Nematodes (Round worms)
General properties
a. Morphology and anatomy.
Nematodes have an anterior mouth and posterior anus and range from
90 µm to 1000 mm in length. They are cylindrical with a thick surface
tegument; their shape is maintained by a positive hydrostatic pressure that
limits movement to a sinusoidal snake-like motion.
b. Life cycle and reproduction
(1) All nematodes hatch from eggs as first-stage larva(L1) and
metamorphose by molting through second-,third- and fourth-stage
larvae (L2, L3, and L4) to become adult worms. Life cycles range from
simple (e.g., ingestion of the egg is followed by development from eggs to
adult in the intestine) to complex (e.g., involving tissue migration or
intermediate hosts or vectors).
(2) All species are diecious but some can reproduce by parthenogenesis.
Intestinal Nematodes
1.Entrobius vermicularis (Pinworm)
a. General properties
E. vermicularis is a small, white round worm. Females average
10mm in length, males 3mm. The worm has a direct life cycle
with no tissue migrate phase.
b. Epidemiology
(1) Distribution. E. vermicularis is a cosmopolitan, though this
helminthes is most common in temperate areas.
(2) Incidence and prevalence. More than 1 billion cases occur world
wide. Peak prevalence is in the 5- to 6-year-old age group; 30% 40% of affected children are white, and 10% - 15% are black.
Adults are more refractory, indicating that resistance may occur
with age (possibly by acquired immunity).
(3) Transmission. eggs deposited in a sticky secretion on the perianal
skin by nocturnally wandering female worms. Eggs also contaminate
bed clothes and can be aerosolized during bed making. Infection
occurs via ingestion or inhalation of eggs.
Entrobius vermicularis eggs, larva & adult
Entrobius vermicularis life cycle
Entrobius vermicularis life cycle
c. Pathogenesis and clinical disease.
The eggs hatch in the large intestine. Worms mature in 2-4 weeks and
live for 2 months. Continuous reinfection is common.
(1) approximately one-third of infections are asymptomatic.
(2) The most common presentation is irritation and pruritus ani.
sometimes itching is severe, and secondary bacterial infection
occurs. Occasionally, necrosis of the mucosal surface produces pain
when nerve endings are exposed.
(3) Worms often occur in the appendix and may be associated with
appendicitis, but causation has not been proved. Rarely, worms may
migrate to ectopic sites, mostly within the female genitourinary tract
d. Laboratory diagnosis
(1) Cellophane tape test. E. vermicularis females lay their eggs on the
perineum during the night. Touching the perianal skin with the sticky
side of the tape will pick up the eggs; the tape is affixed to a
microscope slide and examined. Eggs are oval, approximately 55X25
µm in size, and flattened on one side, and they contain a larva.
(a) Specimens should be collected prior to bathing or using the toilet.
(b) Four to six consecutive negative pinworm tape preparations are
required to rule out infection.
(2) Stool samples. Eggs are only rarely seen in stool, but in patients with
heavy worm burdens, adult female worm may be seen in stool samples.
e. Control and prevention
Pin worm control and prevention measures include practicing proper
personal hygiene, particularly hand washing; apply an ointment or salve
to an infected perianal area to help prevent egg dispersal into the
environment; and avoiding scratching the infected area. Furthermore,
thorough cleaning of all potentially infected environment surfaces,
including linens, and providing treatment to all household members
are important steps to help prevent further infections. Because of the
ease with which this parasite is capable of being transmitted, total
eradication of pin worm is highly unlikely in the near future.
Prevention of reinfection:
Some physicians recommend treating the patient’s entire family.
The eggs are not resistant to desiccation and usually only
survive 6-12 hours, but they may remain viable for a few weeks
in colder, more humid environments, bed lined and towels
should be washed. Single dose of Mebendazol and Pyrantel
pamoate /os (oral) are highly effective. Treatment should be
repeated after 10-14 days to kill the newly acquired developing
adult worms.