P aramyxoviridae encompasses a group of large (300-500 nm in diameter), enveloped, pleomorphic viruses with RNA genomes of 14.6-20.1 kb. The family comprises 4 subfamilies and 17 genera that contain >70 species and includes global human and animal viral pathogens of concern (1). Currently, the genus Morbillivirus, in subfamily Orthomyxovirinae, contains measles virus (MeV), rinderpest virus (RPV), peste des petits ruminants virus (PPRV), canine distemper virus (CDV), phocine distemper virus (PDV), cetacean morbillivirus (CMV), and feline morbillivirus (FeMV) (2,3).Morbillivirus genomes encode 6 structural proteins in the following order: nucleocapsid (N) protein, phosphoprotein (P), matrix (M) protein, hemagglutinin (H) protein, fusion (F) protein, and large polymerase (L) protein (2). Two nonstructural proteins, C and V, are expressed from the P open reading frame and are thought to interfere with the innate immune response in at least a subset of members of the family Paramyxoviridae (4).Morbilliviruses cause respiratory and gastrointestinal disease and profound immune suppression (5). Morbillivirus host species experience a similar pathogenesis; infection occurs through inhalation, direct contact with body fl uids, or fomites or vertical transmission (6-8). Carnivore morbilliviruses readily invade the central nervous system (CNS), and all morbilliviruses produce intranuclear viral inclusion bodies containing nucleocapsid-like structures (1,9,10).Paramyxoviruses known to naturally infect swine include porcine rubulavirus, Menangle virus, Nipah virus, and porcine parainfl uenza virus (11-16). Less well-characterized paramyxoviruses associated with central nervous and respiratory disease in pigs also have been reported (17)(18)(19)(20), but none of these viruses are classifi ed in the genus Morbillivirus. Using histopathology, metagenomic sequencing, and RNA in situ hybridization (ISH), we identifi ed a novel morbillivirus in swine as the putative cause of an outbreak of reproductive disease characterized by fetal mummifi cation, encephalitis, and placentitis.
Materials and Methods
Clinical Background and SamplesIn early 2020, the Iowa State University Veterinary Diagnostic Laboratory (Ames, IA, USA) received 22 porcine fetuses from 6 litters (A-F) that originated from a commercial breeding herd in northern Mexico for routine diagnostic investigation (Table 1). The breeding herd comprised 2,000 sows and reported reproductive clinical signs characterized by an increased percentage (18% reported) of mummifi ed fetuses and stillbirths. For negative controls, we used fetal tissues from 2 litters from a 3,000-head sow farm in the United States that was experiencing increased mummifi ed fetuses and stillborn fetuses.