Abstract

The three-toed sloth (Bradypus variegatus) has been a victim of disorderly human interventions, leading to an increase in accidents and diseases for the species. This study aimed to provide a description of the coronary arteries and to high light the histopathological findings in the sloth’s heart. This information should be beneficial for medical clinics dealing with these species. Eighteen B. variegatus specimens were dissected after natural death. Fifteen of them, comprising five adult males, nine adult females, and one juvenile, were fixed with formaldehyde and preserved in saline solution. In two males and one female, red-coloured latex was injected through the left common carotid artery to make the coronary arteries more visible. In the female, blue-coloured latex was also injected through the right external jugular vein to aid in arterial identification. The chests were opened to evaluate and extract the heart for a detailed description of the coronary arteries. For histopathological analysis, histological slides were prepared from four hearts collected from four animals (three adults and one juvenile). Arterial analyses revealed that sloths possess two coronary arteries: one on the right and one on the left side, each of which supplies blood to their respective surfaces. The heart is composed of the endocardium, myocardium, and epicardium, similar to mammals in general. However, necrosis of endocardial tissue and cardiac muscle fbres, as well as myxomatous degeneration of valves and thrombi in vascular lumens, were observed. These necrotic events, more extensive in nature, are related to infarctions, while the more subtle ones may be associated with stressful situations experienced by the individuals.

Keywords:
Xenarthra; three-toed sloth; coronary arteries; histopathology; heart

Resumo

A preguiça-comum (Bradypus variegatus) tem sido vítima das investidas antrópicas desordenadas, o que tem elevado o número de acidentes e doenças para a espécie. Propôs-se com esse estudo, realizar uma descrição das artérias coronárias, assim como apontar os achados histopatológicos do coração do bicho-preguiça, a fim de favorecer o diagnóstico por imagem, semiologia e clínica médica das espécies. Foram dissecados 18 exemplares de B. variegatus obtidos após morte natural. Quinze deles, sendo 5 machos e 9 fêmeas adultas e 1 jovem, foram fixados com formaldeído e conservados em solução salina. Em 2 machos e 1 fêmea, foi injetado, a partir da artéria carótida comum esquerda, látex corado em vermelho, para tornar as coronárias mais evidentes. Na fêmea também foi injetado através da veia jugular externa direita, látex corado em azul, para evitar equívoco na identificação arterial. Os animais tiveram o tórax aberto a fim de avaliar e retirar o coração para descrição detalhada das artérias coronárias. Para análise histopatológica, foram confeccionadas lâminas histológicas oriundas de corações coletados de 4 animais (3 adultos e 1 jovem). Com base nas análises arteriais, as preguiças possuem duas coronárias: uma a direita e uma a esquerda, que irrigam suas respectivas faces. O coração é composto por endocárdio, miocárdio e epicárdio, semelhante aos mamíferos em geral. Todavia, observou-se necroses endocárdicas, de fibras musculares cardíacas, além de degenerações mixomatosas de válvulas e trombos no lúmen vascular. Esses eventos necróticos, mais extensos, estão relacionados a infartos, enquanto os mais discretos podem ser associados a situações de estresse vivenciadas pelos espécimes.

Palavras-chave:
Xenarthra; preguiça-comum; vascularização; histopatologia; coração

1. Introduction

The heart is a pulsatile muscular organ composed of three layers: the endocardium, which is more internal and formed mainly of endothelial tissue; the myocardium, an intermediate stratum that contains cardiac muscle fibres; and the epicardium, the visceral serous leaflet of the pericardium, which surrounds the organ. From ventricular systole onwards, blood leaves the heart through the vessels towards the organ itself and the entire body, and then returns to it, in atrial diastole(¹1 Miller LM, Gal A. Cardiovascular System and Lymphatic Vessels. Pathologic Basis of Veterinary Disease. 2017; 561–616.e1. Available from: https://doi.org/10.1016/B978-0-323-35775-3.00010-2
https://doi.org/10.1016/B978-0-323-35775... ). Along its path, blood transports respiratory gases, nutritional substances needed by cells, metabolic excreta, and hormones. It also participates in importantactivitiesforthe body, such as thermal regulation and standardising the concentration of ions in body fluids(²2 Mathew J, Sankar P, Varacallo M. Physiology, Blood Plasma. National Center for Biotechnology information. 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK531504/
https://www.ncbi.nlm.nih.gov/books/NBK53... ).

