Overview

Brief Summary

Taxonomy

  • Sac-like stages consist of an outer and inner epithelial tissue layer.
  • Cell doublets and pseudoplasmodial stages develop in fish kidney.
  • Spores consist of valve cells that form the spore wall, capsulogenic cells that produce polar capsules, and infective sporoplasm cells.
  • Polar capsules are intracellular organelles in spores of all myxozoans and which share a common ancestry with nematocysts found in the stinging cells of cnidarians.


Diagnostic description
Sacs are uniformly round and up to 0.35 mm in diameter.This species is commonly found in the bryozoan Fredericella sultana although DNA sequence data indicate that other bryozoans can also act as hosts (Anderson et al. 1999). Since these parasites are highly reduced and lack many distinguishing features, DNA sequence analysis is primarily used for species confirmation.

Lookalikes
Other myxozoan parasites that produce sac-like stages in freshwater bryozoans may be confused with Tetracapsuloides bryosalmonae. However, Buddenbrockia allmani has only been recorded from the bryozoan Lophopus crystallinus while B. plumatellae is found in the bryozoan Cristatella mucedo.Furthermore, the sac-like stages in these two species are generally irregular or oval in shape. Those produced by T. bryosalmonae are uniformly round. DNA sequences can also distinguish species (Tops et al. 2005).
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Introduction

The myxozoan, Tetracapsuloides bryosalmonae, is a multicellular endoparasite of freshwater bryozoans and salmonid fish.Tetracapsuloides bryosalmonae causes the devastating Proliferative Kidney Disease in trout and salmon (Anderson et al. 1999; Canning et al. 1999; Feist et al. 2001), resulting in significant economic losses for aquaculture and threatening wild fish populations.
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Comprehensive Description

Biology

Little is known about the biology of this parasite and its life-cycle has only recently been resolved. Surprisingly, the Myxozoa have recently been shown to be highly reduced animals that are most closely related to cnidarians (jellyfish, hydras and corals) (Jiménez-Guri et al. 2007).

Size
T. bryosalmonae develops into tiny, sac-like stages around 0.35mm in diameter in the body cavity of freshwater bryozoans. Each sac contains 1000’s of infectious stages (spores). Parasite stages in fish consist of only several cells.

Growth
Laboratory studies indicate that sac-like stages can develop within 3 days from single cell stages in bryozoan hosts (Canning & Okamura, 2004). Transmission studies demonstrate that it takes some 3-8 weeks from the time of infection for stages to develop in fish kidney, depending on temperature (Gay et al. 2001).

Life expectancy
Long term infections of bryozoans appear to be maintained by:
  • low virulence (Tops et al. 2009)
  • host-condition dependent cycling between covert and overt infections (Tops et al. 2006; 2009)
  • transmission of infection to new colonies by colony fragmentation (Morris and Adams, 2006) and possibly via bryozoan resting stages (statoblasts) (Hill and Okamura, 2007)
When fish survive infection the parasite may persist for at least one year (Foott and Hedrick, 1987), and perhaps for the lifetime of the fish. Spores are infective for <24 hr (De Kinkelin et al. 2002 ).
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Biology

'The myxozoan, Tetracapsuloides bryosalmonae, exploits freshwater bryozoans as definitive hosts, occurring as cryptic stages in bryozoan colonies during covert infections and as spore-forming sacs during overt infections. Spores released from sacs are infective to salmonid fish, causing the devastating Proliferative Kidney Disease (PKD).' (Tops, Hartikainen, Okamura 2009)

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Ecology

Dispersal

Reproduction dispersal

Reproduction
Very little is known about reproduction in myxozoans. Sexual organs as well as egg and sperm cells are lacking. Instead propagative multicellular spores are produced. Meiosis occurs in cells from which spores are derived in the bryozoan host.

Dispersal
Dispersal should occur via transport of released spores, movements of fish hosts, drifting of detached bryozoan colonies, and possibly via waterfowl that ingest bryozoan dormant stages (statoblasts) (Figuerola et al. 2003).
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General Ecology

Distribution ecology

Distribution
Proliferative kidney disease is reported from many countries in Europe and occurs in the USA and Canada.

Trophic strategy
Dissolved nutrients are absorbed from host tissue through the cellular walls of parasite stages.
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Life History and Behavior

Life Cycle

Lifecycle

The parasite develops first as single cells associated with the bryozoan body wall, causing covert infections (Tops et al. 2006; Morris et al. 2006). The subsequent development of spore-forming sac-like stages (Canning et al. 2000) in the body cavity causes overt infections.Spores released from bryozoans into the water attach to fish via a filament that everts from intracelular organelles (polar capsules) within spores. Attachment to skin or gills allows invasion of fish hosts by amoeboid cells that then proliferate in blood prior to reaching the kidney.The formation and replication of cell doublets occurs in kidney tissue prior to spore production (Morris and Adams, 2008). Spores released in urine of brown and brook trout have been demonstrated to infect bryozoans (Morris and Adams, 2006; Grabner and El-Matbouli, 2008). No fish to fish or bryozoan to bryozoan transmission has been shown.
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Wikipedia

Tetracapsuloides bryosalmonae

PKX redirects here. For the company, see POSCO.

Tetracapsuloides bryosalmonae is a myxozoan parasite of salmonid fishes, which causes Proliferative kidney disease (PKD), one of the most serious parasitic diseases of salmonid populations in Europe and North America[1], which causes losses of up to 90% in infected populations.

