Who is this Togaviridae?

Since its inception, the members of the virus family Togaviridae have been anything but constant. Classified originally on the basis of being small nonsegmented positive sense single strand RNA viruses the classification was anything but precise. Dubbed Togavirus because of the "loose envelope" displayed in electron micrographs, the togaviridae were later shown to have envelopes that are quite tightly bound to the nucleocapsid. So general was the classification criteria of Togavirus, that three virus divisions were discovered within it to be very different from each other. The family Togaviridae was then split into three in 1984, joined by its cousins Flaviviridae and Pestiviridae (often not considered a true viral family). The split was a result of advances in microbiology techniques, and especially the development of powerful genetic analyzation techniques. It was realized that the three groups went about replication in strikingly different ways, and the distinction between the families was established. A major unifying principle between all three groups is that they are all capable of being transmitted by arthropod vectors. The proximity between Togaviridae and Flaviviridae is maintained by their respective vector-borne groups, Alphavirinae and Betavirinae.

Surprisingly, Togaviridae is less known for its arthrpod borne infectivity than for the single member of the Rubivirinae, Rubella. Originally recognized as the "German Measles" in th early nineteenth century, the virus was disregarded as a mild infection of little importance. In an age where small pox, influenza, and pneumonia were the leading killers of man, it is not hard to believe that a virus with an a mortality rate of 0.05% would be overlooked. Rubella outbreaks were common in the spring across the United States with a periodicity of about 3 years. 80% of americans were seropositive for rubellaby child-bearing age. For over a century this highly contagious virus was overlooked, until an astute opthamologist took notice of it.

In 1941, Norman Gregg began to note an unusually high incidence of congenital cataracts. After some research, he found that the cataracts were closely related to a maternal Rubella infection in the first trimester of pregnancy. This was one of the first demonstrations of a virus as a teratogen. Some follow-up studies later showed that first-trimester maternal infections of Rubella were also correlated with more severe birth defects such as deafness and blindness, and also with miscarrage. Children recognized to have congenital abnormalities due to prenatal infection are termed to have congenital rubella syndrome, or CRS.

Public response to the discoverery of a viral teratogen was marked. Parents forced children to attend "rubella parties" with people known to be infected with rubella modelled after "measles parties" intended to ensure children were exposed to the disease and developed immunity at an age where the incidence serious sequella are reduced. Because rubella has only one serotype, lifelong immunity is acquired after the initial immune response. Rubella virus was isolated in 1962, and a live, attenuated vaccine was immediately produced. 1964 marked the last epidemic of Rubella observed in the United States. In 1972 the vaccine was merged with the measles and mumps vaccine to become the MMR vaccine, commonly given to children at the age of 15 months.

The alphavirinae, though far greater in number, and equally, if not more, pathogenic than the rubivirinae, have taken the back seat to rubella. The alphaviruses are transmitted only by arthropod vector bite. Although the viruses replicate in man, the viruses do not become concentrated enough to be transmitted. As a result, alphavirinae rarely reach epidemic proportion, and only as a result of extreme proximity of man and insect population. Most commonly transmitted by various mosquito species, the alphavirinae are capable of replication within the vector. Animal hosts include horses and birds, in which the viruses are mildly pathogenic, presenting little more morbidity than rubella in adult man.

The alphavirinae present two distinct types of disease, one type involves encephalitis and is highly linked with horses, the other is characterized by arthalgia (arthritic pain) and mild fever. Alphavirinae are present throughout the world, and arise periodically in the spring when conditions for viral replication are ideal and vector populations are large. Vaccines have been developed against the encephalitic deiseases, which are characterized by fatality rates of 30-70%.