264 FISH HAICHERY MANAGEMENT 



a diagnosis is required from a pathology laboratory. Broad- spectrum treat- 

 ments based on a poor diagnosis are ill-advised, but treatment based on 

 keen observation and awareness of signs can mean the difference between 

 losing just a few fish or losing tens of thousands. 



Disease Characteristics 



Disease- Ca using Orga n isms 



Organisms that cause diseases in fish include viruses, bacteria, fungi, proto- 

 zoans, and a wide range of invertebrate animals. Generally, they can be 

 categorized as either pathogens or parasites, although the distinction is not 

 always clear. For our purposes, we consider subcellular and unicellular or- 

 ganisms (viruses, bacteria) to be pathogens. Protozoans and multicellular 

 organisms (invertebrate animals) are parasites, and can reside either inside 

 the host (endoparasites) or outside it (ectoparasites). Low numbers of either 

 pathogens or parasites do not always cause disease signs in fish. 



Viruses are neither plant nor animal. They have been particularly suc- 

 cessful in infecting fish. Viruses are submicroscopic disease agents that are 

 completely dependent upon living cells for their replication. All known 

 viruses are considered infective agents and often have highly specific re- 

 quirements for a particular host and for certain tissues within that host. 



Deficiencies or excesses in the major components of the diet (proteins, 

 amino acids, fats, carbohydrates, and fiber) often are the primary cause of 

 secondary bacterial, fungal, and parasitic diseases. Fish with a diet deficient 

 in protein or any of the indispensable amino acids will not be healthy and 

 will be a prime target for infectious agents. The same is true of deficiencies 

 of fatty acids or excesses of digestible carbohydrates. Secondary disease 

 agents may infect a fish in which biochemical functions are impaired. Nu- 

 tritional deficiences are discussed in more detail in Chapter 4. 



Disease Recognition 



Disease can be defined briefly as any deviation of the body from its normal 

 or healthy state causing discomfort, sickness, inconvenience, or death. 

 When parasites become numerous on a fish, they may cause changes in 

 behavior or produce other obvious signs. 



Individual diseases do not always produce a single sign or characteristic 

 that is diagnostic in itself. Nevertheless, by observing the signs exhibited 

 one usually can narrow down the cause of the trouble to a particular type 

 of causative agent. 



Some of the obvious changes in behavior of fish suffering from a disease, 

 parasite, or other physical affliction are (l) loss of appetite; (2) abnormal 



