Keywords:
A checklist of parasites of freshwater fish of the British Isles—Kennedy revisited, reviewed and reassessed Journal Article
M. Longshaw;A. P. Shinn;C. Williams;A. Reading
In: Zootaxa, vol. 5825, no. 1, pp. 1–198, 2026, ISSN: 1175-5334, 1175-5326.
Pathology and parasitology Peer review Animal health Research intelligence
@article{longshaw_checklist_2026,
title = {A checklist of parasites of freshwater fish of the British Isles—Kennedy revisited, reviewed and reassessed},
author = {Matt Longshaw and Chris Williams and Amy Reading and Andrew P. Shinn},
url = {https://mapress.com/zt/article/view/zootaxa.5825.1.1},
doi = {10.11646/zootaxa.5825.1.1},
issn = {1175-5334, 1175-5326},
year = {2026},
date = {2026-06-01},
urldate = {2026-06-05},
journal = {Zootaxa},
volume = {5825},
number = {1},
pages = {1–198},
abstract = {This review addresses the current state of knowledge regarding freshwater fish parasites in the British Isles, highlighting the significant gap since kennedy’s seminal 1974 work (Journal of Fish Biology 6: 613–644). Despite subsequent efforts by others to produce checklists in 1992 and 2016, comprehensive updates to parasite distribution patterns and species records have been limited. The paper emphasises the importance of accurate parasite checklists for understanding the status of and changes to endemic species baselines, climate change impacts, managing fish translocations, and identifying novel infections. Historical records, including paleoparasitological findings from archaeological deposits, provide valuable insights into parasite distribution patterns, though pre-1900s data requires careful interpretation due to taxonomic limitations. This study provides compiled parasite and host lists from published and unpublished records, creating a database that includes host species, parasite nomenclature, organ location, geographical distribution, and reference information. The current compilation details 339 named parasite species in 147 genera from 58 hosts (52 Protista, 3 Microsporidia, 54 Myxozoa, 83 “Monogenea” (79 Monopisthocotyla, 4 Polyopisthocotyla), 58 Digenea and Aspidogastrea, 33 Cestoda, 21 Nematoda, 7 Acanthocephala, 5 Hirudinea, 14 Copepoda, 4 Branchiura, and 5 Mollusca). A further 69 parasites identified to genus or class level have been reported, bringing the total number of parasites reported in the British Isles to 408. Several challenges are identified, including data transposition errors in previous works, sampling method inconsistencies, difficulties in accessing unpublished data, scant repositories to retrospectively evaluate findings, absence of molecular and taxonomic rigour, and the impact of anthropogenic activities on parasite distributions. As a result, any list may represent a significant underestimate of species presence and diversity. The review concludes by recommending future research directions, emphasising the need for combined molecular and morphological approaches, standardised documentation methods, and increased focus on under-represented taxa to enhance our understanding of freshwater fish parasites in the British Isles.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
This review addresses the current state of knowledge regarding freshwater fish parasites in the British Isles, highlighting the significant gap since kennedy’s seminal 1974 work (Journal of Fish Biology 6: 613–644). Despite subsequent efforts by others to produce checklists in 1992 and 2016, comprehensive updates to parasite distribution patterns and species records have been limited. The paper emphasises the importance of accurate parasite checklists for understanding the status of and changes to endemic species baselines, climate change impacts, managing fish translocations, and identifying novel infections. Historical records, including paleoparasitological findings from archaeological deposits, provide valuable insights into parasite distribution patterns, though pre-1900s data requires careful interpretation due to taxonomic limitations. This study provides compiled parasite and host lists from published and unpublished records, creating a database that includes host species, parasite nomenclature, organ location, geographical distribution, and reference information. The current compilation details 339 named parasite species in 147 genera from 58 hosts (52 Protista, 3 Microsporidia, 54 Myxozoa, 83 “Monogenea” (79 Monopisthocotyla, 4 Polyopisthocotyla), 58 Digenea and Aspidogastrea, 33 Cestoda, 21 Nematoda, 7 Acanthocephala, 5 Hirudinea, 14 Copepoda, 4 Branchiura, and 5 Mollusca). A further 69 parasites identified to genus or class level have been reported, bringing the total number of parasites reported in the British Isles to 408. Several challenges are identified, including data transposition errors in previous works, sampling method inconsistencies, difficulties in accessing unpublished data, scant repositories to retrospectively evaluate findings, absence of molecular and taxonomic rigour, and the impact of anthropogenic activities on parasite distributions. As a result, any list may represent a significant underestimate of species presence and diversity. The review concludes by recommending future research directions, emphasising the need for combined molecular and morphological approaches, standardised documentation methods, and increased focus on under-represented taxa to enhance our understanding of freshwater fish parasites in the British Isles.
