Perspectives of Morbillivirus Infections in Cetaceans: A Review Focused on South America Paula A. Angel-Romero1, Dalia C. Barragán-Barrera1,2, Miguel H. Parra Ávila3 1 Laboratorio de Ecología Molecular de Vertebrados Acuáticos, Departamento de Ciencias Biológicas, Facultad de Ciencias, Universidad de los Andes, Bogotá, Colombia. 2 Fundación Macuáticos Colombia, Antioquia, Colombia. 3 Departamento de Ciencias Biológicas, Facultad de Ciencias, Universidad de los Andes, Bogotá, Colombia. Correspondence author: Paula Andrea Angel Romero [email protected] Advisor: Miguel Hernando Parra Ávila, PhD Chair Professor Biological Sciences Department Universidad de los Andes Bogotá, Colombia Co-advisor: Dalia Carolina Barragán Barrera, PhD candidate Laboratorio de Ecología Molecular de Vertebrados Acuáticos - LEMVA Biological Sciences Department Universidad de los Andes Bogotá, Colombia 1 ABSTRACT Cetacean Morbillivirus (CeMV) belongs to the Paramyxoviridae family and different strains have been discovered like porpoise morbillivirus (PMV), dolphin morbillivirus (DMV), pilot whale morbillivirus (PWMV), beaked whale morbillivirus (BWMV), and two recently unestablished lineages. CeMV is responsible for the death numerous individuals from a great variety of species through several outbreaks and mass strandings in several parts of the world. It is probably transmitted through the inhalation of aerosolized virus via the blowhole and it is favored by gregarious species and migratory behavior; besides the viral cellular entry is mediated by the SLAM receptor that provides an interface for morbillivirus H glycoprotein to attach, showing a coevolution process. The disease is categorized as a sub-acute, acute, chronic systemic disease or a chronic localized disease leading to a severe encephalitis and the diagnosis can be made by means of virus isolation, histology and immunohistochemistry (IHC), serology, or different Reverse Transcription Polymerase Chain Reaction (RT-PCR) variants. Most of CeMV reports have occurred in the USA and Europe, and that there is an information gap for South America; therefore, the aim of the current literature review is to present an overview of the CeMV mechanisms and characteristics, the pathology, pathogenesis, epidemiology, diagnosis and affected species of the disease, with a special emphasis on the lack of information and reports of this infectious agent in South America. 2 1. INTRODUCTION Cetacean Morbillivirus (CeMV) belongs to the Order Mononegavirales, family Paramyxoviridae, subfamily Paramyxovirinae and Genus morbillivirus. There are currently seven members of this genus that cause severe diseases in their hosts, which include: measles virus (MV), canine distemper virus (CDV), rinderpest virus (RPV), peste-des-petits ruminants virus (PPRV) (Blixenkrone-Møller, Bolt, Jensen, Harder, & Svansson, 1996), phocine distemper virus (PDV), feline morbillivirus (FmoPV) and cetacean morbillivirus (CeMV) (Cassle et al., 2016; Jo, Osterhaus, & Ludlow, 2018). Regarding CeMV, it includes porpoise morbillivirus (PMV) isolated from harbor porpoises (Phocoena phocoena) from Northern Ireland (McCullough et al., 1991), dolphin morbillivirus (DMV) isolated from Mediterranean striped dolphins (Stenella coeruleoalba) (Van Bressem et al., 1991), pilot whale morbillivirus (PWMV) isolated from a long-finned pilot whale (Globicephala melas) from New Jersey, USA (Bellière, Esperón, & Sánchez-Vizcaíno, 2011; Taubenberger et al., 2000a), beaked whale morbillivirus (BWMV) isolated from a longman's beaked whale (Indopacetus pacificus) from Hawaii (West et al., 2013), and two recently discovered strains which are highly divergent and where identified from Indo-Pacific bottlenose dolphin (Tursiops aduncus) from the West Australian coast (Stephens et al., 2014) and from Guiana dolphin (Sotalia guianensis) from Brazil (Cassle et al., 2016; Groch et al., 2014). Morbilliviruses are known to cause a severe disease due to immunosuppression as they are lymphotropic and replication process starts in lymphoid tissue before infection other tissues and epithelial cells takes place (Barrett et al., 1995; Delpeut, Noyce, & Richardson, 2014; Osterhaus et al., 1995; Shimizu et al., 2013; Van Bressem et al., 2014). CeMV is responsible for the death numerous individuals from a great variety of species through several epidemics and outbreaks, since it was first isolated from S. coeruleoalba in 1990 in Spain as DMV strain, which is one of the highest mortalities reported (Aguilar & Raga, 1993). 3 Here we provide an overview of the CeMV mechanisms and characteristics, the pathology, pathogenesis, epidemiology, diagnosis and affected species of the disease, with a special emphasis on the lack of information and reports of this infectious agent in South America. 