Togaviruses

The togavirus family (Togaviridae) comprises two genera. Alphavirus infec­tions are transmitted by arthropods and are imported to central Europe mainly by travelers to tropical and subtropical countries. Their clinical pictures are variable, but almost always include joint pain (arthralgias). The most important representative of the genus Rubivirus is the rubella virus, the causative agent in German measles. This normally harmless childhood disease can cause severe embryopathies during the first trimester of pregnancy.

Pathogen. The term togaviruses formerly included a variety of viruses, including what we now classify as the flaviviruses. As defined today, the togaviruses include the zoopathic pestiviruses, one species of rubivirus, the rubella virus and the alphaviruses with 25 species. The alphaviruses most important to travelers are the Chikungunya virus (Africa, Asia), the Sindbis virus (Africa, Asia, Australia), the Ross River virus (Australia, Oceania), and the Mayaro virus (South America), which are transmitted to humans by bloodsucking mosquitoes.

Togaviruses possess an icosahedral capsid and a closefitting envelope. The capsid measures 35-40 nm and the entire virion 60-65 nm. The genome of the togaviruses is a single-stranded, polyadenylated, sense RNA. Replication not only produces new 40S genomic RNA, but a subgenomic 26S RNA frag­ment as well, which codes for the capsid proteins. Viral progeny are released by “budding” at the cell surface.

Pathogenesis and clinical picture. The arthropodborne alphaviruses, zoo­noses of the tropical and subtropical regions, frequently cause asymptomatic or benign infections with fever, exanthem, and joint pain. Occasionally, how­ever, persistent arthralgia and polyarthritis (lasting months or even years) do occur, sometimes involving joint destruction. Even rarer, sequelae include en­cephalitis and meningoencephalitis with high lethality rates.

“German measles” is a harmless exanthemous infection in children and youths, caused by a rubivirus, the rubella virus, and transmitted by direct contact. The infections remain inapparent in nearly half the cases. The virus at first replicates in lymphoid organs at the portal of entry and in the naso­pharyngeal space, after which a viremia develops before the exanthem man­ifests. In pregnant women, the virus takes this route through the placenta to the embryo, where it can cause congenital deformities or embryonic death, especially in the first three months of pregnancy. The organs in the develop­mental stage in this trimester are most seriously affected by the rubella in­fection. The most frequent congenital deformities are deafness, cataracts, car­diac defects, microcephaly, and spina bifida. In intrauterine embryo deaths due to rubella infections the immediate cause of death is usually myocardial damage. A measles infection confirmed by IgM detection or a raised antibody count is therefore an indication for a first-trimester abortion.

Diagnosis. Serodiagnosis is the method of choice in suspected alphavirus and rubivirus infections. EIA methods are also available for IgM detection.

Prevention. There are vaccines to protect against alphavirus infections and rubella. The main aim of rubella prophylaxis is to prevent rubella-caused em­bryopathies. Since 10-15% of young adults are still susceptible to rubella in­fections and a live vaccine with few side effects that confers reliable immu­nity is available, serial vaccination of children (boys and girls!) is done before puberty. The vaccine is tolerated so well that prior immune status checks are not required.