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Myxobolus freitasi n. sp. (Myxozoa: Bivalvulida), a parasite of the brain of the electric knifefish in the Brazilian Amazon region

Myxobolus freitasi n. sp. (Myxozoa: Bivalvulida), parasita do cérebro do peixe-faca elétrico na Amazônia brasileira

Abstract

A total of 30 specimens of the Amazonian electric knifefish, Brachyhypopomus beebei Schultz, 1944 (Gymnotiformes: Hypopomidae), were collected from the Peixe-Boi River in the state of Pará, Brazil (1°06’59” S; 47°18’26” W). Fragments of the brain tissue were extracted for analysis via optical microscopy, and 18 specimens (60%) presented microparasites of the genus Myxobolus, with unequal capsules. The spores were 18.6 µm (17.7-19.8 µm) long and 8.6 µm (8.4-9.0 µm) wide; the largest polar capsule was 13.0 µm (12.4-13.4 µm) long and 5.6 µm (5.3-6.0 µm) wide, and the smallest capsule was 5.0 µm (4.5-5.3 µm) long and 2.5 µm (2.3-2.6 µm) wide. Infected brain fragments were extracted for histological processing and staining with hematoxylin-eosin and Ziehl-Neelsen. Some fragments were conserved in ethanol for molecular genetics analysis. A partial sequence of the 18S DNA gene was obtained from the spores, which did not correspond to any other sequences deposited in GenBank, although it did form a clade with other Myxobolus parasites of the nervous system. The morphological data, together with molecular phylogeny, supported the designation of a new species Myxobolus freitasi n. sp.

Keywords:
Cnidaria; Myxozoa; molecular biology; histology

Resumo

Um total de 30 espécimes do peixe-faca elétrico da Amazônia, Brachyhypopomus beebei Schultz, 1944 (Gymnotiformes: Hypopomidae), foram coletados no rio Peixe-Mani, no estado do Pará, Brasil (1 ° 06'59 “S; 47 ° 18 ' 26 “W). Fragmentos de tecido cerebral foram extraídos para análise em microscopia óptica, sendo que 18 espécimes (60%) apresentavam microparasitos do gênero Myxobolus, com cápsulas desiguais. Os esporos apresentavam 18,6 µm (17,7-19,8 µm) de comprimento e 8,6 µm (8,4-9,0 µm) de largura; a maior cápsula polar tinha 13,0 µm (12,4-13,4 µm) de comprimento e 5,6 µm (5,3-6,0 µm) de largura, e a menor cápsula tinha 5,0 µm (4,5-5,3 µm) de comprimento e 2,5 µm (2,3-2,6 µm) de largura. Fragmentos cerebrais infectados foram extraídos para processamento histológico e coloração com hematoxilina-eosina e Ziehl-Neelsen. Alguns fragmentos foram conservados em etanol para análise genética molecular. Dos esporos, foi obtida uma sequência parcial do gene 18S do DNA, que não correspondeu a nenhuma outra sequência depositada no GenBank, embora tenha formado um clado com outros parasitas do gênero Myxobolus do sistema nervoso. Os dados morfológicos, juntamente com a filogenia molecular, apoiaram a designação de uma nova espécie Myxobolus freitasi n. sp.

Palavras-chave:
Cnidaria; Myxozoa; biologia molecular; histologia

Introduction

The hydrographic basin of the Peixe-Boi River is located in the northeastern part of the state of Pará, Brazil, and has a total area of 1,044.32 km2 (Silva & Lima, 2000Silva AM, Lima LC. Caracterização fisiográfica da bacia do rio Peixe-Boi [monografia]. Belém: Centro de Ciências Exatas e Tecnologia, Universidade da Amazônia; 2000.). It is located primarily within the municipality of Peixe-Boi, an area characterized by unregulated settlement, which has caused extensive environmental degradation and modification of the local ecosystems. This has interfered with the biota of the municipality’s rivers. These pressures are a stress factor for the local fish and may, in particular, reduce their immunological response (Dean et al., 2001Dean JH, House RV, Luster MI. Immunotoxicology: effects of an responses to drugs and chemicals. In: Hayes AW, editor. Principles and methods of toxicology. 4th ed. Boca Raton: CRC Press; 2001. p. 1415-1450.), thus leading to increased incidence of infectious diseases (Pavanelli et al., 2002Pavanelli GC, Eiras JC, Takemoto RM. Doenças de peixes. Profilaxia, diagnóstico e tratamento. Maringá: Editora Universidade Estadual de Maringá; 2002.).

