The Borna disease virus is the causative agent of borna disease in horses and other animals. However, recent findings have implicated the borna virus in some human neurological and psychiatric conditions including bipolar disorder.

Borna disease of horses was first described in 1660, and by 1900 was recognised as a disease of economic importance in the Borna region of South East Germany. It acquired its name in 1970.

Borna disease in the horse gives rise to signs like:

• Unusual posture, gait and ear positions
• Movement Disturbances (principally ataxia or excess movement)
• "Pipe smoking" - hay or straw in mouth, but no chewing

The Borna virus was isolated from a diseased horse in the 1970s, but the virus particles were difficult to characterise. Nonetheless the virus' genome has been characterised. It is a single-stranded negative strand RNA virus of the order rabies. A new family named the bornaviridae was created to hold this virus.

Effects in other species

Borna virus appears to have a wide host range, having been detected in horses, cattle, sheep, dogs and foxes. In 1995, the virus was isolated from Sweden. Since that time, the virus has been detected in cats in Japan and Germany, Japan and the USA the virus has been detected in humans, and it has a controversial association with human disease, particularly of the psychiatric kind.

Experimental infection of rats has been demonstrated to lead to learning impairments and altered social behaviour. The virus appears to be distributed primarily in the limbic system of the brain, including the hippocampus and entorhinal cortex. These areas of the brain are considered to be of importance in emotion.

Borna Virus as an agent of human disease

In an experiment in 2000, Ludwig and Bode found that patients suffering from major depression and bipolar disorder were more likely than healthy blood donors to have antibodies to Borna virus (indicative of prior infection) and to have borna virus components present. They have claimed that up to 50% of cases of depression could be attributable to infection with borna virus.

Some other evidence cited in favour of the idea that borna virus could be responsible for human psychiatric disease includes the fact that the drug amantidine, which is used to treat influenza infections, has had some success in treating depression. Nonetheless, there are counter-claims that borna virus infections are not cleared by amantidine. The issue is further complicated by the fact that amantidine is also used in the treatment of Parkinson's Disease, so may have direct effects on the nervous system.

The link between borna virus and human psychiatric disease is not yet conclusively proved, and there is much controversy among researchers about the validity of claims made.