Borna Disease Virus and Psychiatric Disorders : Can Viruses Influence Psychiatric Disorders ?

1.1 Psychiatric disorders and infectious diseases Psychiatric disorders are a wide group of diseases with a heterogeneous aetiology (genetic predisposition, environmental factors, exposure to stress, for example). Several infectious agents preferentially affect the central nervous system and these infections are associated with psychic and neurologic symptomatology. It has been suggested that some infectious diseases can influence the development and the course of several psychiatric disorders. Infectious agents of zoonotic diseases with the ability to cause persistent infections of the central nervous system and influence the development and functions of this system include Toxoplasma gondii, Borrelia burgdorferi and Borna disease virus (BDV). Other neurotropic viruses that affect humans and which are associated with neurologic and psychiatric symptoms include herpes viruses, HIV and rabies virus, among others. Toxoplasma gondii is associated with cognitive dysfunctions in infected subjects (human and animals). Studies have shown that there is a direct statistical link between incidences of schizophrenia and toxoplasmosis infection. Several studies reported significantly higher seropositivity of this infection in schizophenic patients (Torrey & Yolken 2003). Lyme borreliosis is caused by the Gram-negative spirochete Borrelia burgdorferi, which has a high affinity with the central nervous system. There may be a long latent period between infection and the development of clinical neuropsychiatric symptoms. Several studies associated borreliosis with affective, anxiety (panic) and organic disorders and psychosis. A higher seropositivity was demonstrated in psychiatric patients compared to healthy individuals (Hájek et al., 2002).


Epidemiology of Borna disease virus infection
Borna disease virus infection was originally believed to be limited in horses and sheep in endemic areas in Central Europe, especially in Germany.But over the years, the use of new diagnostics methods discovered the presence of natural BDV infections in other regions, such as Australia, the United States of America (Kao et al., 1993;Richt et al., 2000), China (Hagiwara et al., 2001), the United Kingdom (Reeves et al., 1998), Japan (Watanabe et al., 2006), Israel (Teplitski et al., 2003), other European countries (Italy, Poland, Czech Republic, France, Switzerland) (Galabru et al., 2000;Pisoni et al., 2007) and others.Natural BDV infection was detected not only in horses and sheep but also in other animal species: cats (Berg et al., 2001;Reeves et al., 1998), dogs (Weissenbock et al., 1998), cattle (Watanabe et al., 2006), birds (Berg et al., 2001), foxes (Dauphin et al., 2001), and ostriches (Weismann et al., 1994).Borna disease virus infection differs in its course according to the species; the most serious form of Borna disease is described in horses and sheep with severe neurologic symptoms and high mortality rates, in contrast to other species (Ludwig & Bode 2000).

The transmission of BDV infection
There are several supposed routes of transmission of BDV infection between humans and animals: by direct contact with infectious secretions through the nasal mucosa (Durrwald & Ludwig 1997;Hatalski et al., 1997;Ludwig and Bode 2000;Richt et al., 1993Richt et al., , 2000Richt et al., , 2001)), vertically during pregnancy (Hagiwara et al., 2000;Okamoto et al., 2003), and indirectly by infected (contaminated) food or water (Durrwald & Ludwig 1997;Hatalski et al., 1997;Ludwig & Bode 2000;Richt et al., 1993Richt et al., , 2000Richt et al., , 2001)).Findings of BDV RNA and proteins in peripheral blood mononuclear cells indicate the possibility of hematogenous transmission (Solbrig et al., 2003;Vahlenkamp et al., 2000).Pisoni and colleagues reported the possible sexual transmission of BDV infection.They detected higher BDV seropositivity in sexually active female horses compared to lower BDV seropositivity in animals that had never had sexual contact (Pisoni et al., 2007).Human-to-human transmission of BDV infection is supported by several studies that described higher BDV positivity in mental health workers and family members who were in contact with BDV-positive psychiatric patients (Chen et al., 1999a).Some studies described higher BDV positivity in humans who were in contact with infected animals; this BDV positivity was positively correlated with the degree of contact with infected animals.These findings proved the animal-to human transmission of BDV infection, which is typical of zoonotic infections (Takahashi et al., 1997;Thomas et al., 2005;Weismann et al., 1994).Infected rodents and wild birds are considered to be reservoirs of BDV infection (Berg et al., 2001), but pets and farm animals represent a greater risk of human BDV infection because of the closer contact with them.Experimentally infected rodents developed persistent BDV infection, which is associated with the presence of the virus in saliva, urine and faeces.Borna disease virus was also found in the excrement of migrating birds.These reports suggest that wild birds could be a reservoir because of the possibility of water or food contamination by infected secretions.The most frequent route of BDV infection transmission is probably via contact with infected saliva or other secretions through the nasal mucosa.The olfactory route of BDV transmission is efficient; the olfactory bulbs in naturally infected horses show oedema and inflammation early in the course of infection.After infection, BDV initially replicates in the neuroreceptor cells of the olfactory epithelium; then, BDV spreads intra-axonally and transsynaptically towards olfactory structures and then preferentially to the limbic system.In the nuclei of infected neurons, aggregates of virus material form Joest-Degen inclusion bodies that are typical of BDV infection.The spread of this virus is not just restricted to the limbic system; during later stages of infection BDV diffuses though the central nervous system and can be detected in oligodendrocytes, astrocytes, Schwann cells and ependymal cells in the peripheral nervous system.In the late stages of BDV infection the virus spreads centrifugally and virus markers can be detected in the peripheral nerves of all tissues.The spread of BDV infection within the CNS is mediated by ribonucleoprotein particles rather than by the enveloped virus.A minimum incubation period of 3 to 4 weeks is estimated for horses and sheep with nonspecific signs such as hyperthermia, anorexia, colic and constipation in the initial phase (Carbone et al., 1987;Gonzales-Dunia et al., 1997, 2000;Gosztonyi & Ludwig, 1984, 2001).