Interlaboratory variability in screening for Bonamia ostreae, a protistan parasite of the European flat oyster Ostrea edulis. Journal Article
M. Longshaw;P. Martin;G. Flannery;S. Lynch;D. Stone;A. Ramillo;A. Villalba;S. C. Culloty
In: Diseases of Aquatic Organisms, vol. 110, pp. 93–99, 2014.
Pathology and parasitology Peer review Research intelligence
@article{flannery_interlaboratory_2014,
title = {Interlaboratory variability in screening for Bonamia ostreae, a protistan parasite of the European flat oyster Ostrea edulis.},
author = {G Flannery and SA Lynch and M Longshaw and D Stone and P Martin and A Ramillo and Antonio Villalba and Sarah C Culloty},
url = {http://www.int-res.com/articles/dao2014/110/d110p093.pdf},
doi = {10.3354/dao02717},
year = {2014},
date = {2014-01-01},
journal = {Diseases of Aquatic Organisms},
volume = {110},
pages = {93–99},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A review of the infectious agents, parasites, pathogens and commensals of European cockles (Cerastoderma edule and C. glaucum) Journal Article
M. Longshaw;S. K. Malham
In: Journal of the Marine Biological Association of the United Kingdom, vol. 93, no. 1, pp. 227–247, 2013.
Pathology and parasitology Peer review Research intelligence
@article{longshaw_review_2013,
title = {A review of the infectious agents, parasites, pathogens and commensals of European cockles (Cerastoderma edule and C. glaucum)},
author = {M Longshaw and S K Malham},
doi = {10.1017/S0025315412000537},
year = {2013},
date = {2013-01-01},
journal = {Journal of the Marine Biological Association of the United Kingdom},
volume = {93},
number = {1},
pages = {227–247},
abstract = {A systematic review of the parasites, pathogens and commensals of the edible cockle (Cerastoderma edule) and of the lagoon cockle (Cerastoderma glaucum) has been completed. A total of 59 different conditions have been reported throughout the range of both of these hosts; of these 50 have been reported in edible cockles, and 28 in lagoon cockles. Cockles are hosts to viruses, bacteria, fungi (including Microsporidia), Apicomplexa, Amoeba, Ciliophora, Perkinsozoa, Haplosporidia, Cercozoa, Turbellaria, Digenea, Cestoda, Nematoda, Crustacea and Nemertea. A number of these have been reported sporadically although they may be associated with mortalities. In particular, mortalities have been associated predominately with digeneans and some protistan infections. In many cases pathology is marked in affected animals and parasites have been shown to reduce fecundity, alter burrowing behaviour and limit growth. The review provides information on the individual and population effects of these conditions as well as providing suggestions for future research. In particular, there has been a lack of taxonomic rigour applied to many studies and as a result there are a number of erroneous host records. There is a need to re-describe a number of parasite species and to determine the lifecycle of those considered to be important mortality drivers.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A systematic review of the parasites, pathogens and commensals of the edible cockle (Cerastoderma edule) and of the lagoon cockle (Cerastoderma glaucum) has been completed. A total of 59 different conditions have been reported throughout the range of both of these hosts; of these 50 have been reported in edible cockles, and 28 in lagoon cockles. Cockles are hosts to viruses, bacteria, fungi (including Microsporidia), Apicomplexa, Amoeba, Ciliophora, Perkinsozoa, Haplosporidia, Cercozoa, Turbellaria, Digenea, Cestoda, Nematoda, Crustacea and Nemertea. A number of these have been reported sporadically although they may be associated with mortalities. In particular, mortalities have been associated predominately with digeneans and some protistan infections. In many cases pathology is marked in affected animals and parasites have been shown to reduce fecundity, alter burrowing behaviour and limit growth. The review provides information on the individual and population effects of these conditions as well as providing suggestions for future research. In particular, there has been a lack of taxonomic rigour applied to many studies and as a result there are a number of erroneous host records. There is a need to re-describe a number of parasite species and to determine the lifecycle of those considered to be important mortality drivers.
