Series of Presentations on Nematodes from Veterinary Parasitology By Urquhat, Jenning

1. Nematodes
General Characteristics
• Nematodes are commonly called roundworms
because, as the name suggests, they are round when
viewed in cross section.
• They are cylindrical in structure and taper towards
their anterior and posterior ends.
• They are bilaterally symmetrical.
• The sexes are separate in most species, a few are
hermaphrodite.
• Nematodes that parasitize domestic animals are found
in all parts of the body but are most commonly found
in the digestive and respiratory tracts, and in the
circulatory system.

2.  Nematode parasites of domestic animals vary
greatly in size ranging from small hair-like worms
(up to 2 cm long) in the Superfamily
Trichostrongyloidea to large, robust worms ( up to
40cm long) in the Superfamily Ascaridoidea. The
following examples show the considerable
variations in size and appearance seen among the
nematodes.

3. Morphology
External Structures – cuticle
Nematodes are covered by a protective outer skin called
a cuticle which is naturally colorless and partly
translucent. The cuticle also lines the buccal cavity,
esophagus, excretory pore, vagina, cloaca and
rectum.
The cuticle is also resistant to host digestive enzymes
and in most nematodes is relatively impervious,
allowing only the passage of water molecules and
certain small water- soluble ions.
The cuticle also appears to function as "hydrostatic
skeleton of nematodes". Since the body cavities of
nematodes contain pressurized fluids the cuticle
apparently serves to maintain the body at a constant
diameter by resisting the internal pressure of these
fluids.

4. • The cuticle maintains a nematode's form and structure
and also provides an anchor for muscles.
• The patterns of the several cuticular layers and their
contents appear to allow simultaneous radial strength
and longitudinal flexibility. In other words, the cuticle
layers are arranged so as to maintain a constant body
diameter while, at the same time, allowing the
nematode to stretch longitudinally.
• A variety of organic compounds have been identified
in the cuticles of many nematodes. These include
amino acids, proteins, carbohydrates, lipids, RNA,
ascorbic acid, ATP and hemoglobin. Their presence
and variety suggests that the cuticle is far from inert
and is, in fact, metabolically activity most of the time.
• It is also known that the cuticle is antigenic and may
play an important role in eliciting the immune
responses of infected hosts.

5. • In many nematodes the cuticle is a smooth
outer layer but in others the cuticle may
have longitudinal and circular striations
and may be modified to produce a variety
of structures that can be useful in
identification of specific nematodes.
• Cuticular modifications at the anterior ends
of many nematodes may include leaf
crowns, vesicles, alae and papillae.

6. Leaf crowns (A) are rows of finger-like
projections surrounding the rim of the
opening to the buccal cavity. They are
particularly obvious in the strongyles
of horses
Vesicles are inflations of the cuticle
around the mouth - cephalic (B) and
the anterior esophagus -cervical (C).
Alae are, as the name suggests, "wing-
like" expansions of the cuticle.
Cervical alae (D) are located in the
terminal half of the esophageal
region.
Cervical papillae (E) are paired spine-
like projections found in the
esophageal region. Their function is
believed to be sensory.

7. Cuticle modifications - posterior end
Cuticle modifications at the posterior
ends of nematodes may include
caudal papillae and caudal alae.
Caudal papillae are cuticular
protuberances believed to be
sensory in function. They vary in
shape from small "button-like"
protuberances to longer "stalk-like"
structures.
Caudal alae are wing-like expansions
of the cuticle and may be found at
the tail ends of nematodes.
In male nematodes, of some species,
caudal alae are greatly expanded to
form a structure called a copulatory
bursa - so called because it is used
by the male to grasp the female
during copulation

8. • A bursa has two lateral lobes and in some
species, a dorsal lobe. The bursa is supported
by finger-like structures called rays which are
caudal papillae with associated muscle tissue ..
• Each lateral lobe usually contains six rays and
the dorsal lobe has one ray. The number and
shape of these rays may be valuable
characteristics for species identification of
nematodes in the order Strongylida.
• When resting, the bursa looks like a relaxed,
folded hand but during copulation it is greatly
expanded and used to grasp the female.

