Infection of the heart of Pimelodus ornatus ( Teleostei , Pimelodidae ) , by Myxobolus sp . ( Myxozoa , Myxobolidae ) Infecção do tecido cardíaco de Pimelodus ornatus ( Teleostei , Pimelodidae ) , por Myxobolus sp . ( Myxozoa , Myxobolidae )

The phylum Myxozoa Grassé, 1970, consists of a heterogenous group of around 50 genera that are worldwide disseminated in a wide variety of aquatic media. In the present study, 43 specimens of Pimelodus ornatus were collected from an adjacent area to the Cachoeira do Arari municipality on Marajó Island, in the Brazilian state of Pará, in 2013. Macroscopic analysis showed the presence of whitened plasmodia located in the cardiac muscle and also in the region between the bulbus arteriosus and atrium cordis. Microscopic analysis on the parasitized tissues revealed spores that were typically piriform, with the anterior portion slightly narrower than the posterior end. The spore valves were symmetrical. The present species is placed in the genus Myxobolus Butschli, 1882, because of the presence of a pair of equal polar capsules in each spore. The prevalence of parasitism observed was 13.9% (6/43). This research note reports the first occurrence of Myxobolus as a parasite of the heart in the teleostean fish P. ornatus in the Amazon region and confirms the occurrence of secondary myocarditis in this fish, caused by parasitism by Myxobolus sp. The rarity of this parasitic species of Myxobolus at this tissue site, associated with other spore morphology characteristics in the fish, suggests that it is an undescribed species.

The phylum Myxozoa Grassé, 1970, consists of a heterogenous group of around 50 genera (LOM & DYKOVÁ, 1992) that are worldwide disseminated in a wide variety of aquatic media (both freshwater and saltwater).Most of these parasites are considered to be pathogenic acting on many different tissues and organs of fish, such as the liver, kidneys, gills, gonads, intestine, skin and others (LOM & DYKOVÁ, 2006).
Among the myxosporidians, the genus Myxobolus is prominent.This has the following principal characteristics: ellipsoid, oval or rounded spores formed by two valves, two polar capsules, generally piriform, each with a polar filament, bilateral symmetry and mono or binucleated protoplasm (EIRAS et al., 2005).
In Brazil, more than 20 new species of parasites belonging to the genus Myxobolus Bütschli, 1882, have been described, mainly in the Amazon region.Most recently, Myxobolus myleus was described in the gallbladder of Myleus rubripinnis collected near the municipality of Oriximiná in the state of Pará (PA), Brazil (AZEVEDO et al., 2012), and Myxobolus sp. in the palate of Gobioides broussonnetii, from Marajó Island, PA, Brazil (VELASCO et al., 2012).
This article reports the first occurrence of Myxobolus sp. in the heart of P. ornatus Kner, 1858, from the Arari River (Marajó Island, PA, Brazil), with histopathological descriptions of the parasite's interaction with the host's heart tissue and morphological descriptions of the spores under an optical microscope.
Forty-three specimens of P. ornatus (brazilian common name "mandii") were collected from an area adjacent to the municipality of Cachoeira do Arari (01° 00 ' S / 48° 57' W) on Marajó Island, in the Brazilian state of Pará, in 2013.The live specimens were taken to the Carlos Azevedo Research Laboratory at UFRA and were kept in an aquarium at 28-30 °C.The fish were then anesthetized with tricaine methanesulfonate (MS222; Sandoz Laboratories) at a concentration of 50 mg/L and were dissected using a stereomicroscope appropriate for analysis on parasites.For optical microscopy, small fragments (approximately 0.5 cm) of the parasitized tissue from the fish heart were fixed in Davidson solution (neutral buffered formalin, glacial acetic acid, 95% ethyl alcohol and distilled water) for 24 h and were then processed and stained with hematoxylin and eosin, Ziehl-Neelsen, toluidine blue and Masson's trichrome (LUNA, 1968).The measurements of spores were made in accordance with Lom & Dyková (1992) under an optical microscope using a Zeiss Primo Star photographic camera (Zeiss AxioCam ERC 5s) and the AxioVision 5.1 software.
Macroscopic analysis showed the presence of whitened plasmodia located in the cardiac muscle and also in the region between the bulbus arteriosus and atrium cordis of the heart (Figure 1).
Microscopic analysis on the parasitized tissues revealed spores that were typically piriform with the anterior portion slightly narrower than the posterior end (Figure 2 and inset).The spore valves were symmetrical, and had two equal polar capsules.The dimensions of the spores are described in detail in Table 1, with comparisons with other parasites of the genus Myxobolus that have been described in the Amazon region.
The distinctions between the different species of the genus Myxobolus are primarily based on the morphological structure of the spore, and particularly its size and shape.The present parasite was placed in the genus Myxobolus Butschli, 1882, because of the presence of a pair of equal polar capsules in each spore (LOM & DYKOVÁ, 2006).The prevalence of parasitism observed was 13.9% (6/43).
Histological sections through the heart revealed that the spores can disperse into the lumen and wall of the bulbus arteriosus.The presence of free spores in other organs is the result of this dispersal and these organs should not be interpreted as alternative sites of plasmodial development.
The samples sectioning the heart also revealed multifocal infection by Myxobolus sp, organized in the pericardium plasmodia  Myxobolus in the heart of the Amazonian fish  and bulbus arteriosus (Figures 3A, B).The plasmodia had a thin capsule of collagen fibers and there were numerous specimens of the parasite inside the plasmodia (Figures 3C, D).There was a slight inflammatory reaction around the plasmodia.Multifocal degeneration of myocardial fibers around the infected area was also observed (Figure 3E).
Other occurrences of Myxobolus have been described in the Amazon region (AZEVEDO et al., 2002;CASAL et al., 2002;AZEVEDO et al., 2009;AZEVEDO et al., 2012;VELASCO et al., 2012), but there had not previously been any mention of myxosporidiosis in the hearts of fish in this region.
In this regard, the tissue specificity for this species of Myxobolus, in the serosa (pericardium), bulbus arteriosus and cardiac muscle, was similar to that of Myxobolus bulbocordis (MASOUMIAN et al., 1996), which was also reported from the serosa of the atrium and bulbus cordis.
This research note reports the first occurrence of Myxobolus as a parasite of the heart in the Amazon region and confirms the occurrence of secondary myocarditis in P. ornatus associated with parasitism by Myxobolus sp.The rarity of this parasitic species of Myxobolus at this tissue site, associated with other spore morphology characteristics in the fish, suggests that it is an undescribed species.