6.4 Making a Differential Diagnosis

Similar to the categories contributing to disease outbreaks, there are four focal components for creating disease differentials: history, examination, testing, and therapeutic response. A systematic approach involving these components is crucial. Information on the history of the disease can help one discover the specifics to focus on when examining the fish. Testing can be done but is aimless without suitable history and examination.

History

Starting with a comprehensive history from the owner, the veterinary clinician can establish substantial information about how long, how fast, and how severe a disease has been. Depending on the questions and how observant the owner may have been, other information may also come to light during this process..

Examination

Examination is the observation and evaluation of the fish, facility, and records. The information gathered supports and confirms the history and adds new information to the current disease presentation. Examination is handled by observing the fish in its environment as both group and individuals.

A physical examination should start with apparently normal fish first and sick individuals afterwards. Even though the fish are in the same water, the physical handling can move certain pathogens to another fish by the indirect contact of handling. To evaluate the condition of an individual, the examiner needs knowledge of normal and abnormal anatomy as discussed in Chapter 3, Anatomy and Physiology. The gills, skin/scales, eyes, and fins are the most accessible sites to observe. Changes in shape, color, or size of any part of the anatomy should be further investigated.

A facility visit allows the practitioner to directly observe the husbandry, water quality, nutrition, and biosecurity as mentioned in Section 6.3. This provides the best opportunity to review the records kept on daily operations. An important aspect for accessing records entails reviewing and evaluating many categories of production records.

Testing

The disease history guides the examination, and the examination guides the testing needed for diagnosis of the disease. Testing should be aligned with appropriate history and examination findings to avoid aimless diagnostic efforts. Specific testing assays will be covered in the Diagnostics chapter later.

Response to Therapy

Sometimes, the effort to establish a diagnosis is hampered by inconclusive or incomplete information from the previous steps. A differential list of diagnosis in these cases may leave the practitioner with having to choose between two or more possible diseases. If a therapy can be devised to rule out or confirm one of the diagnostic possibilities, the additional information can be turned to effective use in a treatment plan for future outbreaks.