Due to its extremely important role in the cardiovascular system and amid the increasing incidence of heart diseases worldwide, the heart has been the target of research(³3 Gomes FGFLR, Farias ELP, Veronez DAL 2011. Estudo de pontes de miocárdio junto às artérias coronárias em cães. Revista de Educação Continuada em Medicina Veterinária e Zootecnia do CRMV-SP. 2011; 9(2): 36-36. Available from: https://www.revistamvez-crmvsp.com.br/index.php/recmvz/article/view/403
https://www.revistamvez-crmvsp.com.br/in... ,⁴4 Fonfara S, Martínez Pereira Y, Rápido S, Copeland H, López-Alvarez J, Summerfeld N, Cripps P, Dukes-McEwan J. Balloon Valvuloplasty for Treatment of Pulmonic Stenosis in English Bulldogs with an Aberrant Coronary Artery. Journal of Veterinary Internal Medicine. 2010; 24(2): 354-359. Available from: https://doi.org/10.1111/j.1939-1676.2009.0458.x
https://doi.org/10.1111/j.1939-1676.2009... ). In this context, the present study examined the heart of the three-toed sloth (Bradypus variegatus) to describe its coronary arteries as well as histopathological findings. B. variegatus has a strong occurrence in northeastern Brazil(⁵5 Albuquerque PV, Mesquita EP, Alcântara SF, Miranda MELC, Andrade GP, Amorim Júnior AA, Amorim MJAAL. External macroscopic anatomy of the Bradypus variegatus heart. Arquivo Brasileiro de Medicina Veterinária e Zootecnia. 2022; 74(5): 814-824. Available from: https://doi.org/10.1590/1678-4162-12562
https://doi.org/10.1590/1678-4162-12562... ,⁶6 Superina M, Aguiar JM. A reference list of common names for the Edentates. Edentata. 2006; 7(1): 33-44. Available from: https://doi.org/10.1896/1413-4411.7.1.33
https://doi.org/10.1896/1413-4411.7.1.33... ,⁷7 Moraes-Barros N, Chiarello A, Plese T. Bradypus variegatus, 1825. The IUCN Red List of Threatened Species; 2014. Available from: https://doi.org/10.2305/IUCN.UK.2014-1.RLTS.T3038A47437046.en
https://doi.org/10.2305/IUCN.UK.2014-1.R... ). Disorderly anthropogenic actions have promoted an increasing number of accidents and illnesses in the species, which highlights the importance of understanding their organ systems. This information should be beneficial for medical clinics that care for these animals(⁸8 Fuentes A, Hockings KJ. The ethnoprimatological approach inprimatology. American Journal of Primatology. 2010; 72(10): 841-7. Available from: https://doi.org/10.1002/ajp.20844
https://doi.org/10.1002/ajp.20844... ,⁹9 Xavier GAA, Amora TD, Valença YM & Cabral MCC. Apreensões de preguiças Bradypus variegatus SCHINZ, 1825 e casos de acidentes com choques elétricos envolvendo estes animais na Mesorregião Metropolitana do Recife, Pernambuco. In: Seabra GF, Silva JAN & Mendonça ITL (Org.), A conferência da Terra: aquecimento global, sociedade e biodiversidade. Vol.1. Editora Universitária da UFPB, João Pessoa; 2010. p. 301-308. Available from: https://docplayer.com.br/7278347-A-conferencia-da-terra-aquecimento-global-sociedade-e-biodiversidade.html
https://docplayer.com.br/7278347-A-confe... ,¹⁰10 Pereira KF, Silva IO, Silva FFR, Silva VHD, Castro CSS, Boere V. Hematology and serum biochemistry of free-range brown-throated sloths in two urban areas in Brazil. Veterinary World. 2020; 13(9): 2033-2038. Available from: https://doi.org/10.14202/vetworld.2020.2033-2038
https://doi.org/10.14202/vetworld.2020.2... ).

2. Materials and methods

This study was authorised by the Ethics Committee on the Use of Animals (CEUA) of the Federal Rural University of Pernambuco (UFRPE, no. 50/2018). It is registered in the National Genetic Heritage and Associated Traditional Knowledge Management System (SinGen) under number A21069D. It also received approval from the Wild Animal Screening Center (CETAS) of the State Environment Agency (CPRH, no. 02/2017) and the Biodiversity Authorization and Information System of the Chico Mendes Institute (SisBio/ICMBio, no. 46665/10). The anatomical terms used in this paper follow the determinations of the 'International Committee on Veterinary Gross Anatomical Nomenclature', Nomina Anatômica Veterinária, 2017.

Eighteen B. variegatus corpses (after natural death) from CETAS of CPRH were used to describe the coronary arteries. Fifteen animals - five adult males, nine adult females, and one juvenile - were fixed with 20% formaldehyde and preserved in tanks of 30% saline solution, according to the methodology of Alcântara et al.(¹¹11 Alcântara SF, Amorim MJAL, Albuquerque PV, Mesquita EP, Miranda MELC, Andrade GP, Nascimento NEO, Nascimento JCS. Description of brachial plexus of sloth (Bradypus variegatus). Pesquisa Veterinária Brasileira. 2020; 40(10):824-829. Available from: https://doi.org/10.1590/1678-5150-PVB-6597
https://doi.org/10.1590/1678-5150-PVB-65... ). Two adult male cadavers and one female cadaver were injected through the left common carotid artery with latex stained with red chequered paint. The solution filled the arch of the aorta and the ascending aorta, as well as its immediate branches. The female was also injected through the right external jugular vein with latex containing blue chequered paint to highlight the heart bypass veins and thus avoid any mistake in arterial identification.

The animals that were injected with the latex were fixed exactly like the other specimens that were not injected. Thus, the coronary arteries became turgid and marked in red, enabling a better description and the abilityto obtain photographic records of cardiac vascularisation. The heart was accessed through a median sagittal incision of the thorax and reflection of the skin and muscles and removal of ribs. Then, the heart was removed from the cavity to carry out a detailed description of the angioarchitecture of the coronary arteries. The procedures took place in the Anatomy Area of the Department of Animal Morphology and Physiology (DMFA)of UFRPE.

The microscopic characteristics of the heart were also evaluated. For this endeavour, the heart was collected from four specimens, including one juvenile (sloth 1) and three adults (sloths 2, 3, and 4). The corpses were found at CETAS of CPRH shortly after natural death and had not been previously frozen. The heart was removed as described above in the Anatomy Area of DMFA of UFRPE.

The heart of each animal was fixed in 10% buffered formaldehyde with 0.01 M sodium phosphate (PBS, pH 7.3). Then, it was embedded in paraffin and processed in the Pathology Laboratory of the Department of Medicine of the Veterinary Department (DMV) of UFRPE. Four-micron-thick sagittal sections were stained with haematoxylin and eosin. The sections were viewed at 400× total magnification with a MOTIC® BA300 microscope coupled to a digital camera (Moticam® 2300) and connected to a microcomputer.