Mechanism of action, persistent infection, and the course of infection
Borna disease virus can cause persistent infection in the central nervous system.Persistent viral infection is characterized as circumstances in which the virus is not cleared but remains in the cells of infected individuals.There are three types of persistent viral infection: latent, chronic and slow infection.The latent type of persistent infection is typical of BDV.Latent infection is associated with a lack of demonstrable viral particles.The reactivation of persistent latent BDV infection can be triggered by several stimuli: super-infection by other infectious agents, trauma, stress factors, medication or other diseases that lead to changes in immune system.After reactivation, it is possible to detect viral structural proteins that interfere with neurotransmitter receptors and their functions.During infection, BDV influences the central nervous system in several ways: firstly, there is a direct influence through the binding of viral proteins with neurotransmitter receptors (monoamine, serotonin and dopamine systems), and secondly there is an indirect influence though the immune response and inflammatory reactions.Both types of mechanisms contribute to neurotransmitter changes and lead to mood, emotional or behaviour changes in infected subjects, and they may also be associated with psychiatric disorders.The severity of clinical symptoms and the course of BDV infection depend on several factors: immune status and response of the host, the age at infection, and genetic vulnerability (predisposition) to the development of psychiatric disorders, among others (Dietrich et al., 1998).The third mechanism by which BDV can possibly influence CNS functions is due to the fact that viral infections are able to influence the human genome.Some human genetic material originates from viruses and viral sequences being assimilated into the host genome.After infection, BDV sequences are integrated into the genome of brain cells.These sequences are not heritable but they can cause mutations that interfere with brain functioning and can contribute to the development of psychiatric disorders (Feschotte, 2010).

