PATHOLOGICAL EFFECT OF THE PARASITE
UPON THE HOST
n 7.1 INVADING THE HOST’S BODY
The parasites’ habitat, whether it is inter- or intracellular, will be affected by the pres-
ence of the parasite. Simple movement through an organ or tissues is disruptive and
damaging, which has to be corrected. Once a parasite has invaded a cell both the
metabolism and the function of that cell are disrupted. The parasite absorbs its nutrients
from the cytoplasm and then deposits its metabolic waste into the cytoplasm. For
example the Plasmodiumspecies that infects humans feeds on haemaglobin during the
erythrocytic phase. The partially digested pigment is deposited in the liver and spleen as
haematin and the organs take on a characteristic brown coloration. The nitrogenous
waste produced by the parasite’s metabolism is simply passed into the host and, if pre-
sent in sufficient quantities, will become toxic to the host.
The majority of intracellular protozoan parasites, after having invaded a host cell
(an erythrocyte or a gut epithelial cell), undergo a multiplicative phase (eg Plasmodium,
Eimeria, Leishmania). The progeny of the invasive stage escape from the cell to invade
other similar cells. The host cell is irreversibly damaged during this process and, as the
parasitaemia increases, more and more cells are invaded and consequently an increasing
number are destroyed. This leads to either the necrosis of surrounding tissue or the leak-
ing of body fluids, particularly if the epthithelial cells of vessels are damaged, or anaemia
if blood cells are destroyed.
Gut-dwelling nematodes damage the lining of the lumen either by becoming em-
bedded or by their feeding activities. Frequently the epithelial cells are damaged result-
ing in alterations to the gut epithelium architecture and function. Often such changes to
the gut mucosa are the cause of malabsorption syndromes. Accumulation of numerous
large nematodes in the gut apart from causing malfunctioning, can also block the lumen,
a condition that can be fatal.
Individual cells destroyed by intracellular parasites are eventually replaced in a
normal healthy host and normally there is no lasting effect. However when tissue cells
are damaged either by feeding, parasite embedment or migrations, those cells are not
always replaced by identical cells but by fibrous or scar tissue. The immediate host reac-
tion to the presence of the parasites (see section 5.3) is the accumulation of phagocytic
cells and the release of chemical signals that initiate the onset of an inflammatory
response.