Immunohistochemical reactivity of polyclonal antibodies against Sphaerospora testicularis and Ceratomyxa labracis (Myxosporea: Bivalvulida), with other myxosporean parasites

Development of Fish Parasite Vaccines in the OMICs Era: Progress and Opportunities

Globally, parasites are increasingly being recognized as catastrophic agents in both aquaculture sector and in the wild aquatic habitats leading to an estimated annual loss between 1.05 billion and 9.58 billion USD. The currently available therapeutic and control measures are accompanied by many limitations. Hence, vaccines are recommended as the "only green and effective solution" to address these concerns and protect fish from pathogens. However, vaccine development warrants a better understanding of host-parasite interaction and parasite biology. Currently, only one commercial parasite vaccine is available against the ectoparasite sea lice. Additionally, only a few trials have reported potential vaccine candidates against endoparasites. Transcriptome, genome, and proteomic data at present are available only for a limited number of aquatic parasites. Omics-based interventions can be significant in the identification of suitable vaccine candidates, finally leading to the development of multivalent vaccines for significant protection against parasitic infections in fish. The present review highlights the progress in the immunobiology of pathogenic parasites and the prospects of vaccine development. Finally, an approach for developing a multivalent vaccine for parasitic diseases is presented. Data sources to prepare this review included Pubmed, google scholar, official reports, and websites.

Immunohistochemical characterization of polyclonal antibodies against Enteromyxum leei and Enteromyxum scophthalmi (Myxozoa: Myxosporea), intestinal parasites of fish

The enteric myxozoan parasites Enteromyxum leei (Diamant, Lom et Dyková) and Enteromyxum scophthalmi Palenzuela, Redondo et Álvarez-Pellitero are responsible for high weight loss in infected fish, which leads to subchronic disease and low mortality rates in gilthead sea bream (GSB), Sparus aurata L., and to high mortality rates in turbot, Psetta maxima (L.). The detection of initial parasite stages in histological sections is particularly difficult, but can be simplified by means of specific antibodies. Rabbit polyclonal antibodies (pAbs) were raised against E. scophthalmi and E. leei, and direct enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry were used to characterize their sensitivity and specificity. Both pAbs were adsorbed (apAb) with non-infected intestines to avoid non-specific labelling of fish tissues and to improve their specificity. The highest titre obtained in ELISA was 1: 32 000 for apAb-Eleei and 1:16 000 for apAb-Escoph. Working dilutions in immunohistochemistry were 1:1000 for apAb-Eleei and 1:8000 for apAb-Escoph. Both apAbs labelled proliferative and sporogonic stages with high specificity. apAb-Escoph was very specific, whereas apAb-Eleei cross-reacted with Sphaerospora dicentrarchi Sitjà-Bobadilla et Álvarez-Pellitero and Sphaerospora testicularis Sitjà-Bobadilla et Álvarez-Pellitero, suggesting the presence of shared antigens. These pAbs stand as new tools for antigenic characterization and the diagnosis of both Enteromyxum species.

Antigenic characterization of Enteromyxum leei (Myxozoa: Myxosporea)

Enteromyxum leei, an intestinal myxozoan parasite affecting a wide range of fish, was partially purified, and the immunogenic composition of its glycoproteins as well as the proteolytic activity were studied. Parasite extracts, mainly containing spores, were separated by SDS-PAGE, and thereafter, immunoblots were carried out with a polyclonal antiserum (Pab) raised against E. leei. Periodic acid/Schiff staining of gels, periodate- and Proteinase K-treated Western blots and Lectin blots were performed to analyse the terminal carbohydrate composition of the parasite's antigens. Additionally, the cross-reaction of the parasite extracts with a Pab raised against the polar filament of the myxozoan Myxobolus pendula was studied. Both Pabs detected proteic epitopes on antigenic proteins and glycoproteins of E. leei, ranging between 15 and 280 kDa. In particular, 2 glycoproteic bands (15 and 165 kDa), immunoreactive to both Pabs and with glucose and mannose moieties, could correspond to common antigens shared among myxozoans. The 165 kDa band also presented galactose, N-acetyl-galactosamine and N-acetyl-glucosamine, pointing to its possible origin on chitin-built spore valves and to its possible involvement in host-parasite interactions. The molecular weight of the 15 kDa glycoproteic antigen matches that of minicollagen, a cnidarian-specific protein of nematocysts with a myxozoan homologue. Several proteases with apparent molecular weights ranging between 43 and 245 kDa were found in zymographies of E. leei extracts, and these may have a potential role in the parasite's pathogenesis. This is a useful approach for further studies to detect targets for antiparasitic therapy.

Humoral immune responses induced by Kudoa sp. (Myxosporea: Multivalvulida) in BALB/c mice: oral administration, immunization and cross-reactions with Myxobolus aeglefini (Myxosporea: Bivalvulida)

The majority of Kudoa species infect the somatic muscle of fish, establishing cysts. Because there is no effective method to detect infected fish without destroying them, these parasitized fish reach the consumer. The elevated humoral responses detected previously by us in BALB/c mice immunized with Kudoa sp. pseudocyst extracts showed the possible immunopathological effects in man from the ingestion of Kudoa-infected fish. In this work, the high IgG1 and IgE levels induced by the oral administration of Kudoa pseudocysts to BALB/c mice confirmed the allergenic nature of some of their components. An alternative way of preparing the soluble extract by using a FastPrep' shaker indicated the inconvenience of using sonication to prepare the Kudoa sp. extract. IgG+M, IgG, IgG3 and IgA cross-reactions of Kudoa sp. with another myxosporean, Myxobolus aeglefini, were found.

Recent advances in our knowledge of the Myxozoa

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.