9. Internal Morphology
• The basic nematode body consists of an outer tube
(the body wall) enclosing an inner tube (the digestive
tract).
• The fluid-filled body cavity lies between these two
tubes and contains the reproductive tract. This body
cavity is a pseudocoelom because, unlike a true
coelom, it does not possess a cellular lining or
peritoneum. There is no vascular system in the
nematodes, instead the circulation of nutrients in the
pseudocoelom is assisted by body movements and
locomotion.
• The body wall has three layers: cuticle, hypodermis
and an inner layer of muscle cells. The hypodermis,
lying beneath the cuticle, is relatively thin. The
primary function of the hypodermis is to secrete the
cuticle.

10. • The hypodermis has four longitudinally
thickened areas or cords that protrude into the
body cavity in the mid-dorsal, mid-ventral and
lateral regions.
• The dorsal and ventral cords contain
longitudinal nerve trunks while the lateral cords
contain excretory canals.
• Internal to the hypodermis are one or more
layers of longitudinally arranged striated
muscles. These are closely associated with the
hypodermis and also connect to the cuticle by
fibers passing from the contractile part of each
muscle cell.

11. Movement of Nematodes
• Nematodes are covered by an outer cuticle that is
structured so as to maintain the body at a constant
diameter while allowing longitudinal flexibility.
• Since nematodes have longitudinally arranged but
no circular muscles, contraction of muscles bend
the body dorsally or ventrally.
• Since the cuticle prevents radial swelling,
contraction of one muscle group will stretch
another. This situation ensures that the dorsal and
ventral muscles will act antagonistically to each
other, allowing nematodes to move in a sinusoidal
manner by undulating waves of muscle
contractions.
• These movements allow nematodes to move
among soil particles and swim in body fluids of a
host.

12. Digestive System
• The digestive system of nematodes is usually a
simple tube with the majority of variations
occurring in the sizes of the mouth
opening, buccal capsule (syn = buccal cavity)
and esophagus.
• The mouth may be a simple opening with a tiny
buccal cavity leading directly to the esophagus.
At the other extreme, the mouth opening may
be quite large leading to a prominent buccal
cavity with thick walls and often containing
teeth.

13. • The type of mouth opening and
accompanying buccal cavity appears to be
related to diet. A large mouth opening and
prominent buccal cavity is found in those
nematodes that feed by taking a bite of
mucosa, and drawing it into the buccal cavity
where it is digested. These nematodes are
often called "plug feeders".

14. • In nematodes that feed by simple ingestion of host
fluids the mouth opening and accompanying buccal
cavity are generally quite small.
• In nematodes that are described as mucosal grazers,
rather then plug feeders, the mouth opening and its
buccal cavity are usually intermediate in size.
• In most nematodes, the esophagus is muscular and is
used to pass food into the intestine by a pumping
action. The shape of the esophagus may be used as
an initial screening characteristic for identification. For
example, adult nematodes belonging to the
Superfamily Strongyloidea have a club-shaped
(strongyliform) esophagus which, as the name
suggests, is shaped like a club.
• Adult members of the Superfamily Oxyuroidea have
an esophagus which has a prominent posterior bulb
separated from the rest of the esophagus by a narrow
isthmus.

15. • The intestine is a straight tube roughly circular in
cross section and with a wall consisting of a single
layer of columnar (rectangular-shaped) cells.
• The lumen surfaces of these cells consist of
multitudes of projecting microvilli used in
absorption and because of their vast numbers
provide an immense absorptive capacity.
• The intestine terminates in a rectum in females
and a cloaca in males. The cloaca is a common
termination for the intestinal tract and the vas
deferens in adult males. In either case, the cloaca
(males) and rectum (females) leads to an anus
which usually opens to the outside at the posterior
end of the body.