2. CETACEAN MORBILLIVIRUS CHARACTERISTICS 2.1. Antigenic and molecular characteristics Morbilliviruses are characterized by a lipid envelope that contains the helical nucleocapsid with the linear non-segmented, negative-sense, single-stranded RNA (Barrett, 1999). RNA viruses have a very high mutation rate (i.e. from ~10−2 to ~10−5 mutations/site/replication) which leads them to genotypic and phenotypic variations causing the appearance of variants or strains that have differences regarding the immunological properties, the virulence, the host rage and tropism and the epidemiology (Beffagna, Centelleghe, Franzo, Di Guardo, & Mazzariol, 2017). The DMV is 15,702-bp long and is composed of six different structural proteins each of them codified by a transcription unit or gene and two virulence factor proteins. The principal protein is the nucleocapsid protein (N) which contains the viral genetic material in a ribonucleoprotein complex (RNP), that protects the RNA from enzymes present in the cytoplasm of the host cell that can cause its degradation. N protein also acts as an acceptor molecule for the RNA-dependent-RNA polymerase allowing it to attach to mediate replication and transcription processes (Jo et al., 2018). It has 523 amino acids and presents high variability at the C-terminus. On the contrary, the N protein is highly conserved at the amino terminus but displays a region of high variability at the carboxyl terminus (Banyard, Tiwari, & Barrett, 2011). The phosphoprotein (P) and the large protein (L) are also contained in the RNP complex (Jo et al., 2018). The proteins associated to the viral membrane are the matrix protein (M), the fusion protein (F) and hemagglutinin glycoprotein (H). M protein is found in the inside of the viral envelope, which is hydrophobic, enriched positively charged residues that are believed to interact with the RNP and it does not seem to be modified post-translationally (Barrett, 4 1999) and it allows the assembly of the virus and the exit of the cell (Jo et al., 2018); F protein is a conserved acylated protein and H glycoprotein is more variable, both proteins are responsible for the attachment and fusion with the host cell (Barrett, 1999). The H protein is a crucial structure to perform the cell entry, and associated with the morbillivirus F protein, cause cell-to-cell fusion and therefore a cytopathic effect (CPE) (Banyard et al., 2011). Regarding the virulence factors, non-structural V and C proteins are derived from the P gene, by editing of the mRNA by insertion of a G residue and by translation of an overlapping reading frame (ORF) (Bellière et al., 2011). Advances in molecular biology techniques have allowed the diagnosis of CeMV infections, in order to stablish phylogenetic relationships between different morbillivirus strains. It is of high importance to clarify the differences between strains as it was proven that the amino acids which differentiated DMV isolated from S. coeruleoalba and fin whale (Balaenoptera physalus) from different epidemics outbreaks where under diversifying selection, which can lead eventually to a potential host switch (Beffagna et al., 2017). Likewise, these techniques provide valuable information about the virus epidemiology and are important to develop candidate vaccines (Barrett, 1999). 2.2. Mechanisms of Cellular Entry and Receptors The general cellular entry process starts with the H glycoprotein, which is responsible for the attachment to the host cell membrane to initiate the cell entry process (Banyard et al., 2011), and together with the F protein, the process of cell-to-cell fusion is induced causing the fusion of the cell membrane and M protein (Banyard et al., 2011; Barrett et al., 1995; Delpeut et al., 2014). Morbilliviruses are characterized by the ability to infect immune cells like B and T-lymphocytes, macrophages, activated monocytes and dendritic cells. the last two cells mentioned, express the cellular receptor signaling lymphocytic activated molecule (SLAM or CD150) which is the receptor of the morbilliviruses H protein and the infection with CeMV usually leads to an acute disease, lymphopenia or low lymphocyte count in the blood and immunosuppression, leaving organisms vulnerable to secondary infections and also affecting the central nervous system (CNS) causing a severe encephalitis (Jo et al., 2018; 5 Ludlow, Rennick, Nambulli, De Swart, & Paul Duprex, 2014; Ohishi et al., 2010; Ohishi, Suzuki, & Maruyama, 2012; Tatsuo, Ono, & Yanagi, 2001). The SLAM receptors family share similar domain structures, in which the ectodomain has a distal membrane immunoglobulin variable (IgV) domain and a proximal membrane immunoglobulin constant-2 (IgC2) domain that provides a target for morbillivirus H glycoprotein