3. Results

Based on the analysis of the coronary arteries, in sloths, these vessels emerge from the ascending aorta, in the sinus, one right and one left. They provide a balanced circulation, without one standing out in relation to the other. Immediately after the origin of the right coronary artery, a vessel emerges, giving rise to branches that irrigate the cranial portion of the right side of the heart. From this point, a circumflex vessel originates, encircling the right auricle and giving off a branch to it. Descending vessels also emerge from the circumflex vessel, reaching the right cardiac side, including the subsinuosal interventricular branch (Figure 1). A paraconal interventricular branch arises from the left coronary artery. It irrigates the left side of the heart, reaching the apex. A circumflex vessel also originates from this coronary artery, from which a branch exits to the left atrium and branches to the cranial and middle portions of the left heart surface (Figure 1 ).

Figure 1
Photomacrograph of the heart of an adult three-toed sloth (Bradypus variegatus) with reduced angioarchitecture of the coronary arteries. (A and B) Origin and branching of the right coronary artery (→). (C and D) Origin and branching of the left coronary artery (→). Right atrium (AUD), left atrium (AUE), aortic arch (AC), brachiocephalic trunk (TB), pulmonary trunk (TP), right pulmonary artery (APD), left pulmonary artery (APE), right pulmonary ventricle (VD), and left ventricle (VE). A vessel emerges from the right coronary arteries directed towards the cranial portion of the right side of the heart (→), where it gives of branches (→). Circumfex branch of the right coronary arteries (→), from which an artery arises to the right atrium (→) and branches to the right cardiac surface (→). Subsinuosal branch (→). Paraconal branch (→), from which vessels depart that are distributed across the left side of the heart (→). Circumfex branch of the left coronary arteries (→), which originates an artery to the left atrium (→) and vessels to the corresponding cardiac surface (→).

According to histological data, the B.variegatus heart does not demonstrate discrepancies in relation to the general characteristics observed for other mammals. The organ has a typical epithelial lining, a typical endothelial lining (endocardium), a cardiac muscle layer (myocardium), and, externally, a layer of connective tissue with serous lining (epicardium). There is little accumulation of adipose tissue surrounding the heart.

In terms of histopathological findings, sloth 1 displayed degenerated cardiac muscle fibres that formed a thick area. The left atrioventricular valve, in turn, demonstrated thickening of its wall through a myxomatous degenerative process called endocardiosis, with collagen flexibility and deposition of fibrous material (Figure 2). In sloth 2, areas of necrosis were evident in the internal metallic layer of the thoracic aorta artery, which demonstrated the presence of a thrombus inside it. There was also necrosis of Purkinje nerve fibres and evidence of an inflammatory process, namely an increase in perivascular nuclei between cardiac muscle fibres, associated with fibroblasts/inflammatory cells. Congested cardiac vessels could also be observed (Figure 3). Sloths 3 and 4 showed extensive necrosis of many cardiac muscle fibres. In sloth 3, there was loss of the heart's architecture. Sloths 3 and 4 exhibited valvular endocardial necrosis and the presence of thrombi adhered to the endothelium and free in the vascular lumen, as well as damage to the muscle fibres surrounding the thrombotic vessels. These histological findings are indicative of infarction (Figure 4).

Figure 2
Photomicrographs of the heart of three-toed sloth (Bradypus variegatus) 1, which was routinely processed in paraffin and whose sagittal sections were stained with haematoxylin and eosin. (A) Interventricular region with septum separating the two cardiac ventricular chambers: right (VD) and left (VE). Thick area of myocardium (mio) with degenerated cardiac muscle fibres (→). Scale bar represents 200 μm. (B) Detail of the cardiac muscles of the left ventricle (VE); the muscles of the trabeculae carneae cannot be visualised. Scale bar represents 200 μm. (C) Atrial region showing presence of adipose tissue in the epicardium (▲). Scale bar represents 200 μm. (D) Left atrioventricular valve with thickening of its wall resulting from a myxomatous degenerative process called endocardiosis (*). Scale bar represents 200 μm. (E) Detail of the valve endothelium with thickening of the left atrioventricular valve, displaying collagen degeneration and deposition of fibrous material (→). Scale bar represents 50 μm. (F) Degeneration of cardiac muscle fibres (→). Scale bar represents 50 μm.

Figure 3
Photomicrographs of sagittal sections of the three-toed slot (Bradypus variegatus) heart, which was routinely processed in paraffin and stained with haematoxylin and eosin. (A) Thoracic aorta with its tunica intima (Tl) intact and metallic fibres (FE) covering the entire media layer. However, in some areas, there was necrosis (N) in the internal metallic layer. Scale bar represents 200 μm. (B) Lumen of the thoracic aorta artery with presence of arterial thrombus. Scale bar represents 200 μm. (C and D) Sinoatrial node (NSA) and details of Purkinje cells (FP) with necrotic nerve fibres (→). Scale bar represents 50 μm. (E and F) Increase in number of perivascular nuclei between cardiac muscle fibres (fibroblasts/inflammatory cells) and congestion of cardiac vessels (c). Scale bar represents 50 μm.

Figure 4
Photomicrographs of sagittal sections of the three-toed sloth (Bradypus variegatus) heart, which was routinely processed in paraffin and stained with haematoxylin and eosin. (A and B) Lower magnification view of the heart of sloth 3, showing loss of organ architecture and details of necrosis of numerous cardiac muscle fibres (NMC), associated with infarction. Scale bar represents 200 μm and 50 μm, respectively. (C) Focus of valve-related endocardial necrosis (VAL). Scale bar represents 200 μm. (D) Necrotic lesion of cardiac muscle fibres (NMC) in sloth 4. Scale bar represents 200 μm. (E and F) Presence of a thrombus attached to the vascular endothelium and free in the vascular lumen, respectively. A necrotic lesion of cardiac muscle fibres (NMC) surrounded by thrombotic vessels is also visible. Scale bar represents 200 μm.

