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Submitting Fish Specimens for Diagnostic Evaluation

By: Dr. Al Camus

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Millions of people enjoy keeping ornamental fish, a hobby that is both popular and rewarding. While many older, well-conditioned ponds and aquariums are relatively trouble free, problems of disease do periodically arise, particularly in newly established systems. The artificial conditions and high stocking densities found in many ponds and aquariums create a stressful environment that can be ripe for the rapid spread of disease.

Unfortunately, many pet fish owners are often frustrated by the difficulty in finding competent veterinarians trained to diagnose disease in fish. Sometimes owners try to make their own diagnoses, which often leads to inappropriate treatments that result in increased mortality, as well as wasted time and money. This situation is changing for the better. In recent years, many practicing veterinarians and veterinary schools have shown an increased interest in aquatic medicine, and diagnostic services are becoming more widely available.

If you are experiencing a mortality (fish death) problem with a pond or aquarium, call your local veterinarian or veterinary school to inquire about diagnostic services they may offer. Help may also be found through some state agricultural extension services and university aquaculture and fisheries departments.

When problems do occur, it is essential to identify the specific disease or diseases involved in order to choose an appropriate treatment. The necropsy (post mortem examination) is an important diagnostic tool in aquatic medicine. Aquarists can save valuable time and minimize losses by following proper procedures for submitting fish samples for disease diagnosis. The following list will serve as a general guideline for submitting samples.

Any disease investigation begins with an accurate history and you should be prepared to provide the diagnostician with a detailed account of the problem. Accurate historical information is essential for several reasons. First, it may determine the types of samples to be collected (bacterial, viral, histopathology, etc.). Secondly, it may provide clues necessary to correctly interpret a case that may not be revealed by necropsy findings and laboratory samples alone.

Historical information should include

  • Any available water quality data
  • Date and volume of last water change
  • Source of the water used
  • Size of the pond or tank
  • How long the system has been in use
  • Species of fish stocked
  • Number of fish stocked
  • Size or approximate weight of the fish
  • Feeding activity
  • Type of feed
  • Duration of the problem
  • Number of deaths (mortality) and number of fish sick (morbidity)
  • Any history of recent additions to the system (fish, plants, etc.)
  • Any recent changes in feeding practices or maintenance
  • Clinical signs of disease (behavioral and physical)
  • Any previous treatments attempted

    You should also be prepared to provide a complete description of the system, including filters, pumps, aeration, location, substrate, etc.

    Which Fish to Submit

    The best specimen to submit for disease evaluation is one that is alive and exhibiting signs typical of the diseased population as a whole. Unfortunately, in the case of live animals, euthanasia may be necessary for complete evaluation of the case. This may be a difficult decision for the pet owner, but if large numbers of fish are at risk, the information gained may well outweigh the sacrifice.

    How Many Fish to Submit

    Whenever possible, three or more specimens should be submitted for necropsy (this would normally apply to a large population of pet fish). Examination of multiple samples greatly improves the chances of discovering the offending agent. When more than one problem is identified, diagnosticians will always look for a common thread among the multiple samples, which will typically be the most significant finding.

    Handling and Storage

    Proper handling, storage and transport of specimens are essential to get an accurate diagnosis. Post mortem changes begin immediately after death and occur rapidly at the warm water temperatures at which many fish are maintained. Decomposition occurs in both fish and parasite tissues after death, making assessment of pathological changes and identification of organisms more difficult, if not impossible. In general, the smaller the fish, the more rapidly decomposition will occur.

  • Dead fish should never be allowed to float in water. Fish floating in water for more than a few hours are often unsuitable for histopathologic (microscopic tissue evaluation) examination and parasites may die or leave the dead fish. Bacteria can often be recovered internally for longer periods, but contamination of tissues by bacteria leaking from the gastrointestinal tract becomes a complicating factor in the isolation of pathogens. Bloated or malodorous specimens are typically of little use for diagnostic evaluation.

  • Dead fish should never be frozen. Freezing destroys tissue morphology (structure) and makes recovery of bacteria and parasites difficult or impossible.

  • Fish that cannot be necropsied immediately should be placed in individually labeled plastic bags. Never add water or ice to the bags. Refrigerate or place the bags in a small ice chest with a large quantity of ice packed around the bags. Do not use dry ice, as it tends to freeze samples. Avoid using gel cold packs when transporting over long distances or for extended periods of time, as they do not remain cold for long. If you do not live within convenient driving distance of a diagnostic facility, many laboratories will accept samples sent by overnight mail, delivery service, or bus. Some laboratories may also provide a pick-up service in large metropolitan areas. Always consult with the service being used to make certain the sample is properly packaged to prevent leakage.

    Submit a Water Sample

    Always remember that fish, more than any other domestic animal, are negatively impacted by poor environmental quality. A separate water sample should always be submitted along with the diseased fish. Approximately one pint submitted in a clean jar is usually sufficient for routine testing.

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