Introduction
Borna disease (BD) is a sporadically occurring, usually fatal disorder
caused by a neurotropic RNA virus, the Borna disease virus (BoDV)
recently renamed as mammalian orthobornavirus (Rott and Becht, 1995,
Richt, 2007, Heinig, 1969). Even though horses and sheep are the main
natural hosts of BoDV, other Equidae , farm animals and companion
animals (cats, dogs) have been diagnosed with natural BD infection
(Richt, 2007, Staeheli et al., 2000). Furthermore, psittacine birds can
be infected with an avian Bornavirus (PaBV, psittaciform
orthobornavirus) associated with proventricular dilatation disease
(Honkavuori et al., 2008).
The route of infection with BoDV occurs most likely via open nerve
endings in the nasal and pharyngeal mucosa (Dürrwald et al., 2007,
Priestnall et al., 2011). Shrews have been identified as carriers of the
virus, whereas BoDV-infected horses do not seem to play a role in virus
transmission (Nobach et al., 2015, Hilbe et al., 2006, Staeheli et al.,
2000).
In horses, natural infection with BoDV can take an inapparent, subacute,
acute or peracute course (Richt, 2007). The incubation period ranges
from 2 to several months (Schmidt, 1952). Depending on the affected
brain area, the clinical signs of classic BD in horses can range from
changes in behaviour and consciousness to slow motion eating, fever,
somnolence, stupor, hyperexcitability, or aggressiveness (Richt, 2007,
Bilzer et al., 1996, Grabner and Fischer, 1991). Other signs of a more
advanced stage in BD are hypokinesia, ataxia and hyporeflexia, leading
to compulsive circular walking, head tremor, blindness, or convulsions
and coma in final stages of the disease (Bilzer et al., 1996, Grabner
and Fischer, 1991). The recent association of BoDV-1 with eight cases of
fatal encephalitis in humans shows the zoonotic potential of the virus
(Niller et al., 2020). The deaths of three people after a BoDV-1
infection, all of them solid organ recipients from a single donor,
suggest another way of BoDV-1 transmission (Anon, 2018). While the
zoonotic potential of BoDV is confirmed, its association with certain
human psychiatric diseases remains controversial (Lieb and Staeheli,
2001, Staeheli and Lieb, 2001, Dürrwald et al., 2007).
BoDV infection in horses is endemic in Central Europe (Germany, Austria,
Switzerland and Liechtenstein), while there are reports of sporadic
clinical disease in other countries (Rott and Becht, 1995, Staeheli et
al., 2000, Dürrwald et al., 2007).
In Spain, the only animals analysed for Bornavirus infection were
captive birds (psittacines), which were tested for Avian Bornavirus
(PaBV) (RNA, virus, antigen) and PaBV-specific antibodies
(Heffels-Redmann et al., 2011); 14.3% of the samples were positive to
PaBV. BoDV infection is currently regarded as an exotic disease in
horses; however, serological investigations in these animals have never
been performed.
Given this background, the present work aimed to carry out a serological
survey on susceptible breeding and sport horses in Spain in order to
retrospectively determine the degree of exposure to BoDV in asymptomatic
horses and to determine risk factors for the exposure to BoDV in
breeding horses.