be recognized by their ‘rhabditiform esophagus'

General Biology

• Many gastrointestinal nematodes apply closely to the mucosal surface where oxygen tension is highest.
• Some swallow part-digested alimentary contents, and most feed on host secretions.
• Some take in desquamated epithelial cells or graze on mucosal lining cells.
• Plug feeders exhibit a more aggressive feeding behavior.
• They suck a mouthful of mucosa into their buccal cavity.
• Teeth and enzymes ingested reduce host tissue to a pulp which is swallowed.
• The worm then releases its grip and moves to a fresh site.
• Some species bury their heads deep into the mucosa to suck arterial blood.
• Different feeding mechanisms provoke different types of pathogenicity.
• Nematodes also produce excretory and secretory (ES) substances that may be pharmacologically active, immunogenic, or immunomodulatory.

Life-cycle

• The basic nematode life-cycle is straightforward, with many variations.
• Parasitic females produce eggs that pass out of the host, usually in feces.
• Four larval stages: first-stage larva, second-stage larva, etc., or L1, L2, etc.
• The stage that hatches out of the egg and the stage that is infective for the next host vary between nematode groups.
• Each larval stage develops until it outgrows its cuticle, which is then discarded.
• This process is called 'molting'.
• When it emerges from its final molt, the nematode is an immature adult, either male or female.
• After a maturation period, mating occurs.
• The whole progression: Egg → L1 → L2 → L3 → L4 → Adult.
• Asexual multiplication does not take place during larval phases.
• Each nematode egg has the potential to produce just one adult worm.
• Nematode eggs vary greatly in appearance, hence are useful for diagnosing parasitic infections as they can be detected in fecal samples.
• Animal parasitic nematodes are divided into bursate and nonbursate nematodes.
• To identify parasitic nematodes, look at the tail-end of the male worms.
• Presence of a bursa narrows the choice to the bursate superfamilies.

Bursate Superfamilies

• There are four bursate nematode superfamilies.
• Trichostrongyloidea, Strongyloidea, and Ancylostomatoidea (hookworms) are closely related as 'the strongyles'.
• They are predominantly gastrointestinal parasites with direct life-cycles.
• Metastrongyloidea are associated with the respiratory tract and typically utilize an intermediate host in their life-cycle.

Strongyles

• The informal term 'strongyle' has no zoological validity
• Veterinary Parasitology uses the term due to the species' biology, epidemiology and pathogenicity and its association with parasitic gastroenteritis
• Their eggs are indistinguishable, so laboratory diagnostic reports will often report the number of 'strongyle eggs per gram feces'.
• A 'typical strongyle egg' is approximately 80 µm long, oval, thin shelled and generally contains 4–16 cells when passed in feces.

General Strongyle Life-cycle

• Strongyle worms have similarities and follow a general pattern.
• The strongyle egg leaves the host with the feces.
• The L1 develops within the egg and hatches.
• L1 feeds on bacteria, molts and sheds the castoff cuticle to become an L2.
• The L2 also feeds on bacteria and molts to become the L3, but this time the shed skin is retained as a protective envelope, which is now called an 'ensheathed third-stage larva'.
• The L3 cannot feed but uses stored glycogen for locomotion.
• L3 have to leave the manure, cross the surrounding 'zone of repugnance' and climb onto more distant vegetation.
• L3 swim along films of moisture which is described as 'translation'.
• The ensheathed L3 is the infective stage to be swallowed for it to colonize a suitable host.
• On reaching its predilection site within the alimentary tract, the L3 burrows into the mucosa, loses its sheath, starts feeding again and molts.
• The L4 or immature adult breaks out of the mucosa (a damaging process called ‘emergence') and spends the rest of its life lying on the luminal surface.
• Egg-laying starts once adult worms have grown to maturity and mated.
• The prepatent period varies between superfamilies.
• Some strongyles can enter the host by skin penetration as well as by ingestion and some migrate through body tissues before settling in their predilection site.

