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		Submission of Fish for Diagnostic Evaluation
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		The Diagnostic Laboratory Once the case history is recorded, a general evaluation by the diagnostic laboratory is initiated. First, the diagnostician will test water chemistry parameters, such as total and unionized ammonia, nitrite, pH, total hardness, total alkalinity, and chloride. (Water chemistry measurements for marine systems include ammonia, nitrite, nitrate, salinity or specific gravity, and pH). Measurements of chlorine, dissolved oxygen, hydrogen sulfide, iron, and copper are sometimes warranted. Fish are measured and weighed and abnormal changes to the external body surface (see physical changes above) are recorded. A complete fish necropsy will include biopsies of external tissues (skin, gill, and fin). These tissues are examined under the microscope, primarily looking for parasites, although some bacteria (i.e. Flexibacter columnaris or Flavobacterium columnare), fungus, and effects of water quality and nutrition can sometimes be assessed. The fish are opened to expose the internal organs and examined for lesions and growths. The kidney is sampled for bacterial culture (the brain, liver, spleen, swimbladder, and other organs may also be cultured, as deemed necessary). Tissues may be taken for histological processing and virology (see Table 1 below). Squash preparations of internal organs are examined under the microscope, primarily for observation of parasite infestation. Bacterial and fungal agents and nutritional imbalances can often be suspected microscopically but must be confirmed by other methods. In general, the laboratory can make an initial diagnosis of water quality and parasitic infection within 24 hours of receipt of a sample. Most cases can be adequately assessed at that time and treatment can be recommended. However, in some cases, more time will be needed to determine the causative agents. Growth of bacteria on specialized media usually requires 24 to 48 hours, and sensitivity testing to determine the proper antibiotic to use takes an additional 24 hours. If tissues must be submitted for histology or virology, one to two weeks turn around time is common. The Value of the Sample: What Can It Tell Us? Fish diseases are often caused by a combination of factors, such as water quality, nutrition, and/or infection by pathogens (bacteria, parasites, or viruses). Therefore, starting any treatments before accurately diagnosing the problem will make it more difficult to pinpoint the primary cause. Furthermore, the fish's immune system becomes compromised, leading to secondary infections and masking the original problem. By knowing the history and keeping good records of your system, you may be able to eliminate common problems that are the underlying cause or that allow infection by secondary disease agents. One way to aid the fish health specialist in determining a cause of death is to map a mortality curve ( Figure 1 ). A mortality curve is a graph that indicates how many fish died at different points in time, from the beginning of the disease outbreak until the present time, or the end of the outbreak. If fish demonstrate abnormal behavior or the majority of fish in your system die within 24 hours, it follows a "Type I mortality curve." Agents that create this peracute reaction are environmental, namely a change in water quality. For example, low dissolved oxygen in ponds can result in peracute mortality (see IFAS fact sheet FA-27). Toxins such as chemicals and drug over-treatment can also cause a peracute response. A "Type II mortality curve" occurs when fish appear debilitated over a few days to a week, with progressively higher mortalities. Causes of this acute mortality are generally due to infectious agents, such as bacteria or viruses. Fish that act lethargic, decrease feeding, or die over a period of a week to a few weeks undergo a chronic reaction. A number of factors, including parasites, bacteria, viruses, toxins (e.g. heavy metals) or poor nutrition can cause this "Type III mortality curve." Figure 1: By graphing the percent mortality (number of dead fish/total number of fish in the population) over time, one can determine if affected fish are suffering from a peracute, acute, or chronic type pathogen. more ...

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