PHYLUM NEMATODE

This phylum is usually ubiquitous. Nematodes are free-living in marine, freshwater and land
habitats and parasitic in animals and plants. They are clearly of great economic importance as they
exist in extraordinary numbers and play a significance role in the total matter and energy cycle of
the biosphere. Nematodes are pseudocoelomate, bilaterally symmetrical, triploblastic, un-
segmented, vermiform, organ-system grade of organization with complete digestive tube. The
worms are enclosed in a tough but flexible cuticle. There are no respiratory, blood or other
circulatory organs.

Distinguishing characteristics of this large group of animals are their cylindrical shape; their
flexible, non-living cuticle; their lack of motile cilia or flagella; the muscles of their body wall,
which have several unusual features, such as running in a longitudinal direction only, and eutely.
Correlated with their lack of cilia, nematodes do not have protonephridia; their excretory system
consists of one or more large gland cells, or both cells and canals together. Their pharynx is
characteristically muscular with a triradiate lumen.

Body wall muscles of nematodes are very unusual. They lie beneath the hypodermis and contract
longitudinal only. There are no circular muscles in the body wall. The muscles are arranged in four
bands, namely ventral, lateral and two dorsal epidermal chords, separated by four hypodermal
cords. The fluid-filled pseudocoel, in which the internal organs lie, constitutes a hydrostatic
skeleton. The alimentary canal of nematodes consists of a mouth, a muscular pharynx, a long non-
muscular intestine, a short rectum, and a terminal anus. The nervous system is made up of a nerve
ring around the oesophagus which connects to six anterior nerves and six or more posterior nerves.
Sensory papillae are concentrated around the head and tail. Most nematodes are dioecious. Males
are smaller than females, and their posterior end usually bears a pair of copulatory spicules.
Fertilization is internal. There is no asexual reproduction or regeneration.

There are two basic types of excretory systems in nematodes viz:

Glandular/Non-tubular type- This is called renette and consists of a large cell or sub-ventral glands
in the coelomic cavity leading to the excretory pore by duct, terminating in an ampulla.

Tubular type- This is made up of two long canals, lying in the lateral hypodermal cords, connected
anteriorly by a transverse canal, opening into excretory pore.

The phylum is initially divided into two- the Aphasmidia and the phasmidia. These were later
renamed Adenophorea (gland bearers) and Secernentea (secretors) respectively. The Secernentea
share several characteristics, including the presence of phasmids, a pair of sensory organs located
in the lateral posterior region, and this is used as the basis for this classification. Most common
nematodes belong to Secernentea. Examples are Ascaris, Ancylostoma, Wuchereria bancrofti.
Ascaris: It is an endoparasite in the intestine of man. It is one of the most common nematode
parasites of humans. It is cylindrical, and tapering at both ends. The sexes are separate and there
is sexual dimorphism. The body is covered with a smooth, tough and elastic cuticle which is
striated transversely and gives the pseudo-segmented appearance to the worm. The mouth is
triradiate aperture with three lips, one dorsal and two ventro-lateral. The dorsal lip has double pair
of papillae and ventro-lateral lips have a pair of papillae each. All the papillae are sensory in nature.
The lips have teeth. Sexes are morphologically different. The male has a sharply curved posterior
end with a main genital pore from which two spicules or penial setae project. The female is
straighter and the genital pore is is mid-ventral on the body about one third the distance from the
anterior end. There are two distinct longitudinal lateral lines on the cuticle marking the position of
the excretory canals. The body wall is made up of outer cuticle, middle epidermis and inner muscle
layer.

LOCOMOTION: Movement is aided by the contraction of longitudinal muscles, which produce

produces thrashing movements. The presence of a stiff cuticle and absence of circular muscles
results in poor power of locomotion. The pseudocoelom provides tugor pressure, while the somatic
muscles contract in a coordinated manner. Alternate contraction of dorso-lateral and ventro-lateral
muscle generate a series of backwardly moving undulatory dorso-ventral flexures of the body in a
snake-like movements, which result in wriggling motions.

NUTRITION: It has a complete alimentary canal of mouth to anus. Its food comprises blood;
tissue exudes and partly or fully digested food of the host. Food is digested extracellularly in its
intestine. Intracellular digestion has been reported to occur in the cells of intestinal wall as they
engulf solid particles to digest intracellularly. The undigested items are egested through the anus.

RESPIRATION: There are no respiratory organs, but it carries on anaerobic respiration as oxygen
content in the intestine of host is very poor.

EXCRETION: The excretory system is made of some glandular structures and a system of canals.
The glandular portion known as renette, opens into two major lateral ducts joined at the anterior
region in an H-shaped fashion. The excretory pore is located ventrally to the mouth.

