PARASITOLOGY
n BOX 6.1 FEEDINGnThe cestodes’ only means of feeding is by absorption through the tegument.
nTrematodes have a gut and a tegument but it still is a matter for discussion and experi-
mental observation as to how much food is absorbed directly through the tegument and
how much via the gut epithelium.
nNematodes have an outer non-permeable cuticle, an alimentary canal with a ‘mouth’ and
a well developed pharynx to assist in the uptake of food and its distribution through the
gut system. The mouthparts of various species are adapted to the different types of feed-
ing, ie whether they feed on tissue or body fluids etc.
nProtozoa fall into two basic categories: those which engulf the food by phagocytosis or
pinocytosis; and those which absorb their food through the outer membrane.n 6.2 UPTAKE OF NUTRIENTS
A number of studies on the kinetics of the uptake of nutrients have been carried out using
two species of adult tapeworm, namely Hymenolepis diminutaand H. microstoma. Both of
these worms can be removed from the gut of their host, a rat and mouse respectively,
and maintained in vitro. The cysticerci of Taenia crassicepshave also been used to study
the uptake of nutrients.
The results obtained from those experiments indicate that the uptake of nutrients by
those parasites follow similar pathways to the manner in which nutrients are absorbed
into the gut epithelial cells. Parameters such as pH, temperature, oxygen, carbon dioxide
and enzymes can all be influential in the absorption of soluble nutrients. The methods
used for the study of rates of enzyme reaction such as Lineweaver–Burke plots, from which
maximum uptake and affinity rates can be calculated, have been applied to the study of
a parasite’s nutrient uptake.n Transport kinetics reveal that the uptake of carbohydrates is dependent on the molecu-
lar shape and structure of the carbohydrate molecule, and often there is competition
for the identical transport locus (competitive inhibition).
n Glucose absorbed by active transport in the presence of galactose or fructose may lead
to competitive inhibition of the uptake of compounds, particularly if one of them is at
a higher concentration.n 6.2.1 UPTAKE OF AMINO-ACIDS
Amino-acids, from the gut amino-acid pool, are absorbed by tapeworms. There appears
to be neutral, basic and acidic amino-acid transport systems. The amino-acids enter
the worms via mediated transport systems and accumulate against a concentration
gradient. Cestodes unlike mammals require sodium and chloride ions coupled to
the amino-acid transport system. The cestodes have an equal affinity for the absorption
of both Dand Lamino-acids. However apparently a two-way process is in operation.
Molecules leak from as well as enter the parasite. A pool of amino-acids is usually
present in the lumen of the host’s gut and a similar ‘pool’ exists within the tissues of
the parasite. The movement of amino-acids in and out of the parasite maintains an
equilibrium between the amino-acid pool in the host’s gut lumen and the pool within
the parasite.