Cryptobia and cryptobiosis in fishes

Euglenozoa: taxonomy, diversity and ecology, symbioses and viruses

Euglenozoa is a species-rich group of protists, which have extremely diverse lifestyles and a range of features that distinguish them from other eukaryotes. They are composed of free-living and parasitic kinetoplastids, mostly free-living diplonemids, heterotrophic and photosynthetic euglenids, as well as deep-sea symbiontids. Although they form a well-supported monophyletic group, these morphologically rather distinct groups are almost never treated together in a comparative manner, as attempted here. We present an updated taxonomy, complemented by photos of representative species, with notes on diversity, distribution and biology of euglenozoans. For kinetoplastids, we propose a significantly modified taxonomy that reflects the latest findings. Finally, we summarize what is known about viruses infecting euglenozoans, as well as their relationships with ecto- and endosymbiotic bacteria.

Cryptobia iubilans Infections in Discus Fish in Trinidad and Tobago

Discus (Symphysodon spp.) are costly and prized specimens in the international ornamental fish trade. The majority of discus submitted to the Aquatic Animal Health Unit at the University of the West Indies School of Veterinary Medicine for necropsy between September 2010 and September 2015 had lesions consistent with Cryptobia iubilans infection, thus prompting this study. To determine the prevalence of the flagellated gastrointestinal protozoan C. iubilans in discus fish, 32 discus were sourced from 10 suppliers, including breeders, importers, and hobbyists across Trinidad. Fish were euthanized, and the internal organs, particularly the stomach and intestine, were observed under a light microscope for characteristic granulomatous lesions and/or live C. iubilans parasites. All wet-mount slides on which granulomas were observed were also Ziehl-Neelsen acid-fast stained to presumptively exclude the presence of Mycobacterium spp., the main differential when diagnosing C. iubilans-associated granulomatous gastritis or to determine the presence of dual infections. Further histological analyses were performed on stomach and intestinal sections, and transmission electron microscopy was used to confirm the parasite in stomach sections. The prevalence of C. iubilans infection was found to be 81.3%, and the prevalence of presumptive dual infections with Mycobacterium spp. was found to be 21.9%. To the best of our knowledge, this is the first documented study of C. iubilans infections in the wider Caribbean region.

Taming Parasites by Tailoring Them

The next-generation gene editing based on CRISPR (clustered regularly interspaced short palindromic repeats) has been successfully implemented in a wide range of organisms including some protozoan parasites. However, application of such a versatile game-changing technology in molecular parasitology remains fairly underexplored. Here, we briefly introduce state-of-the-art in human and mouse research and usher new directions to drive the parasitology research in the years to come. In precise, we outline contemporary ways to embolden existing apicomplexan and kinetoplastid parasite models by commissioning front-line gene-tailoring methods, and illustrate how we can break the enduring gridlock of gene manipulation in non-model parasitic protists to tackle intriguing questions that remain long unresolved otherwise. We show how a judicious solicitation of the CRISPR technology can eventually balance out the two facets of pathogen-host interplay.

An updated view of kinetoplastid phylogeny using environmental sequences and a closer outgroup: proposal for a new classification of the class Kinetoplastea

Given their ecological and medical importance, the classification of the kinetoplastid protists (class Kinetoplastea) has attracted much scientific attention for a long time. Morphology-based taxonomic schemes distinguished two major kinetoplastid groups: the strictly parasitic, uniflagellate trypanosomatids and the biflagellate bodonids. Molecular phylogenetic analyses based on 18S rRNA sequence comparison suggested that the trypanosomatids emerged from within the bodonids. However, these analyses revealed a huge evolutionary distance between the kinetoplastids and their closest relatives (euglenids and diplonemids) that makes very difficult the correct inference of the phylogenetic relationships between the different kinetoplastid groups. Using direct PCR amplification of 18S rRNA genes from hydrothermal vent samples, several new kinetoplastid-like sequences have been reported recently. Three of them emerge robustly at the base of the kinetoplastids, breaking the long branch leading to the euglenids and diplonemids. One of these sequences belongs to a close relative of Ichthyobodo necator (a fish parasite) and of the 'Perkinsiella amoebae'-like endosymbiont of Neoparamoeba spp. amoebae. The authors have studied the reliability of their basal position and used all these slow-evolving basal-emerging sequences as a close outgroup to analyse the phylogeny of the apical kinetoplastids. They thus find a much more stable and resolved kinetoplastid phylogeny, which supports the monophyly of groups that very often emerged as polyphyletic in the trees rooted using the traditional, distant outgroup sequences. A new classification of the class Kinetoplastea is proposed based on the results of the phylogenetic analysis presented. This class is now subdivided into two new subclasses, Prokinetoplastina (accommodating the basal species I. necator and 'Perkinsiella amoebae') and Metakinetoplastina (containing the Trypanosomatida together with three additional new orders: Eubodonida, Parabodonida and Neobodonida). The classification of the species formerly included in the genus Bodo is also revised, with the amendment of this genus and the genus Parabodo and the creation of a new genus, Neobodo.

