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.