Borna disease virus 1 (BoDV-1; species Mammalian 1 orthobornavirus, family Bornaviridae) is the causative agent of Borna disease, a mostly fatal neurological disorder of horses, sheep, and other domestic mammals in southern and eastern Germany, Austria, Switzerland, and Liechtenstein.1, 2 The bicoloured white-toothed shrew (Crocidura leucodon) is the only known natural reservoir host of BoDV-1.3, 4, 5, 6, 7 In this host, BoDV-1 establishes a persistent infection with remarkably broad tissue tropism, but without apparent clinical disease.4, 5, 7 By contrast, the virus is almost exclusively neurotropic and causes a T lymphocyte-mediated encephalitis in erroneous spillover hosts, such as horses and sheep.1, 2, 8
The zoonotic potential of BoDV-1 has been a matter of an unresolved scientific dispute for decades. Worldwide distribution of human BoDV-1 infection had been postulated mostly in the context of affective or psychiatric disorders, such as depression or schizophrenia, whereas severe encephalitis cases were rarely investigated.1, 2, 9, 10, 11, 12, 13, 14 However, BoDV-1 sequences and isolates from these studies were demonstrated to be the likely result of laboratory contamination because these sequences were almost identical to sequences of laboratory strains used in the respective institutions, and epidemiological links to the endemic area could not be presented.14, 15, 16 Furthermore, reported bornavirus-reactive antibodies and viral antigens in serum samples could not be confirmed by independent methods.1, 13 In addition, bornavirus-reactive antibodies were shown to be highly cross-reactive within the genus Orthobornavirus, and therefore serological findings alone could not be interpreted as proof of BoDV-1 infection.17
Research in context
Evidence before this study
We searched PubMed for research articles published in English and German between inception and July 11, 2019, with the following search terms: “Bornaviridae” OR “Borna disease” OR “bornavirus”. The search yielded 1042 results from 1931 to 2019. We focused on studies reporting animal and human bornavirus infections since 1985. The zoonotic potential of Borna disease virus 1 (BoDV-1) has been a matter of controversy for decades. Studies reporting the identification of BoDV-1 infection markers in humans were demonstrated by subsequent studies to be the likely result of laboratory contamination, and serological tests were often not validated or irreproducible. In addition, bornavirus-reactive antibodies were shown to be highly cross-reactive within the genus Orthobornavirus, and therefore serological findings alone could not be interpreted as proof of BoDV-1 infection. Thus, evidence for any zoonotic risk posed by bornaviruses had not been presented until 2015, when zoonotic transmission of the newly discovered variegated squirrel bornavirus 1 (VSBV-1) resulted in fatal encephalitis in at least four exposed people. Finally, three publications confirmed, by independent methods, for the first time that BoDV-1 infection induced fatal encephalitis in a cluster of solid organ transplant recipients and two additional unrelated cases in Germany. Thus, bornavirus infections have to be considered as hitherto unknown lethal zoonoses.
Added value of this study
Our study provides comprehensive data about the occurrence, course of infection, and epidemiology of zoonotic bornavirus infections, and the identification of eight further BoDV-1 infections in humans between 1999 and 2019 emphasises the zoonotic potential of BoDV-1 in endemic regions. We found BoDV-1 infection in two further immunocompromised patients and in six patients with no record of immunosuppression. Seven of nine fatal encephalitis cases in one diagnostic centre were BoDV-1 positive, whereas none of the surviving patients in this study tested positive. All patients lived within the known endemic area of BoDV-1 in central Europe. Phylogenetic analysis indicates multiple independent infections from a local reservoir without further transmission of the virus (so-called spillover infections to dead-end hosts).
Implications of all the available evidence
Based on these new data, BoDV-1 infection has to be considered as a severe and potentially lethal zoonosis. It does not represent a newly emerging entity in the known BoDV-1 endemic region but appears to have occurred unnoticed in humans for at least decades. Particularly in severe and fatal non-purulent encephalitis of unknown aetiology, bornaviruses have to be considered as potential causative agents. Our data confirm the role of BoDV-1 as a relevant zoonotic pathogen, provide the basis for a case definition, and emphasise the importance of differential diagnostics of severe and fatal encephalitis.
The discovery of variegated squirrel bornavirus 1 (VSBV-1; species Mammalian 2 orthobornavirus) in Germany in 2015, which causes fatal encephalitis after transmission from exotic squirrels to humans, moved the zoonotic potential of mammalian bornaviruses back into focus.18, 19 In 2018–19, BoDV-1 was reported in three cases of encephalitis in solid organ transplant recipients infected via organs received from the same donor and two further transplantation-independent cases.20, 21, 22 The organ donor had shown no signs of neurological disease and had died of suspected sudden cardiac arrest. Both kidney graft recipients died from BoDV-1-induced polyradiculoneuritis and encephalitis or encephalomyelitis, whereas the liver recipient survived and recovered with sequelae from leukencephalopathy and visual constraint due to optic nerve atrophy.21 The donor and the patients who had not received transplants lived in known BoDV-1-endemic regions in Bavaria, southern Germany.20, 21, 22
In the first part of this study, we focused on the occurrence of BoDV-1-associated encephalitis cases in one large university hospital within the endemic area. In the second part, we provide a detailed description of eight newly identified cases of BoDV-1-induced encephalitis. We present and discuss the results of clinical and virological investigations as well as epidemiological and phylogenetic analyses.