4. Discussion

Coronary arteries in sloths arise from the aortic sinus, one on the right and one on the left, as in the crab-eating raccoon (Procyon cancrivorus)(¹²12 Pereira KF, Terra DRS, Ferreira LS, Sabec-Pereira DK, Lima FC, Santos OP. Anatomia do coração e vasos da base de Procyon cancrivorus. Arquivos do MUNDI. 2016; 20(3): 1-12. Available from: https://doi.org/10.4025/arqmudi. V20i3.34343
https://doi.org/10.4025/arqmudi. V20i3.3... ), the paca (Agouti paca)(¹³13 Ávila BHP, Machado MRF, Oliveira FS. Descrição anátomo-topográfica do coração da paca (Agouti paca). Acta Scientiae Veterinariae. 2010; 38(2): 191 -195. Available from: http://hdl.handle.net/11449/2821
http://hdl.handle.net/11449/2821... ), the South American fur seal (Arctocephalus australis)(¹⁴14 GuimarãesJP. Análise morfológica e ultra-estrutural do coração do lobo-marinho-do-sul (Arctocephalus australis, Zimmermamm, 1793). Tese de Doutorado. (Programa de Pós-Graduação em Anatomia dos Animais Domésticos e Silvestres). Universidade de São Paulo, São Paulo; 2009. p. 99. Available from: https://doi.org/10.11606/T.10.2009. tde-03122010-105433
https://doi.org/10.11606/T.10.2009. tde-... ), the southern tamandua (Tamandua tetradactyla)(¹⁵15 Pinheiro GS, Branco E, Pereira LC, Lima AR. Morfologia, topografia e irrigação do coração do Tamandua tetradactyla. Arquivo Brasileiro de Medicina Veterinária e Zootecnia. 2014; 66(4): 1105-1111. Available from: https://doi.org/10.1590/1678-6844
https://doi.org/10.1590/1678-6844... ), and birds such as ostrich (Struthio camelus)(¹⁶16 Soares GL, Oliveira D & Baraldi-Artoni SM. Aspectos da anatomia do coração do avestruz. Ars Veterinaria. 2010; 26(1 ): 38-42. Available from: https://doi.org/10.15361 /2175-0106.2010v26n1 p038-042
https://doi.org/10.15361 /2175-0106.2010... ). In the last two, as well as in the sloth, the coronary arteries give off circumflex branches. However, in anteaters, these vessels originate the paraconal and subsinuosal branches, from which vessels arise that vascularise the ventricles(⁵5 Albuquerque PV, Mesquita EP, Alcântara SF, Miranda MELC, Andrade GP, Amorim Júnior AA, Amorim MJAAL. External macroscopic anatomy of the Bradypus variegatus heart. Arquivo Brasileiro de Medicina Veterinária e Zootecnia. 2022; 74(5): 814-824. Available from: https://doi.org/10.1590/1678-4162-12562
https://doi.org/10.1590/1678-4162-12562... ). In bradypodids, the paraconal branch emerges directly from the left coronary artery and not from its circumflex branch; this anatomy also occurs in the South American fur seal(¹⁴14 GuimarãesJP. Análise morfológica e ultra-estrutural do coração do lobo-marinho-do-sul (Arctocephalus australis, Zimmermamm, 1793). Tese de Doutorado. (Programa de Pós-Graduação em Anatomia dos Animais Domésticos e Silvestres). Universidade de São Paulo, São Paulo; 2009. p. 99. Available from: https://doi.org/10.11606/T.10.2009. tde-03122010-105433
https://doi.org/10.11606/T.10.2009. tde-... ). Correia-Oliveira et al.(¹⁷17 Correia-Oliveira M, Oliveira IMS, Roza MS, Abidu-Figueiredo M. Morfometria cardíaca e distribuição das artérias coronárias em coelhos Nova Zelândia (Oryctolagus cunniculus). Revista Brasileira de Medicina Veterinária. 2014: 36(2): 159-166. Available from: https://rbmv.org/BJVM/article/view/506
https://rbmv.org/BJVM/article/view/506... ) noted the absence of the right circumflex coronary branch in 100% of the European rabbits (Oryctolagus cuniculus) they trained; moreover, 86.66% of males and 93.33% of females lacked the left branch. In the capuchin monkey (Sapajus apella), it was possible to notice the subsinuosal branch originating from the right coronary artery and anastomosing with the paraconal branch, which also occurred for the right and left circumflex branches. However, the left circumflex and paraconal branches are not branches of the right coronary artery(¹⁸18 Rade W 2004. Origem, trajeto, ramificações e distribuição dos ramos ventriculares da artéria coronária direita no macaco-prego (Cebus apella). Dissertação de Mestrado. (Programa de Pós-graduação em Ciências Veterinárias). Universidade Federal de Uberlândia, Minas Gerais; 2004. p. 34. Available from: https://repositorio. ufu.br/handle/123456789/30170
https://repositorio. ufu.br/handle/12345... ).

