1.5 The structure and biology of rubella virus

Part B. Rubella

Rubella virus is an enveloped, positive-stranded RNA virus and a member of the genus Rubivirus and belongs to the Matonaviridae family. Ruhugu virus and Rustrela virus joined Rubella virus as second and third of only three members of the genus Rubivirus. The rubella virus is roughly spherical with a diameter of 60–70 nm. It is composed of a pleomorphic nucleocapsid containing a single-stranded, positive-sense RNA genome with 9,762 nucleotides. The virus contains three structural proteins, two in the envelope (E1 and E2) and one in the core (capsid or C protein) surrounding the RNA. 

Refer to Figure 1.4, Structure of rubella virus.

The cellular receptor for rubella has not yet been identified. The envelope proteins, E1 and E2, are glycoproteins that exist as heterodimers that project from the virus to form 6 to 8 nm surface spikes [20]. E1 appears to be the dominant surface molecule and is associated with neutralizing and antigenic epitopes. There is only one serotype of the virus. As is true for the measles virus, the host range of rubella virus is limited to humans.

Genetic characterisation of rubella viruses has identified two distinct genetic groups (clades) of rubella viruses. The two clades, clade 1 and clade 2, differ by 8-10% at the nucleotide level. As of 2015, there are 10 genotypes (including 1 provisional genotype) described for clade 1 and 3 genotypes are recognized in clade 2 based on phylogenetic analysis of 739 nucleotides within the E1 coding region [21]. The data available from the genetic analysis of circulating rubella viruses are limited due to collection of fewer specimens for molecular characterisation compared to measles. In addition, rubella sequences from many regions of the world are either underrepresented in the sequence database for rubella or remain unavailable. It is possible that more genotypes may be identified as surveillance improves. More information on the molecular epidemiology, genetic characteristics of rubella virus and rubella genotypes is provided in Chapter 7. Molecular epidemiology of measles and rubella.

The rubella virus is somewhat more stable than measles virus at ambient temperatures. The virus is heat-labile and can be inactivated after 30 minutes at 56°C. When stabilised with protein it can be repeatedly frozen (at -60°C or below) and thawed without loss of titre. Infectivity is rapidly lost at -20°C. Lipid solvents, weak acids and alkalis, and UV light inactivate the rubella virus. It is also susceptible to a wide range of disinfectants and is inactivated by 1% sodium hypochlorite, 70% ethanol and formaldehyde.