Fish are the most ancient and diverse group of vertebrates, and they include more than half of the vertebrate species described to date, i.e. approximately 28,000 out of an estimated total of 54,711 vertebrate species (Nelson, 2006Nelson JS. Fishes of the world. 4th ed. New York: John Wiley & Sons Inc.;2006.). The aquatic lifestyle of these animals facilitates dispersal and transmission of parasites (Ahid et al., 2009Ahid SMM, Filgueira KD, Fonsêca ZAAS, Soto-Blanco B, Oliveira MF. Ocorrência de parasitismo em Mola mola (Linnaeus, 1758) por metazoários no litoral do Rio Grande do Norte, Brasil. Acta Vet Bras 2009; 3(1): 43-47. http://dx.doi.org/10.21708/avb.2009.3.1.1118.
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).

In the Amazon region, Gymnotiformes fish are small organisms, with commercial importance in aquarium, the species of the freshwater benthopelagic genus Brachyhypopomus Schultz, 1944 (Hypopomidae) present a variety of pathogens, which exercise different levels of pathogenicity in relation to their natural habitat. it is due to homeostasis between the nutritional and physiological status of the fish being in balance with the environment (Mendoza-Franco & Reina, 2008Mendoza-Franco EF, Reina RG. Five new species of Urocleidoides (Monogenoidea) (Mizelle and Price 1964) Kritsky, Thatcher, and Boeger, 1986, parasitizing the gills of Panamanian freshwater fishes. J Parasitol 2008; 94(4): 793-802. http://dx.doi.org/10.1645/GE-1438.1. PMid:18837577.
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; Azevedo et al., 2011Azevedo C, Casal G, Matos P, Alves A, Matos E. Henneguya torpedo sp. nov. (Myxozoa), a parasite from the nervous system of the Amazonian teleost Brachyhypopomus pinnicaudatus (Hypopomidae). Dis Aquat Organ 2011; 93(3): 235-242. http://dx.doi.org/10.3354/dao02292. PMid:21516976.
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; Casal et al., 2016Casal G, Matos ER, Rocha S, Neto JS, Al-Quraishy S, Azevedo C. Ultrastructure and Phylogeny of Pleistophora beebei sp. nov. (Microsporidia) Infecting the Amazonian Teleostean Brachyhypopomus beebei (fam. Hypopomidae). Acta Protozool 2016; 55(4): 259-268. http://dx.doi.org/10.4467/16890027AP.16.022.6009.
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).

