For Central European veterinarians, Borna disease (BD) has been known for a long time as a sporadically occurring, progressive viral polioencephalomyelitis predominantly affecting horses and sheep and-as discovered in the last decade-an increasing number of domestic and zoo animals. The aetiological agent, the Borna disease virus (BDV), a negative-sense, single-stranded RNA virus classified in the new virus family Bornaviridae within the order Mononegavirales, can induce severe clinical signs typically of a viral encephalitis with striking behavioural disturbances. After an incubation period lasting a few weeks to several months, BDV-infection causes locomotor and sensory dysfunctions followed by paralysis and death. Natural infections seem to be subclinical in most cases. BD received world-wide attention when it was reported that sera and/or cerebrospinal fluids from neuro-psychiatric patients can contain BDV-specific antibodies. Since infected animals produce BDV-specific antibodies only after virus replication, it was assumed that the broad spectrum of BDV-susceptible species also includes man. However, reports describing the presence of other BDV-markers, i.e. BDV-RNA or BDV-antigen, in peripheral blood leukocytes or brain tissue of neuro-psychiatric patients are highly controversial and, therefore, the role of BDV in human neuro-psychiatric disorders is questionable. (c) 2001 Harcourt Publishers Ltd.