xiv INTRODUCTION 
Environment and season have a direct influence on 
the prevalence and vitality of parasites. A wet season 
predisposes to fluke disease, providing the locality is one 
in which flukes and their intermediate hosts (L. ¢runcatu- 
lata) are already present. 
Effects of Parasites.—Parasites produce their effects 
in various ways, according to their position and nature. 
In the bowel, worms may produce anemia through 
draining away blood from their host, and even per- 
foration of the bowel-wall may result, with consequent 
peritonitis. Flukes at the time of migration act 
mechanically by blocking the bile-ducts, and so jaundice 
and ascites result. Strongyles are common in the 
respiratory tract of young ruminants, and produce 
bronchitis and even pneumonia. 
Some parasites produce toxins, which exert a poisonous 
effect on their host, but these are rare. Ascaris mar- 
ginata has been credited with the power of excreting a 
toxin which produces fits in the dog, but this is very 
doubtful, and it is more probable that the actual cause 
is reflex action, set up by irritation of nerve-endings in 
the bowel. Ascarides also may produce mechanical 
obstruction of the bowel by grouping together into 
bundles or balls. 
Tenia cenurus will serve as an example of a parasite 
which produces its effects by reason of the position it 
takes up in the host’s body. The Cenurus cerebralis, 
which is its cystic stage, exists in some part ‘of the cere- 
brum or cerebellum, and thereby produces nervous 
symptoms, which vary according to the exact site. 