Diseases of crayfish: A review Journal Article
M. Longshaw
In: Journal of Invertebrate Pathology, vol. 106, no. 1, pp. 54–70, 2011.
Pathology and parasitology Peer review Animal health Research intelligence
@article{longshaw_diseases_2011,
title = {Diseases of crayfish: A review},
author = {M Longshaw},
url = {http://www.sciencedirect.com/science/article/B6WJV-51W6NYJ-7/2/6efe55992b4967afd31cba5be0211a33},
doi = {10.1016/j.jip.2010.09.013},
year = {2011},
date = {2011-01-01},
journal = {Journal of Invertebrate Pathology},
volume = {106},
number = {1},
pages = {54–70},
abstract = {A systematic review of parasites, pathogens and commensals of freshwater crayfish has been conducted. All major groups of disease causing agents have been covered including viruses, bacteria, fungi, protistans and metazoans. Most agents tend to cause limited problems for crayfish. Exceptions to this include fungi, bacteria and viruses. However, in many cases, these tend to be isolated reports in either a specific geographical location or in individual animals. The apparent absence of pathology associated with these agents in crayfish should not be taken to suggest that movements of crayfish to new geographical areas is necessarily acceptable. Several examples are given where seemingly healthy animals have been moved to new areas leading to mortality of other crayfish within the same area as a direct result of transmission of pathogens to na�ve hosts. Some future research needs are proposed, including the need for pathogen characterisation and production of disease-free crayfish for aquaculture},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
A systematic review of parasites, pathogens and commensals of freshwater crayfish has been conducted. All major groups of disease causing agents have been covered including viruses, bacteria, fungi, protistans and metazoans. Most agents tend to cause limited problems for crayfish. Exceptions to this include fungi, bacteria and viruses. However, in many cases, these tend to be isolated reports in either a specific geographical location or in individual animals. The apparent absence of pathology associated with these agents in crayfish should not be taken to suggest that movements of crayfish to new geographical areas is necessarily acceptable. Several examples are given where seemingly healthy animals have been moved to new areas leading to mortality of other crayfish within the same area as a direct result of transmission of pathogens to na�ve hosts. Some future research needs are proposed, including the need for pathogen characterisation and production of disease-free crayfish for aquaculture
The British river of the future: How climate change and human activity might affect two contrasting river ecosystems in England Journal Article
M. Longshaw;S. W. Feist;R. E. Gozlan;A. C. Johnson;M. C. Acreman;M. J. Dunbar;A. M. Giacomello;S. A. Hinsley;A. T. Ibbotson;H. P. Jarvie;et al.
In: Science of the Total Environment, vol. 407, pp. 4787–4798, 2009.
Pathology and parasitology Biology / Non-native studies Peer review Animal health Research intelligence
@article{johnson_british_2009,
title = {The British river of the future: How climate change and human activity might affect two contrasting river ecosystems in England},
author = {A C Johnson and M C Acreman and M J Dunbar and S W Feist and A M Giacomello and R E Gozlan and S A Hinsley and A T Ibbotson and H P Jarvie and I Jones and M Longshaw and S C Maberly and T J Marsh and C Neal and J R Newman and M A Nunn and R W Pickup and N S Reynard and C A Sullivan and J P Sumpter and R J Williams},
doi = {https://doi.org/10.1016/j.scitotenv.2009.05.018},
year = {2009},
date = {2009-01-01},
journal = {Science of the Total Environment},
volume = {407},
pages = {4787–4798},
abstract = {The possible effects of changing climate on a southern and a north-eastern English river (the Thames and the Yorkshire Ouse, respectively) were examined in relation to water and ecological quality throughout the food web. The CLASSIC hydrological model, driven by output from the Hadley Centre climate model (HadCM3), based on IPCC low and high CO2 emission scenarios for 2080 were used as the basis for the analysis. Compared to current conditions, the CLASSIC model predicted lower flows for both rivers, in all seasons except winter. Such an outcome would lead to longer residence times (by up to a month in the Thames), with nutrient, organic and biological contaminant concentrations elevated by 70-100% pro-rata, assuming sewage treatment effectiveness remains unchanged. Greater opportunities for phytoplankton growth will arise, and this may be significant in the Thames. Warmer winters and milder springs will favour riverine birds and increase the recruitment of many coarse fish species. However, warm, slow-flowing, shallower water would increase the incidence of fish diseases. These changing conditions would make southern UK rivers in general a less favourable habitat for some species of fish, such as the Atlantic salmon (Salmo salar). Accidental or deliberate, introductions of alien