Cryptobia (Trypanoplasma) salmositica and salmonid cryptobiosis

Salmonid cryptobiosis is caused by Cryptobia (Trypanoplasma) salmositica. The haemoflagellate has been reported from all species of Pacific Oncorhynchus spp. on the west coast of North America. It is normally transmitted by the freshwater leech, Piscicola salmositica, in streams and rivers, and sculpins, Cottus spp., are considered important reservoir hosts. The pathogen can also survive on the body surface of fish because it has a contractile vacuole to osmoregulate when the fish is in fresh water. This allows for direct transmission between fish, especially in aquaculture facilities. The parasite divides rapidly by binary fission in the blood to cause disease, the severity of which is directly related to parasitaemia. Cryptobia salmositica has a mitochondrium and it normally undergoes aerobic respiration; however, if its mitochondrium is damaged it will switch to glycolysis. Its glycolytic enzymes and catalase are contained in glycosomes. Cysteine protease is a metabolic enzyme, and its neutralization inhibits oxygen consumption and multiplication of the parasite. An important virulent factor in cryptobiosis is a secretory metalloprotease. The protective mechanism involves production of complement fixing antibodies, phagocytosis by macrophages, and cell-mediated cytotoxicity. Recovered fish are protected, probably for life as the immunity is non-sterile. Clinical signs of the disease include anaemia, anorexia, splenomegaly, general oedema and abdominal distension with ascites. The metabolism and swimming performance of infected fish are significantly reduced and the bioenergetic cost of the disease is very considerable. Fish are susceptible to hypoxia and their immune system is depressed during acute cryptobiosis. Severity of the disease and mortality rates vary significantly between species and stocks of salmon. Protective strategies include selective breeding of Cryptobia-resistant fish. This is innate resistance to infection and it is controlled by a dominant Mendelian locus. In these fish the parasite is lysed via the alternative pathway of complement activation. In Cryptobia-tolerant fish (infected with the pathogen but which do not suffer from disease) the metalloprotease secreted by the parasite is neutralized by alpha2 macroglobulin. Hence, the production of a transgenic Cryptobia-tolerant salmon is an option. This strategy has the advantage in that human intervention (e.g. vaccination, chemotherapy) is not required once the transgenic fish is produced. Acquired immunity is another option; a single dose of the attenuated live vaccine protects fish for at least 2 years. The protective mechanism in vaccinated fish is similar to that in recovered fish. The trypanocidal drug, isometamidium chloride, is an effective therapeutic and prophylactic agent. It accumulates in the mitochondrium of the parasite and significantly disrupts aerobic respiration by causing lesions in the organelle. Efficacy of the drug is significantly increased after its conjugation to antibodies. This immuno-chemotherapeutic strategy has the advantage in that it will lower the drug dosage and hence side-effects of chemotherapy. It will probably reduce the accumulation of the drug in fish, an important consideration in food fish.

Daily handling stress reduces resistance of carp to Trypanoplasma borreli: in vitro modulatory effects of cortisol on leukocyte function and apoptosis

Carp subjected to daily handling stress were much more susceptible to Trypanoplasma borreli infection than control fish. In a search for the cellular mechanisms involved, it was observed that cortisol suppressed T. borreli-induced expression of interleukin-1beta, tumor necrosis factor-alpha, serum amyloid A and inducible nitric oxide synthase. An NF-kappaB-inhibitor could replicate cortisol-induced apoptosis of activated peripheral blood leukocytes. In contrast, although this NF-kappaB-inhibitor induced apoptosis of neutrophilic granulocytes, cortisol prevented apoptosis of these cells, suggesting the latter process to be NF-kappaB-independent. Carp leukocytes, upon induction of apoptosis, exhibit a number of sequential metabolic alterations. First, the mitochondrial transmembrane potential (DeltaPsi(m)) is disrupted and glutathione levels are depleted, followed by exposure of phosphatidylserine on the outer cell membrane. In vitro, cortisol could inhibit NO production induced by low concentrations of lipopolysaccharide (LPS), but remarkably, enhanced NO production induced by high concentrations of LPS. However, no differences in NO production were observed in stressed versus non-stressed infected carp.

