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Members of the genus Morbillivirus lack a neuraminidase activity and exhibit greater amino acid sequence relatedness within the genus than with other genera. They have a gene order, number of transcriptional elements and length of intergenic sequences identical to those of members of the genera Respirovirus, Aquaparamyxovirus, Henipavirus and Narmovirus (Figure 2.Paramyxoviridae). All morbilliviruses have a P/C/V transcription unit with RNA editing, as do respiroviruses and henipaviruses, namely the templated exact copy mRNA encodes a phosphoprotein (P) and the predominant edited mRNA form (1 G added) encodes a Zn2+-binding cysteine-rich protein (V). All members encode a non-structural protein (C). All morbilliviruses produce both intracytoplasmic and intranuclear inclusion bodies containing nucleocapsid-like structures. Viruses cross-react in serological tests. Sialic acid does not appear to be a receptor for morbilliviruses. Narrow host-range distribution of receptor defines susceptibility of organisms to infection. For measles virus (MV) the receptors are signaling lymphocytic activation molecule family member 1 (SLAMF1, aka CD150) and nectin cell adhesion molecule 4 (NECTIN4); for the vaccine virus an additional receptor is CD46 (Griffin 2007). SLAMF1 also appears to be a receptor for canine distemper virus (CDV) and rinderpest virus (RPV), which have a preference for canine and bovine SLAMF1, respectively. Each species is a significant cause of disease in its respective host.
See discussion under family description.
Species demarcation is now entirely based on the distance in the phylogenic tree based on the comparison of complete large (L) protein amino acid sequences. Since the primary criterion is the tree topology, whether or not a virus belongs to the same species becomes a matter of branch length between the nearest node and the tip of the branch. This length is defined as 0.03 in the trees generated as described in the legend to Figure 3.Paramyxoviridae.
Morbilliviruses are distinguished by host range, genetic (sequence) and antigenic differences. There is a low to moderate degree of sequence relatedness between members, depending on the protein (for example, the nucleoprotein (N) protein of MV is 65% related to that of CDV, viruses that represent two lineages of the genus). Cross-neutralization and cross-protection also occur between members of the genus, although members can be distinguished readily by monoclonal antibodies. MV infects primates, CDV infects carnivores, and RPV and peste-des-petits-ruminants virus (PPRV) infect artiodactyls, especially ruminants and suids. PPRV is distinguished from RPV by sequence analysis (the N protein of PPRV shares 68–72% identity with that of MV, RPV or CDV), and because it does not readily infect cattle. Phocine distemper virus (PDV) is most closely related to CDV and is distinguished by host range and sequence divergence. Cetacean morbilliviruses have been isolated from dolphins and porpoises but antibody prevalence studies suggest infection in other whale species. Members of this species are most closely related to RPV and MV: these viruses are distinguished by host range and sequence divergence. Feline morbillivirus (FeMV) is associated with a persistent infection in cats (Felis catus L.) causing tubule interstitial nephritis.