Borna disease virus (BDV) is a unique pathogen that has long been studied. Recent research has confirmed its distinctive biological features and its role in severe human illness.
Understanding Borna Virus
Borna virus is a linear, negative-sense, single-stranded RNA virus belonging to the Bornaviridae family within the order Mononegavirales. It possesses the smallest genome among viruses in its order, approximately 8.9 kilobases in length. A distinguishing feature of bornaviruses, unique among animal viruses in the Mononegavirales order, is their ability to replicate within the host cell nucleus.
The virus is characterized by its neurotropic nature, meaning it primarily targets and affects the nervous system. It also exhibits a non-cytolytic replication strategy, meaning it does not directly kill infected cells. This allows the virus to cause persistent infections, remaining in the host for extended periods.
Animal Hosts and Natural Transmission
Borna viruses naturally circulate within specific animal populations, acting as reservoir hosts. The bicolored white-toothed shrew (Crocidura leucodon) is recognized as the main natural reservoir for Borna Disease Virus 1 (BoDV-1). These shrews can carry and shed the virus without showing obvious signs of disease themselves.
Transmission among these shrew populations is thought to occur through bodily fluids, such as saliva or urine, which can contaminate the environment. Humans and other accidental hosts, like horses and sheep, can contract the virus through contact with infected shrews or their excretions. Another type, Variegated Squirrel Bornavirus 1 (VSBV-1), has been detected in variegated and Prevost’s squirrels (Sciurus variegatoides and Callosciurus prevostii), and infections have been linked to contact with these exotic squirrels, often in breeding settings.
Borna Virus and Human Disease
Historically, BoDV-1 was investigated for a potential link to psychiatric disorders in humans, though this was not definitively established. Recent discoveries confirm that specific bornaviruses, namely BoDV-1 and Variegated Squirrel Bornavirus 1 (VSBV-1), can cause severe, often fatal, encephalitis in humans. These are zoonotic infections, transmitted from animals to humans. VSBV-1 infections have been observed in individuals with direct contact with infected variegated squirrels, such as private breeders or zoo caretakers.
Human infection with BoDV-1 is primarily linked to exposure to infected bicolored white-toothed shrews, which are native to endemic regions in central Europe, including parts of Germany, Austria, and Switzerland. Contact with shrew excretions or carcasses is a likely route of transmission.
Recognizing and Managing Human Infection
Human bornavirus infection, particularly with BoDV-1 and VSBV-1, typically manifests as encephalitis, which is an inflammation of the brain. Initial symptoms can be non-specific, including fever, headache, and general malaise, but rapidly progress to more severe neurological signs. These can include confusion, disorientation, psychomotor slowing, myoclonic jerks (sudden muscle spasms), ataxia (lack of coordination), epileptic seizures, and ultimately, a decline in consciousness leading to coma.
Diagnosing bornavirus infections can be challenging due to the rapid progression of the disease. Diagnosis often involves detecting viral RNA in cerebrospinal fluid (CSF) or brain tissue using PCR testing. Serological tests for antibodies in serum can also be used, though CSF antibody detection may appear later. Currently, there is no specific antiviral cure for bornavirus encephalitis; treatment is primarily supportive, often requiring intensive care to manage symptoms and complications.