NERVOUS SYSTEM: The nervous system is well developed and complicated and like the
excretory system, it is situated in the body wall. At the region of the pharynx is a nerve ring with
two paired ganglia. There are two major fibres namely, a dorsal and a ventral nerves interconnected
by transverse fibre. On the body surface are various papillae which are sensory in function.

SENSE ORGANS: Sense organs are very simple and they are either as minute elevations or pits
in the cuticle of the body. They are the labial papillae, amphids, phasmids, cervical papillae,
cephalic papillae and genital papillae.

REPRODUCTION: The sexes are separate and gonads are well developed. The gonads and their
ducts form continuous tubes. It is monogenetic. In males, testes lead to vas deferens which leads
to ejaculatory duct that opens out at the curved posterior end. In females, the ovary leads to the
oviduct, then to the seminal receptacle and then to the uterus, which leads to the vagina that opens
out at the anterior part of the body.

Fertilization through copulation leads to production of several eggs stored at uterus. The eggs are
discharged into intestine of the host who expel them with faeces to the outside. A female Ascaris
may lay 200,000 eggs a day, carried by the host’s faeces. Given suitable soil conditions, embryos
develop into infective juveniles within two weeks. Direct sunlight and high temperatures are
rapidly lethal, but the eggs have an amazing tolerance to other adverse conditions, such as
desiccation or lack of oxygen. Shelled juveniles can remain viable for many months or even years
in soil. The eggs are un-segmented when they leave the host. The larva develops within the eggs
but does not hatch out until the mature egg is swallowed. The juvenile or rhabditiform larva moults
to become the second stage juvenile or second stage rhabditoid. This is the infective stage.
Moulting of the first stage juvenile to 2nd stage juvenile occurs in the open. Transmission of
infection is when a new host swallows the infective eggs under condition of poor sanitation usually
by drinking contaminated water or eating contaminated vegetable matter. The larva is thus released
when the infective eggs reach the small intestine of the host and the egg shells are dissolved by
action of host’s digestive juices and develops into adult nematode. The larva does not develop in
the intestine of host. Before the larva becomes adult, it undergoes a typical wandering tour. It bores
through the intestine to the liver, the heart, the lungs, the trachea and eventually back into the
intestine where another life cycle begins. Nematodes grow by moulting. In the alveoli of the lungs,
the larva moults to become 3rd stage larva; in the intestine the fourth and final moulting takes place
and it grows into adult and attains maturity.

The adverse effects of Ascaris can be grouped into two: the effects due to the adults and those due
to the migrating larvae. The consumption of semi-digested food from the host may contribute to
malnutrition and underdevelopment in children. Adult worms interfere with normal metabolism
through blockage of the lumen, reducing the absorptive area; absorption of intestinal contents;
ingestion of blood by destruction of intestinal mucosa; neutralizing pepsin and trypsin with an anti-
enzyme, ascarase. All allergic responses to waste products of the parasites are evident in most
patients. These present in form of abdominal pain, rashes, insomnia, restlessness etc. It can
completely block the intestinal lumen with fatal consequences. Adult worms produce pathological
effects when they wander into abnormal sites. Thus adult worms have been found clogging the
appendix, bile and pancreatic ducts, trachea, Eustachian tube and urino-genital ducts with grave
consequences. Worms can also produce bizarre effects when they migrate upwards to the stomach,
the resulting nausea causing the host to vomit live worms through the mouth. Aspiration of worm
can cause death. They can also emerge involuntarily through the nose or anus to the chagrin and
embarrassment of the unsuspecting host. Infection rates tend to be highest in children, and males
tend to be more heavily infected than females, presumably because boys are more likely to ingest
dirt.
 Life cycle of Ascaris lumbricoides, the cause of human ascariasis as shown in the diagram
below:

Adult Ascaris lumbricoides worms (1) live in the lumen of the small intestine. A female may
produce 200,000 eggs each day, which are passed with the feces (2) of the host. Ingested
unfertilized eggs are not infective, but fertile eggs begin to develop and become infective after 18
days to several weeks (3), depending on environmental conditions (an optimal environment being
moist, warm, shaded soil). After infective eggs are swallowed (4), the larvae hatch (5), invade the
intestinal mucosa, and are carried via first the portal and then the systemic circulation to the lungs
(6). The larvae mature further in the lungs for 10 to 14 days, then penetrate the alveolar walls,
ascend the bronchial tree to the throat, and are swallowed (7). Upon reaching the small intestine,
they develop into adult worms (1). Between two and three months are required from ingestion of
infective eggs to oviposition (egg-laying) by the adult female. Adult worms can live one to two
years.

