PARASITOLOGY

PARASITE EXAMPLES GROUPED ACCORDING TO LIFE-CYCLE

A general increase in the number of eosinophils (eosinophilia) and the serum antibodies
IgG and IgE is often symptomatic of this infection. Large numbers of mature adults can
cause mechanical blockages of the bowels.

n 4.1.3ANCYLOSTOMASPP (HOOKWORMS)
The adult hookworms live in the mucosa of the small intestine. They become embedded
within the villi and use their biting mouthparts to feed off blood. They are able to
secrete an anti-coagulant to keep the blood flowing.

n The adult females are about 12 ×0.6 mm and the males are 10 ×0.45 mm. An adult

female produces about 28,000 eggs per day which pass out of the body via the faeces.
n Once the eggs make contact with warm moist soil they embryonate. Within 48 h the

first stage larvae (rhabditiform) hatch out and feed on soil bacteria and debris.
n After two further moults while in the soil they develop into the infective stage, the

filariform larvae. These larvae crawl into a position, such as on blades of grass or a point
high enough to make contact with humans.
n The filariform larvae can actively penetrate skin through hair follicles or damaged

skin. Once they reach the dermal layers of the skin they first migrate along through
the dermal layers and eventually enter a blood vessel.
n After moulting into fourth stage larvae they enter into the bronchi of the lungs. From

the lungs they crawl up the throat and are swallowed and the fifth stage larvae emerge
into the gut (see Fig. 4.2).
n Adult females produce eggs within about 40 days.

The migrations through the skin cause a skin irritation known as cutaneous larvae
migrans. During the lung phase these parasites have an effect upon the host similar to Ascaris.
Ascarisand Ancylostomaare characterised by having larvae that migrate from the site
of infection to the gut where they mature into adult worms. The eggs of Ascaris, once

Larval
migrations

Larvae
hatch out

Eggs are

swallowed

Lungs

Alimentary

canal

Heart

Liver

nA. The life-cycle of Ascaris spp

Lungs

Heart Skin penetrating

larvae

Adult worms

in gut

Migration

route

Eggs pass out into soil

nB. The life-cycle of Ancylostoma spp

Adult
worm

• Figure 4.2A. Ascarisspp. A large intestinal nematode with a direct life-cycle. Larvae hatch out of the egg in the small intestine, burrow
  through the wall of the gut, migrate through the liver to the heart, lungs and back to the gut where they develop into adult worms.
  During the migrations the larvae undergo a series of moults. The migration process is known as visceral larvae migrans.
  B. Ancylostoma spp. Nematodes commonly known as the hookworms. Infective stages are skin-penetrating larvae that survive in damp soil.
  Once the larvae have penetrated the skin, they migrate through dermal layers of the skin (cutaneous larvae migrans) and then via the blood to the heart, lungs
  and finally settle in the small intestine. The mouths of the adults are equipped with biting mouthparts. They secrete anti-coagulants and feed on
  blood from the intestinal wall. If the larvae are accidentally swallowed they can apparently avoid the migration.