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		Stress - Its Role in Fish Disease
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		Resistance: An animal is able to adapt to stress for a finite period of time. During this period the animal may look and act normal, but is depleting energy reserves because of the extra requirements placed upon it (See Figure 1). Exhaustion: The animal's reserves have been depleted and adaptation fails because the stress was too severe or lasted too long (see Figure 1). What Is Disease? Disease is an abnormal condition characterized by a gradual degeneration of a fish's ability to maintain normal physiologic functions. The fish is not "in balance" with itself or its environment. Disease Resistance All fish do not get sick and die each time a disease outbreak occurs. There are many factors which affect how an individual responds to a potential pathogen. The pathogen (bacteria, parasite, or virus) must be capable of causing disease. The host (fish) must be in a susceptible state, and certain environmental conditions must be present for a disease outbreak to occur ( Figure 2 ). Figure 2 Defense Against Disease Protective barriers against infection Mucus (slime coat) is a physical barrier which inhibits entry of disease organisms from the environment into the fish. It is also a chemical barrier because it contains enzymes (lysozymes) and antibodies (immunoglobulins) which can kill invading organisms. Mucus also lubricates the fish which aids movement through the water, and it is also important for osmoregulation. Scales and skin function as a physical barrier which protects the fish against injury. When these are damaged, a window is opened for bacteria and other organisms to start an infection. Inflammation (nonspecific cellular response) is a cellular response to an invading protein. An invading protein can be a bacteria, a virus, a parasite, fungus, or a toxin. Inflammation is characterized by pain, swelling, redness, heat, and loss of function. It is a protective response and is an attempt by the body to wall off and destroy the invader. Antibodies (specific cellular response) are molecules specifically formed to fight invading proteins or organisms. The first time the fish is exposed to an invader, antibodies are formed which will protect the fish from future infection by the same organism. Exposure to sublethal concentrations of pathogens is extremely important for a fish to develop a competent immune system. An animal raised in a sterile environment will have little protection from disease. Young animals do not have an immune response which works as efficiently as the immune response in older animals and therefore, may be susceptible to disease. more ...

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