Borna disease virus and its influence on the central nervous system
Animal models have been used to study and help explain the influence of BDV infection on the central nervous system.Changes in several neurotransmitter receptors have been described.Research on adult experimental rats has shown how BDV infection causes longterm changes in cognition, emotions and behaviour.Research on neonatal animals has explained how BDV infection influences brain development with subsequent changes in behaviour and cognition.
Changes in the dopamine system of infected adult rats were described, where disturbances were found in the levels of dopamine in the caudate-putamen (D2 receptors) and the nucleus accumbens (D2 and D3 receptors): pre-and postsynaptic sites of dopamine receptors were damaged in the striatum, dopamine reuptake sites were reduced in the caudate-putamen and nucleus accumbens, postsynaptic D2 receptors were reduced in the caudate-putamen and D2 and D3 receptor binding was decreased in the nucleus accumbens.Postsynaptic dopamine receptors (D1 and D2) remain intact in the prefrontal cortex; this imbalance leads to D1 hypersensitivity and neurobehavioural disturbances in BDV-infected animals.Partial dopamine deafferentation and compensatory hyperactivity of the remaining striatal nerve terminals in the nigrostriatal projections are associated with locomotor activity and stimulant sensitivity.Decreased numbers of dopamine D2 and D3 receptors and the remaining numbers of D1 receptors in the striatum are associated with dyskinesias and dystonia (Solbrig et al., 1994(Solbrig et al., , 1996(Solbrig et al., , 2000(Solbrig et al., , 2010)).These changes in dopamine neurotransmission support a connection between BDV and neuropsychiatric disorders such as schizophrenia and addiction and extrapyramidal disorders.In addition to the altered dopamine system the dysfunction of serotonin and noradrenergic systems is present and is associated with other symptoms.There is evidence of a serotonin system dysfunction in the striatum and norepinephrine dysregulation in the prefrontal and anterior cingulate cortex.Changes caused by BDV infection include reduced serotonin transmission, which is associated with autistic disorders and depression.The course of BDV infection in adult rats is more severe in comparison to infected neonate rats, which have a milder symptomatology in BDV infection.These animals developed behaviour and cognitive changes, learning difficulties, increased motor activity, abnormal anxiety responses, deficits in motor coordination and postural stability and impairments in social behaviour, and they also showed abnormally early locomotor development.They also showed altered circadian rhythms and appetite changes.Damage to or dysfunction in the CNS of these animals is associated with direct viral effects on the morphogenesis of the hippocampus and cerebellum (Dietz et al., 2004;Pletnikov et al., 2000Pletnikov et al., , 2001Pletnikov et al., , 2002;;Solbrig et al., 2010).Borna disease virus infection impairs synaptic plasticity, which is important for learning and memory (Volmer et al., 2007).Behaviour disturbances were reported in other experimentally infected animals: altered social and sexual behaviours such as abnormal dominance relationships and a failure to mate were described in primates (tree shrews and rhesus monkeys).Rhesus monkeys were initially hyperactive and later became apathetic (Sprankel et al., 1978;Stitz et al., 1980).Neonatal Borna disease virus infection in rats is associated with the activation of microglia and astrocytes and the loss of neurons in the dentate gyrus in the hippocampus, cortex and cerebellum (Gonzales-Dunia et al., 2000;Pletnikov et al., 2002;de la Torre, 2002).Ovanesov and colleagues were the first to find a significant increase in microglial activation and secondary neuronal loss (Ovanesov et al., 2008).Borna disease virus infection also affects astrocytes, which play an essential role in the maintenance of homeostasis in the CNS.Borna disease virus infection is also associated with impairment in the ability of astrocytes to take up glutamate; this impairment leads to increased levels of extracellular glutamate, the activation of NMDA receptors and an increased calcium influx, and results in neurotoxicity and cell death (Billaud et al., 2000).In the pathogenesis of Borna disease, Borna disease virus infection also plays an important role in the inflammatory reaction (the BDV-specific T-cell response and the activity of CD8+ T-cells cause the destruction of virus-infected neurons).The inflammatory reaction is associated with the symptomatology of infection (Stitz et al., 1995).

Viro-psycho-immunological disease model and schematic (in connection with psychiatric disorders)
The connections between viral infections, stress and immune functions are described and explained by the viro-psycho-immunological disease model.Acute or chronic stress or other factors (other infections, diseases, immunosupressive medications, for example) cause changes in immune functions.These alterations in the immune system are responsible for the reactivation of latent forms of BDV infection in the central nervous system.The reactivation of BDV directly influences CNS function due to the affinity of viral proteins to neurotransmitter receptors and indirectly influences it via the inflammatory reaction, which also leads to changes in neurotransmission.Individuals with a greater vulnerability (predisposition) for developing psychiatric disorders (genetic disposition) can develop psychiatric symptoms (Dietrich et al., 1998) (see Figure 1).

Laboratory diagnosis of BDV infection
The aims of laboratory diagnostics in BDV are the detection of BDV infection and the determination of infectious activity and its severity.Several questions can be asked: is the patient infected by BDV?Is this infection active?How intense or severe is this infection?Is treatment of BDV infection suitable?Antigenaemia indicates an acute and productive phase of infection.During this phase of BDV infection antibodies bind to the viral antigens and form CIC, which are measurable for weeks or months.The frequency and stability of BDV CIC make them easily available screening markers of BDV infection.By using the ELISA method, it is possible to detect viral Ag in plasma.The disadvantage of this method of detection is the very short period of antigenaemia in the acute phase of BDV infection and the high risk of false negative results after the formation of BDV CIC.
Bode and colleagues developed an ELISA (triple ELISA) method that mainly detects BDV CIC but also free BDV antigens and antibodies (Bode et al., 2001).
Several authors used the detection of viral RNA in PBMCs (peripheral blood mononuclear cells) in brain tissue via polymerase chain reaction (PCR) for the diagnosis of BDV infection.However, other researchers did not use this method for the diagnosis of BDV infection and do not recommend RNA detection as the best diagnostic tool.The first reason is due to the possibility of sample contamination during the laboratory procedure (although contamination should not occur when the detection of BDV RNA is performed according to international security instructions).The second reason is because the absence of BDV RNA in samples does not exclude the possibility of BDV infection, since RNA detection is less reliable than the detection of other virus particles (Ag or CIC) because of the low replication rate of this virus and the small amount of RNA in PBMCs.Also, the presence of BDV RNA in the brain does not necessarily reflect an active state of BDV infection (Bode et al., 2001, Sauder & de la Torre, 1998;Thakur et al., 2009;Wolff et al., 2006).Currently, the detection of BDV CIC by ELISA and viral isolation from PBMCs are recommended as the best screening methods of active BDV infection because of their relative stability and frequency.The best indicators (markers) of viral disease are antigen ELISA and CIC ELISA, which also both correlate well with the severity of the BDV infection.
The ELISA method for the detection of BDV CIC was developed by Bode and Ludwig (Bode et al., 2001).Table 1 shows a summary of the diagnostic methods.1. Diagnostic tools for the detection of Borna disease virus infection in humans (Thakur et al., 2009), abbreviations explained in text

