1.1 The structure and biology of measles virus

Measles virus is a paramyxovirus belonging to the genus Morbillivirus. It is a pleomorphic virus ranging in diameter from 100 to 300 nm. Within the morbilliviruses, measles is most closely related to the rinderpest virus group, and more distantly related to the canine distemper virus group [1]. Two envelope glycoproteins are important in pathogenesis. These are the F (fusion) protein, which is responsible for fusion of virus and host cell membranes, viral penetration, and haemolysis, and the H (haemagglutinin) protein, which is responsible for binding of virus to cells. The measles genome consists of six genes, each encoding a single structural protein. One of these genes, the phosphoprotein (P) gene, also encodes two non-structural proteins (V and C). Refer to Figure 1.1 Structure of measles virus. 

The measles genome consists of 15,894 nucleotides although some variation in genome length has been described [2]. Measles virus has a single, non-segmented, negative-sense RNA genome with a linear arrangement of genes that are separated by an intergenic trinucleotide, GAA. Each gene contains a single open reading frame (except P), transcriptional start and stop signals, and a polyadenylation signal. The encapsidated genomic RNA is termed the nucleocapsid (NC) and serves as a template for transcription and replication. The nucleoprotein (N) mRNA is transcribed first and N is the most abundant of the viral proteins.

Measles virus, as an RNA virus, has the potential for generation of variants during replication due to the lack of proof-reading capacity of RNA-dependent polymerases. This has implications for the molecular epidemiological interpretation of nucleotide differences that may be observed from viruses collected during outbreaks. Although there is only one serotype of measles, there are distinct genetic lineages of wild-type measles viruses based on the nucleotide sequence of the 450 nucleotides that code for the carboxyl-terminal region of the nucleoprotein.

As of 2015, there are 24 genotypes of measles virus with reference strains recognized by WHO. Only six of the genotypes have been detected since 2011 [3]. Most importantly, the genetic variation does not appear to be biologically significant, as all genotypes are neutralized by measles vaccine-induced antibodies [4]. More information on the molecular epidemiology, genetic characteristics of measles virus and measles genotypes is provided in Chapter 7. Molecular epidemiology of measles and rubella.

Measles virus is viable for less than 2 hours at ambient temperatures on surfaces and objects, while the aerosolized virus remains infectious for 30 minutes or more. It is heat-labile and is inactivated after 30 minutes at 56°C. However, the virus appears to survive freeze-drying relatively well and, when freeze-dried with a protein stabilizer, can survive storage for decades at -70°C. The virus is inactivated by solvents, such as ether and chloroform, by acids (pH less than 5), alkalis (pH greater than 10), and by UV and visible light. It is also susceptible to many disinfectants, including 1% sodium hypochlorite, 70% alcohol and formalin.