Chapter 14 Selected Fish Diseases in Wild Populations
Fishing is a vital source of income for local communities and a popular pastime in many countries of the world. An initial report of wild fish health problems usually originates from the fishing community. The majority of reports therefore concern fish of economic or sporting interest. The most obvious indicator of health difficulties in wild fish populations is mass mortality, but decreased numbers, abnormal behavior, or visible gross external lesions in wild fish are also reported.
The central tenet of disease investigation in wild fish is to establish a thorough systematic routine to collect and record evidence, in dramatically differing settings ranging from mountain streams to the open sea, that will eventually lead to a diagnosis. Establishing a disease diagnosis is strengthened by the supporting history, identification and prevalence of known pathogens or environmental factors that cause disease, associated clinical signs and pathology, investigator experience, and knowledge of the scientific literature.
Veterinarians understand the concept that disease agents are influenced by environmental factors. Within many animal production systems, however, the relevance of environmental effects is often alleviated by attention to husbandry, housing, nutrition, and other disease prevention techniques. In the wild, environmental factors influence the occurrence of disease unabated, and this concept must be further considered in any investigation of wild fish health problems.2,3
Many natural processes affect the physical and biochemical parameters of rivers, lakes, estuaries, and oceans, and fluctuations may occur seasonally, diurnally, or tidally, depending on the parameters in question. Box 14-1 lists the main parameters that vary naturally in aquatic systems and have a role in the overall health of animals living in this environment. Populations of wild fish live in equilibrium in the presence of many disease agents. Interactions and equilibriums are a feature of the “big picture” within which the living animal strives to maintain homeostasis. Stressful changes in the environment allow pathogens to become established and to become detrimental to the health of the fish. Disease investigation involves the collation of information to ascertain which environmental and pathogenic elements have caused the problem.
In all situations, you should remember that the evidence and results of an investigation might be used in legal proceedings, so it is important to develop techniques that will withstand the judiciary process. This includes accurate sample identification, the collection of photographic data, and development and thorough recording of the chain of evidence.
Record details of the person reporting the problem and ascertain the site of the problem accurately, with map references if possible. Some areas are remote, and because it is impossible to contain the site, changes often may occur before any investigation begins, and what is witnessed on one day may be different on another. Therefore you should obtain a good description of exactly what the initial reporter has seen. What is the obvious problem: dead fish, live fish swimming abnormally, lesions noted on live fish? When did the problem start? What type of fish are involved, and how many are affected? If the principal investigator cannot reach the site for some time, and if reporting personnel appear competent or are trained (e.g., fisheries staff), initial on-site data collection may begin immediately.
The aquatic environment and surrounding terrain may vary dramatically, and human safety is paramount. Although you may be well prepared in terms of equipment, it is folly to investigate water bodies alone without firsthand knowledge. Local fisheries personnel generally have good information about the regional terrain and should accompany the investigation. Some means of contact with a central control point, with details of the investigator’s whereabouts and expected contact times, should be prearranged.
Attempt to circumscribe the affected area in order to estimate the extent of the problem and investigate it thoroughly. In freshwater environments this involves heading upstream beyond the boundaries of the problem area initially and then at least an equal distance downstream. In the marine environment, inspect nearby coastal areas and waters. Evaluate the main use of the surrounding area, which could be agriculture, aquaculture, forestry, industry, recreation, or urban. Take account of seasonal and routine practices related to such operations. Note any input, such as tributaries, drains, and sources of discharges, associated with the surrounding land use. If necessary, obtain records regarding discharges and factory activities.
Check for unusual changes, such as land subsidence, and examine surrounding vegetation for evidence of death of other animals or abnormal behavior, such as snails having exited the water. Check for color changes in the water and nearby surrounding land, and note the presence of any pungent smells. Take photographs from various angles, and obtain relevant water and sediment samples from different locations, including control samples, to help avoid misinterpretation of natural variation in measured parameters. Note and photograph labels of chemical contents from any rubbish dumped near the site. Ascertain recent weather conditions, and inquire about recent changes in water levels and land use. Also, obtain past water-quality data for the area, if available.
In examining the affected population, note the general state of the fish. If they are dead, note any decay and changes in their general condition that may help in estimating the duration of the ongoing problem. Also note the physical appearance: gills flared, mouths agape, twisted spines, excess mucus, and gross external lesions. In live fish, note their color and fin position, and assess any unusual behavior, such as loss of equilibrium or attempts to exit water.
Decide which fish are most representative of the overall problem and should be collected for further examination. In wild fish, one may assume that any causative disease agent will be present in affected fish at a high prevalence rate. For this reason, in the initial stages of a disease investigation, a thorough examination of a small representative sample is more informative than a select examination of large numbers of fish. A sample size of five is a good starting point. Samples are then taken based on species and age or size. If possible, samples from early-stage and late-stage disease are useful. Thus, in any one investigation, a number of samples of five fish may be taken.
Some tasks should be done on-site during the gross postmortem examination (Figure 14-1). Other samples may be appropriately stored for further laboratory testing. Parasitologic examinations and swabs for microbiologic culture provide more meaningful results when taken from fresh fish, so sample sick fish, which are humanely euthanized, on-site. If sick fish cannot be caught, sample the freshest dead fish.
Weigh and measure each fish, and take skin scales for aging, if possible. Examine skin scrapes, gill squashes, and intestinal smears for parasites. Some parasites (e.g., Argulus spp.) tend to abandon fish as soon as they are removed from the water, and if care is not taken, the extent of such infestations maybe underestimated.
Open the fish using a sterile technique, and look for any abnormal swellings, accumulations of fluid, hemorrhages, adhesions, or other lesions. Take swabs from the posterior kidneys and any visible lesions for culture. Make impression smears of individual organs and lesions for cytologic examination, where appropriate. A selection of tissues and visible lesions that include normal surrounding tissue should be placed in formalin for histologic processing and examination (Box 14-3). Collect further tissue samples, or freeze some whole fish, for toxicologic and viral testing (Box 14-4).
Box 14-3 Fish Tissues to Sample for Histology