There are variations in coronary branches between groups and even between individuals of the same species. As noted by Pinto Neto et al.(¹⁹19 Pinto Neto JL, Leão CES, Vieira THM, Lopes AKMS, Vieira SRC, Silva NC, Wafae GC, Ruiz CR, Wafae N. Indicadores anatômicos de dominância entre as artérias coronárias em caprinos. Brazilian Journal of Veterinary Research and Animal Science. 2009; 46: 48-63. Available from: https://pesquisa.bvsalud.org/portal/resource/pt/vti-5165
https://pesquisa.bvsalud.org/portal/reso... ) in goats (Capra aegagrus hircus), the left coronary artery can originate the paraconal or subsinuosal branches, which are located in the subsinuosal groove. In the xenarthrans used in this study, on the cardiac surface, the subsinuosal and paraconal branches occupy the grooves that receive these same names and irrigate, respectively, the right and left ventricle, as seen in the paca(¹³13 Ávila BHP, Machado MRF, Oliveira FS. Descrição anátomo-topográfica do coração da paca (Agouti paca). Acta Scientiae Veterinariae. 2010; 38(2): 191 -195. Available from: http://hdl.handle.net/11449/2821
http://hdl.handle.net/11449/2821... ) and the southern tamandua(¹⁵15 Pinheiro GS, Branco E, Pereira LC, Lima AR. Morfologia, topografia e irrigação do coração do Tamandua tetradactyla. Arquivo Brasileiro de Medicina Veterinária e Zootecnia. 2014; 66(4): 1105-1111. Available from: https://doi.org/10.1590/1678-6844
https://doi.org/10.1590/1678-6844... ). In the ringed seal (Phoca hispida), the subsinuosal branch originates from a circumflex branch of the left coronary artery(²⁰20 Smodlaka HH, Henry RW, Schumacher J, Reed RB. Macroscopic anatomy of the heart of the Ringed Seal (Phoca híspida). Anatomia, Histologia, Embryologia. 2007; 37(1): 30-35. Available from: https://doi.org/10.1111/j.1439-0264.2007.00791.x
https://doi.org/10.1111/j.1439-0264.2007... ). For crossbred cattle (Bos taurus), the paraconal and subsinuosal branches originate exclusively from the left coronary artery and send vessels to both ventricles(²¹21 Correia-Oliveira M, HernandezJMF, Abidu-Figueiredo M 2013. Morfometria cardíaca e distribuição das artérias coronárias em bovinos mestiços. Biotemas. 2013; 26(2): 199-207. Available from: https://doi.org/10.5007/2175-7925.2013v26n2p199
https://doi.org/10.5007/2175-7925.2013v2... ). In the marsh deer (Blastocerus dichotomus), however, the paraconal and subsinuosal grooves are occupied by the branches of the left coronary artery, the paraconal and circumflex branches, respectively. Avessei emerges from the right coronary artery and ends cranially in the heart(²²22 Machado MRF, Borges EM, Oliveira FS, Filippini-Tomazini M, Melo APF, Duarte JMB. Intramyocardial course of the coronary arteries in the marsh deer (Blastocerus dichotomus). Brazilian Journal of Veterinary Research and Animal Science. 2002; 39(6): 285-287. Available from: https://doi.org/10.1590/S1413-95962002000600002
https://doi.org/10.1590/S1413-9596200200... ).

In sloths, the paraconal branch is longer, reaching the cardiac apex. In the southern tamandua, the circumflex branch of the right coronary artery reaches the apex and, less frequently, the paraconal groove(¹⁵15 Pinheiro GS, Branco E, Pereira LC, Lima AR. Morfologia, topografia e irrigação do coração do Tamandua tetradactyla. Arquivo Brasileiro de Medicina Veterinária e Zootecnia. 2014; 66(4): 1105-1111. Available from: https://doi.org/10.1590/1678-6844
https://doi.org/10.1590/1678-6844... ). Based on the data obtained in the present study for B. variegatus as well as the study by Pinheiro et al.(¹⁵15 Pinheiro GS, Branco E, Pereira LC, Lima AR. Morfologia, topografia e irrigação do coração do Tamandua tetradactyla. Arquivo Brasileiro de Medicina Veterinária e Zootecnia. 2014; 66(4): 1105-1111. Available from: https://doi.org/10.1590/1678-6844
https://doi.org/10.1590/1678-6844... ) with T. tetradactyla, coronary irrigation is balanced in Pilosa animals, with no predominance of one coronary artery over the other in terms of their irrigation areas. There was an exception in one of the tamanduas investigated: the authors found that the right coronary artery protruded from the left, which has also been recorded in human hearts(²³23 Kaimkhani ZA, Ali MM, Faruqi AM. Pattern of coronary arterial distribution and its relation to coronary artery diameter. Journal of Ayub Medical College Abbottabad, 2005; 17(1): 40-43. Available from: https://ayubmed.edu. pk/JAMC/PAST/17-1 /ZahidKaimkhani.htm
https://ayubmed.edu. pk/JAMC/PAST/17-1 /... ). This anatomy differs from domestic mammals such as the cat (Felis catus)(²⁴24 Vladova D. Ventricular coronary pattern in the cat. Trakia Journal of Science. 2005; 3(2): 44-49. Available from: http://www.uni-sz.bg
http://www.uni-sz.bg... ,²⁵25 Biase C, Borelli V, Benedicto HG, Pereira MR, Favaron PO, Bombonato PP. Análise comparativa entre a vascularização ventricular e do nó sinoatrial em gatos. Pesquisa Veterinária Brasileira. 2012; 32(1 ): 78-82. Available from: https://doi.org/10.1590/S0100-736X2012000100013
https://doi.org/10.1590/S0100-736X201200... ) and the goat(¹⁹19 Pinto Neto JL, Leão CES, Vieira THM, Lopes AKMS, Vieira SRC, Silva NC, Wafae GC, Ruiz CR, Wafae N. Indicadores anatômicos de dominância entre as artérias coronárias em caprinos. Brazilian Journal of Veterinary Research and Animal Science. 2009; 46: 48-63. Available from: https://pesquisa.bvsalud.org/portal/resource/pt/vti-5165
https://pesquisa.bvsalud.org/portal/reso... ) and wild mammals such as the capybara (Hydrochoerus hydrochaeris)(²⁶26 Tetani SC, Melo APF, Rodrigues RF. Estudo da vascularização arterial em corações de capivara (Hydrochaeris hydrochaeris - CARLETON, M. D. 1984). Brazilian Journal of Veterinary Research and Animal Science. 2010; 47: 204-208. Available from: https://pesquisa.bvsalud.org/portal/resource/pt/vti-4913
https://pesquisa.bvsalud.org/portal/reso... )and the tiger (Panthera tigris)(²⁷27 Perez W, Lima M. Brief description of the cardiac anatomy in a tiger (Pantheratigris, Linnaeus, 1758): a case report. Veterinární Medicina. 2007; 52: 83-86. Available from: https://doi.org/10.17221/2054-VETMED
https://doi.org/10.17221/2054-VETMED... ), where the predominance is from the left coronary artery.

