Environmental Engineering Reference
In-Depth Information
initiated in the integument followed by the death
of the fi sh before the infecting fungus may pro-
ceed beneath the skin or the underlying muscula-
ture. Do certain mycotoxins, produced by the
fungal pathogen, adversely affect the metabolism
of the host, sometimes resulting into a seize?
On the basis of his work on Pacifi c salmon,
Neish (
1976
,
1977
) has emphasized the role of
'stress' in initiating saprolegnian infections.
Although more empirical evidences are required
to substantiate this hypothesis, the available evi-
dence is persuasive and provides a mechanism,
which explains how physiological changes,
which occur in fi sh, can predispose them to infec-
tion by parasites to which they are normally
resistant. Various stressors, which include crowd-
ing, injury, suboptimal water temperature, han-
dling, presence of noxious chemicals in sublethal
concentrations, etc., acting singly or synergisti-
cally, operate through the pituitary-interrenal
axis to produce an increase in the level of plasma
corticosteroids. An increase in plasma corticoste-
roid levels can impair the infl ammatory responses
(McLeay
1975
) and lead to an increase in
corticosteroid-regulated protein catabolism and
gluconeogenesis (Woodhead
1975
). This may
ultimately result into a protein defi ciency, which
contributes to the wasting of skeletal muscles and
leads to a decrease in antibody production and
collagen synthesis. Lack of collagen, in turn,
impairs the ability of a fi sh to heal wounds and
ulcers. High levels of plasma corticosteroids
might also be associated with the fi shes' osmo-
regulatory function (Olivereau
1962
; Utida et al.
1972
; Woodhead
1975
), with the necessity to
catabolize protein to obtain energy with the
inability of fi shes to clear the hormone (Woodhead
1975
). Another important factor to be considered
is the ascorbic acid metabolism of a fi sh. Fish, in
general, have a dietary requirement for vitamin C
(Ashley et al.
1975
). It should be noted that an
increase in the levels of plasma corticosteroids
may also cause depletion of ascorbic acid reserves
(Wedemeyer
1969
,
1970
). It should be empha-
sized that the 'stress hypothesis', given, is
intended to be complimentary to other observa-
tions on mucus production. There is evidence
which suggests that mucus production is
controlled by the endocrine system. The main
hormone involved in this activity appears to be
prolactin; however, there is also evidence which
does not support this viewpoint (Lam
1972
).
Presumably there might be an interaction between
prolactin and the interrenal corticosteroids (Utida
et al.
1972
; Meier
1972
). There may be a direct
link between increased plasma corticosteroid lev-
els in fi sh and their susceptibility to infection.
These higher hormone levels may occur in
response to the physiological requirements of a
fi sh at certain period of its life like smoltifi cation
and sexual maturation.
The histopathological investigations (Sinha
1985
; Prabhuji and Sinha
2010
) on some fresh-
water fi shes, viz.,
Anabas testudineus, Channa
punctatus, Chela laubuca
and
Colisa lalius,
have
shown varying degrees of destruction of epider-
mis, hypodermis and underlying musculature by
oömycetous fungi.
During December 1990 to January 1995,
Prabhuji and Sinha (unpublished data) had the
opportunity to make observations on more than
four hundred diseased (in an epizootic form) fi sh
specimens of
Channa punctatus, Puntius
sophore, Catla catla, Colisa fasciatus
and
Carassius carassius.
The affected individuals
had shown the symptoms of tail rot, bloated
abdomen and the development of several degen-
erative lesions on the body. Although oömycetes
have been found to be associated with the body
lesions, as the study indicated, the fungal mem-
bers have been designated to be the secondary
pathogens; bacteria, probably, being the primary
infecting agents. Bucke Maff et al. (
1979
), in
their studies on an epizootic of perch (
Perca fl u-
viatilis
L.), have reported the occurrence of large
ulcerative lesions on the body and fi ns of the fi sh,
culminating in deep necrotic areas associated
with oedema and haemorrhage exposing the
underlying musculature and skeletal structures.
The fungal and bacterial populations, isolated
from the lesions, have been the members of
Saprolegniaceae and bacterial species of
Aeromonas, Pseudomonas
and occasionally
myxobacteria. However, Bucke Maff et al. (
1979
)
failed to reveal any virus particles, even after
using electron microscopy.
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