Rabies is very unique virus in many ways. One unusual aspect of rabies is the amount of time that can elapse between exposure and development of clinical disease (illness). This is called the incubation period of virus. Usually people develop rabies within six months of exposure (e.g. from a bite from an infected animal like a dog or a bat). However, longer incubation periods can occur.
Some rabies cases with purportedly long incubation periods can’t be confirmed because the person may have been exposed to the virus multiple times. In countries where rabies virus is not present, it is much easier to tell when a person is exposed because they had to have been traveling abroad. Such a case was reported in the December 2008 edition of the journal Emerging Infectious Diseases. The case described was that of a ten-year-old girl that died of rabies in Australia, a country which is rabies-free. The child had lived in Australia for five years, but previously lived in Vietnam and Hong Kong, where rabies is common. Since she had not left Australia in the last five years, it is almost certain that she was exposed to the virus at least five years before developing disease.
One potential complicating factor in some rabies-free countries is the presence of one or several bat lyssaviruses, which are very closely related to the rabies virus (in fact, the rabies virus itself belongs to the Lyssavirus genus). These viruses can also cause disease that looks very similar to rabies. Human cases of infection by bat lyssavirus were reported in Australia after the child in the report died of rabies, but analysis of viral RNA collected from the girl recently confirmed that it was rabies, and not another lyssavirus. Also, typing of the rabies virus showed that it was most consistent with a Chinese strain, which suggests that she acquired the infection in Hong Kong. This demonstrates that rabies can have a very long incubation period.
Rabies is a devastating disease but one that is largely preventable in people, given proper attention and access to post-expsoure treatment. More information about rabies can be found in on the Worms & Germs Resources page and in our rabies archives.
Photo: Electron micrograph of a rabies virus (source: Tektoff-RM/CNRI/Science Source/Photo Researchers, Inc.).