One important group of fish parasites are those of the phylum Cnidaria Hatschek, 1888, which belong to the unclassified subphylum Myxozoa (Cnidaria), a group that includes approximately 2402 species, in 64 genera (Fiala et al., 2015Fiala I, Bartošová-Sojková P, Whipps CM. Classification and phylogenetics of Myxozoa. In: Okamura B, Gruhl A, Bartholomew J, editors. Myxozoan evolution, ecology and development. Cham: Springer; 2015. p. 85-110. http://dx.doi.org/10.1007/978-3-319-14753-6_5.
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; Zhang, 2011Zhang Z. Animal biodiversity: an introduction to higher-level classification and taxonomic richness. Zootaxa 2011; 3148(1): 7-12. http://dx.doi.org/10.11646/zootaxa.3148.1.3.
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). Fish are the intermediate vertebrate hosts of most myxozoans. These parasites are responsible for myxosporidiosis, a disease that causes high mortality rates and which is found worldwide (Lom & Dyková, 2006Lom J, Dyková I. Myxozoan genera: definition and notes on taxonomy, life-cycle terminology and pathogenic species. Folia Parasitol (Praha) 2006; 53(1): 1-36. http://dx.doi.org/10.14411/fp.2006.001. PMid:16696428.
http://dx.doi.org/10.14411/fp.2006.001...
). Myxobolus Bütschli, 1882, is the most diverse group of this phylum, with approximately 850 species described to date (Abdel-Ghaffar et al., 2017Abdel-Ghaffar F, Abdel-Gaber R, Maher S, El Deeb N, Kamel R, Al Quraishy S, et al. Morphological and ultrastructural characteristics of Myxobolus ridibundae n. sp. (Myxosporea: Bivalvulida) infecting the testicular tissue of the marsh frog Ranaridibunda (Amphibia: Ranidae) in Egypt. Parasitol Res 2017; 116(1): 133-141. http://dx.doi.org/10.1007/s00436-016-5269-x. PMid:27757539.
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). This genus is a member of the order Bivalvulida Shulman, 1959, and is characterized by an ellipsoidal, ovoid or rounded body in the valvular view, which is biconvex in the sutural view. Itcomprises two valves surrounding two pyriform polar capsules that are sometimes unequal in size, and it has binucleated sporoplasm (Matos et al., 2001Matos E, Corral L, Matos P, Casal G, Azevedo C. Incidência de parasitas do Phylum Myxozoa (Sub-reino Protozoa) em peixes da região amazônica, com especial destaque para o gênero Henneguya. Rev Ciênc Agrár–. Amaz J Agricult Environ Sci 2001; 36: 83-99.; Kent et al., 2001Kent ML, Andree KB, Bartholomew JL, El-Matbouli M, Desser SS, Devlin RH, et al. Recent advances in our knowledge of the Myxozoa. J Eukaryot Microbiol 2001; 48(4): 395-413. http://dx.doi.org/10.1111/j.1550-7408.2001.tb00173.x. PMid:11456316.
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; Lom & Dykoá, 2002Lom J, Dykoá I. Ultrastructure of Nucleospora secunda n. sp. (Microsporidia), parasite of enterocytes of Nothobranchius rubripinnis. Eur J Protistol 2002; 38(1): 19-27. http://dx.doi.org/10.1078/0932-4739-00844.
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, Lom & Dyková, 2006Lom J, Dyková I. Myxozoan genera: definition and notes on taxonomy, life-cycle terminology and pathogenic species. Folia Parasitol (Praha) 2006; 53(1): 1-36. http://dx.doi.org/10.14411/fp.2006.001. PMid:16696428.
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).

Parasites of the genus Myxobolus infect a wide range of organs, such as the gills, liver, musculature, skin and nervous system. They are of considerable clinical interest, given their potential as a source of disease, as in the case of Myxobolus cerebralis (Hoffer, 1903), which causes whirling disease (Gilbert & Granath, 2001Gilbert MA, Granath WO Jr. Persistent infection of Myxobolus cerebralis, the causative agent of salmonid whirling disease, in Tubifex tubifex. J Parasitol 2001; 87(1): 101-107. PMid:11227872.) and may have been responsible for the decline in the population of Oncorhynchus mykiss Walbaum, 1792, in the western United States (Nehring et al., 2003Nehring RB, Thompson KG, Shuler DL, James TM. Using sediment core samples to examine the spatial distribution of Myxobolus cerebralis actinospore production in Windy Gap Reservoir, Colorado. N Am J Fish Manage 2003; 23(2): 376-384. http://dx.doi.org/10.1577/1548-8675(2003)023<0376:USCSTE>2.0.CO;2.
http://dx.doi.org/10.1577/1548-8675(2003...
).

Given the importance of Myxobolus, parasites of this genus that were found in specimens of the electric knifefish, Brachyhypopomus beebei Schultz, 1944, were analyzed in the present study. This species has considerable potential as an ornamental fish, due to its behavior, body size and attractiveness.