Ancylostoma: This is the common hookworm man and it is so named because the anterior end
curves dorsally suggesting a hook. It is a small and cylindrical in shape. Sexes are separate. The
posterior of the female worm tapers bluntly in a short post-anal tail while that of the male ends in
copulatory bursa that surrounds the cloaca and it has six muscular rays. The bursa has three lobes,
two lateral and one dorsal. The anterior end of both sexes is slightly bent dorsally and has a large
buccal capsule that is lined with hard substance and is provided with six teeth. The body of
hookworm is covered externally by cuticle. This is followed internally by the epidermis and the
longitudinal musculature. It is pseudocoelomate.
The mode of respiration, excretory system, locomotion, nervous system and receptors are like
those of Ascaris.

NUTRITION: The alimentary canal is simple and complete. It has mouth, buccal capsule,
muscular pharynx having a triradiate lumen lined by cuticle, oesophagus bulb, intestine, rectum
and cloaca in male and anus in female. It feeds on intestinal mucous membrane and blood. It has
oesophageal gland that secretes a ferment which prevent the clotting of blood when it feeds on the
blood of the host.

Eggs are passed in the stool (1), and under favorable conditions (moisture, warmth, shade), larvae
hatch in 1 to 2 days. The released rhabditiform larvae grow in the feces and/or the soil (2) and after
5 to 10 days (and two molts) they become become filariform (third-stage) larvae that are infective.
(3) These infective larvae can survive 3 to 4 weeks in favorable environmental conditions. On
contact with the human host, the larvae penetrate the skin and are carried through the veins to the
heart and then to the lungs. They penetrate into the pulmonary alveoli, ascend the bronchial tree
to the pharynx, and are swallowed. (4) The larvae reach the small intestine, where they reside and
mature into adults. Adult worms live in the lumen of the small intestine, where they attach to the
intestinal wall with resultant blood loss by the host (5). Most adult worms are eliminated in 1 to 2
years, but longevity records can reach several years.

REPRODUCTION: Sexes are separate and sexual dimorphism is well distinct. The male
reproductive organs open into the cloaca while that of the female opens out by the gonophore
situated at the junction of the posterior and the middle third of the body. Life cycle is monogenetic.
No intermediate host. Copulation occurs in the intestine of the host and the fertilized eggs then
pushed into the uteri for laying through vagina and gonopore into the intestine of the host. Eggs
pass out with the host’s faeces and these eggs are not infective to man. They require well aerated,
moist, and loamy or humus soils with natural pH and dense shade to embryonate rapidly. The eggs
hatch into rhabditiform larvae and moult twice to become the filariform larvae which are infective
to man. The filariform larva penetrates the skin of the feet, hands, buttocks etc- areas which are in
frequent contact with the soil. They undergo behavioural responses like thermotaxis, negative
geotaxis and vertical migration up and down and not laterally to enhance their chance of infection.
In the subcutaneous tissues, it enters the lymphatic and small venules. Then, it goes through the
heart, lung, trachea, stomach and finally into the intestine. After the larval migration, the larvae
undergo the final moult in the intestine to produce the adults. In few weeks, it becomes sexually
mature and the cycle begins again.

The hallmark of hookworm disease is iron deficiency anaemia, a condition which should give no
cause for concern when nutrition is adequate, or could be readily reversed by iron therapy.
Morbidity in hookworm infections can be either acute or chronic; acute infection is due to activities
of migrating larva and the adults recently reaching the intestine, while morbidity due to chronic
infections arise from the activities of established adults in the intestine as well as the consequences
of the physiological, biochemical, and haematological disturbances they cause. Penetrating larvae
produce inflammation (dermatitis) which is intensified by secondary bacterial infection causing
irritation of varying severity known as ground itch or dew itch. During escape from capillary in
the lung alveoli and progress up the bronchi to the throat, they produce haemorrhage when large
number of worms are involved; otherwise, a dry cough and sore throat may be the only symptoms.
On arriving in the intestine the worms cause severe abdominal pain, loss of appetite, nausea, and
vomiting, flatulence, etc. associated with abdominal pain are indigestion, craving to eat soil
(geophagy) and diarrhea produced on arrival of the fourth stage larva, and dysentery and
constipation arising from the bloodsucking activities of adult worms in the intestine. The adult
worms in the intestine cause anaemia and such chronic malnutrition, particularly in the young,
often causes stunted growth and below-average intelligence. It also causes loss of energy in
children. This can be removed by restoring the haematological level with iron therapy. It also
secretes neutrophil factor that interferes with activities of neutrophil. Heavy infections often
produce severe anaemia, protein deficiency, dry skin and hair, facial and peripheral edema,
distended abdomen (in children), stunted growth, delayed puberty, mental dullness, cardiac failure,
even death.

