The causative factors of fish diseases can be grouped under six broad categories:
1. Internal to the individual – genetic diseases
2. Environment associated diseases (such as
critical intensities of light/temperature/pH/ dissolved gases) 3. Physical
injuries (ex: handling/transporting)
4. Nutritional diseases (ex: deficiency
syndromes)
5.
Co-existing organisms (biological entities)
6. A combination of a few or all factors indicated above.
General methods to detect the health of
fish in field conditions:
- Visual examination is one of
the quickest and least costly and requires a well-trained eye. But it need not
be highly reliable. Some of the quick indicators are given below:
External: Reflexes: In healthy fishes,
reflexes will be quick such as: Escape reflex, Eye reflex and Tail reflex. The
other types of symptoms may be: Sluggish behaviour Twirling, spiral or erratic
movements Faded or darkened pigmentation Exophthalmus or ‘pop eye’ condition
Hemorrhages Erosion of jaw or mouth Gill parasites, gill erosions, white
nodules Tailor fin rot Distended abdomen (Dropsy) Protruded anus (vent) Blood
oozing Ulcers/boils (furuncles) External parasites Cotton wool like growth.
Internal: Gas . filled
hollows Ascitic fluid in the abdominal cavity Hemorrhages in the muscle
wall/air bladder/ internal organs Liquid in the air bladder White nodules in
internal organs Swelling of organs (Kidneys, liver etc.,)
- Microscopic and Histologic
examination: Impression smear or wet mount preparation can be examined using a
light microscope. This is a rapid and inexpensive diagnostic tool. This method
is good for observing motile bacteria and protozoa. More specific information
can also be obtained from histological method when special stains are applied
to the tissue sections. But it is slow and expensive. It also requires a
trained technician, and some times fails to yield a definitive diagnosis. - Bacterial isolation: A sample
is taken and either streaked on agar – based medium or introduced into a liquid
broth containing a mixture of specially designed ingredients. Some media are
also designed to allow the selective growth of certain bacteria from a
potential mixture of pathogenic and non-pathogenic bacteria. General
morphologic classification of the bacteria can be made based on the colony
size, shape, color and smell. For more exact identification, biochemical
characterization is often used. In this, a single purified colony is assayed
for its ability to metabolize a variety of different minerals, chemicals and
food sources. This is a very specific diagnostic method. However, it requires days
or even weeks obtaining the results, and not all bacteria will grow on defined
medium. - Tissue culture: As viruses
cannot grow or multiply unless they are within living cells, cell cultures
allow the diagnostician to grow many types of viruses in the laboratory. A
sample is homogenized and added to the cells in the tissue culture flask. If
virus is present in the sample, it causes the Cytopathic Effect (CPE). However,
cell cultures have been developed only for some fint’ish, but not for shellfish.
Rapid diagnostic tests for detecting fish pathogens: In order to prevent
outbreaks, minimize the presence of pathogens and to reduce the use of
antimicrobial compounds, rapid detection of pathogen is essential. They are
further advantageous since the tests are: a. speedy, sensitive and accurate, b.
presumptive and/or confirmatory, c. be micro-modified for inexpensive handling
of large number of individuals and small volume samples, d. require
non-destructive samples, e. yield qualitative and quantitative results. The
results obtained from such tests can be correlated with the other clinical
symptoms of the fish. Some of the important rapid diagnostic tests are given
below:
1. Immuno-diagnostic assays such as Monoclonal & Polyclonal antibody assays.
2. Direct fluorescent antibody test (d-fat)
3. Enzyme immuno assays
(EIAs) or ELISA
4. Dot immunobinding assay
5. Western Blotting
technique
6. The Latex agglutination
assay
7. DNA – based diagnostic tests
8. Polymerase Chain Reaction (PCR) tests.
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