Factors influencing BDV positivity
Borna disease virus positivity in psychiatric patients ranges from negative to highly positive.These differences in positivity can be caused by several factors; features of the psychiatric population (age, diagnosis, severity of the psychopathology, immune status), geographical region, differences in specificity and sensitivity of the laboratory methods used and which diagnostic parameters (Ab, Ag, CIC or RNA) are detected and which biological material (brain tissue, cerebrospinal fluid, serum) is used.Other factors include the seasonal occurrence of BDV infection and contact with animals.

BDV positivity and age
Several studies showed a higher rate of BDV positivity in younger individuals.Patti and colleagues investigated BDV CIC positivity in children in Italy; BDV positivity was detected in 57%.The prevalence of BDV infection was found to be significantly greater in children, particularly in the third year of life; then it decreased until 15 years of age, where another increase was observed (Patti et al., 2008).Another study performed by Scholbach and colleagues demonstrated higher rates of BDV Ag and CIC positivity in children.There were two age intervals of peak BDV positivity: the first was a peak at 6 months old and the second was a peak around 2-3 years old.These findings support the possible vertical transmission of BDV infection.Two other possible explanations for the greater prevalence of BDV infection in children than in adults are the less well-developed immune status of children and the fact that children of 2-3 years old are more likely to have greater contact with animals and their secretions, which are associated with a greater risk of BDV transmission (Scholbach et al. 2008).

BDV positivity and the types of psychiatric disorder
Borna disease virus infection was detected in psychiatric patients with various disorders; the majority of these studies detected BDV in patients with affective (bipolar and depressive) disorders and psychotic disorders (schizophrenia and schizoaffective disorders).The highest rate of BDV positivity was found in patients with bipolar and depressive disorders (Ferszt et al., 1999).Rybakowski and colleagues reported significantly higher rates of BDV Ab seropositivity in Polish psychiatric patients with affective-anxiety spectrum disorders and mental retardation than in healthy controls, and the rate of BDV seropositivity was significantly higher in patients with recent disease onset compared to past disease onset (10.2% vs. 1.6%) (Rybakowski et al., 2001).

BDV positivity and psychopathology
Several research groups reported an association between BDV positivity and the type or severity of the psychopathology.In a study performed by Iwahashi and colleagues, a significantly higher rate of BDV seropositivity was detected in schizophrenic patients with a negative symptomatology than in patients with positive symptoms (Iwahashi et al., 1998).
Waltrip and colleagues detected a higher rate of BDV seropositivity in schizophrenic patients with deficit syndrome than in patients with non-deficit syndrome (Waltrip et al., 1997).Bode and colleagues detected a higher rate of BDV seropositivity (more than 30%) in patients with major depressive disorder and a lower rate of positivity (8%) in patients with dysthymia (Bode et al., 1993).Ferszt and colleagues confirmed the higher rate of BDV antigenaemia in patients with affective disorders.The number of previous depressive episodes with symptoms including fatigue and concentration difficulties was positively related to BDV Ag positivity (Ferszt et al., 1999).Bode and colleagues found significantly higher rates of BDV Ag and CIC positivity (including higher values) in severely depressed patients during an acute crisis and lower rates of BDV Ag and CIC positivity (lower rates and lower amounts of Ag and CIC) in patients with moderate depression.The severity of depressive symptoms correlated with the concentration and duration of antigenaemia (Bode et al., 2001).In study reported by Rackova and colleagues, a significantly higher rate of BDV CIC positivity was found in psychiatric patients with a more severe psychopathology than in patients with a milder symptomatology, as measured by psychiatric scales (Rackova et al., 2009).

Regional occurrence of BDV infection
Naturally occuring Borna disease virus infection is still confined to several areas of Central Europe, these regions are endemic for this infection.The presence of BDV infection in animals and humans has been detected not only in European areas but across the world (China, Japan, Israel and several other countries) (Durrwald & Ludwig 1997, Ludwig & Bode 2000).