Material and Methods

A total of 30 specimens of Brachyhypopomus beebei were collected from the Peixe-Boi River in the municipality of Peixe-Boi, located in the northeastern extremity of the state of Pará, in northern Brazil (1°06’59” S; 47°18’26” W), in 2015 and 2016. The specimens were collected using fishing nets, and were transported live in aerated plastic bags containing river water to the Carlos Azevedo Research Laboratory (LPCA-UFRA) and the Edilson Matos Research Laboratory (LPEM-UFPA), both in Belém, Pará, Brazil. In these laboratories, the specimens were transferred to aquaria containing water from the species’ natural habitat at temperatures of 28-30°C, as observed in the field, for two days (Ethic Committee Approval number 013/2014 CEUA/UFRA). The fish were analyzed, anesthetized using tricainemethanesulfonate (MS222 Sigma) at a concentration of 50 mgL-1 and then euthanized by means of myelotomy and necropsied to detect the presence of parasites using a stereomicroscope.

During this analysis, the braincase was opened and small fragments of the cerebral tissue were extracted for observation via optical microscopy. After confirmation of the presence of Myxobolus spores, images were obtained using a Zeiss Axiocam ICc 5 camera attached to a Zeiss Primo Star microscope. A total of 30 spores were measured using the AxioVision LE software.

For the histological analyses, small fragments were extracted from infected organ and fixed in Davidson solution for 24 h. The fragments were then embedded in paraffin in order to cut sections of thickness 5 µm. These were then stained using standard (hematoxylin-eosin, HE) and special (Ziehl-Neelsen) histological techniques (Luna, 1968Luna LG. Manual of histologic staining methods of the armed forces institute of pathology. 3rd ed. New York: MacGraw-Hill Book Company; 1968.).

For the molecular analyses, fragments were removed from the brain and fixed in 80% alcohol. The total DNA was extracted using PureLink® Genomic DNA mini-kit (Invitrogen, Carlsbad, California, USA), following the manufacturer’s instructions. The small subunit ribosomal DNA (18s DNA) was amplified using the MX5/MX3 and MC5/MC3 primers (Andree et al., 1999Andree KB, Székely C, Molnár K, Gresoviac SJ, Hedrick RP. Relationships among members of the genus Myxobolus (Myxozoa: Bivalvidae) based on small subunit ribosomal DNA sequences. J Parasitol 1999; 85(1): 68-74. http://dx.doi.org/10.2307/3285702. PMid:10207366.
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; Molnár, 2002Molnár K. Site preference of fish myxosporeans in the gill. Dis Aquat Organ 2002; 48(3): 197-207. http://dx.doi.org/10.3354/dao048197. PMid:12033706.
http://dx.doi.org/10.3354/dao048197...
). The final volume of the polymerase chain reaction (PCR) was 25 μL, containing 5-10 ng of the template DNA, 20 mM of Tris (pH 8.4), 50 mM of KCl, 4 mM of dNTPs (Invitrogen®), 2 mM of MgCl2, 5 pmol of each primer and 1.2 units of Taq DNA polymerase (Invitrogen®). The amplification protocol consisted of 35 cycles of 1 min at 95°C, 1 min at 66°C and 2 min at 72°C, preceded by 5 min at 95°C, and followed by 5 min at 72°C.

The amplicons were electrophoresed on 1.5% agarose gel, purified using GFX PCR DNA and the gel purification kit (GE Healthcare, Chicago, Illinois, USA), following the manufacturer’s instructions. The samples were then sequenced in an ABI 3130 automatic DNA analyzer (Applied BiosystemsTM, Foster City, California, USA) with BigDye® Terminator v3.1, following the manufacturer’s specifications. The MC5 and MC3 primers, which were used to obtain the amplicons, were also used for sequencing. The nucleotide sequences were edited and aligned using the BioEdit software (Hall, 2007Hall T. BioEdit. Biological sequence alignment editor for Win95/98/NT/2K/XP. Carlsbad, CA: Ibis Biosciences; 2007.).