16. Reproductive System
• The sexes are separate in most species of nematodes and
males are always smaller than females because females need
to accommodate the production of large quantities of eggs.
• This disparity in size between male and female nematodes is
shown in the accompanying image of Syngamus trachea in
copulation: the smaller, pale blue, male (0.5 cm long) is dwarfed
by the larger female (2.0 cm long) whose body is packed with
coils of its yellowish-white reproductive tract.
• The female reproductive system is tubular and in most
nematodes of veterinary importance consists of two ovaries,
each of which connects to an oviduct, and a uterus. The two
uteri end in a common vagina which opens to the outside by a
vulva which is often covered by a protective flap of cuticle, the
vulva flap. In female members of the order Strongylida a
muscular structure called an ovejector controls the exit of eggs
from the uterus.

17. • The male reproductive system in nematodes of veterinary
importance is a single tube differentiated into testis, seminal
vesicle and vas deferens and terminating in a muscular
ejaculatory duct which empties into the cloaca.
• In most nematode species there are two accessory male
organs, spicules and a gubernaculum.
• Spicules are made of cuticle, are often paired and used in
copulation to dilate the female vulva.
• The gubernaculum is a cuticular modification of the dorsal wall
of the cloaca and is used to guide the spicules down through
the cloaca so as to penetrate the female vulva. In this particular
species, the spicules normally protrude from the cloaca even at
rest.
• In some nematodes a copulatory bursa is used to grasp the
female at the level of the vulva, the rigid spicules are then
extruded and inserted into the vagina to keep it open so that
amoeboid sperm can be transferred for fertilization to occur.

18. Basic Nematode Life Cycle
• Despite the diversity and complexity of many nematode life cycles, all of
them can be related to the same basic pattern.
• This pattern is illustrated by the adjacent figure and consists of two
phases, parasitic and pre-parasitic. The parasitic phase takes place inside
the definitive host while the pre-parasitic phase occurs either as a free living
phase in the external environment or inside a second host, called
an intermediate host. This basic life cycle also consists of seven stages, an
egg, four larval stages (L2, L2, L3, L4) and two adult stages comprising
separate males and females. Sometimes the sexually immature adult stages
are called L5's
• In most species sexual reproduction by adult nematodes occurs within an
infected definitive host. Eggs are laid by the female and pass from this host
into the external environment. These eggs must pass through the three
developmental stages (L1, L2, and L3) before the nematode is again
infective for another host.
• This is an important point to emphasize - In the vast majority of
nematode life cycles the stage that passes from the definitive host is
not the same stage that is infective for another definitive host. The
nematode stage (usually an egg or L1) that passes from a definitive
host must develop through to a stage (usually the L3) that can then
infect another host.

19. • A first stage larva develops inside an egg, then hatches.
• Initiation of the hatching process is controlled by several factors
including temperature and moisture levels in the external
environment. Hatching occurs only when environmental
conditions are favorable for survival of hatched larvae. These
conditions stimulate the enclosed larvae to assume its own role
in hatching by secreting enzymes to digest the surrounding egg
membranes, then exerting pressure against the weakened
membranes to rupture them and escape.
• This newly hatched L1 feeds on bacteria and grows until
constrained by its outer skin or cuticle. At this point further
growth is possible only if the larva grows a new, more flexible,
cuticle and casts off its old outer cuticle. This process is called
molting and involves two steps.
1. Synthesis of a new cuticle by the hypodermis. At this stage the
larva, with a new cuticle is completely enclosed by its old
cuticle.
2. Exsheathment - a process by which the old cuticle is
loosened and ruptured followed by the larva moving out of the
casing of the old cuticle.

20. • Nematodes molt four times during each life
cycle with a molt occurring at the end of
each larval stage. Therefore, molts separate
the first and second larval stages (L1 and
L2), the second and third larval stages (L2
and L3), the third and fourth larval stages
(L3 and L4) and also the fourth larval
stages and immature adults (L4 and L5).
• The L5 grows to the limit of its new cuticle,
at the same time developing into a sexually
mature adult male or female.

21. •.