In general, the aspects highlighted regarding the sloth's coronary arteries are very similar to the characteristics of these vessels in the ostrich, except for the subsinuosal branch reaching the apex and the fact that the right circumflex artery produces branches directed to the left cardiac face in the ostrich(¹⁶16 Soares GL, Oliveira D & Baraldi-Artoni SM. Aspectos da anatomia do coração do avestruz. Ars Veterinaria. 2010; 26(1 ): 38-42. Available from: https://doi.org/10.15361 /2175-0106.2010v26n1 p038-042
https://doi.org/10.15361 /2175-0106.2010... ). This different morphological approach between birds and mammals is related to the fact that these animals are at the same evolutionary stage(²⁸28 Ruben JA, Jones TD. Selective Factors Associated with the Origin of Fur and Feathers. American Zoologist. 2000; 40(4): 585-596. Available from: https://doi.org/10.1093/icb/40.4.585
https://doi.org/10.1093/icb/40.4.585... ), even though they have different reptilian ancestors.

The histopathological investigation of the three-toed sloth heart revealed that although the types of tissues observed were similar to those seen for other mammals, necrotic areas could be described in different regions and there were related thrombi in the aorta and smaller vessels associated with cardiac vascularisation. Endocardiosis of the left atrioventricular valve was also identified. Researchers indicated that this pathology involving the valves of both antimeres of the heart was the most prevalent heart disease among dogs (Canis lupus familiaris)with and withouta defined breed treated atthe University Veterinary Hospital of the Federal University of Santa Maria(²⁹29 Marques JO. Perfil clínico e ecocardiográfico de 60 cães atendidos no Hospital Veterinário da Universidade Federal de Santa Maria [undergraduate thesis]. Santa Maria: Universidade Federal de Santa Maria; 2020. p. 27. Available from: https://repositorio.ufsm.br/bitstream/handle/1/20551/TCCE_RAPSMVCPA_2020_MARRQUES_ JOSIANE.pdf?sequence=1
https://repositorio.ufsm.br/bitstream/ha... ). The high occurrence of myxomatous valve degeneration has also been described by Yamato et al.(³⁰30 Yamato RJ, Larsson MHMA, Mirandola RMS, Pereira GG, Yamaki FL, Pinto ACBCF, Nakandakari EC. Parâmetros ecocardiográficos em modo unidimensional de cães da raça Poodle miniatura, clinicamente sadios. Ciência Rural. 2006; 36(1):142-148. Available from: https://doi.org/10.1590/S0103-84782006000100021
https://doi.org/10.1590/S0103-8478200600... ) for poodles and by Castro et al.(³¹31 Castro MG, Veado JCC, Silva EF, Araújo RB. Estudo retrospectivo ecodopplercardiográfico das principais cardiopatias diagnosticadas em cães. Comunicação - Arquivo Brasileiro de Medicina Veterinária e Zootecnia. 2009; 61 (5):1238-1241. Available from: https://www.scielo.br/j/abmvz/a/6WvWBLZy4DXk9XBtdv7Jppf/?format=p df⟨=pt
https://www.scielo.br/j/abmvz/a/6WvWBLZy... ) for various dog breeds. Involvement of the right atrioventricular valve is often considered a secondary condition due to left heart failure; it results in changes in the left ventricle and pulmonary hypertension(³²32 O'grady MR, Bonagura JD, Powers JD, Herring DS. Quantitative cross-section echocardiography in the normal dog. Veterinary Radiology & Ultrasound. 1986; 27:34-39. Available from: https://doi.org/10.1111 /j.1740-8261.1986. tbOOOO1.x
https://doi.org/10.1111 /j.1740-8261.198... ). This relationship explains the occurrence of bilateral valve impairment, as seen in dogs(²⁹29 Marques JO. Perfil clínico e ecocardiográfico de 60 cães atendidos no Hospital Veterinário da Universidade Federal de Santa Maria [undergraduate thesis]. Santa Maria: Universidade Federal de Santa Maria; 2020. p. 27. Available from: https://repositorio.ufsm.br/bitstream/handle/1/20551/TCCE_RAPSMVCPA_2020_MARRQUES_ JOSIANE.pdf?sequence=1
https://repositorio.ufsm.br/bitstream/ha... ,³⁰30 Yamato RJ, Larsson MHMA, Mirandola RMS, Pereira GG, Yamaki FL, Pinto ACBCF, Nakandakari EC. Parâmetros ecocardiográficos em modo unidimensional de cães da raça Poodle miniatura, clinicamente sadios. Ciência Rural. 2006; 36(1):142-148. Available from: https://doi.org/10.1590/S0103-84782006000100021
https://doi.org/10.1590/S0103-8478200600... ,³¹31 Castro MG, Veado JCC, Silva EF, Araújo RB. Estudo retrospectivo ecodopplercardiográfico das principais cardiopatias diagnosticadas em cães. Comunicação - Arquivo Brasileiro de Medicina Veterinária e Zootecnia. 2009; 61 (5):1238-1241. Available from: https://www.scielo.br/j/abmvz/a/6WvWBLZy4DXk9XBtdv7Jppf/?format=p df⟨=pt
https://www.scielo.br/j/abmvz/a/6WvWBLZy... ), but also points to left unilateral primary damage, which is reported more commonly, as shown in one of the sloth hearts examined in the present study.