Following a BLAST search of GenBank sequences, the partial SSU rDNA sequence obtained from the samples was aligned with 42 Myxobolus, Thelohanellus and Henneguya sequences with similarity greater than 85%, which are available in GenBank. Sphaeromyxa kenti (JX443489) and Sphaeromyxa zaharoni (AY538662) sequences were also included as outgroups of the database for the phylogenetic analysis, just as they were used by Zatti et al. (2018)Zatti SA, Atkinson SD, Maia AAM, Bartholomew JL, Adriano EA. Ceratomyxa gracillima n. sp. (Cnidaria: Myxosporea) provides evidence of panmixia and ceratomyxid radiation in the Amazon basin. Parasitology 2018; 145(9): 1137-1146. http://dx.doi.org/10.1017/S0031182017002323. PMid:29338808.
http://dx.doi.org/10.1017/S0031182017002...
.

Bayesian inference (BI) analysis was used based on Markov Chain Monte Carlo (MCMC) tree searches, which were run in MrBayes 3.1.2 (Ronquist & Huelsenbeck, 2003Ronquist MAF, Huelsenbeck JP. MrBayes 3: bayesian phylogenetic inference under mixed models. Bioinformatics 2003; 19(12): 1572-1574. http://dx.doi.org/10.1093/bioinformatics/btg180. PMid:12912839.
http://dx.doi.org/10.1093/bioinformatics...
). Two parallel runs of four simultaneous MCMC searches, each with five million generations, were conducted, with one tree being sampled every 500 generations.The results from the first 1000 trees were discarded as burn-in. The remaining trees were analyzed in MrBayes to estimate the posterior probability of each node in the phylogenetic reconstruction. Tracer v1.4.1 (Rambaut et al., 2018Rambaut A, Drummond AJ, Xie D, Baele G, Suchard MA. Posterior summarisation in Bayesian phylogenetics using Tracer 1.7. Syst Biol 2018; 67(5): 901-904. http://dx.doi.org/10.1093/sysbio/syy032. PMid:29718447.
http://dx.doi.org/10.1093/sysbio/syy032...
) was used to check the stationarity of all the parameters sampled in the chains. The BI analysis was based on the GTR + G model of nucleotide substitution, as indicated by jModelTest 2.0.2 (Posada, 2008Posada D. jModelTest: phylogenetic model averaging. Mol Biol Evol 2008; 25(7): 1253-1256. http://dx.doi.org/10.1093/molbev/msn083. PMid:18397919.
http://dx.doi.org/10.1093/molbev/msn083...
). This model has base frequencies of A = 0.2614, C = 0.2007, G = 0.2869 andT = 0.251; a substitution model of AC = 1.4849, AG = 3.7832, AT = 3.0822, CG = 0.627, CT = 6.5267 and GT = 1; and variable site rates (G = 0.2840) that follow the gamma distribution. Genetic distances were determined using PAUP 4.0b (Swofford, 1998Swofford DL. Phylogenetic analysis using parsimony (*and other methods). Sunderland:Sinauer Associates; 1998.) with SSU rDNA gene sequences from only one fish parasite, Myxobolus sp., which was grouped in the same clade as Myxobolus freitasi n. sp.

Results

In the present study, a 60% prevalence of infection was recorded for myxozoa in the central nervous system (CNS) of the host, Brachyhypopomus beebei. The spore morphology was suggestive of myxosporids of the genus Myxobolus (Figure 1AB).Hematoxylin-eosin staining (Figure 1C) revealed the cysts and their proximity to the pyramidal neuron layer, while Ziehl-Neelsen staining (Figure 1D) provided a more detailed view of the arrangement of the spores and, in particular, the polar capsules.

Figure 1
A and B - Fresh Myxobolus freitasi n. sp. spores (arrowhead), showing the larger and smaller polar capsules (PC). Scale bars: 20 μm (A) and 5 μm (B); C - Photomicrograph of histological sections stained with Haematoxylin and Eosin ofcysts with spores of Myxobolus freitasi n. sp. (*) and pyramidal neurones (arrowhead) in the cerebral parenchyma of the B. beebei. Scale bar: 80μm; D - Histological sections stained with Ziehl-Neelsenof cyst (*) with spores of Myxobolus freitasi n. sp. (arrowhead) infecting the cerebral parenchyma (B), highlighting spores (inset) with large and small polar capsules (PC). Scale bar: 150 μm (D) and 80 μm (inset).