Molecular taxonomy of the suborder Bodonina (Order Kinetoplastida), including the important fish parasite, Ichthyobodo necator

Ichthyobodo necator is an important fish ectoparasite with a broad host and ecological range. A novel method, involving the use of an anesthetic, allowed the collection of large numbers of parasites from the skin and gills of hybrid striped bass (Morone saxatilis male x M. chrysops female). Genomic DNA from these samples was used to amplify and clone the 18S rRNA gene. The 18S rRNA gene was similarly cloned from Bodo caudatus, Bodo edax, Bodo saltans, an unidentified Bodo species, and Dimastigella trypaniformis. The resulting sequences were aligned with other representative kinetoplastid species using pileup and similarities in secondary structure. Phylogenetic relationships within the suborder Bodonina and representatives of the suborder Trypanosomatina were determined using maximum-likelihood statistics. The phylogenetic analyses strongly supported the order Kinetoplastida as a monophyletic assemblage consisting of at least two major lineages. One lineage consisted exclusively of L. necator, indicating that it may represent a new suborder. The second lineage consisted of all other kinetoplastid species. This second lineage appeared to contain at least 8 bodonine sublineages, none of which correlated with currently recognized families. For three sublineages, there was a close correspondence between the 18S phylogeny and the classical taxonomy of Dimastigella, Rhynchobodo, and Rhynchomonas. In contrast, Bodo and Cryptobia were polyphyletic, containing species in two or more sublineages that may represent separate genera.

Phylogenetic position of the kinetoplastids, Cryptobia bullocki, Cryptobia catostomi, and Cryptobia salmositica and monophyly of the genus Trypanosoma inferred from small subunit ribosomal RNA sequences

Phylogenetic relationships within the kinetoplastid flagellates were inferred from comparisons of small-subunit ribosomal RNA gene sequences. These included three new gene sequences from Cryptobia bullocki, (2091 bp), Cryptobia catostomi (2090 bp), and Cryptobia salmositica (2091 bp). Trees produced using maximum parsimony and distance-matrix methods (least squares and neighbor-joining) demonstrated with strong bootstrap support, that the kinetoplastids are a monophyletic group divided into two major lineages consistent with the two suborders, Trypanosomatina and Bodonina. Within the trypanosomatid clade, the genus Trypanosoma is a monophyletic group that divides into two groups, the salivarian trypanosomes and the stercorarian trypanosomes. Dimastigella and Rhynchobodo, currently classified in the Bodonina, are basal to the trypanosomatid-bodonid clade, suggesting that the suborder Bodonina is paraphyletic. Further, Trypanoplasma borreli grouped within the Cryptobia clade, and was more closely related to C. salmositica than to either C. bullocki or C. catostomi. This new molecular evidence, coupled with morphological similarities of the two genera, again calls into question the validity of the genus Trypanoplasma.

In vitro and in vivo effects of rabbit anti-thymocyte serum on circulating leucocytes and production of complement fixing antibodies in thymectomized Oncorhynchus mykiss (Walbaum) infected with Cryptobia salmositica Katz 1951

Rabbit anti-thymocyte serum (RATS) against thymocytes of rainbow trout was toxic to leucocytes from intact and thymectomized rainbow trout at 10 °C under in vitro conditions. The total number of leucocytes decreased significantly in 24 h after RATS was injected intraperitoneally into intact rainbow trout, but the number returned to pre-injection level within 1 week. RATS destroyed a lower percentage of leucocytes in thymectomized fish than in intact fish under both in vitro and in vivo conditions and the recovery in the number of leucocytes was slower in thymectomized fish. The parasitaemia, packed cell volume and production of complement fixing antibody in thymectomized and intact fish (injected with RATS before Cryptobia salmositica infection) were not significantly different from control fish (not injected with RATS), and they both acquired protective immunity against cryptobiosis on recovery. This indicates that RATS is not cytotoxic to B-like cells in the lymphoid tissue which produce complement fixing antibody against C. salmositica and that the protective antigen in C. salmositica seems to be thymus-independent.

In vitro secretion of metallo-protease (200 kDa) by the pathogenic piscine haemoflagellate, Cryptobia salmositica Katz, and stimulation of protease production by collagen

Cryptobia salmositica cultured in minimum essential medium secreted metallo-protease, and a significantly higher amount of the protease was found in media supplemented with either type I or type IV collagen. The enhancement of the protease secretion by type I collagen was dose-dependent. Using haemoglobin (substrate) SDS-PAGE, the secreted protease was detected as a single clear band (about 200 kDa). The 200 kDa metallo-protease was also detected on the C. salmositica surface membrane and the amount was increased by incubating the parasite with collagen. Collagen as a specific substrate may enhance the role of the C. salmositica metallo-protease in the disease process in infected fish.

