Rubella virus is an enveloped, positive-sense single-stranded RNA virus that causes rubella, also known as German measles. It is the sole member of the genus Rubivirus in the family Matonaviridae. The virus has a spherical, pleomorphic virion about 60 to 70 nanometres in diameter surrounded by a lipid envelope embedded with two glycoproteins, E1 and E2. Its genome is approximately 9.8 kilobases long and encodes both nonstructural and structural proteins via two open reading frames.
Genome and Virology
The rubella virus genome contains a 5′ untranslated region, a replicase open reading frame encoding the nonstructural proteins P150 and P90, an intergenic region and a second open reading frame encoding the structural proteins—capsid, E2 and E1. Upon entry, the viral RNA serves as mRNA and is translated into the nonstructural polyproteins that form the replicase complex. Replication occurs on cytoplasmic membranes derived from endosomes and lysosomes, where a negative-sense template is synthesised and used to generate genomic and subgenomic RNAs. The subgenomic RNA directs synthesis of structural proteins in the rough endoplasmic reticulum. Virion assembly involves association of capsid protein with genomic RNA and budding into intracellular membranes. The E1 glycoprotein mediates attachment to cell receptors and low-pH-triggered fusion; E2 assists in assembly. Rubella virus has slower replication kinetics than alphaviruses and exhibits a high degree of host specificity, replicating mainly in human epithelial and placental cells. The virus is stable at a narrow pH and temperature range and is sensitive to lipid solvents due to its envelope.
Epidemiology and Congenital Rubella Syndrome
Rubella is a mild, self-limited exanthematous disease characterised by rash, lymphadenopathy and fever. However, infection during early pregnancy can lead to congenital rubella syndrome (CRS), causing fetal death or severe birth defects such as deafness, cataracts and congenital heart disease. Before the introduction of vaccination, rubella epidemics occurred every 6–9 years and large outbreaks caused thousands of CRS cases worldwide. A live attenuated rubella vaccine, often combined with measles and mumps vaccines (MMR), has dramatically reduced incidence; endemic rubella transmission has been eliminated in the Americas and several other regions. Nonetheless, rubella remains endemic in parts of Africa and Asia where vaccination coverage is low, and imported cases can spark outbreaks in susceptible populations. Humans are the only reservoir, and the virus is transmitted by respiratory droplets. Control relies on maintaining high immunisation coverage, surveillance for rubella and CRS, and screening women of childbearing age for immunity.
Rubella virus demonstrates how an RNA virus that is generally mild can have devastating consequences for the developing fetus, highlighting the importance of vaccination.
Related Terms: Measles virus, Mumps virus, Congenital rubella syndrome, Alphavirus, Togaviridae