particularly
when used as early as possible, and have wide spectrum
of pathogen control. However, several difficulties are
often encountered by chemotherapy as, the cost of drugs
is expensive, the resistant strains of pathogens are easily
induced in water and the drug residues may deposit in
fish body may introduce potential hazard to public
health and to the environment by the emergence of drug
resistant microorganisms and antibiotic residues and
retain in water system as toxicants or pollutants Sugita
et al ( 1991). In order to avoid the side effects of
chemotherapy, the control measure by immunization of
fish with vaccines gains the effort and rapid