Protists
8
Abstract
The Protists are a large group of eukaryotic microorganisms with a taxonomy that is under
constant revision. Molecular studies show the group includes diverse and sometimes
distantly related phyla that share relatively simple levels of organization that can be
unicellular or multicellular, but without specialized tissues. In fish, Protists range from
true parasites that may cause significant mortality, to those that show commensalism.
Conditions such as overcrowding or poor water quality, as well as other changes in
environmental conditions may allow parasites to rapidly increase in number and as a result
fish become vulnerable to infection with an increased chance of invasion by secondary
pathogens. This chapter presents a selection of the most common Protists reported from
salmonid species.
Keywords
Protist Salmon Trout
The Protist are a group of eukaryotic microorganisms with a
changing taxonomy. Current molecular information
indicates that the group includes diverse and sometimes
distantly related phyla, that share relatively simple levels
of organization and can be unicellular or multicellular but
without specialized tissues. In general they are not closely
related through evolution and have different life cycles,
trophic levels, modes of locomotion and cellular structures.
In fish, Protists range from true parasites that may cause
significant mortality, to those that show commensalism.
Conditions such as overcrowding or poor water quality and
other changes in environmental conditions may allow
parasites to rapidly increase in number and as a result, the
host may lose weight, suffer from osmotic distress and
become susceptible to predation with an increased chance
of invasion by secondary pathogens. Chosen examples from
this group affecting salmonids are presented with emphasis
on the host effects, while details of the parasite ecology and
life cycle are only included where relevant to the understanding of the infection or pathogenesis. Common Protists,
the Phyla or class, location on the fish and environment are
summarised in Table 8.1.
8.1
Amoebozoa
8.1.1 Paramoeba perurans
Amoebic gill disease (AGD) is attributed to colonization by
Paramoeba perurans and of global significance to the aquaculture industry. Neoparamoeba has recently been reclassified
to Paramoeba. Infestation is associated with a severe gill
proliferative response and consequently, significant
mortalities can occur in Atlantic salmon reared in sea water.
AGD outbreaks normally occur from late summer to early
winter with 10–12
C believed to be the threshold for clinical
signs. However, outbreaks have occurred at lower temperatures and recent studies show high salinity as a more relevant
risk factor. The amoeba is usually confined to the gill surface
but can penetrate the epithelium and occur internally. Heavily
infected fish are lethargic and congregate at the water surface.
The opercula are flared with excessive mucous from the gills
and the presence of focal, whitish patches (Fig. 8.1). Fresh
preparations of amoebae removed from infected gills show
large distinct pseudopodia, a well-delineated hyaloplasm and
measure approximately 15–40 μm. High salinity (low rainfall),
D.W. Bruno et al., A Colour Atlas of Salmonid Diseases,
DOI 10.1007/978-94-007-2010-7_8, # Springer Science+Business Media B.V. 2013
107
8
Abstract
The Protists are a large group of eukaryotic microorganisms with a taxonomy that is under
constant revision. Molecular studies show the group includes diverse and sometimes
distantly related phyla that share relatively simple levels of organization that can be
unicellular or multicellular, but without specialized tissues. In fish, Protists range from
true parasites that may cause significant mortality, to those that show commensalism.
Conditions such as overcrowding or poor water quality, as well as other changes in
environmental conditions may allow parasites to rapidly increase in number and as a result
fish become vulnerable to infection with an increased chance of invasion by secondary
pathogens. This chapter presents a selection of the most common Protists reported from
salmonid species.
Keywords
Protist Salmon Trout
The Protist are a group of eukaryotic microorganisms with a
changing taxonomy. Current molecular information
indicates that the group includes diverse and sometimes
distantly related phyla, that share relatively simple levels
of organization and can be unicellular or multicellular but
without specialized tissues. In general they are not closely
related through evolution and have different life cycles,
trophic levels, modes of locomotion and cellular structures.
In fish, Protists range from true parasites that may cause
significant mortality, to those that show commensalism.
Conditions such as overcrowding or poor water quality and
other changes in environmental conditions may allow
parasites to rapidly increase in number and as a result, the
host may lose weight, suffer from osmotic distress and
become susceptible to predation with an increased chance
of invasion by secondary pathogens. Chosen examples from
this group affecting salmonids are presented with emphasis
on the host effects, while details of the parasite ecology and
life cycle are only included where relevant to the understanding of the infection or pathogenesis. Common Protists,
the Phyla or class, location on the fish and environment are
summarised in Table 8.1.
8.1
Amoebozoa
8.1.1 Paramoeba perurans
Amoebic gill disease (AGD) is attributed to colonization by
Paramoeba perurans and of global significance to the aquaculture industry. Neoparamoeba has recently been reclassified
to Paramoeba. Infestation is associated with a severe gill
proliferative response and consequently, significant
mortalities can occur in Atlantic salmon reared in sea water.
AGD outbreaks normally occur from late summer to early
winter with 10–12
C believed to be the threshold for clinical
signs. However, outbreaks have occurred at lower temperatures and recent studies show high salinity as a more relevant
risk factor. The amoeba is usually confined to the gill surface
but can penetrate the epithelium and occur internally. Heavily
infected fish are lethargic and congregate at the water surface.
The opercula are flared with excessive mucous from the gills
and the presence of focal, whitish patches (Fig. 8.1). Fresh
preparations of amoebae removed from infected gills show
large distinct pseudopodia, a well-delineated hyaloplasm and
measure approximately 15–40 μm. High salinity (low rainfall),
D.W. Bruno et al., A Colour Atlas of Salmonid Diseases,
DOI 10.1007/978-94-007-2010-7_8, # Springer Science+Business Media B.V. 2013
107