BDV positivity and laboratory methods used
The high variability found for BDV positivity in humans can be influenced by the laboratory method used since they differ in terms of sensitivity and specificity.Another factor that can influence the detection of BDV infection is the infectious marker used: antibody, antigen, circulating immunocomplex or RNA.
The detection of BDV CIC by ELISA has shown a 10-fold higher incidence of BDV infection than was estimated for BDV Ab positivity by the immunofluorescence method in psychiatric patients with affective disorders.The rates of positivity of BDV Ab by IFA were 11% and 20% in patients with depression and 2% in blood donors vs. CIC positivity of 62%, 52% and 24% in the same groups (Bode et al., 2001).Similar results were reported by Bode and colleagues in 1994, where BDV Ab positivity was detected in 20% of psychiatric patients compared to the higher positivity of BDV Ag in 40-50% of the same psychiatric patients (Bode et al., 1994).Wolff and colleagues did not confirm these results.They analysed plasma samples with a high reactivity in the ELISA assay (high positivity of BDV antigens) by immunoaffinity purification and highly sensitive real-time RT-PCR (polymerase chain reaction): neither method provided any evidence for the presence of viral proteins or nucleic acids (Wolff et al., 2006).

Seasonal occurrence of BDV infection
Borna disease virus infection shows a seasonal prevalence, being more frequent in spring and early summer.Significantly higher numbers of seropositive animals (especially sheep and horses) have been detected during these periods and a higher occurrence of clinical cases was also observed (Durrwald & Ludwig 1997;Ludwig & Bode 2000;Staeheli et al., 2000;Vahlenkamp et al., 2002).The number of animals that become infected and succumb to the disease differs each year, but no correlation has been described between the seasonal occurrence of BDV in animals and humans.

Contact with animals
Several studies have described an association between the prevalence of BDV infection in humans and contact with animals, which are potential reservoirs of BDV infection.
Weismann and colleagues reported a significantly higher rate of BDV antibody positivity (46%) in workers exposed to infected ostriches compared to controls (10%).There was a strong positive correlation between the intensity of exposure and the rate of seropositivity (Weismann et al., 1994).Takahashi and colleagues found a significantly higher rate of BDV seropositivity (from 2.6% to 14.8%) in blood donors from regions containing concentrations of horse farms compared to the BDV seropositivity of 1% in blood donors from other regions (Takahashi et al., 1997).These findings support the possible animal-to-human transmission of BDV infection.In contrast, another study from Bangladesh did not confirm this hypothesis.The authors surveyed horses and their caretakers for BDV antibody positivity and found a BDV positivity of between 25-30% in the horses but none of caretakers were positive for BDV (Khan et al., 2000).Thomas and colleagues measured BDV seroprevalence in agricultural workers in the United Kingdom.The seroprevalence was 2.3% in 1994, 3.1% in 1996 and 2.6% in 1999.People living or working on livestock farms had a higher rate of seroprevalence (2.6%) than those on mixed (2.3%) or arable (1.6%) farms, but this was not statistically significant.Exposure to horses, sheep and cats did not increase the risk of seropositivity.Furthermore, the seropositive people were not more likely to report symptoms of psychiatric morbidity (Thomas et al., 2005).

Other factors and BDV positivity
The activation of persistent BDV infection can be triggered by several stimuli (superinfection by other infectious agents and immunosuppressive medication, among others) that affect the immune system.This hypothesis (claim) is supported by studies reporting a higher rate of BDV positivity in HIV patients (Auwanit et al., 1996;Bode et al., 1992;Cotto et al., 2003).A lower rate of BDV seropositivity (4-8%) was detected in the early stages of HIV infection, which then increased (13.9%) during later stages of this disease (Bode et al., 1992).

BDV and psychiatric disorders
Borna disease virus infection in animals is characterized by various behavioural changes, such as changes in social behaviour, apathy, aggressive behaviour, changes in appetite and weight, and cognitive impairment, amongst others.Because these behavioural disturbances of naturally and experimentally BDV-infected animals resemble psychiatric symptoms and disorders in humans, especially affective disorders, early studies investigated a possible link between patients with these diagnoses and BDV infection.The possible connection between BDV infection and psychiatric disorders is explained in the viro-psycho-immunological model, shown in Figure 1 (Dietrich et al., 1998).
The earliest work that suggested a link between BDV infection and human psychiatric disorders was in 1985.Rott and colleagues examined serum samples from 979 psychiatric patients and 200 healthy volunteers for the presence of BDV Ab by indirect immunofluorescence.Borna disease virus antibodies were found in 16 of the psychiatric patients but none of the healthy volunteers.The patients who had positive serum samples also had a history of affective disorders (Rott et al., 1985).Since 1985, several studies have demonstrated significantly higher rates of BDV infection positivity in psychiatric patients compared to healthy individuals, but several studies did not confirm these results.The rate of BVD positivity in psychiatric patients ranged from negative to highly positive (almost 100% in patients with affective disorders).
Several researchers found a significant association between BDV positivity (levels of BDV Ag and CIC) and severity of the psychopathology (Bode et al., 2001;Ferszt et al., 1999;Rackova et al., 2009) and between BDV seropositivity and negative symptoms in schizophrenia (Iwahashi et al., 1998;Waltrip et al., 1997).Borna disease infection is associated with a chronic course of the disease without full remission and with recurring psychiatric disorders.The reactivation of persistent BDV infection is caused by immune changes and influences neurotransmitter systems and, in vulnerable individuals, it can contribute to the onset of a new phase of a psychiatric disorder and influence its course.

