PARASITOLOGY

PARASITOLOGY

n Development of the worm takes place within the subcutaneous tissues of the human

host. The adult females can grow to a length of 40 ×0.3 cm whereas the males are by

comparison very small, 0.3 ×0.15 cm. In young infections the worms migrate

through the subcutaneous tissue. In older established infections the worms slow and

eventually cease moving and coil up within a subcutaneous nodule (see Fig. 4.7).

Normally a nodule contains a single male and female. The female continues to pro-

duce larvae many of which remain just below the surface of the skin.

Brugia malayi, Loa loa, Dipetalonema perstans, and Mansonella ozzardiare further ex-

amples of filarian nematodes which have arthropod vectors.

n 4.5.3LOA LOA

The adult worm (a nematode) migrates through the subcutaneous tissues of its human

host. The mature female is 60 ×0.5 mm and the male 32 ×0.4 mm. The female pro-

duces live microfilariae which penetrate capillaries and then circulate round the body via

the bloodstream.

n A small blood sucking fly (Chrysops) pierces the skin of the host and the microfilariae

are taken up with a blood meal. The larvae first reside in fat bodies inside the stomach

of the fly. The L 3 larvae, the infective stage, emerges within 10 days and migrates to

the biting mouthparts.

Sections through female

worm containing live

larvae in uterus

nSection through a skin

nodule caused by

Onchocerca volvulus

Female worm

Cuticle

Uterus with

live larvae

Ground substance in

which cells are embedded

Mononuclear

cells attached

to worm cuticle

Fibrocytes

Deposits of fibrin

collagen forming part

of ground substance

Inner fibrous layer

with inflammatory cells

Middle fibrous layer

Outer fibrous capsule

nEnlarged area showing cells attached to ageing worm

• Figure 4.7The adults of
  the nematode Onchocerca
  spp live in the
  subcutaneous tissues of
  mammals. Old established
  worms become enclosed
  in skin nodules. Each
  nodule normally contains a
  small male and a large
  female worm and
  numerous larvae in the
  nodular tissue.