Taxonomy[edit]

Until the late 1990s, the organism which caused PKD was enigmatic. The "PKX organism", the causative agent of the disease, had been recognized as some form of Malacosporean[2], but the absence of mature spores in salmonid hosts, the lack of fish to fish transmission, and seasonality of the disease suggest that the life cycle of PKX was completed in another host and that infection of salmonids could be accidental. Korotneff observed a myxozoan in the bryozoan, Plumatella fungosa, in 1892, which he described as Myxosporidium bryozoides[3]. Myxozoan infection of bryozoans were not reported again until 1996. Ecological investigations of freshwater bryozoans in North America discovered parasitic sacs of a myxozoan species, freely floating in the body cavities of several bryozoans. Molecular analyses indicated that the 18S rDNA sequences of these sacs were indistinguishable from those of PKX[4], and the PKX organism was scientifically described as Tetracapsuloides bryosalmonae Canning, Curry, Feist, Longshaw & Okamura 1999[5], which has been assigned to a new class, the Malacosporea within the phylum Myxozoa[6]. Around the same time, another group described the PKX organism from Arctic char, Salvelinus alpinus, as Tetracapsuloides renicola Kent, Khattra, Hedrick & Devlin 2000[7], but the first given name has priority according to the rules of the binomial nomenclature.

Life cycle[edit]

T. bryosalmonae is highly unusual amongst the myxosporea, in that it uses a bryozoan as an alternate host, rather than an oligochaete or polychaete worm. To date, T. bryosalmonae has been found to parasitize five bryozoan species belonging to the genera Fredericella and Plumatella, all considered to be primitive genera[8]. Problems have occurred in determining the fish host for this species. However, recent work has demonstrated that the parasite cycles between bryozoa and native salmonid species.

Pathology[edit]

Proliferative Kidney Disease is characterised by a swollen kidney and spleen, bloody ascites, and pale gills, which indicate the fish is anaemic. Note that these symptoms are common amongst many diseases of fish and do not specifically indicate an infection with Tetracapsuloides bryosalmonae.

Distribution[edit]

T. bryosalmonae has been recorded in Europe and North America. Phylogenetic analyses of internal transcribed spacer 1 sequences revealed a clade composed of all North American sequences plus a subset of Italian and French sequences. High genetic diversity in North America and the absence of genotypes which are characteristic of the North American clade in the rest of Europe implies that southern Europe was colonized by immigration from North America; however, sequence divergence suggests that this colonization substantially pre-dated human movements of fish. Furthermore, the lack of southern European lineages in the rest of Europe, despite widespread rainbow trout farming, indicates that T. bryosalmonae is not transported through fisheries activities. This result contrasts with the commonness of fisheries-related introductions of other pathogens and parasites such as Myxobolus cerebralis and Ceratomyxa shasta[9].

Cited literature[edit]

  1. ^ Hedrick R., McConnell E., de Kinkelin P (1993). "Proliferative kidney disease of salmonid fish". Annual Review of Fish Diseases 3: 277–290. doi:10.1016/0959-8030(93)90039-E. 
  2. ^ Kent, M.L., and R.P. Hedrick (1985). "PKX the causative agent of proliferative kidney disease (PKD) in Pacific salmonid fishes and its affinities with the Myxozoa". Journal of Protozoology 32 (2): 254 260. PMID 4009511. 
  3. ^ Korotneff, A. (1892). "Myxosporidium bryozoides". Zeitsch. Wiss. Zool. 53: 591–596. 
  4. ^ Anderson, C.L., Canning, E.U. & Okamura, B. (1999). "18S rDNA sequences indicate that PKX organism parasitizes Bryozoa". Bulletin of the European Association of Fish Pathologists 19: 94–97. 
  5. ^ Canning, E.U., Curry, A., Feist, S.W., Longshaw, M., & Okamura, B. (1999). "Tetracapsula bryosalmonae n.sp. for PKX organism the cause of PKD in salmonid fish". Bulletin of the European Association of Fish Pathologists 19 (2): 203–206. 
  6. ^ Canning, E.U., Curry, A., Feist, S.W., Longshaw, M., & Okamura, B. (2000). "A new class and order of myxozoans to accommodate parasites of bryozoans with ultrastructural observations on Tetracapsula bryosalmonae (PKX organism)". Journal of Eukaryotic Microbiology 47 (5): 456–468. doi:10.1111/j.1550-7408.2000.tb00075.x. PMID 11001143. 
  7. ^ Kent, M.L. J. Khattra, R.P. Hedrick, and R.H. Devlin (2000). "Tetracapsula renicola (Myxozoa: Saccosporidae); the PKX myxozoan – the cause of proliferative kidney disease of salmonid fishes". Journal of Parasitology 86 (1): 103–111. doi:10.1645/0022-3395(2000)086[0103:TRNSMS]2.0.CO;2. PMID 10701572. 
  8. ^ Anderson, C.L., Canning, E.U. & Okamura, B. (1999). "18S rDNA sequences indicate that PKX organism parasitizes Bryozoa". Bulletin of the European Association of Fish Pathologists 19: 94–97. 
  9. ^ Henderson, M. & Okamura, B. (2004). "The phylogeography of salmonid proliferative kidney disease in Europe and North America". Proceedings of the Royal Society B 271 (1549): 1729–1736. doi:10.1098/rspb.2004.2677. PMC 1691782. PMID 15306294. 
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