1.3.2.1 Possible reasons for parasite migration within
the host
n The point of infection is not in the area of the parasite’s organ or tissue.
The metacercariae of Fasciola hepaticaare swallowed and once they excyst and the
juvenile form is released into the gut lumen, it has first to penetrate through the gut
wall to enter the body cavity. Once in the peritoneal cavity the young flukes move
over the viscera until they reach the liver. They then burrow into a liver lobe and
migrate through the hepatic tissue to enter the bile duct.
The tetrathyridia of Mesocestoides cortiafter been swallowed migrate through the
intestinal wall. Some remain in the body cavity but others actively penetrate the liver
tissue.
The infective larvae of the hookworms Ancylostoma duodenaleundergo a process of
cutaneous larvae migransbefore they reach their definitive site in the gut.
n The developing stages or larval stages each require a different physiological environ-
ment to develop.The eggs of Ascaris lumbricoidesand Toxocara canisare swallowed and the larvae
hatch out into the small intestine, the preferred site of the adult worms. Both types
of larvae undergo a process of visceral larvae migransand eventually return to the small
intestine.
Plasmodiumspp (those specific to humans) once inoculated into the bloodstream
rapidly find their way into the liver cells. This type of migration is thought to be related
to the parasite trying to find a safe hiding place from the host’s immune defences.
The microfilaria larvae of filarian nematodes circulate round in the body fluids. They
often can lodge in nervous tissue including the eyes.
n 1.3.3 THE PARASITE’S FEEDING ACTIVITY
Further aspects of parasite nutrition are dealt with in Chapter 6.
Feeding activities of the parasite can have a profound effect on the host by depleting
or denying the host of essential nutrients and in the process causing damage to sur-
rounding tissues.
Those parasitic helminths that have a gut and mouth feed upon the surrounding
tissues, body fluids or even host cell debris which may have been caused by their pre-
sence. Parasites that have dietary preferences, such as mucosal blood, invariably evolve
appropriate mouth parts to be able to puncture capillary blood vessels and can secrete
anti-coagulants to maintain the blood flow.
The majority of the protozoa, the cestodes (the tapeworms) and most of the trema-
todes (the flukes) absorb their nutrients either through their cell membrane or via the
outer tegument. They absorb molecules into their tissues from the surrounding host
fluid and could deprive the host of certain essential nutrients.
In some instances, especially with gut parasites, they often compete with the host for
essential nutrients. The tapeworm Diphyllobothrium latumcan absorb vitamin B 12 from the
contents of the host’s gut against a concentration gradient and deprive the host.
Schistosoma mansonifeeds mainly on blood proteins; and a heavy infection can lead to the
host suffering from malnutrition.
INTRODUCTION TO PARASITOLOGY