Metazoa
9
Abstract
Metazoa parasites are multicellular organisms where cells have differentiated into organised
tissues and organs. These parasites may be found in all fish organ systems both in wild and in
farmed salmonids, in fresh and sea water. Recently, parasites that would have been included
as Protists are now considered metazoans, as the case of Myxozoa. The use of molecular tools
are able to identify many Metazoan parasites and 12S ribosomal DNA primers have significant potential for metagenetic analysis, but should not detract or replace traditional histology
to asses their impact on the host. In this book we have used the term infestation to represent
ectoparasitic conditions and infection, as referring to those that are endoparasitic. This
chapter will cover a selection of the many species of Metazoa that can be found in salmonids.
Keywords
Metazoa Salmon Trout
Metazoa parasites are multicellular organisms where cells have
differentiated into organised tissues and organs. These parasites
occur in all organ systems of wild and farmed salmonids worldwide, in both fresh and sea water. Molecular methods are
available for many Metazoaparasites and 12S ribosomal DNA
primers have significant potential for metagenetic analysis.
However, this should not detract or replace traditional histology. Recently, parasites that would have been included in
Protist are now considered as Myxozoa and therefore they are
covered in this chapter. We have used infestation to represent
external ectoparasitic infestations and infection as referring to
internal endoparasitic conditions. Where appropriate, Metazoa
life cycles are summarised to improve understanding and the
examples covered in this chapter are summarized in Table 9.1.
9.1
Myxozoa
9.1.1 Ceratomyxa shasta
Ceratomyxa shasta is a significant parasite of hatchery and
wild juvenile anadromous salmonids on the Pacific coast of
North America. Differences in susceptibility are reported
between species of salmon as well as the range of responses
to infection. Significant losses can occur in hatcheries, but
C. shasta is also implicated as a primary cause of mortality
among wild stocks. For example, the survival of Chinook
salmon is reduced in locations where parasite densities are
highest. Infection results in exophthalmia, lethargy and a
dark body. Abdominal distension and haemorrhaging are
common around the vent region with severe inflammation
and necrosis of the intestine. Infection starts in the epithelium of the posterior intestine and progresses towards a
multifocal inflammatory response with sloughing and mucosal necrosis. Histologically, an acute inflammatory reaction
in the intestine can be observed with proliferation of the
connective tissue of the intestinal caeca and massive infiltration by the developing trophozoites and other developmental
stages with subsequent spread to other organs. The occlusion
and destruction of the lumen is considered to be the cause of
the mortality among infected fish. A limited granulomatous
inflammation may develop in the viscera with consequential
peritonitis.
The actinosporean stage of C. shasta develops in a polychaete, Manayunkia speciosa. Fish become infected by
coming into contact with water containing the infective
D.W. Bruno et al., A Colour Atlas of Salmonid Diseases,
DOI 10.1007/978-94-007-2010-7_9, # Springer Science+Business Media B.V. 2013
123
9
Abstract
Metazoa parasites are multicellular organisms where cells have differentiated into organised
tissues and organs. These parasites may be found in all fish organ systems both in wild and in
farmed salmonids, in fresh and sea water. Recently, parasites that would have been included
as Protists are now considered metazoans, as the case of Myxozoa. The use of molecular tools
are able to identify many Metazoan parasites and 12S ribosomal DNA primers have significant potential for metagenetic analysis, but should not detract or replace traditional histology
to asses their impact on the host. In this book we have used the term infestation to represent
ectoparasitic conditions and infection, as referring to those that are endoparasitic. This
chapter will cover a selection of the many species of Metazoa that can be found in salmonids.
Keywords
Metazoa Salmon Trout
Metazoa parasites are multicellular organisms where cells have
differentiated into organised tissues and organs. These parasites
occur in all organ systems of wild and farmed salmonids worldwide, in both fresh and sea water. Molecular methods are
available for many Metazoaparasites and 12S ribosomal DNA
primers have significant potential for metagenetic analysis.
However, this should not detract or replace traditional histology. Recently, parasites that would have been included in
Protist are now considered as Myxozoa and therefore they are
covered in this chapter. We have used infestation to represent
external ectoparasitic infestations and infection as referring to
internal endoparasitic conditions. Where appropriate, Metazoa
life cycles are summarised to improve understanding and the
examples covered in this chapter are summarized in Table 9.1.
9.1
Myxozoa
9.1.1 Ceratomyxa shasta
Ceratomyxa shasta is a significant parasite of hatchery and
wild juvenile anadromous salmonids on the Pacific coast of
North America. Differences in susceptibility are reported
between species of salmon as well as the range of responses
to infection. Significant losses can occur in hatcheries, but
C. shasta is also implicated as a primary cause of mortality
among wild stocks. For example, the survival of Chinook
salmon is reduced in locations where parasite densities are
highest. Infection results in exophthalmia, lethargy and a
dark body. Abdominal distension and haemorrhaging are
common around the vent region with severe inflammation
and necrosis of the intestine. Infection starts in the epithelium of the posterior intestine and progresses towards a
multifocal inflammatory response with sloughing and mucosal necrosis. Histologically, an acute inflammatory reaction
in the intestine can be observed with proliferation of the
connective tissue of the intestinal caeca and massive infiltration by the developing trophozoites and other developmental
stages with subsequent spread to other organs. The occlusion
and destruction of the lumen is considered to be the cause of
the mortality among infected fish. A limited granulomatous
inflammation may develop in the viscera with consequential
peritonitis.
The actinosporean stage of C. shasta develops in a polychaete, Manayunkia speciosa. Fish become infected by
coming into contact with water containing the infective
D.W. Bruno et al., A Colour Atlas of Salmonid Diseases,
DOI 10.1007/978-94-007-2010-7_9, # Springer Science+Business Media B.V. 2013
123