PARASITOLOGY

PARASITOLOGY

desiccation. Apart from causing a ‘itchiness’ in the anal region and slight behavioural

problems, the parasite has little or no pathological effect upon the host.

n 4.1.2ASCARISSPP

This genus contains some the most commonly found large nematodes that inhabit the small

intestine of vertebrates, especially mammals. Ascaris lumbricoideslives in humans and

A. suumin pigs. Nearly all vertebrates have a specific species of Ascaris.

n The adult females of A. lumbriciodescan be up to 35 cm long and the males up to 30 cm;

and both can be about 1 cm wide. Both live within the upper reaches of the small

intestine.

n A mature female can produce over a 100,000 eggs per day which pass out of the body

via the faeces. If the eggs are deposited in suitable soil they embryonate and remain

until swallowed by the next host. If the eggs are deposited in a non-soil environment,

they can survive for a long period (provided that they are not subject to continuous

desiccation) and once back in the soil they embryonate.

n If the eggs are swallowed by a suitable host, the gut environment (pH, temperature,

O 2 concentration, digestive juices etc) stimulates the larvae to hatch out.

n The first stage larvae moult into second stage larvae which penetrate the walls of

the intestine (the mucosa) and enter into the lamina propria of the villi. The larvae

penetrate into capillary blood vessels and then migrate to the liver via the portal blood

vessels.

n From the liver the larvae travel via the pulmonary circulatory system and enter the lungs

as third stage larvae. These migrate up the epiglottis and are swallowed and re-enter

the small intestine where they moult twice before maturing into adult worms.

A relatively large number of larvae migrating at any one time can be the cause of a

certain amount of pathology, known as visceral larvae migrans. The migrations through

the lungs can lead to pneumonia-like symptoms. As the parasites move through the

tissues they cause both local and general inflammation, partly due to their physical

presence and partly to various secretions (mainly metabolic waste products).

Eggs are swallowed

Adult worms in the gut

Female worm moves to

perianal region

to deposite eggs

• Figure 4.1Enterobius
  vermicularis, a nematode
  commonly known as the
  human pinworm, has a
  direct life-cycle and lives
  in the lower regions of
  the alimentary canal. The
  females emerge out of
  the anus to deposit eggs
  around the anal opening
  causing an ‘itchy bottom’.