Description of Myxobolus freitasi n. sp.

Vegetative stages - The histological analyses revealed the presence of irregular cysts in the cerebral parenchyma, with fine fibrous capsules containing spores. The cystswere 130-770 µm (N = 8) in length. The cerebral tissue was replaced by a mass of Myxobolus spores, with multifocal necrosis in the areas adjacent to the cyst.

Mature spores - The spores had a piriform body and two polar capsules of unequal size, (Figure 2), which were 18.6 µm (17.7-19.8 µm) long and 8.6 µm (8.4-9.0 µm) wide. The larger polar capsule was 13.0 µm (12.4–13.4 µm) long and 5.6 µm (5.3-6.0 µm) wide, while the smaller polar capsule was 5.0 µm (4.5-5.3 µm) long and 2.5 µm (2.3-2.6 µm) wide. A helicoidal filament was observed within each capsule, with 14-15 coils in the larger capsule and 4-5 coils in the smaller capsule. Comparisons with the measurements of other Myxobolus species with unequal capsules are shown in Table 1.

Figure 2
Diagram of Myxobolus freitasi n. sp. spore, in the valvular view.
Table 1
Comparison of the measurements of the spore of Myxobolus freitasi n. sp. with those of the other Myxobolus species that have unequal capsules. All measurements are given in micrometers.

Type locality - Peixe-Boi River (01º7’ S; 47º18’ W) in the municipality of Peixe-Boi, in northeastern Pará, Brazil.

Voucher specimens - Histological sections, on glass slides with coverslips, of the brain of B. beebei, containing spores of Myxobolus freitasi n. sp., are deposited in the International Biological Collection of Protozoan Type Slides at the Brazilian National Institute of Amazon Research (INPA), Manaus, state of Amazonas, Brazil (INPA 28).

Prevalence - A total of 18 (60%) of the 30 specimens analyzed were infected.

Etymology - The species epithet honors Pedro Gonçalves de Freitas, an employee of the Federal Rural University of Amazonia, in Belém, for his invaluable contribution, through 30 years of specimen collection, to the work of the two parasitological research laboratories (LPEM-UFPA and LPCA-UFRA) in which the present study was developed.

Phylogeny - The partial 18S DNA gene sequence obtained here contains approximately 1,400 bp (GenBank accession number MG250286). The Bayesian analysis indicated that the species of the genera Myxobolus, Thelohanellus and Henneguya revealed the existence of two major clades, A and B, such that clade A was formed by Myxobolus and Thelohanellus species, and clade B mainly by Henneguya species (Figure 3). Myxobolus freitasi n. sp., which parasitizes the central nervous system (CNS) of Brachyhypopomus beebei (Gymnotiformes), is the basal Myxobolus species of subclade A1, which is formed by the Myxobolus parasites of the CNS of salmonids in North America, Europe and Asia. Although Myxobolus freitasi n. sp. and Myxobolus axelrodi are similar in their morphology, infection site and geographical region, they are phylogenetically distinct.This may be related to the fact that their hosts are members of different fish families, as well as occurring in different mesoregions (Figure 3). The smallest genetic distance was 9.7%, recorded in relation to Myxobolus neurotropus (DQ846661), and all other sequences presented distances greater than 10.0% (Table 2).

Figure 3
Bayesian inference tree showing Myxobolus freitasi n. sp. (in bold type) and the closely-related Myxobolus species from around the world that infect the nervous system, and other Myxobolus, Thelohanellus and Henneguya species from GenBank with a high degree of affinity.
Table 2
Genetic distances (p-distances) between all Myxobolus species of Clade A (Figure 3).