Cases of necrosis in cardiac tissues of different species are sometimes associated with the action of pathogens or food poisoning(³⁴34 Peixoto PV, Brust LAC, Brito MF, França TN, Cunha BRM, Andrade GB. Intoxicação natural por Amaranthus spinosus (Amaranthaceae) em ovinos no Sudeste do Brasil. Pesquisa Veterinária Brasileira. 2003; 23(4):179-184. Available from: https://doi.org/10.1590/S0100-736X2003000400007
https://doi.org/10.1590/S0100-736X200300... ,³⁵35 Rodrigues AS, Chaves NST, Damasceno AD, Souza MA, Rocha Júnior LH, Gonzaga Júnior WC. Aspectos anatomohistopatológicos da intoxicação experimental de bovinos pela ingestão de frutos de Stryphnodendron fissuratum Mart, ("rosquinha"). Ciência Animal Brasileira. 2005; 6(3):195-202. Available from: https://revistas.ufg. br/vet/article/view/364/339
https://revistas.ufg. br/vet/article/vie... ,³⁶36 Costa TLC. Detecção molecular de Neospora caninum em macaco da noite (Aotus azarae) de vida livre no estado do Mato Grosso. Arquivo Brasileiro de medicina Veterinária e Zootecnia. 2018; 70(4):1227-1232. Available from: https://doi.org/10.1590/1678-4162-9900
https://doi.org/10.1590/1678-4162-9900... ,³⁷37 Montgomery GG, Sunquist ME. Impact of sloths on neotropical forest energy flow and nutrient cycling. In: Golley F, Medina E, organizators. Tropical ecological systems: trends in terrestrial and aquatic research. New York: Springer-Verlag; 1975. p. 69-98. Available from: file:///C:/Users/Marleyne/Downloads/978-3-642-88533-4_7.pdf). However, in the necrotic events seen in the sloth hearts, there were no structures compatible with the stage of development of any infectious agent. These mammals are well adapted to consuming toxic leaves as a way to reduce competition with other folivores(³⁸38 Cork SJ, Foley WJ. Digestive and metabolic strategies of arboreal mammalian folivores in relation to chemical defenses in temperate and tropical forests. In: Palo RT, Robbins CT, organizators. Plant defenses against mammalian herbivory. Flórida: CRC Press; 1991. p. 133-166.Available from: file:///C:/Users/Marleyne/Downloads/ CorkandFoleyl 991 PIDefenses.pdf,³⁹39 Pereira Júnior HRJ. Evolução cromossômica na ordem Xenarthra [dissertation]. São Paulo: Universidade Estadual Paulista; 2007. p. 180. Available from: pereirajunior_hrj_dr_botib.pdf (1.606Mb),⁴⁰40 Charmandari E, Constantine T, George C. Endocrinology of the stress response. Annual Review of Physiology. 2005; 67(1 ):259-284. Available from: https://doi.org/10.1146/annurev.physiol.67.040403.120816
https://doi.org/10.1146/annurev.physiol.... ).

Stress has also been indicated as a triggering factor for cardiovascular problems(⁴¹41 Batista De Deus JC, Silva WP, Soares GJD. Efeito da distância de transporte de bovinos no metabolismo post mortem. Revista Brasileira de Agrociências. 1999; 5(2):152-156. Available from: file:///C:/Users/Marleyne/ Downloads/264-Article%20Text-520-1 -10-20120806.pdf). In adverse conditions, living beings develop strategies to maintain haemostasis, which are reflected in physiological compensations related, above all, to the heart, the respiratory rate, blood pressure, and body temperature(⁴²42 Moberg GP, Mench JA. The biology of animal stress: basic principles and implications for animal welfare. New York: CABI Publishing; 2000. Available from: https://doi.org/10.1079/9780851993591.0001
https://doi.org/10.1079/9780851993591.00... ). However, when stress is temporary, somatic and psychological damage can be noticed(⁴³43 Batista JS, Bezerra FSB, Lira RA, Orpinelli SRT, Dias CEV, Oliveira AF. Síndrome do estresse em catetos (Tayassu tajacu) submetidos à captura e contenção em diferentes horários da manhã em Mossoró, RN. Ciência Animal Brasileira. 2008; 9(1 ):170-176. Available from: https://revistas.ufg.br/vet/article/view/989/3443
https://revistas.ufg.br/vet/article/view... ). Batista et al.(⁴³43 Batista JS, Bezerra FSB, Lira RA, Orpinelli SRT, Dias CEV, Oliveira AF. Síndrome do estresse em catetos (Tayassu tajacu) submetidos à captura e contenção em diferentes horários da manhã em Mossoró, RN. Ciência Animal Brasileira. 2008; 9(1 ):170-176. Available from: https://revistas.ufg.br/vet/article/view/989/3443
https://revistas.ufg.br/vet/article/view... ) provided a good description of this phenomenon in the collared peccary (Tayassu tajacu). They reported stress syndrome: after necropsy and guidance for microscopic analysis of the organs, there was degeneration, necrosis, and retraction of skeletal and cardiac muscle fibres.

