Page 107 - Medical Parasitology_ A Textbook ( PDFDrive )

100 10 Nematodes: Roundworms

Fig. 10.14 Wuchereria
bancrofti microfilaria

Habitat
The adult worms reside in the lymphatic system of human. The microfilariae are
found in blood.

Morphology
The adults are whitish, thread-like worms with smooth cuticle and tapering ends. The
female is larger (70–100 mm × 0.25 mm) than the male (25–40 mm × 0.1 mm).
Microfilariae measure 250–300 μm in length (Fig. 10.14). It has a body sheath.
When stained with Giemsa, morphological details can be made out. Body nuclei are
seen and they are discrete and countable. The sheath does not take up stain with
Giemsa. At the cephalic end is a clear space called the cephalic space. The microfi-
laria of W. bancrofti has a cephalic space as long as it is broad (ratio is 1:1), while
in B. malayi, it is longer than its breadth (ratio is 2:1). In W. bancrofti microfilaria,
there are no terminal nuclei while in B. malayi microfilaria, there are presence of 2
terminal nuclei. Microfilariae do not multiply or undergo any further development
in the human body. If they are not taken up by a female vector mosquito, they die.
Their lifespan is about 2–3 months. The microfilariae show nocturnal periodicity in
peripheral circulation and are present in peripheral blood only at night (between 10
pm and 2 am). This coincides with the night biting habit of the vector mosquito.

Life Cycle (Fig. 10.15)
(1) When vector mosquito takes a blood meal, the infective L3 larvae enter the human
skin. (2) They enter the circulation and develop into adults in the lymphatics. (3) The
female adult worm produces sheathed microfilariae that migrate into lymph and blood
channels. (4) When vector mosquito takes a blood meal, it ingests microfilariae. (5)
The microfilariae shed sheaths, penetrate the midgut of the mosquito, and migrate to
the thoracic muscles. (6) It develops into L1 larva. (7) It moults twice and develops
into L3 larva. (8) The L3 larvae migrate to the head and proboscis of the mosquito.
The vector or intermediate host of W. bancrofti is Culex quinquefasciatus.