Fungal and Related Oomycete Infections
7
Abstract
The oomycetes (fungal-like) occur as primary and secondary agents of infection among
wild and farmed salmonids in fresh and sea water. These infections can prove fatal, at least
for farmed fish, if preventative measures are not undertaken. Molecular sequencing has
improved our understanding of the phylogenetic relationships within this taxonomically
diverse group with current evidence indicating these organisms evolved from simple
holocarpic marine parasites. This chapter covers the commonly encountered oomycetes
infections in salmonids.
Keywords
Oomycetes Fungus Salmon Trout
The oomycetes (fungal-like) are common and occur as
primary and secondary agents of infection among wild and
farmed salmonids in fresh and sea water (Fig. 7.1). These
infections can prove fatal, at least for farmed fish, if preventative measures are not undertaken. Molecular sequencing
has improved our understanding of the phylogenetic
relationships within this taxonomically diverse group with
current evidence indicating these organisms evolved from
simple holocarpic marine parasites. Examples of key
infections are summarised in Table 7.1.
7.1
Saprolegnia spp.
Oomycete infections (family Saprolegniaceae) may be commensal or act directly as primary infectious agents and this
group, in general terms, is represented by two species,
S. parasitica and S. diclina affecting wild and farmed fish
at all developmental stages in fresh water (Figs. 7.2, 7.3, 7.4
and 7.5). The presence of superficial cotton-wool-like tufts,
particularly on the integument and gills of host fish or eggs,
is likely to be the result of Saprolegnia infection. Thin, white
or grey threads with circular or crescent-shaped colonies
grow by radial extension until adjacent lesions merge
(Figs. 7.6 and 7.7). Diseased fish become increasingly
lethargic with a loss of equilibrium shortly before death,
following lethal haemodilution. Respiratory difficulties
may also feature when infection is associated with the gills.
Microscopic examination shows the characteristic filamentous, coenocytic mycelium of non-septate hyphae with
many zoosporangia. An early infection comprises rapid
degenerative changes in the muscle resulting in a diffuse
oedema. As the infection radiates from the focus of infection, more of the epidermis is destroyed, and consequently
hyphae can penetrate the basement membrane, with growth
continuing into the dermis, hypodermis and musculature
(Fig. 7.8). Thrombi are frequently observed in blood-vessels
as a result of the penetrating hyphae. There is loss of integrity of the integument and oedema of the hypodermis
accompanied by marked myofibrillar degenerative changes.
Severe infection causes swelling in the inter-myotomal
connective tissue, loss of nuclei and a minor host reaction.
More aggressive lesions show deeper myofibrillar and focal
cellular necrosis, spongiosis or intracellular oedema and
ultimate sloughing of the epidermis.
A marked lymphocytopenia and significant impairment of
haematopoietic tissue is reported. Lymphoid cell degeneration, cell depletion, vascular alterations within blood-vessels
D.W. Bruno et al., A Colour Atlas of Salmonid Diseases,
DOI 10.1007/978-94-007-2010-7_7, # Springer Science+Business Media B.V. 2013
99
7
Abstract
The oomycetes (fungal-like) occur as primary and secondary agents of infection among
wild and farmed salmonids in fresh and sea water. These infections can prove fatal, at least
for farmed fish, if preventative measures are not undertaken. Molecular sequencing has
improved our understanding of the phylogenetic relationships within this taxonomically
diverse group with current evidence indicating these organisms evolved from simple
holocarpic marine parasites. This chapter covers the commonly encountered oomycetes
infections in salmonids.
Keywords
Oomycetes Fungus Salmon Trout
The oomycetes (fungal-like) are common and occur as
primary and secondary agents of infection among wild and
farmed salmonids in fresh and sea water (Fig. 7.1). These
infections can prove fatal, at least for farmed fish, if preventative measures are not undertaken. Molecular sequencing
has improved our understanding of the phylogenetic
relationships within this taxonomically diverse group with
current evidence indicating these organisms evolved from
simple holocarpic marine parasites. Examples of key
infections are summarised in Table 7.1.
7.1
Saprolegnia spp.
Oomycete infections (family Saprolegniaceae) may be commensal or act directly as primary infectious agents and this
group, in general terms, is represented by two species,
S. parasitica and S. diclina affecting wild and farmed fish
at all developmental stages in fresh water (Figs. 7.2, 7.3, 7.4
and 7.5). The presence of superficial cotton-wool-like tufts,
particularly on the integument and gills of host fish or eggs,
is likely to be the result of Saprolegnia infection. Thin, white
or grey threads with circular or crescent-shaped colonies
grow by radial extension until adjacent lesions merge
(Figs. 7.6 and 7.7). Diseased fish become increasingly
lethargic with a loss of equilibrium shortly before death,
following lethal haemodilution. Respiratory difficulties
may also feature when infection is associated with the gills.
Microscopic examination shows the characteristic filamentous, coenocytic mycelium of non-septate hyphae with
many zoosporangia. An early infection comprises rapid
degenerative changes in the muscle resulting in a diffuse
oedema. As the infection radiates from the focus of infection, more of the epidermis is destroyed, and consequently
hyphae can penetrate the basement membrane, with growth
continuing into the dermis, hypodermis and musculature
(Fig. 7.8). Thrombi are frequently observed in blood-vessels
as a result of the penetrating hyphae. There is loss of integrity of the integument and oedema of the hypodermis
accompanied by marked myofibrillar degenerative changes.
Severe infection causes swelling in the inter-myotomal
connective tissue, loss of nuclei and a minor host reaction.
More aggressive lesions show deeper myofibrillar and focal
cellular necrosis, spongiosis or intracellular oedema and
ultimate sloughing of the epidermis.
A marked lymphocytopenia and significant impairment of
haematopoietic tissue is reported. Lymphoid cell degeneration, cell depletion, vascular alterations within blood-vessels
D.W. Bruno et al., A Colour Atlas of Salmonid Diseases,
DOI 10.1007/978-94-007-2010-7_7, # Springer Science+Business Media B.V. 2013
99