Protection against Cryptobia (Trypanoplasma) salmositica and Salmonid Cryptobiosis

Cryptobia (Trypanoplasma) salmositica is a haemoflagellate that causes morbidity and mortality in salmon, Oncorhynchus spp, on the Pacific coast of North America. In this review, Patrick Woo briefly describes the pathogen, its transmissions (either indirectly via its leech vector, Piscicola salmositica, or directly between fish) and the clinical signs of the disease. He then outlines strategies that have been developed to protect fish against the pathogen, and the mechanism of innate resistance to disease in Cryptobia-tolerant fish. Protective strategies include the breeding of fish that are resistant to infection, and the use of an attenuated C. salmositica strain to protect susceptible fish from disease for at least two years. He ends the review with suggestions for further research that include the use of the leech vector to deliver the vaccine and the development of more novel protective strategies (eg. immunochemotherapy, anti-idiotype vaccine) against cryptobiosis.

Biochemical characterisation of an epitope on the surface membrane antigen (Cs-gp200) of the pathogenic piscine haemoflagellate Cryptobia salmositica Katz 1951

A protective surface antigen (200 kDa) on C. salmositica was detected using a monoclonal antibody (mAb-001). Enzymatic studies on the epitope indicated that it was sensitive to nonspecific protease K and to site-specific trypsin and protease V8 but not to alpha-chymotrypsin. The reactivity of the epitope with mAb-001 was not affected when the antigen was denatured with 8 M urea; however, reduction of the antigen with dithiothreitol destroyed the epitope. The epitope was susceptible to sodium m-periodate oxidation and N-glycosidase F, but not to O-glycosidase or neuraminidase. It was also sensitive to mild potassium hydrochloride hydrolysis and to phospholipase C, which is specific for phosphatidylinositol. These results suggest that the epitope consists of a polypeptide, a carbohydrate, and probably a phospholipid. The asparagine-bound N-glycosidically linked hybrid-type carbohydrate chain has the minimum length of a chitobiose core unit. There is probably a phosphatidylinositol residue which anchors the polypeptide to the surface membrane. The antigen is extensively posttranslationally modified.

Proliferation of pronephric lymphocytes of carp, Cyprinus carpio induced by extracts of Bothriocephalus acheilognathi

The interaction between Bothriocephalus acheilognathi Yamaguti, 1934 (Cestoda) and pronephric lymphocytes of carp, Cyprinus carpio L. was studied by examining proliferation of lymphocytes isolated from both naïve fish and fish injected intraperitoneally with cestode extract. Lymphocytes from naïve hosts were stimulated to proliferate in the presence of the extract depending upon the extract protein concentrations; lower concentrations (0.01-0.05 microgram/ml) induced the greatest response, and immunosuppression occurred at higher concentrations. Significant differences were noted in fish that received intraperitoneal injections of parasite extracts. Five days post-injection, lymphocyte proliferation was significantly greater in these individuals compared with sham injected or untreated controls. This difference was reduced at 10 days post-injection, although the response was dependent on the concentration of the parasite extract. The possible significance of the observed stimulation/suppression of lymphocyte activity to establishment of the parasite in the wild is discussed.

The origins of parasitism in the protists

The origins of parasitism among the protists are, like the group itself, polyphyletic. Probably the majority of present-day parasitic forms evolved from free-living ancestors which were ingested as part of the food of their hosts, though origins from ectoparasitic forms and via a phase of facultative parasitism are possibilities, particularly among the ciliated protozoa and (for ectoparasitism) the Kinetoplasta. Sporozoan parasites most probably developed via a stage which was ingested and became adapted to life in the host's gut. Further developments in parasitism involved deeper penetration into the host's tissues and the adoption of more than one host in the life cycle, thus avoiding entirely the potentially hazardous phase of existence outside the host.