Therapy of BDV infection
Non-pharmacological procedures that could lead to the elimination of BDV infection were used in the past.The separation of infected animals and improvements in hygiene were recommended, but these procedures only decreased the risk of BDV infection spreading but did not eliminate the infection.Vaccination with killed vaccines and then live vaccines became available in the twentieth century, but both types of vaccine were shown to be ineffective against the Borna disease virus.Vaccination against persistent viral infection in the central nervous system cannot be recommended (Ludwig & Bode, 2000).Antiviral medications were also tested for the treatment of BDV infection.Hexamethylenetetramine was used for the treatment of horses, and then later hexamine (Ludwig & Bode, 2000), ribavirine (Jordan et al., 1999), interferon alpha or beta (Hallensleben et al., 1999;Staeheli et al., 2001) and cyclosporine A (Stitz et al., 1989), but no effect was found on BDV infection.Some of these preparations showed a decrease in BDV replication in vitro, but they did not show efficacy in vivo.
Bechter and colleagues described several cases where an improvement in the psychic status of psychiatric patients with schizophrenia and depression was shown after cerebrospinal fluid filtration (Bechter et al., 2000).Currently, amantadine is used for the treatment of BDV infection.Amantadine is a noncompetitive N-methyl D-aspartate-type glutamate receptor antagonist that also binds to nicotinic acetylcholine receptors.Amantadine inhibits BDV replication.Initially, amantadine-sulphate was used in the prophylaxis of influenza A (Kandel et al., 2001), in the treatment of Parkinson's disease (Brenner et al., 1989), hepatitis C (Adinolfi et al., 2003) and cocaine dependence (Kampman et al., 2000).A mild antidepressant effect of amantadine was found in a double-blind study when compared to amitriptyline and a placebo in patients with depression without BDV infection (Vale et al., 1971).Amantadine is a virostatic agent effective in the treatment of BDV infection, since it influences BDV replication in vivo.The use of amantadine was first described in two case reports in 1991 and 1997 in the treatment of two psychiatric patients with bipolar affective disorder, where improvements in the psychiatric status of both patients were associated with the decrease and elimination of BDV positivity (Bode et al., 1997).Later, several open trials were performed to test the efficacy of this drug in psychiatric patients and amantadine-sulphate was proven to be effective in the treatment of depression in BDV positive patients (Dietrich et al., 2000;Ferszt et al., 1999); the improvement in the depressive symptomatology was correlated with a decrease in BDV infection positivity.The patients were treated with 200 mg of amantadine per day for 12 weeks, and the majority showed significant improvements (clinical response) within the first three weeks (Dietrich et al., 2000).Ohlmeier and colleagues published the results of a double-blind trial that evaluated the effect of amantadine in the treatment of patients with the manic phase of bipolar disorder and amantadine reduced manic symptoms in BDV-infected bipolar patients (Ohlmeier et al., 2008).

Aims
Because of the marked differences in BDV positivity in patients with different psychiatric disorders, we decided to review available studies on the detection of BDV infection in psychiatric disorders and attempt to explain these differences.

Method
We undertook a systematic review to determine the rate of positivity of Borna disease virus infection in psychiatric patients.

Data sources
PubMed, Medline, Journals@Ovid Full Text, Evidence-Based Medicine and the Cochrane database  were searched using the keywords 'Borna disease virus', 'psychiatric disorders' and 'prevalence' in conjunction with each of the following organic disorders, addictions, psychotic disorders, affective disorders and anxiety disorders.Due to variations in the laboratory methods, materials and study groups used, a meta-analysis was not possible.

Study selection
The inclusion criteria were studies published from 1985 to January 2011, the study participants: psychiatric patients, investigation: antibody, antigen, circulating immunocomplexes or viral RNA, material: serum, peripheral blood cells, cerebrospinal fluid, brain tissue, and outcomes: prevalence data.

Results
We  Between 1985 and1993 the presence of BDV infection in groups of patients with various neuropsychiatric disorders was detected using IFA for BDV Ab.The majority of the studies identified found BDV in patients with schizophrenia and affective disorders.

Borna disease virus infection and organic disorders
Borna disease virus infection causes cognitive impairment (memory and learning impairment) in infected experimental animals.This impairment is linked to cholinergic loss in the forebrain and is associated with the pathology of Alzheimer's disease (Solbrig et al., 2010).Only a few studies reported BDV positivity in organic disorders, especially dementia.We identified five studies that detected the presence of BDV infection in patients with vascular and Alzheimer's dementia; the rate of BDV positivity ranged from 0% to 1% (see Table2).

