General information

The Borna virus is a non-segmented, enveloped RNA virus which constitutes the only member of the family Bornaviridae. This virus triggers the co-called Borna disease in mammals and the proventricular dilatation disease in birds.

Mammals

Numerous species of mammals are susceptible to this virus. It is of clinical relevance particularly in horses, in cats (where the disease is also called “staggering disease”) and in sheep.

The virus has a strong neurotropism and triggers non-purulent meningoencephalitis, associated with anorexia, apathy, somnolence and multiple neuronal dysfunctions. Animals suffering from Borna disease develop motor and behavioural disorders. In horses and sheep, in addition to the symptoms listed above, a lowered head posture, separation from the herd, empty chewing and salivation have been described and, at a later stage, recumbency and flailing movements. Cats frequently suffer from hind-leg ataxia and lumbosacral pain.

There is often little or no immune response, which makes it difficult to diagnose by testing for antibodies. The incubation period is unknown. The progression of a clinically manifested infection is lethal (duration of the disease usually 1 – 3 weeks).

Clinically inapparent infections are also possible. It has been a controversial issue whether this virus also infects humans and is there linked to neuropsychiatric disorders.

A seasonal increase of the disease from March to September has been described in horses and sheep; in cats, increases can be found from December to May.

The modes of transmission have not yet entirely been clarified; infection probably occurs through the nerve endings of the nasal and pharyngeal mucosa. Infections from horse to horse (sheep to sheep, cat to cat) are experimentally possible, but very unlikely. Shrews constitute the virus reservoir.

Birds

Proventricular dilatation disease (PDD) is a globally distributed, serious disease particularly affecting psittacines (large parrots) like macaws, amazons or grey parrots. In 2008, identification of the previously unknown avian Borna virus (ABV) in birds infected with PDD was achieved for the first time. An etiological relationship could then be proven in infection experiments.

PDD either affects the gastrointestinal tract, the central nervous system or both areas. This means, on the one hand, there may be digestive disorders such as diarrhoea, vomiting or regurgitation as well as anorexia and the excretion of undigested seeds in faeces. On the other hand, PDD can manifest itself through neurological dysfunctions like ataxia and coordination disorders, tremor or paresis. Both complexes of symptoms are associated with depression, general weakness and excessive loss of weight.

In addition to peracute and acute deaths, especially in older birds chronic progressions of the disease can also be observed. Moreover, clinically inapparent birds can be infected with the virus. Breeding flocks and new additions should thus be tested for an infection with ABV.

Avian Borna viruses are RNA viruses which show high genetic divergence. A negative result does therefore not entirely rule out PDD, as there may be other virus variants, which have not yet been described, leading to this disease.

The safest way to detect an ABV infection requires a combination of antibody and pathogen detection. In some birds, only viral RNA can be detected, in others only anti-ABV antibodies are detectable, while others again react positively on both tests.