Discussion

Myxobolus freitasi n. sp. has a shape similar to that described for other myxozoans of this genus, such as M. axelrodi, described by Camus et al. (2017)Camus AC, Dill JA, Rosser TG, Pote LM, Griffin MJ. Myxobolus axelrodi n. sp. (Myxosporea: Myxobolidae) a parasite infecting the brain and retinas of the cardinal tetra Paracheirodon axelrodi (Teleostei: Characidae). Parasitol Res 2017; 116(1): 387-397. http://dx.doi.org/10.1007/s00436-016-5301-1. PMid:27796562.
http://dx.doi.org/10.1007/s00436-016-530...
infecting Paracheirodon axelrodi, M. toyamai, found in Cyprinus carpio in Japan (Yokoyama & Ogawa, 2015Yokoyama H, Ogawa K. The resurrection of Myxobolus toyamai with a validation of a stunted polar capsule based on morphological evidence. Parasitol Int 2015; 64(4): 43-47. http://dx.doi.org/10.1016/j.parint.2015.01.008. PMid:25640192.
http://dx.doi.org/10.1016/j.parint.2015....
) and M. stellatus infecting Thoracocharax stellatus from Colombia (Stilwell et al., 2020Stilwell JM, Petty BD, Camus AC, Woodyard ET, Griffin MJ, Rosser TG. Characterisation of Myxobolus stellatus n. sp. (Cnidaria: Myxobolidae) infecting the cranial nerves and ganglia of the spotfin hatchetfish Thoracocharax stellatus (Kner) (Characiformes: Gasteropelecidae) from Colombia. Syst Parasitol 2020; 97(3): 305-314. http://dx.doi.org/10.1007/s11230-020-09911-x. PMid:32253575.
http://dx.doi.org/10.1007/s11230-020-099...
).

The presence of unequal polar capsules in M. freitasi n. sp. is a characteristic that had already been observed in other parasites of the genus Myxobolus described in previous studies: M. inaequus in Eigenmannia virescens (Kent&Hoffman, 1984), M. absonus in Pimelodus maculatus (Cellere et al., 2002Cellere EF, Cordeiro NS, Adriano EA. Myxobolus absonus sp. n. (Myxozoa: Myxosporea) parasitizing Pimelodus maculatus (Siluriformes, Pimelodidae), a South American freshwater fish. Mem Inst Oswaldo Cruz 2002; 97(1): 79-80. http://dx.doi.org/10.1590/S0074-02762002000100012. PMid:11992152.
http://dx.doi.org/10.1590/S0074-02762002...
) and M. desaequalis in Apteronotus albifrons (Azevedo et al., 2002Azevedo C, Corral L, Matos E. Myxobolus desaequalis n. sp. (Myxozoa, Myxosporea), Parasite of the Amazonian Freshwater Fish, Apteronotus albifrons (Teleostei, Apteronotidae). J Eukaryot Microbiol 2002; 49(6): 485-488. http://dx.doi.org/10.1111/j.1550-7408.2002.tb00233.x. PMid:12503685.
http://dx.doi.org/10.1111/j.1550-7408.20...
). These three Myxobolus species with unequal polar capsules were most similar to M. freitasi n. sp. in terms of the width and length of the spore, although they were considerably different in terms of the dimensions of the polar capsules, with M. freitasi n. sp. having by far the largest capsules. The number of coils in the polar filament of M. axelrodi was within the range of values recorded in M. freitasi n. sp., in both capsules. In this context, it may be important to note that both of these species are parasites of the CNS.

