Disease outbreaks and mass mortalities of fish generate notable public concern and often require investigation on the part of fish heath diagnosticians and fishery managers.
External lesions on fish are commonly observed in association with field morbidity or mortality events, and descriptions of these lesions can be helpful in diagnosing the cause of the etiologic agent(s), i.e., the cause of the event.
However, the terminology used to describe external lesions is variable and may differ between agencies and individuals. Further, and most importantly, lesion observations are often nonspecific with regard to the etiologic agent(s).
This means that certain types of lesions can be caused by a variety of different bacteria, viruses, fungi or parasites, and you cannot tell by simply looking what the cause of a lesion is.
For example ulcerative lesions can be associated with all of the aforementioned examples. With the support of the US Environmental Protection Agency, Office of Wetlands, Oceans and Watersheds, this guide to fish lesion observations was developed.
This guide was developed to present an overview of observation techniques and provide terminology that can accurately describe external fish lesions. You do not need to be a fish pathologist to make observations that can be used to make a diagnosis. Using a consistent approach, and with a little practice, field technicians, fisheries biologists and environmental managers can make relevant observations, take high quality tissue samples, and make substantial contributions to field fish health studies.
To make good observations they should not only make sense to you, but the descriptions should not be misunderstood by others reviewing your data. Further, it is helpful to be systematic (i.e., follow a prepared, itemized “game plan” for making observations) and consistent (treat all observations and samples in the study the same way) in your sampling strategy. This guide was intended to facilitate observations both in the field as well as in the laboratory.
This guide contains common terminology, sample descriptions and data sheets to assist with lesion observations. The data sheets have been provided for you to use “as is” or as a template to utilize data entry items or formats on your own data sheets. Good planning and use of appropriate data sheets plays an important role in determining the final outcome of your observations. Remember, you can never go back and get data that was not collected!!!
Protective Clothing:
Although making visual observations of lesions does not necessarily involve direct, physical contact with animals, careful inspection or disposal of individual fish carcasses often does involve some form of contact. Therefore, it is prudent to use protective clothing to avoid contact with potential disease agents. Gloves, lab coat, apron, goggles, and other protection should be worn as needed. A list of vendors that supply these items is included at the end of this guide.
Case Identity and History:
When making collections or lesion observations each animal should have a unique ID number using a defined accession system. Useful information to include with your gross pathology observations includes: source of fish, water quality from source including temperature, source location, time and weather conditions during collection, collection method, general environmental observations, anomalous behaviour (i.e., gulping at surface, flashing, morbidity, etc.), an estimate of the number of affected fish, relationship to previous reports, and nearby landmarks (i.e., marinas, sources of potential contamination, etc.).
Review of External Anatomy:
This section will allow you to clearly specify the anatomical location of any gross pathology observed. Examples of four fishes with varying anatomical features are annotated below for reference.
External Examination (checklist of parameters that may be relevant for comment):
- Identify the species, length and weight.
- Is the animal alive, weak, moribund or dead? How many animals are affected?
- Are there any areas of abnormal (or different) coloration? Does the fish appear generally pale or have areas of dark coloration?
- What colour are the gills? (healthy gills are usually a bright cherry red)
- Is there excessive mucus on the skin or gills?
- Is the body shape normal? Abdominal distension, skeletal deformities, bulging eyes?
- Are observations of abnormalities on one side (left or right) or bilateral?
- Are there any sores, areas of reddening, lesions or frayed fins? (focus of this guide)
- Parasite survey: skin scrape, gill biopsy, faecal sample results.
Making observations:
Good lesion observations are descriptive and systematic. Record detailed observations of any lesion or area that may be abnormal. In any samples taken for histopathology, include pieces of the abnormal tissue with a bit of surrounding, apparently normal tissue. To provide maximal imagery of your observations, include comments on:
- size (provide measurement or estimate dimensions )
- number (how many)
- location (where on body)
- presentation (focal, multifocal, diffuse);
- shape (round, oval, thread-like, ribbon or tube-like, etc.)
- texture (smooth, granular, nodular, cheesy)
- colour and opacity (transparent, translucent, opaque)
- firmness (hard, soft)
- 3-D description (depressed, flat, raised)
- severity (minimal, mild, moderate, marked, severe)
- extent (% body coverage; Note that this is a difficult estimate to make accurately and can easily be over-estimated. Consider that if one entire side of a fish is affected, that would equate to 50% coverage)
Use the above comment categories and “food descriptions” to convey the lesion imagery, particularly when the anomaly which you are describing is unknown to you. Examples of material or food imagery which can be used to describe lesions include: cauliflower, cottage cheese, cream cheese, pea soup, cream and coffee, marbles, sandpaper, etc. Imagery can also be used for size comparisons if no ruler is available, e.g., size of a pin head, pencil eraser, nickel, dime, quarter, orange, basketball, etc.
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