macrophytes and fish may change the range of species in the rivers. In some areas, it is possible that a concurrence of different pressures may give rise to the temporary loss of ecosystem services, such as providing acceptable quality water for humans and industry. An increasing demand for water in southern England due to an expanding population, a possibly reduced flow due to climate change, together with theWater Framework Directive obligation to maintain water quality, will put extreme pressure on river ecosystems, such as the Thames},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The possible effects of changing climate on a southern and a north-eastern English river (the Thames and the Yorkshire Ouse, respectively) were examined in relation to water and ecological quality throughout the food web. The CLASSIC hydrological model, driven by output from the Hadley Centre climate model (HadCM3), based on IPCC low and high CO2 emission scenarios for 2080 were used as the basis for the analysis. Compared to current conditions, the CLASSIC model predicted lower flows for both rivers, in all seasons except winter. Such an outcome would lead to longer residence times (by up to a month in the Thames), with nutrient, organic and biological contaminant concentrations elevated by 70-100% pro-rata, assuming sewage treatment effectiveness remains unchanged. Greater opportunities for phytoplankton growth will arise, and this may be significant in the Thames. Warmer winters and milder springs will favour riverine birds and increase the recruitment of many coarse fish species. However, warm, slow-flowing, shallower water would increase the incidence of fish diseases. These changing conditions would make southern UK rivers in general a less favourable habitat for some species of fish, such as the Atlantic salmon (Salmo salar). Accidental or deliberate, introductions of alien macrophytes and fish may change the range of species in the rivers. In some areas, it is possible that a concurrence of different pressures may give rise to the temporary loss of ecosystem services, such as providing acceptable quality water for humans and industry. An increasing demand for water in southern England due to an expanding population, a possibly reduced flow due to climate change, together with theWater Framework Directive obligation to maintain water quality, will put extreme pressure on river ecosystems, such as the Thames
Histopathology of fish parasite infections - importance for populations Journal Article
M. Longshaw;S. W. Feist
In: Journal of Fish Biology, vol. 73, pp. 2143–2160, 2008.
Pathology and parasitology Peer review Animal health Research intelligence
@article{feist_histopathology_2008,
title = {Histopathology of fish parasite infections - importance for populations},
author = {S W Feist and M Longshaw},
doi = {https://doi.org/10.1111/j.1095-8649.2008.02060.xDigital Object Identifier (DOI)},
year = {2008},
date = {2008-01-01},
journal = {Journal of Fish Biology},
volume = {73},
pages = {2143–2160},
abstract = {There is a bewildering array of fish parasites and probably all fish species harbour one or more species. Under normal conditions hosts often show little or no signs of infection, either clinically or at the organ and tissue level. All tissues can be infected, including the blood. Coelozoic species generally do not elicit significant host responses. Any developmental stages, however, where these migrate through the host tissues may produce a cellular response if resident for sufficient time. Histozoic parasites are generally more likely to evoke a pathological response since they are invariably in more intimate contact with immunocompetent host cells such as lymphocytes and phagocytes. In some cases, particularly with protistan infections, host response can be dramatic and cause significant disease due to organ dysfunction, with subsequent mortalities. Infections of the external epithelia caused by a variety of parasites including Monogenea and Copepoda are also significant, especially when large numbers are present. Resulting tissue necrosis compromises osmoregulation and facilitates entry of prokaryote infections. Parasites have numerous strategies to evade detection within the host, but even protected intracellular forms, e.g. microsporeans and muscle-invading myxozoans are recognized when they outgrow their accommodation or large cysts rupture, presenting parasite antigens to the host. In most instances, an evolutionary balance has been achieved between the host and the parasite and even when histopathology is evident, this is frequently localized and does not unduly impair performance of the affected organ. Examples include chronic inflammation, granuloma formation and focal fibrosis. This paper outlines the principle histopathological responses to a variety of parasite groups and provides a more detailed treatment of selected parasitic infections in marine and freshwater fish species to illustrate the relative importance of acute and chronic parasitic infections for host survival},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