Borna disease virus infection and addiction
Borna disease virus influences dopaminergic neurotransmission, including the reward system, which plays a crucial role in the pathology of addictive disorders.We found four studies that detected BDV infection in addicted patients.Only one study detected BDV positivity in addicted patients at the start of detoxification and at the end of treatment.Rackova and colleagues demonstrated BDV CIC positivity in 36.6% and 42.9% of patients with alcohol and drug abuse; the positivity of BDV infection was not significantly higher than in healthy individuals (Rackova et al., 2010).The rate of BDV positivity ranged from negative results to positivity in 42.9% (see Table 3).

Borna disease virus infection and psychotic disorders (schizophrenia, schizoaffective disorders)
Borna disease virus infection causes changes in dopaminergic neurotransmission in adult experimental animals and influences brain development in infected neonatal animals; these changes are important in the aetiology of psychotic disorders in humans.We identified 29 studies that detected BDV infection in patients with schizophrenia or schizoaffective disorders, 13 studies that detected BDV RNA, 9 that detected BDV Ab and 7 that detected both infectious markers: Ab and RNA.The rate of BDV infection positivity ranged from 0% to 63.6% in schizophrenic patients and from 0% to 13.86% in control groups of healthy individuals (see Table 4 and 5).

Borna disease virus infection and affective disorders (depressive disorder, bipolar affective disorders)
The first studies to detect BDV positivity in psychiatric patients reported higher rates of BDV positivity in patients with recurring affective disorders.We found 19 studies that described BDV positivity in patients with affective disorders (depressive and bipolar disorders, dysthymia), 8 studies that examined BDV RNA, 7 studies that detected BDV Ab, 2 studies that detected BDV RNA and Ag and 1 study that detected BDV CIC, Ag and Ab.The rate of BDV infection positivity ranged between 0% and 90% in patients with affective disorders.The highest positivity rate was reported in patients with bipolar depression and severe psychopathology.The rate of BDV infection was from 0% to 32% in the control group of healthy individuals (see Table 6).

Borna disease virus and unspecified psychiatric disorders
Several studies examined psychiatric patients with various diagnoses and it was not possible to divide these groups according the diagnosis.We found 17 studies that detected BDV infection in unspecified psychiatric (or neuropsychiatric) patients, 5 studies that detected BDV Ab, 1 study that detected Ab and Ag, 1 that only detected Ag, 7 studies that detected BDV RNA, 2 that detected BDV CIC and 1 study detected BDV Ab and RNA.The rate of BDV positivity ranged between 0% and 66.7% in psychiatric patients and between 0% and 37.3% in healthy individuals (see Table 8).

Borna diasease virus and other disorders
The influence of BDV infection on other disorders was also studied, particularly on neurological and infectious diseases.Prudlo and colleagues did not find any increase in BDV Ab positivity in neurologic patients (with amyotrophic lateral sclerosis) compared to surgical patients (Prudlo et al., 2002).Li and colleagues detected BDV RNA in patients with viral encephalitis, but no BDV positivity was found in other neurological disorders (Li et al., 2009;Salvatore et al., 1997).Flower and colleagues reported higher rates of BDV CIC positivity in multitransfused patients (Flower et al., 2008), and BDV RNA and Ag were detected in the cerebrospinal fluid of patients with multiple sclerosis (Deuschle et al., 1998).

Conclusions
The Borna disease virus is a neurotropic RNA virus belonging to the family Bornaviridae, order Mononegavirales, which has a high affinity for the central nervous system, especially for limbic structures.The Borna disease virus causes an infection in birds and mammals including humans.Some infected animals develop symptoms which are very similar to human psychiatric disorders (cognitive impairment, behavioural changes, changes in social behaviour, appetite, sleeping...).Because of this similarity BDV has begun to be associated with several psychiatric disorders, especially with mood and psychotic disorders.The association of BDV and psychiatric disorders is explained by the viro-psycho-immunological disease model.A BDV persistent infection of the central nervous system is activated by changes in the immune system, an activated BDV infection influences neurotransmitter functions and contributes to the onset of psychiatric disorders in vulnerable individuals (with a genetic predisposition).
In the 1980s the presence of a BDV infection was found in patients with neuropsychiatric disorders.Since the 1980s many studies have detected a BDV infection in patients with psychiatric disorders (the majority of them tested BDV positive in affective disorders and schizophrenia) and in patients with neurological disorders, HIV infection and others.The positivity of a BDV infection ranges from negative results to very high positivity rates, more than 90% in patients with affective disorders in comparison with BDV positivity between 0%-40% in healthy individuals.The Borna disease virus positivity in psychiatric patients ranges from negative to highly positive.These differences in positivity can be caused by several factors; features of the psychiatric population (age, diagnosis, severity of the psychopathology, immune status), geographical region, differences in specificity and sensitivity of the laboratory methods used and others.The significantly higher rates of BDV positivity were detected in younger individuals, especially in children.Several research groups reported an association between higher BDV positivity and the severity of the psychopathology.The high variability of BDV positivity in humans can be influenced by the laboratory method used.The detection of BDV CIC by ELISA has shown a 10-fold higher incidence of BDV infection than was estimated for BDV Ab positivity by the immunofluorescence method.

