RELATIONSHIP BETWEEN HOST FISHES AND THEIR PARASITES
The influence of parasite on its host
Every parasite living in or a fish exerts some degree of harmful influence on its host. This influence may result in extensive change in individual organs or tissues, or it can take on the character of a general effect. In either case the parasites causing these changes is regarded as pathogenic. On the other hand, this influence can be so slight that it does not cause the appearance of any external sings. In such cases parasites are commonly considered as non-pathogenic. However, small the noxious effect, every parasite is harmful to its host. It must also be remembered that any deflection from the parasite of an unusual site within the host, or sudden increase in the numbers of the parasite or changes in the physiological condition of the host can cause supposedly non-pathogenic parasite to assume a strongly pathogenic character. For example, Dactylogyrus minutum, a small and allegedly harmless monogenean infesting the gills of the carp occurs only in small numbers in natural waters but is capable of prolific reproduction in aquaria, where it might cause the death of its host.
Parasite can influence the body of the fish in many different ways. Usually one type of activity is responsible for various pathological changes, indicating the extent to which the fish as a whole has been affected. For example, the mechanical obstruction caused by the cysts of Myxobolus cyprinid in the renal blood vessels of the carp results both in disturbances of the blood functions and of the water exchange processes. The ways on which parasites may affect the body of the fish can be grouped under several headings.