There is a bewildering array of fish parasites and probably all fish species harbour one or more species. Under normal conditions hosts often show little or no signs of infection, either clinically or at the organ and tissue level. All tissues can be infected, including the blood. Coelozoic species generally do not elicit significant host responses. Any developmental stages, however, where these migrate through the host tissues may produce a cellular response if resident for sufficient time. Histozoic parasites are generally more likely to evoke a pathological response since they are invariably in more intimate contact with immunocompetent host cells such as lymphocytes and phagocytes. In some cases, particularly with protistan infections, host response can be dramatic and cause significant disease due to organ dysfunction, with subsequent mortalities. Infections of the external epithelia caused by a variety of parasites including Monogenea and Copepoda are also significant, especially when large numbers are present. Resulting tissue necrosis compromises osmoregulation and facilitates entry of prokaryote infections. Parasites have numerous strategies to evade detection within the host, but even protected intracellular forms, e.g. microsporeans and muscle-invading myxozoans are recognized when they outgrow their accommodation or large cysts rupture, presenting parasite antigens to the host. In most instances, an evolutionary balance has been achieved between the host and the parasite and even when histopathology is evident, this is frequently localized and does not unduly impair performance of the affected organ. Examples include chronic inflammation, granuloma formation and focal fibrosis. This paper outlines the principle histopathological responses to a variety of parasite groups and provides a more detailed treatment of selected parasitic infections in marine and freshwater fish species to illustrate the relative importance of acute and chronic parasitic infections for host survival
Effect of microbial pathogens on the diversity of aquatic populations, notably in Europe Journal Article
M. Longshaw;S. W. Feist;E. J. Peeler;R. E. Gozlan;S. St-Hilaire
In: Microbes and Infection, vol. 8, no. 5, pp. 1358–1364, 2006.
Pathology and parasitology Peer review Research intelligence
@article{gozlan_effect_2006,
title = {Effect of microbial pathogens on the diversity of aquatic populations, notably in Europe},
author = {R E Gozlan and E J Peeler and M Longshaw and S St-Hilaire and S W Feist},
url = {http://www.sciencedirect.com/science/article/pii/S1286457906000207},
doi = {https://doi.org/10.1016/j.micinf.2005.12.010},
year = {2006},
date = {2006-04-01},
journal = {Microbes and Infection},
volume = {8},
number = {5},
pages = {1358–1364},
abstract = {The expansion of aquaculture and the demand for ornamental fish have resulted in the large-scale movements of aquatic animals and their pathogens. Here we review the most important non-native fish and shellfish pathogens in European waters and their global impacts on wild fish host populations. The role of theoretical models in the study of the impact of microbial pathogens is discussed, including its integration into risk assessments},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The expansion of aquaculture and the demand for ornamental fish have resulted in the large-scale movements of aquatic animals and their pathogens. Here we review the most important non-native fish and shellfish pathogens in European waters and their global impacts on wild fish host populations. The role of theoretical models in the study of the impact of microbial pathogens is discussed, including its integration into risk assessments
Recent advances in our knowledge of the Myxozoa Journal Article
M. Longshaw;S. W. Feist;M. L. Kent;K. Andree;J. L. Bartholomew;M. El-Matbouli;S. S. Desser;R. H. Devlin;R. P. Hedrick;R. W. Hoffman;et al.
In: Journal of Eukaryotic Microbiology, vol. 48, no. 4, pp. 395–413, 2001.
Pathology and parasitology Peer review Animal health Research intelligence
@article{kent_recent_2001,
title = {Recent advances in our knowledge of the Myxozoa},
author = {M L Kent and K Andree and J L Bartholomew and M El-Matbouli and S S Desser and R H Devlin and S W Feist and R P Hedrick and R W Hoffman and J Khattra and S L Hallett and R J G Lester and M Longshaw and O Palenzuela and M E Siddall and C Xiao and K B Andree and M El Matbouli and R W Hoffmann and Al Khattra et},
url = {http://www.sciencedirect.com/science/article/B6WVB-45GNBMG-112/2/4dd2e4eced62cacfcb416c6e054b70c8},
doi = {10.1111/j.1550-7408.2001.tb00173.x},
year = {2001},
date = {2001-01-01},
journal = {Journal of Eukaryotic Microbiology},
volume = {48},
number = {4},
pages = {395–413},
