Togaviruses

This family contains the genera Alphavirus (former group A arboviruses) and Rubivirus (rubella virus).

The Alphavirus genus contains 27 viruses, all mosquitoborne. The most important viruses are the 'New World alphaviruses, eastern, western and Venezuelan equine encephalitis viruses that have caused epizootics in horses and human epidemics of encephalitis in the Americas; and the "Old World' alphaviruses chikungunya and O‘nyongnyong virus in Africa, Ross River virus in Australia and the Pacific islands, and Sindbis virus in Africa, Scandinavia, the former Soviet Union and Asia. These cause epidemic disease characterised by fever, rash and polyarthritis. The arthropathy may be severe and can last for months, or up to 3 years in the case of Ross River virus.

Rubella is caused by a togavirus (genus Rubivirus) which spreads by droplet infection. One attack confers a high degree of immunity. It tends to affect older children, adolescents and young adults and spreads less readily than measles. The incubation period is usually about 18 days. The disease in children is trivial. In adults the illness may be more severe, but is of short duration and little importance except when it develops in a woman during the first 4 months of pregnancy. In such cases the child may be born with one or more congenital malformations.

Clinical features In children the constitutional symptoms are so slight that the illness is rarely suspected until the rash is seen. The spots are pink macules which appear first behind the ears and on the forehead. The rash spreads rapidly, first to the trunk and then to the limbs. Tender enlargement of the suboccipital lymph nodes is usual. In adolescents and adults the onset may be acute, with fever and generalised aches, but even then the illness lasts for only 2 or 3 days. Polyarthritis is the most common complication and may occur in up to one-third of adult women. Encephalomyelitis and thrombocytopenic purpura are very rare. Complete recovery from all of these complications is the rule.

The rash of rubella is very similar to that due to certain drugs, enteroviruses and also parvovirus B19 which causes erythema infectiosum (fifth disease). Serological tests are necessary for a definitive diagnosis of rubella.

No treatment is available. If infection is known to have occurred during the first 16 weeks of pregnancy there is such a high chance of fetal abnormality that termination should be discussed.

Rubella vaccine should be given to all children at the age of 12–15 months, and again at about 4 years, with measles and mumps vaccine (as MMR vaccine). Rubella vaccine should also be offered to adolescents who have not previously been immunised. Women of childbearing age who are found to be serologically negative should also be offered vaccine, provided that they are not pregnant and are willing to avoid pregnancy for 12 weeks after vaccination. (A history of prior rubella is unreliable and only serological testing reliably detects immunity.)