Wuchereria bancrofti: This is the nematode found in the lymphatic systems (lymphatic vessels
and lymph nodes) of man. Adult worms are long and slender, hair-like, transparent and often
creamy-white in colour with a smooth cuticle. They taper towards both ends and bluntly rounded
ends. Their head is slightly swollen and bears two circles of well-defined papillae. Their mouth is
small and unarmed without lips; a buccal capsule is lacking. The oesophagus consists of muscular
anterior and glandular posterior regions. Males are about 40 mm and 100 µm wide. Their posterior
end is ventrally curved sharply and they possess 2 distinctly unequal copulatory spicules. Females
are 6 cm to 10 cm long and 300 µm wide. Their genitals open through vulva which is near the
level of the middle of their oesophagus.

The adults live in the lymph glands of human where they form tight coils. The females are
ovoviviparous, producing thousands of pre-larval stage called microfilariae which are ensheathed
in egg membrane. The microfilariae pass from the lymphatic vessels into the blood stream through
the thoracic duct. The microfilariae exhibit nocturnal periodicity in the peripheral blood: they
accumulate in the peripheral circulation around the midnight (between the hours of 2200 and
0200), but remain in the deep circulation particularly in the pulmonary vessels of the lungs during
the daytime. Concentration of microfilariae in the peripheral blood at night is correlated with the
peak feeding period of the mosquito vector, but this does not explain why the microfilariae should
migrate to the lungs in the day. However, the periodicity of the microfilariae is reversible by
changing the sleeping time of the infected person, vigorous exercises at night or a move by twelve
hours in geographic time zone.

When a mosquito bites man, it sucks blood containing microfilariae, which lose their sheath in the
stomach and migrate to the thoracic muscles within few hours. They undergo the first moult and
become the second larva stage which is the characteristic sausage shape. Two weeks after, the
second moult occurs producing the third stage (filariform) larva which is the infective stage. The
larvae migrate to the proboscis sheath and when the mosquito bites man, the larva escape onto the
skin and gain entrance into the body through the bite wound; thus gaining entry into the lymphatic
system through the subcutaneous tissues. They migrate through the lymphatic system, finally
settling in their definitive site where they moult twice and become sexually mature in about 9
months. The life span of adult worm is between 5-10 years and microfilariae that are produced
after one year of infection can survive between 6-24 months.
During a blood meal, an infected mosquito introduces third-stage filarial larvae onto the skin of
the human host, where they penetrate into the bite wound (1). They develop in adults that
commonly reside in the lymphatics(2). The female worms measure 80 to 100 mm in length and
0.24 to 0.30 mm in diameter, while the males measure about 40 mm by .1 mm. Adults produce
microfilariae measuring 244 to 296 μm by 7.5 to 10 μm, which are sheathed and have nocturnal
periodicity, except the South Pacific microfilariae which have the absence of marked periodicity.
The microfilariae migrate into lymph and blood channels moving actively through lymph and
blood (3). A mosquito ingests the microfilariae during a blood meal (4). After ingestion, the
microfilariae lose their sheaths and some of them work their way through the wall of the
proventriculus and cardiac portion of the mosquito's midgut and reach the thoracic muscles (5).
There the microfilariae develop into first-stage larvae (6) and subsequently into third-stage
infective larvae (7). The third-stage infective larvae migrate through the hemocoel to the
mosquito's prosbocis (8) and can infect another human when the mosquito takes a blood meal (1).

Before microfilariae appear in the blood, the infection is symptomless. The pathogenesis of the
disease (elephantiasis) is determined mainly by adult worms. The accumulation of living and
worms eventually obstructs the lymph circulation and the blockage may result in the disease in
which the limbs, mammary glands, scrotal sac and other parts grow to enormous size. This acute
stage is marked by acute inflammation of various parts of lymphatic system which become
swollen and painful, accompanied by headache, nausea, elephantoid or filarial fever with high
temperature and sweating. The initial inflammatory stage is characterized by swelling
granulomatous lesions, swelling, and impaired circulation. Adult worms in lymph cause dilation
of the channels and interfere with lymph flow, resulting in lymphedema. Patients with
lymphedema have periodic attacks of adenolymphangitis (inflammation of lymph channels),
lymphadenitis (inflammation and enlargement of lymph nodes and lymphatic glands) and
lymphangitis (inflammatory reactions of lymphatic system) This stage is followed by
enlargement of the lymph nodes and dilation of the lymph channels, which, over the years,
harden and become infiltrated and clogged with fibrous tissues elements, resulting in some of the
untreated cases in the condition known as elephantiasis, which is typically associated with the
gross expansion of the tissues of the lower limbs, vulva, breasts, arm and scrotum which become
swollen. Occasionally, hyperplasia of muscle fibres may be observed.