abstract = {In the last few years two factors have helped to significantly advance our understanding of the Myxozoa. First, the phenomenal increase in fin fish aquaculture in the 1990s has lead to the increased importance of these parasites; in turn this has lead to intensified research efforts, which have increased knowledge of the development, diagnosis, and pathogenesis of myxozoans. The hallmark discovery in the 1980s that the life cycle of Myxobolus cerebralis requires development of an actinosporean stage in the oligochaete, Tubifex tubifex, led to the elucidation of the life cycles of several other myxozoans. Also, the life cycle and taxonomy of the enigmatic PKX myxozoan has been resolved: it is the alternate stage of the unusual myxozoan, Tetracapsula bryosalmonae, from bryozoans. The 18S rDNA gene of many species has been sequenced, and here we add 22 new sequences to the data set. Phylogenetic analyses using all these sequences indicate that:1) the Myxozoa are closely related to Cnidaria (also supported by morphological data); 2) marine taxa at the genus level branch separately from genera that usually infect freshwater fishes; 3) taxa cluster more by development and tissue location than by spore morphology; 4) the tetracapsulids branched off early in myxozoan evolution, perhaps reflected by their having bryozoan, rather than annelid hosts; 5) the morphology of actinosporeans offers little information for determining their myxosporean counterparts (assuming that they exist); and 6) the marine actinosporeans from Australia appear to form a clade within the platysporinid myxosporeans. Ribosomal DNA sequences have also enabled development of diagnostic tests for myxozoans. PCR and in situ hybridisation tests based on rDNA sequences have been developed for Myxobolus cerebralis, Ceratomyxa shasta, Kudoa spp., and Tetracapsula bryosalmonae (PKX). Lectin-based and antibody tests have also been developed for certain myxozoans, such as PKX and C. shasta. We also review important diseases caused by myxozoans, which are emerging or re-emerging. Epizootics of whirling disease in wild rainbow trout (Oncorhynchus mykiss) have recently been reported throughout the Rocky Mountain states of the USA. With a dramatic increase in aquaculture of fishes using marine netpens, several marine myxozoans have been recognized or elevated in status as pathological agents. Kudoa thyrsites infections have caused severe post-harvest myoliquefaction in pen-reared Atlantic salmon (Salmo salar), and Ceratomyxa spp., Sphaerospora spp., and Myxidium leei cause disease in pen-reared sea bass (Dicentrarchus labrax) and sea bream species (family Sparidae) in Mediterranean countries.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
In the last few years two factors have helped to significantly advance our understanding of the Myxozoa. First, the phenomenal increase in fin fish aquaculture in the 1990s has lead to the increased importance of these parasites; in turn this has lead to intensified research efforts, which have increased knowledge of the development, diagnosis, and pathogenesis of myxozoans. The hallmark discovery in the 1980s that the life cycle of Myxobolus cerebralis requires development of an actinosporean stage in the oligochaete, Tubifex tubifex, led to the elucidation of the life cycles of several other myxozoans. Also, the life cycle and taxonomy of the enigmatic PKX myxozoan has been resolved: it is the alternate stage of the unusual myxozoan, Tetracapsula bryosalmonae, from bryozoans. The 18S rDNA gene of many species has been sequenced, and here we add 22 new sequences to the data set. Phylogenetic analyses using all these sequences indicate that:1) the Myxozoa are closely related to Cnidaria (also supported by morphological data); 2) marine taxa at the genus level branch separately from genera that usually infect freshwater fishes; 3) taxa cluster more by development and tissue location than by spore morphology; 4) the tetracapsulids branched off early in myxozoan evolution, perhaps reflected by their having bryozoan, rather than annelid hosts; 5) the morphology of actinosporeans offers little information for determining their myxosporean counterparts (assuming that they exist); and 6) the marine actinosporeans from Australia appear to form a clade within the platysporinid myxosporeans. Ribosomal DNA sequences have also enabled development of diagnostic tests for myxozoans. PCR and in situ hybridisation tests based on rDNA sequences have been developed for Myxobolus cerebralis, Ceratomyxa shasta, Kudoa spp., and Tetracapsula bryosalmonae (PKX). Lectin-based and antibody tests have also been developed for certain myxozoans, such as PKX and C. shasta. We also review important diseases caused by myxozoans, which are emerging or re-emerging. Epizootics of whirling disease in wild rainbow trout (Oncorhynchus mykiss) have recently been reported throughout the Rocky Mountain states of the USA. With a dramatic increase in aquaculture of fishes using marine netpens, several marine myxozoans have been recognized or elevated in status as pathological agents. Kudoa thyrsites infections have caused severe post-harvest myoliquefaction in pen-reared Atlantic salmon (Salmo salar), and Ceratomyxa spp., Sphaerospora spp., and Myxidium leei cause disease in pen-reared sea bass (Dicentrarchus labrax) and sea bream species (family Sparidae) in Mediterranean countries.