BORNA DISEASE
Herbert Weissenb ock
Pathology and Forensic Veterinary Medicine, Department of Pathobiology, University of Veterinary Medicine, Vienna, Austria
There is only one virus species, Borna disease virus (BDV), within the genus Bornavirus of the family Bornaviridae.
Recently, several novel viral genotypes were recovered from birds, which have been provisionally assigned to a new Bornavirus species, Avian bornavirus (ABV).Infection of susceptible hosts with BDV can lead to a severe neurological condition called Borna disease (BD). ABVs are the aetiological agent of a common psittacine disease, proventricular dilatation disease (PDD) and of clinically and morphologically similar diseases in the canary ( Serinus canaria), Canada geese (Branta canadensis) and trumpeter swans (Cygnus buccinator)4124. Confirmed cases of BD have been found only in endemic areas of Germany, Switzerland and Austria. The disease mainly affects horses (Equus caballus) and sheep ( Ovis aries aries), but sporadic cases have been found in cattle (Bos primi- genus), goats (Capra hircus), rabbits (Oryctolagus cunicu- lus), dogs (Canis familiaris) and certain zoo animals, including alpacas (Lama pacas'), southern two-toed sloths (Choloepus didactylus) and pygmy hippopotamus ( Choerop- sis liberiensis)4124. Reports of cases of classical BD or detection of bornaviral genome sequences from other parts of the world are a matter of scientific controversy. Also, the association of BDV with disease in other animal species, such as cats (Felis catus), lynx (Lynx lynx), fox (Vulpes vulpes) and ostrich (Struthio camelus) is not unequivocally proven. The mentioned lynx was shot in Sweden in 1999 because of highly apathetic behaviour with a staring, expressionless gaze. At pathological examination, the emaciated animal displayed a moderate to severe nonsuppurative encephalitis and showed positive results in several diagnostic assays for BD(26).
In endemic areas, the virus seems to be maintained by persistently infected reservoir hosts, such as the bicoloured white-toothed shrew (Crocidura leucodon). In Sweden, detection of BDV (not ABV) nucleic acid was reported from faecal samples of mallards (Anas platyrhynchos) and jackdaws (Corvus monedula), and it was postulated that wild birds could act as reservoirs of the virus(27).ABV seems to be distributed worldwide, and cases of PDD infected with different ABV genotypes have been reported widely, including from Europe. Psittacine birds seem to be the natural hosts and, as they shed virus in the faeces, transmission between birds is very likely. A condition similar to PDD that occurs sporadically in canaries (Serinus canaria) has been attributed to a divergent genotype of ABV, provisionally termed canary bornavirus.
In natural hosts, BDV infection most likely occurs via the olfactory tract with subsequent spread of the virus to the brain. Non-suppurative encephalitis in many cases with characteristic intranuclear inclusion bodies, and viral antigen are most prevalent in the rostral brain regions, such as the olfactory cortex, the limbic system and the brainstem. In immunocompetent hosts the virus remains restricted to the central nervous system (CNS) and shedding of virus does not occur. BDV itself is not cytolytic and clinical disease and pathological changes are due to the effect of the immune response. In horses, BDV induces a severe neurological disease, with depression, repetitive behaviour and finally paresis, with a mortality of at least 80%.
ABV have a different distribution pattern, as they are present in the CNS and also in the peripheral and autonomic nerve system as well as in many non- neural cell populations. The major lesions of PDD are nonsuppurative encephalitis, and ganglioneuritis of the ganglia and nerves of the upper digestive tract, resulting in prov- entricular dilatation and upper digestive tract impaction in birds.
Psittacine birds with PDD show regurgitation, passage of undigested seeds in the faeces and weight loss. Although specific treatment is not possible, clinical signs can be alleviated in some cases by suppressing inflammation with the use of COX-2 inhibitors such as celecoxib.Abundant viral antigen or viral RNA are easily demonstrated in the nervous system, in both BDV and ABV infection. In live animals the diagnosis of BDV is difficult, as presence of viral signals in extraneural locations, including blood, has never been convincingly proven and the widely used serological assays are not entirely reliable. In cases of ABV infection, PCR assays for detection of ABV nucleic acid fragments in faeces are a promising approach.
BDV in horses and sheep has become a rare, sporadic disease in recent decades and requires no specific control measures. For PDD, the recognition of the aetiology paves the way towards efficient control measures, such as eliminating the virus from breeding facilities by PCR- based screening programmes.
A potential association of BDV with human psychiatric diseases has been discussed, but convincing proof is missing.