need for migration. Parasites in the wrong host continue their migrations, ‘lose their
way’ and end up in situations where they cause considerable necrosis of tissues.
n The physical presence of a parasite and its metabolic waste products can stimulate a
negative host response, eg production of alkaline phosphatase, corticosteroids, etc.
n The age of the parasite; a young parasite may be able to avoid the host’s response. An
older parasite may die and the presence of the dead parasite may become problematic.Any parasite once established within a host can become pathological in either a major
or minor way. The site where the parasite finally settles will almost invariably be altered
due to localised tissue damage caused by the parasite and the subsequent host reaction
to control the parasite and repair the tissue damage.
Pathological damage to the host as a result of parasite invasion could be due to any
one or a multiple combination of the following described below.n 1.3.1 MECHANICAL DAMAGE
Once a parasite has actually penetrated a host, whether it is intracellular or extracel-
lular (but within a tissue) or in one of the many body cavities (lumens), its physical
presence will disturb the local homeostatic balance.
Physical features of the parasite such as hooks, protruding hardened structures and
biting mouth parts damage the cells immediately surrounding the parasite.
Body fluids — including blood — and cellular debris accumulate around the parasite.
The immediate effect is swelling and redness.
Apart from the gut and some of the lung-dwelling parasites, very few parasites are at
their preferred site immediately after infection. The parasite then attempts to migrate
from the site of infection to its predetermined site.n Blockages; the filarial nematodes Wuchereria bancroftiand Brugia pahangilive in the sub-
cutaneous tissue of the lower limbs. The result is oedema (a fluid buildup) of the lower
limbs and also the enlargement of the genitalia.
n Heavy infections of Ascaris lumbricoidesin children can block the small and large
intestine.
n Erythrocytes infected with Plasmodium falciparumcan become sequestered in the blood
capillaries of the brain, causing cerebral malaria.
n The eggs of schistosomes can damage small blood vessels and block ducts within the
portal system and blood vessels of the bladder.n 1.3.2 MIGRATION WITHIN THE HOST
Parasites can move through the intercellular matrix between the cells and cell walls. The
parasitic helminths progress through the tissues by active body movements, use of their
mouth parts, hooks etc, and/or by the secretion of proteolytic enzymes. The larger the
parasite the greater will be the physical damage to the tissue caused by the movement of
the parasite.
The motile protozoan parasites use flagellae, cilia or an amoeboid type of locomotion
to move through body fluids or tissue. Most protozoa generally travel via body fluids and
often depend upon the blood circulation to reach their destination if it is not the blood
itself. The intracellular parasites, once they have invaded their host cell, remain there until
they reach a reproductive phase. To release the next phase into the host, the parasites
burst out of the cell, which invariably destroys the host cell.PARASITOLOGY