Sloths are very specific eutherians in terms of habits and morphophysiological characteristics. In recent years, they have been losing a substantial part of their territory due to anthropic actions. Hence, they are increasingly found closer to urban centres, where they are exposed to many dangers and are at an increased risk of illnesses and accidents(⁸8 Fuentes A, Hockings KJ. The ethnoprimatological approach inprimatology. American Journal of Primatology. 2010; 72(10): 841-7. Available from: https://doi.org/10.1002/ajp.20844
https://doi.org/10.1002/ajp.20844... ,⁹9 Xavier GAA, Amora TD, Valença YM & Cabral MCC. Apreensões de preguiças Bradypus variegatus SCHINZ, 1825 e casos de acidentes com choques elétricos envolvendo estes animais na Mesorregião Metropolitana do Recife, Pernambuco. In: Seabra GF, Silva JAN & Mendonça ITL (Org.), A conferência da Terra: aquecimento global, sociedade e biodiversidade. Vol.1. Editora Universitária da UFPB, João Pessoa; 2010. p. 301-308. Available from: https://docplayer.com.br/7278347-A-conferencia-da-terra-aquecimento-global-sociedade-e-biodiversidade.html
https://docplayer.com.br/7278347-A-confe... ,¹⁰10 Pereira KF, Silva IO, Silva FFR, Silva VHD, Castro CSS, Boere V. Hematology and serum biochemistry of free-range brown-throated sloths in two urban areas in Brazil. Veterinary World. 2020; 13(9): 2033-2038. Available from: https://doi.org/10.14202/vetworld.2020.2033-2038
https://doi.org/10.14202/vetworld.2020.2... ,⁴⁴44 Peres MA. Colheita e avaliação do sêmen do bicho-preguiça (Bradypus sp.) [master's thesis]. São Paulo: Universidade de São Paulo; 2005. 74 p. Available from: https://doi.org/10.11606/D.10.2005.tde-27062006-164456
https://doi.org/10.11606/D.10.2005.tde-2... ,⁴⁵45 Burke A, Virmani R. Significance of multiple coronary artery thrombi. A consequence of diffuse atherosclerotic disease? Ital Heart J. 2000; 1(12):832-834. Available from: https://ifcardio.org/pdf/italian-heart-journal/2000/12/20000329.pdf
https://ifcardio.org/pdf/italian-heart-j... ). The constant stress that these animals experience might have an intimate relationship with the various necroses, inflammatory processes, and degeneration seen in the sloths' hearts in the present study.

The existence of thrombi in the vessels responsible for cardiac circulation has a specific association with myocardial infarction(⁴⁶46 Jinnouchi H, Finn AV, Virmani R. Histopathology of Cardiovascular Thrombus. Cardiovascular Thrombus. 2018; 1 -13p. Available from: https://doi.org/10.1016/B978-0-12-812615-8.00001 -6
https://doi.org/10.1016/B978-0-12-812615... ). This relationship could be seen in the sloth: in specimens whose hearts exhibited vascular thrombotic events, there were many necrotic muscle fibres, which denote infarction. Thrombosis is considered one of the most relevant cardiovascular complications and is related to morbidity and mortality. Atheromas are considered an important predisposing factor to the development of thrombi(⁴⁷47 Foley WJ, Engelhardt WV, Charles-Dominique P. The passage of digesta, particle size, and in vitro fermentation rate in the three-toed sloth Bradypus tridactylus (Edentata: Bradypodidae). Journal Zoology. 1995; 236:681-696. Available from: http://doi.org/10.1111/j.1469-7998.1995.tb02739.x
http://doi.org/10.1111/j.1469-7998.1995.... ). In sloths, however, thrombotic events seem to be conditioned by other conditions because atheromas were not observed, probably due to the slow absorption of nutrients caused by low metabolism that does not favour the accumulation of fat in these animals(⁴⁸48 Gilmore DP, Costa CP, Duarte DPF. Sloth biology: an update on their physiological ecology, behavior and role as vectors of arthropods and arboviruses. Brazilian Journal of Medical and Biological Research. 2001; 34(1): 9-25. Available from: https://doi.org/10.1590/S0100-879X2001000100002
https://doi.org/10.1590/S0100-879X200100... ,⁴⁹49 Sholander PF, Krog J. Countercurrent heat exchange and vascular bundles in sloths. Journal of Applied physiology. 1957; 10:405-411. Available from: https://doi.org/10.1152/jappl.1957.10.3.405
https://doi.org/10.1152/jappl.1957.10.3.... ). When analysing the anatomical and physiological aspects and lifestyle habits of sloths, they spend long periods in safety, motionless, which could compromise the circulatory system if it were not for the presence of admirable networks on the thoracic and pelvic limbs and the duplicity of the caudal vena cava that fragments the passage of blood, guaranteeing excellent tissue transparency and drainage. However, these vascular projects also function as blood reservoirs(⁵⁰50 Goffart M. Function and form in the sloth. Oxford: Pergamon Press; 1971. Available from: https://www.jstor. org/stable/2808832?seq=13
https://www.jstor. org/stable/2808832?se... ,⁵¹51 Albuquerque PV, Sena DGF, Braz RS, Mesquita EP, Lacerda MAS, Silva WJ, Sá FB, Amorim MJAAL. Ramos do arco aórtico e da aorta torácica em bicho-preguiça (Bradypus variegatus). Arquivo Brasileiro de Medicina Veterinária e Zootecnia. 2018; 70(4): 1203-1211. Available from: https://doi.org/10.1590/1678-4162-9963
https://doi.org/10.1590/1678-4162-9963... ). This anatomy may favour stasis, which is also linked to thrombus formation. Additional research is needed to identify the exact causes related to the serious cardiovascular problems of sloths identified in this study. Nevertheless, the peculiar vascular characteristics associated with the stress resulting from anthropogenic tensions are a reasonable cause.

5. Conclusion

Based on the analysis of cardiac vascularisation, there was not a predominance of one coronary artery of the ascending aorta over the other. The right and left coronary arteries supply blood to their respective sides of the heart. Necrosis of cardiac tissues is a recurring concern in B. variegatus that is associated with thrombosis, infarction, and stress. Heart diseases, such as endocardiosis, are less common. This information is crucial to understanding the heart health of sloths.

Acknowledgements

The authors acknowledge the Postgraduate Program in Animal Bioscience at DMFA/UFRPE and the Anatomy Area and the Pathology Laboratory at UFRPE for making this work possible. CETAS/CPRH donated B. variegatus corpses. Professor Fabrício Bezerra de Sá provided instruction in angiotechnics. Professor Adelmar de Amorim Júnior reviewed the anatomical terms. Finally, veterinarian Sandra Maria de Torres provided valuable contributions to the histopathology of hearts.

References

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