Page 101 - Medical Parasitology_ A Textbook ( PDFDrive )

94 10 Nematodes: Roundworms

a b

c

Fig. 10.10 Enterobius vermicularis. (a) Adult anterior section with bulb shaped oesophagus, (b)
Egg, (c) Cross sections of adult

Habitat
Adult worms are found in the caecum and appendix.

Morphology
The adults are short, white, fusiform worms with pointed ends. The mouth is sur-
rounded by 3 wing-like cuticular expansions (cervical alae). It has a bulb-shaped
oesophagus (Fig. 10.10a). The female is 8–13 mm long. Its posterior third is pointed
(pin-like). The worm is oviparous. The male worm is 2–5 mm long. Its posterior end
is curved ventrally and carries a copulatory spicule. The lifespan of the adults is
about 7–8 weeks.
Enterobius vermicularus egg has a characteristic D-shape, flattened on one side
and convex on the other measuring 50–60 μm (Fig. 10.10b). The shell is thick and
the egg contains a fully formed coiled embryo. It becomes infectious 4–6 h after
being deposited on the perianal skin. Under cool moist conditions, the egg remains
viable for about 2 weeks.

Life Cycle (Fig. 10.11)
(1) Eggs are laid by the gravid female worm in perianal region. Larvae inside the
eggs mature within 4–6 h. (2) Embryonated eggs are ingested by humans (fingers to
anus to mouth or via inhalation). (3) Eggs hatch in the intestine. (4) Adults develop