Fish Diseases and Management

7.1.1.Principles of disease diagnosis

Diagnosis of the aetiological agent is the most important aspect of the health management of any animal against any of the infectious diseases. Apart from searching for the causative agents of infection, some diagnostic techniques are also used to screen healthy animals to certify that they are not having any sub-clinical infections. Investigation of the possible disease problems is frequently initiated by changes in feeding or swimming behaviour (Level I) followed by appropriate sampling and analysis. As many symptoms are common to a variety of diseases, it is usually necessary to carry out extensive laboratory analysis, which may include traditional diagnostic techniques like histopathology, bacteriology, virology and water analysis (Level II). Advanced techniques like PCR using molecular biological methods are also used for fast and confirmatory diagnosis of infections (Level III). Mortality pattern gives a fairly good idea whether the problem is due to an infectious agent, water quality or toxicity. In general, infectious diseases are marked by gradual acceleration in morbidity and mortality. Exception may occur in cases of highly virulent virus or bacterial infections leading to 90 – 100% mortality in highly susceptible fish. Normally if majority of fish dies overnight or in 24 h period, it could be due to oxygen depletion or other water quality parameters, chemical toxicants or other environmental causes. Mortality due to nutritional deficiency would be protracted and less pronounced.