Azevedo et al. (2011)Azevedo C, Casal G, Matos P, Alves A, Matos E. Henneguya torpedo sp. nov. (Myxozoa), a parasite from the nervous system of the Amazonian teleost Brachyhypopomus pinnicaudatus (Hypopomidae). Dis Aquat Organ 2011; 93(3): 235-242. http://dx.doi.org/10.3354/dao02292. PMid:21516976.
http://dx.doi.org/10.3354/dao02292...
also described a myxosporean, Henneguya torpedo, in the CNS of Brachyhypopomus, with a prevalence of 33.3%. Azevedo et al. (2018)Azevedo C, Feltran R, Rocha S, Matos E, Maciel E, Oliveira E, et al. Simultaneous occurrence of two new myxosporean species infecting the central nervous system of Hypopygus lepturus from Brazil. Dis Aquat Organ 2018; 131(2): 143-156. http://dx.doi.org/10.3354/dao03283 PMid:30460920.
http://dx.doi.org/10.3354/dao03283...
subsequently described the species H. lepturus and Thelohanellus lepturus, which are both myxosporean parasites of the CNS of gymnotiform fishes from the Brazilian Amazon region, with a prevalence of 29.6%. In all cases, the prevalence recorded in the previous studies was lower than that found in the present study on B. beebei (60%). Despite the prevalence of M. freitasi n. sp. infection and the extent of the lesions in the cerebral tissue, the host did not present clinical symptoms. Infections of the CNS by myxosporeans have been reported from many regions of the world, and typically result in direct or indirect damage to the health of the host (Lom et al., 1987Lom J, Pavlásková M, Dyková I. Brain thelohanellosis due to Thelohanellus oculileucisci (Myxozoa: Myxosporea) in Gobiogobio. Folia Parasitol (Praha) 1987; 34(4): 375-377. PMid:3428769.; Meng et al., 2011Meng F, Yokoyama H, Shirakashi S, Grabner D, Ogawa K, Ishimaru K, et al. Kudoa prunusi n. sp. (Myxozoa: Multivalvulida) from the brain of Pacific bluefin tuna Thunnus orientalis (Temminck& Schlegel, 1844) cultured in Japan. Parasitol Int 2011; 60(1): 90-96. http://dx.doi.org/10.1016/j.parint.2010.11.006. PMid:21112413.
http://dx.doi.org/10.1016/j.parint.2010....
; Sindeaux et al., 2016Sindeaux JL No, Velasco M, Silva JMV, Santos PF, Sanches O, Matos P, et al. Lymphocytic meningoencephalomyelitis associated with Myxobolus sp. (Bivalvulidae: Myxozoa) infection in the Amazonian fish Eigenmannia sp. (Sternopygidae: Gymnotiformes). Rev Bras Parasitol Vet 2016; 25(2): 158-162. http://dx.doi.org/10.1590/S1984-29612016023. PMid:27096533.
http://dx.doi.org/10.1590/S1984-29612016...
). This highlights the importance of the present study for understanding myxosporidiosis in the neural tissue of Amazonian fish.

Taken together, the morphological, morphometric and molecular data obtained in the present study support the description of the new species, denominated Myxobolus freitasi n. sp. This taxon is clearly distinct from all other forms described previously in the genus Myxobolus.

Acknowledgements

This study was supported financially by the Brazilian Coordination for Higher Education Personnel Training (CAPES) and the National Council for Scientific and Technological Development (CNPq) programmes CNPq/Universal (441645-2014-3), CNPq/Productivity in Research (301497/2016-8) and CNPq-PDJ (151439/2019-2).

  • How to cite: Sindeaux-Neto JL, Velasco M, Silva DT, Matos P, Silva MF, Gonçalves EC, et al. Myxobolus freitasi n. sp. (Myxozoa: Bivalvulida), a parasite of the brain of the electric knifefish in the Brazilian Amazon region. Braz J Vet Parasitol 2021; 30(1): e020920. https://doi.org/10.1590/S1984-296120201081

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Publication Dates

  • Publication in this collection
    12 Mar 2021
  • Date of issue
    2021

History

  • Received
    31 Aug 2020
  • Accepted
    04 Nov 2020
Colégio Brasileiro de Parasitologia Veterinária FCAV/UNESP - Departamento de Patologia Veterinária, Via de acesso Prof. Paulo Donato Castellane s/n, Zona Rural, , 14884-900 Jaboticabal - SP, Brasil, Fone: (16) 3209-7100 RAMAL 7934 - Jaboticabal - SP - Brazil
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