Submitting Fish Specimens for Diagnostic Evaluation

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

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.

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.