PARASITOLOGY

The effects of protozoan parasites are varied; they could be localised as with Eimeria

tenella, a parasite of the gut mucosal cells. The resultant pathology disrupts the bowel

functions. Trypanosoma cruziin the extracellular phase has a systemic effect upon the host,

whereas when it is in the tissue phase it damages local tissues, usually the heart muscle

cells. Plasmodium, a parasite which is systemic, has an overall effect upon the host rather

than a specific site effect. Whether the actual damage is local or systemic the presence

of parasites has an overall effect upon the host, usually in the form of fever, nausea, pain,

dyspepsia or immunosuppression.

n 7.5 HUMAN TREMATODE PATHOGENS

7.5.1 Paragonimus westermani (lung fluke)

The adult Paragonimuslives in the human lung and produces eggs after 16 days. Eggs pass

out of the body via the sputum. The eggs need to make contact with water before they

hatch and, like most trematode eggs, require an optimum temperature of 25–28°C to

hatch (always different from that of the host). In water miracidia hatch out of the eggs

and swim around until they make contact with a gastropod (Pleuroceridae and Thiaridae

only). Once contact is made the miracidia penetrate through the foot and migrate

toward the digestive gland. Within the miracidium (see section 3.10) are germ cells

which develop into sporocysts (the so-called mother sporocysts) and likewise the sporo-

cysts contain germ cells which develop into the next phase, the rediae. From within the

rediae cercariae of the small microcercous type are shed into the surrounding aquatic en-

vironment. The cercariae have a small tail, a well developed stylet and oral sucker, and

can penetrate into freshwater crab and crayfish muscles where they encyst into meta-

cercariae. Once the crustacean’s muscles are eaten the cysts are digested and release

juvenile flukes into the intestinal lumen. The juvenile forms burrow through the intest-

inal wall into the body cavity. They migrate toward the thorax and then through the

diaphragm into the pleural cavity and settle in the lungs, where they develop and mature

into adult forms.

n 7.5.2OPISTHORCHIS VIVERRINI(THE CAUSE OF

CHOLANGIOCARCINOMA) AND CLONORCHIS SINENSIS

(THE CAUSE OF CARCINOMA OF THE BILE DUCT)

O. viverrini

The adult parasites live in the human bile duct. Eggs pass out via the faeces but only

hatch when eaten by a snail (Bythnia spp). A sporocyst (the mother sporocyst) develops

within the snail. The sporocyst produces rediae and they develop into cercariae which

pass out of the snail. These cercariae have a longitudinal tail fin (gymnocephalous) and

swim freely until they make contact with a fish. The tail drops off and they penetrate into

the fish muscles and encyst (a metacercaria). The metacercariae are ingested when raw

or semi-cooked fish is eaten. The metacercariae excyst in the lumen of the small intes-

tine, releasing juveniles that then migrate up the biliary tree to the small intrahepatic bile

ducts, where they mature.

The infection is entirely mucosal and there are no migration phases. Mature worms

never actually enter the tissues. Mature worms produce eggs which move down the

biliary tract and are excreted with the faeces.

Prevalence in snails is about 1% but nevertheless it was found that in certain areas of

South East Asia 100% of certain cyprinoid species harbour metacercariae. Reports of con-

siderable variation in the prevalence and number of metacerariae in fish are associated

with the time of year and the species.

PARASITOLOGY

Both parasites respond
to praziquantel
treatment.