Fish Diseases and Management

1.1.2 Host, pathogen and environment interaction in the disease development in fish and shellfish and role of stress in disease development

As could be seen from the classical description of Snieszko in 1970, the incidence of disease is due to the breakdown of the delicate balance between host, pathogen and environment. The host in many cases is able to resist invasion of most of the pathogens if it is healthy and free from stress. In many of the aquaculture systems, however, such a situation rarely exist, as there are a multitude of variables that need to be kept under check for a balanced and optimum requirement of the cultured species. The aquatic environment by itself is highly dynamic and the body functions of fishes which are poikilothermic are controlled by temperature and other water quality parameters. Fish has to continuously adjust to the changing environmental parameters, else it leads to low productivity, reduced weight gain, reduced feed conversion, decreased immunity, reduced natural disease resistance, increase in infectious diseases, death and reduced profits for commercial fish farmer.

Irresponsible use of chemical disinfectants and antibiotics is also a potential environmental hazard. In fish, stress is usually related to stocking density, environmental quality, handling or transport. Additionally, intensive culture practices with poorly controlled feed use and waste production adversely affect the environment. Variations in the environmental and biological parameters from the optimum range introduce stress to the animals under culture, which has a direct bearing on their immune system. In fish, stress response is initiated by adaptive changes due to the stimulation of hypothalamus-pituitary-inter renal (HPI) axis and consequent production of corticosteroids. Although these stress hormones help the animal in its effort to regain internal homeostasis, they significantly lower the defence mechanisms due to their immuno suppressive nature.