The genus Sphaerothecum is closely related to
Dermocystidium and classified as a member of the Class
Mesomycetozoea (formerly Ichthyosporea) based on phylogenetic analyses of the small subunit ribosomal DNA. Detection
of the parasite can be recorded histologically and in tissues of
naturally exposed adult fish using a nested PCR test.
8.3.3 Dermocystidium salmonis
Dermocystidium spp. affects many fish in fresh and sea
water. D. salmonis occurs in all life stages and affects Atlantic salmon but also reported in Chinook, coho, and sockeye
salmon. Although generally not fatal, Dermocystidium has
been associated with mortality in young or juvenile fish,
particularly when water temperatures are low.
The disease usually affects the gill lamellae, oral cavity
and skin, and occasionally, systemic infections have been
reported. When large cyst form in the gills, they may prevent
closure of the opercula resulting in anoxia and mortality. In
fresh mounts of infected gills or skin, numerous spherical to
oval spores (7–12 μm across) can be seen. The morphological distinctive kinds of spores probably representing different developmental stages of the parasite but the most
characteristic type are the so called ‘signet ring cell’ or
hypnospore, with a large refringent vacuole and a narrow
rim of cytoplasm. Other spores are irregularly vacuolated
and equipped with one or multiple prominent nuclei.
Internally, the swollen abdomen harbours small, round
(~1 mm in diameter) white cysts visible within the cavity,
and large numbers of cysts may protrude through the body
wall (Fig. 8.15). Splenic and hepatic lesions are
characterised by well-circumscribed granulomas and an
increase in leucocytes (Fig. 8.16). Each cyst contains a
number of uninucleate spores which evoke a granulomatous
dermatitis, with marked inflammatory response, haemorrhage, hyperplasia and hydropic degeneration.
Dermocystidium are currently classified under the Class
Mesomycetozoea (formerly Ichthyosporea), a rather diverse
group situated near the evolutionary division between
animals and fungi, within the recently assigned assemblage
Opisthokonta. Dermocystidium stain PAS positive and can
be diagnosed microscopically (Fig. 8.17).
Fig. 8.15 Dermocystidium in rainbow trout. The abdominal wall has been opened to reveal cysts and marked inflammatory response
Fig. 8.16 Dermocystidium in liver of rainbow trout. Low power
114
8 Protists
Dermocystidium and classified as a member of the Class
Mesomycetozoea (formerly Ichthyosporea) based on phylogenetic analyses of the small subunit ribosomal DNA. Detection
of the parasite can be recorded histologically and in tissues of
naturally exposed adult fish using a nested PCR test.
8.3.3 Dermocystidium salmonis
Dermocystidium spp. affects many fish in fresh and sea
water. D. salmonis occurs in all life stages and affects Atlantic salmon but also reported in Chinook, coho, and sockeye
salmon. Although generally not fatal, Dermocystidium has
been associated with mortality in young or juvenile fish,
particularly when water temperatures are low.
The disease usually affects the gill lamellae, oral cavity
and skin, and occasionally, systemic infections have been
reported. When large cyst form in the gills, they may prevent
closure of the opercula resulting in anoxia and mortality. In
fresh mounts of infected gills or skin, numerous spherical to
oval spores (7–12 μm across) can be seen. The morphological distinctive kinds of spores probably representing different developmental stages of the parasite but the most
characteristic type are the so called ‘signet ring cell’ or
hypnospore, with a large refringent vacuole and a narrow
rim of cytoplasm. Other spores are irregularly vacuolated
and equipped with one or multiple prominent nuclei.
Internally, the swollen abdomen harbours small, round
(~1 mm in diameter) white cysts visible within the cavity,
and large numbers of cysts may protrude through the body
wall (Fig. 8.15). Splenic and hepatic lesions are
characterised by well-circumscribed granulomas and an
increase in leucocytes (Fig. 8.16). Each cyst contains a
number of uninucleate spores which evoke a granulomatous
dermatitis, with marked inflammatory response, haemorrhage, hyperplasia and hydropic degeneration.
Dermocystidium are currently classified under the Class
Mesomycetozoea (formerly Ichthyosporea), a rather diverse
group situated near the evolutionary division between
animals and fungi, within the recently assigned assemblage
Opisthokonta. Dermocystidium stain PAS positive and can
be diagnosed microscopically (Fig. 8.17).
Fig. 8.15 Dermocystidium in rainbow trout. The abdominal wall has been opened to reveal cysts and marked inflammatory response
Fig. 8.16 Dermocystidium in liver of rainbow trout. Low power
114
8 Protists