General Strongyle Epidemiology

• The condition is parasitic gastroenteritis, diarrhe, weight-loss, or suboptimal weight-gains.
• PGE is a seasonal disease driven by climatic factors.
• Ambient temperature is the dominant force in temperate regions, while rainfall patterns are of greater significance in the tropics.
• Strongyle eggs and larvae will not develop at temperatures below about 8-10°C, and growth and development accelerate with the increasing warmth.
• The energy reserves of ensheathed third-stage larvae becomes more active at higher temperatures as its entirely dependent on stored glycogen

Type 1 and Type II Disease

• Once the L3 has infected a susceptible host, it is in a consistently warm environment with an abundant food supply and the parasitic phase can proceed without interruption.
• If this provokes clinical signs, the process has Type I' disease.
• Many strongyle and other nematode species can pause parasitic development for weeks.
• Arrested, inhibited development, or hypobiosis enables survival for the next generation by synchronizing the parasitic life-cycle with external events.
• Arrested development is genetically based and may be strain variation, Populations may behave differently in different localities.
• L3 may be able to switch on the mechanism while on pasture
• Arrested larvae can accumulate in host tissues over time, but may 'wake up' almost simultaneously.
• Damage caused can be extensive if large numbers of larvae emerge from the alimentary mucosa,
• This is known as 'Type II' disease.
• Cattle and sheep are able to develop protective immunity against many strongyle parasites if they are exposed for a long time.
• Calves and lambs stay vulnerable in their first grazing season, and disease is relatively uncommon later in life.
• Goats are always susceptible and horses occupy an intermediate position on this scale.
• Larvae can overwinter as preparasitic L3 on grass or as arrested larvae within host tissues.
• The proportion of the parasite population depends on the worm species and their susceptibility to local climatic conditions.
• What triggers the PGE epidemiological cycle each year depends on the animal.
• Typically there is normally only one significant source of pasture contamination for cattle and this is from new eggs three weeks after the calves ingested small numbers of overwintered I3.

Trichostrongyloidea

• Most trichostrongyloids are small worms, 0.5-2 cm long and they look like short lengths of cotton.
• Each has its own predilection site in the host, usually the stomach, abomasum or small intestine.
• The head end has no distinctive features and so male tail is used for identification as use is made of spicules
• The life cycle is typically 'strongyle'.
• The parasitic L3 and L4 stages develop within the lumen of a gastric gland or intestinal crypt before they emerge from the mucosa.
• The prepatent period is typically about 3 weeks.
• Trichostrongyloids are mostly of importance in ruminants but occur in other hosts like pigs, horses and poultry
• All contribute to PGE except Haemonchus, which is a blood-sucker provoking anemia instead of diarrhea.
• Some are primary pathogens e.g. Ostertagia and Teladorsagia and others (e.g. Cooperia) are less pathogenic.

Strongyloidea

• Strongyloid worms are often longer (1-5 cm) and always firmer than the trichostrongyloids.
• They reside in the caecum or colon.
• This life cycle is generally similar to trichostrongyloids, L3 that penetrate tissues of intestinal wall to provoke a nodule and L3 return directly to lumen and called ‘nonmigratory’.
• The larvae of some species, however, are 'migratory' and migrate around body.
• The prepatent periods that are species of strongyloid vary at three weeks to a year.
• Horse owners around the world are familiar with the ‘large and small strongyles,(‘red-worms’) are colloquial terms migratory genus Strongylus
• Another equine genus, Triodontophorus, is also nonmigratory,large strongyle like

Important facts for Strongyloidea

• Species of the 'nodular worm' Oesophagostomum have the same characteristics as ruminants and pigs, and diarrhea will sometimes occur.
• Ruminants also harbor Chabertia but it's minor clinic
• The Strongyloidea has two genera, the poultry gapeworm, and the swine kidney worm.

Ancylostomatoidea (Hookworm)

• HOokworm is a members of this superfamily, are similar in appearance to other Strngyloidea
• Have a large buccal cavity armed with teeth and/or cutting plates
• Their head is bent dorsally
• Hookworms live in their small intestine.
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Metastrongyloidea (Lungworm)

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