The evolutionary expansion of the trypanosomatid flagellates

The trypanosomatids combine a relatively uniform morphology with ability to parasitise a very diverse range of hosts including animals, plants and other protists. Along with their sister family, the biflagellate bodonids, they are set apart from other eukaryotes by distinctive organisational features, such as the kinetoplast-mitochondrion and RNA editing, isolation of glycolysis enzymes in the glycosome, use of the flagellar pocket for molecular traffic into and out of the cell, a unique method of generating cortical microtubules, and bizarre nuclear organisation. These features testify to the antiquity and isolation of the kinetoplast-bearing flagellates (Kinetoplastida). Molecular sequencing techniques (especially small subunit ribosomal RNA gene sequencing) are now radically reshaping previous ideas on the phylogeny of these organisms. The idea that the monogenetic (MG) trypanosomatids gave rise to the digenetic (DG) genera is losing ground to a view that, after the bodonids, the African trypanosomes (DG) represent the most ancient lineage, followed by Trypanosoma cruzi (DG), then Blastocrithidia (MG), Herpetomonas (MG) and Phytomonas (DG), with Leptomonas (MG), Crithidia (MG), Leishmania (DG) and Endotrypanum (DG) forming the crown of the evolutionary tree. Vast genetic distances (12% divergence) separate T. brucei and T. cruzi, while the Leishmania species are separated by very short distances (less than 1% divergence). These phylogenetic conclusions are supported by studies on RNA editing and on the nature of the parasite surface. The trypanosomatids seem to be able to adapt with ease their energy metabolism to the availability of substrates and oxygen, and this may give them the ability to institute new life cycles if host behaviour patterns allow. Sexual processes, though present in at least some trypanosomatids, may have played only a minor part in generating diversity during trypanosomatid evolution. On the other hand, the development of altruistic behaviour on the part of some life cycle stages may be a hitherto unconsidered way of maximising fitness in this group. It is concluded that, owing to organisational constraints, the trypanosomatids can undergo substantial molecular variation while registering very little in the way of morphological change.

Further observations on Cryptobia dahli (Mastigophorea: Kinetoplastida) parasitizing marine fish

Studies were conducted primarily to ascertain the mode of transmission of Cryptobia dahli parasitizing the digestive tract of lumpfish (Cyclopterus lumpus). Another flagellate, morphologically similar to C. dahli, was also observed in the gut of a deepsea fish (Macrourus berglax). Several invertebrates, which are food for lumpfish, were examined for flagellates, but were neither infected nor showed evidence of cystic stages. Parasites were more abundant in the stomach, especially at about pH 5, than in other areas of the digestive tract. Transmission was achieved by pipetting the parasites into the stomach of uninfected fish, by feeding food contaminated with flagellates, and also by holding infected and uninfected fish in the same aquarium. In nature, lumpfish probably acquire parasites during winter when they aggregate and regurgitate into seawater because parasites can survive for short periods outside their host.

Growth, morphology and division of flagellates of the genus Trypanoplasma (Protozoa, Kinetoplastida) in vitro

Nine strains of trypanoplasms were grown in axenic culture. Cultures of Trypanoplasma borreli Laveran and Mesnil, 1901 from fish hosts Blicca bjoerkna, Cyprinus carpio, Scardinius erythrophthalmus and Tinca tinca and of T. guerneyorum Minchin, 1909 from Esox lucius and Trypanoplasma spp. from the leech Piscicola geometra were maintained in biphasic blood-agar medium SNB-9 supplemented with vitamins and antibiotics. In culture, the flagellates transformed into smaller, elongated stages with a little-developed undulating membrane and into short flagella that were morphologically similar to stages in the leech vector. The cultures were passaged weekly at 17-20 degrees C, but they also grew at 4 degrees C. The flagellates divided by binary fission, which was initiated by the formation of two new anterior flagella. The original anterior flagellum of the mother individual was gradually apposed to the cell surface and became the recurrent flagellum of one of the daughter individuals. In the meantime, nuclear division took place, followed by transverse cleavage of the kinetoplast. The division was completed by longitudinal fission of the mother individual into two offspring. Multiple fission that resulted in rosettes, which then cleaved into several daughter cells, was also observed, as well as some dyskinetoplastic and other anomalous forms. In cultures isolated from tenches with high parasitaemia, non-dividing, long filiform stages were observed. Culture stages were not infective for susceptible fishes.

Immune response of fish to parasitic protozoa

Epizootic outbreaks of fish diseases are increasingly common as a result of intensive aquaculture, fish farming and sea ranching. Very few drugs are available for treatment or prophylaxis against fish diseases, and development of such compounds is inhibited by different national regulations governing the use of chemicals in fish for human or animal consumption. Alternative approaches are urgently needed. But although the taxonomy and biology of fish parasites have been extensively studied, relatively little is known about protective immunity in fish and the effects of parasites on the piscine immune system. In this article, Patrick Woo discusses the immune responses of fish to parasitic protozoa, showing that vaccination is a viable control strategy, and stressing the need for a coordinated global research programme on fish diseases.