Arguments supporting the association of BDV infection with psychiatric disorders include
Characteristics of the Borna disease virus -its ability to infect and spread through the central nervous system, causing persistent infection and its activation, have led to the connection with psychiatric disorders.An experimental BDV infection shows the influence of BDV on neurotransmitter receptors which lead to behavioural changes, cognitive impairment and neurological disturbances.Many studies have detected the significantly higher BDV positivity in psychiatric patients in comparison with healthy individuals.The Borna disease virus was isolated from PBMCs in plasma, cerebrospinal fluid and brain tissue.Higher levels of BDV antigens and circulating immunocomplexes have been found in acutely depressed patients than in patients with mild depression.Levels of BDV antigens and circulating immunocoplexes correlate with the severity of psychiatric symptoms.Depressed patients who were treated with virostatic medication -amantadine, improved their psychopathology significantly and more quickly.The improvement of depression symptoms correlates with the decrease or disappearance of BDV positivity (levels of antigens).

Arguments against the BDV role in ethiopathology in psychiatric disorders include
Many studies have failed to detect BDV infection in psychiatric patients or did not find significant differences in BDV positivity between psychiatric patients and healthy individuals.Some researchers detected high BDV positivity in healthy individuals and considered BDV infection to be a normal part of human life without any influence on health.
There are reports about the effect of antiviral treatment with amantadine in BDV positive patients with depression disorders.The improvement of depression symptomatology during this treatment could be explained by the antidepressive effect of amantadine.Some studies did not prove the virostatic effect of amantadine in the treatment a BDV infection.
Several studies criticize the unreliability of some laboratory methods, in these studies BDV was not isolated from BDV CIC positive samples.
What can we conclude?The role of BDV in psychiatric disorders is still unclear.But very probably this infection can be involved in the onset and the course of psychiatric disorders in a determinate subpopulation of psychiatric patients.German and Italian researchers examined the presence of BDV infection in children.These studies are crucial to obtaining information about the possible influence of neurotropic viruses on brain development.A BDV infection in early life can contribute to the onset of psychiatric disorders in adults in predisposed individuals.For future research, which is important for answering still unclear questions about the Borna disease virus we need more and larger studies testing comparable patient groups and using comparable laboratory methods.We are also missing more double-blind studies with amantadine in BDV positive psychiatric patients.Studies testing for a BDV infection from childhood to adulthood are necessary to answer the question: What is first?: BDV infection and secondly the development of psychiatric disorders or first psychiatric disorders which secondarily leads to several changes in the immune system and then the activation of the BDV infection.

Fig. 1 .
Fig.1.Viro-psycho-immunological disease model of BDV infection and psychiatric disorders adapted fromDietrich et al., 1998 The absence of BDV Ab in the serum does not mean the BDV infection result is negative; antibodies bind antigens and form circulating immunocomplexes.

Table
identified 53 studies published from 1985 to July 2011 describing Borna disease virus positivity in psychiatric patients (psychiatric disorders, affective and psychotic disorders, addictions, neurotic disorders), 29 studies investigated viral antibodies in the serum, 23 BDV RNA in peripheral blood cells, 4 detected circulating immunocomplexes, viral antigens and antibodies in serum, 2 detected BDV RNA in cerebrospinal fluid (CFS), 5 detected BDV RNA in brain tissue and 8 studies detected BDV RNA and Ab.The prevalence of BDV infection in these studies ranged from 0 to 100% for patients with bipolar affective disorder.

Table 2 .
Studies that detected BDV infection in patients with organic disorders

Table 3 .
Studies that detected BDV infection in patients with alcohol and drug addictions

Table 4 .
Studies that detected BDV Ab in patients with schizophrenic disorders

Table 5 .
Studies that detected BDV RNA in patients with schizophrenic disorders

Table 6 .
Studies that detected BDV infection in patients with affective disorders3.5 Borna disease virus infection and neurotic disordersOnly a few studies reported BDV positivity in neurotic disorders: we identified 10 studies and 7 of them examined patients with chronic fatigue syndrome.Six studies detected BDV Ab, two detected BDV RNA and one detected both RNA and Ab markers.The rate of BDV positivity ranged from 0% to 34% in psychiatric patients and between 0% and 5.9% in healthy controls (see Table7).

Table 7 .
Studies that detected BDV infection in patients with anxiety disorders and OCD

Table 8 .
Studies that detected BDV infection in psychiatric patients