[Emerging Infectious Diseases * Volume 3 * Number 3 * July - September 1997]

Synopses

Borna Disease Virus Infection in Animals and Humans

Jrgen A. Richt,* Isolde Pfeuffer,* Matthias Christ,* Knut Frese, Karl
Bechter, and Sibylle Herzog*
*Institut fr Virologie, Giessen, Germany; Institut fr
Veterinr-Pathologie, Giessen, Germany; and Universitt Ulm, Gnzburg,
Germany
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      The geographic distribution and host range of Borna disease
      (BD), a fatal neurologic disease of horses and sheep, are
      larger than previously thought. The etiologic agent, Borna
      disease virus (BDV), has been identified as an enveloped
      nonsegmented negative-strand RNA virus with unique properties
      of replication. Data indicate a high degree of genetic
      stability of BDV in its natural host, the horse. Studies in the
      Lewis rat have shown that BDV replication does not directly
      influence vital functions; rather, the disease is caused by a
      virus-induced T-cell mediated immune reaction. Because
      antibodies reactive with BDV have been found in the sera of
      patients with neuropsychiatric disorders, this review examines
      the possible link between BDV and such disorders.
      Seroepidemiologic and cerebrospinal fluid investigations of
      psychiatric patients suggest a causal role of BDV infection in
      human psychiatric disorders. In diagnostically unselected
      psychiatric patients, the distribution of psychiatric disorders
      was found to be similar in BDV seropositive and seronegative
      patients. In addition, BDV-seropositive neurologic patients
      became ill with lymphocytic meningoencephalitis. In contrast to
      others, we found no evidence is reported for BDV RNA, BDV
      antigens, or infectious BDV in peripheral blood cells of
      psychiatric patients.

Borna disease (BD), first described more than 200 years ago in southern
Germany as a fatal neurologic disease of horses and sheep, owes its name to
the town Borna in Saxony, Germany, where a large number of horses died
during an epidemic in 1885. The virus etiology of BD was proven in the
early 1900s when Zwick and coworkers (1) in Giessen, Germany, successfully
transmitted brain homogenates from infected horses to experimental animals.
Other milestones in BD-related research were the demonstration of virus
growth in cell cultures (2-4); the finding that the pathogenesis of BD is
caused by a T-cell-dependent immune mechanism (5-8); and most recently, the
molecular characterization of the etiologic agent of BD, the highly
neurotropic Borna disease virus (BDV) (9-17).

BD is characterized by a disseminated nonpurulent meningoencephalomyelitis
with infiltration of mononuclear cells (1,8,18,19) and a predilection for
the gray matter of the cerebral hemispheres and the brain stem (8,19). In
neurons, sometimes in glia cells, acidophilic intranuclear inclusion
bodies, called Joest-Degen inclusion bodies, are occasionally found. BD
occurs sporadically in Germany and Switzerland; its presence in other
countries has not yet been substantiated. Natural infections in other
Equidae, ruminants, rabbits, cats, and ostriches have also been described
(19-21).

This review discusses the etiology of Borna disease, the natural and
experimental infection in various animal species, the pathogenesis of the
disease in the experimental rat model, the genetic stability of BDV, and
the possible link between BDV or a similar agent and human neuropsychiatric
disorders.

Etiology

The etiologic agent of BD, BDV, has been recently characterized as an
enveloped, nonsegmented, negative-stranded RNA virus with a genomic size of
approximately 9 kb and a nuclear site for replication and transcription
(14-17). The genomic organization is similar to that of members of the
Mononegavirales order; therefore, BDV is the prototype of the new family
Bornaviridae within this order. The Mononegavirales also include
Filoviridae (e.g., Marburg and Ebola viruses), Paramyxoviridae (e.g.,
mumps, measles virus), and Rhabdoviridae (e.g., rabies, vesicular
stomatitis virus). Six major open reading frames (ORFs)(I,II,III, IV,V,x1)
are predicted in the genome sequence (16,17). Only five ORFs correspond to
previously identified proteins with molecular weights of 10 (ORF x1: p10
BDV gene; 22), 18 (ORF III: gp18 BDV gene; 23), 24 (ORF II: p24 BDV gene;
24), 38/39 (ORF I: p38 BDV gene; 25), and 94 (ORF IV: p57 BDV gene; 26) or
84 (ORF IV: p57 BDV gene; 27, Richt et al., unpub. obs.) kDa.

Natural and Experimental Infection

Natural Infection

Extensive epizootiologic studies in horses have shown that BD is rare but
occurs all over Germany, extending beyond the classic disease-endemic
regions (28-30). Furthermore, BDV-specific antibodies were detected in
horses in several European countries, Israel (28,29), Japan (31), Iran
(32), and the United States (33). Since BDV-specific antibodies are
frequently found in clinically healthy horses (20,28,29), natural
infections in horses seem to remain subclinical in most cases. Unknown
exogenous and endogenous factors might influence the genesis of the disease
(20).

In addition to its predominant natural host, the horse, other Equidae,
sheep, cattle, rabbits, goats, deer, alpacas, llamas, cats, pigmy
hippopotamus, sloth, vari monkeys (memur variegatus), and ostriches have
become naturally infected with BDV (8,19,20,34,35). In sheep flocks,
clinical BD can affect large numbers of animals; however, in horse stables,
usually only a few animals show clinical signs. The virus is assumed to be
transmitted through salival, nasal, or conjunctival secretions because
BDV-specific RNA has been found in these secretions (20,28,29,36). Animals
become infected by direct contact with these secretions or by exposure to
contaminated food or water. A minimum incubation period of 4 weeks is
estimated for horses and sheep with nonspecific signs such as hyperthermia,
anorexia, colic, and constipation in the initial phase of the disease.
During the acute phase of disease, neurologic signs such as ataxia,
depression, circular movement, standing in awkward positions, collapsing,
running into obstacles, and paralysis, result from nonpurulent
meningoencephalomyelitis. Clinical illness usually lasts 1 to 3 weeks, and
death rates for diseased horses are 80% to 100% (18-20,37). In surviving
animals, recurrent episodes are possible, especially after stress (18).
Clinical manifestations, however, may vary among individual animals and
various species.

BD tends to occur in spring and early summer and is more frequent in some
years than in others; therefore, arthropods have been discussed as a
potential vector. BDV, however, has never been isolated from insects in
Europe. In the Near East, ticks have been associated with transmission of
an equine encephalomyelitis similar to BD (38). A definite virus reservoir
for BDV has not been found; various rodents most likely represent such a
reservoir. In addition, since many seropositive horses with subclinical
infections have infectious virus or BDV-specific RNA in various secretions,
they can be potential sources of infection for other animals and humans
(29,36).

Experimental Infection

BDV can be experimentally transmitted to a wide variety of animal species,
from chickens to nonhuman primates (1,8,19,20,37). Incubation periods,
clinical signs, and severity of the disease depend on the animal species
and the virus variants. Rabbits, Lewis rats, and guinea pigs are highly
susceptible, whereas chickens, monkeys, cattle, and tree shrews (Tupaia
glis;39) are less susceptible (8,19). Some animals (hamsters, black-hooded
rats, mice, ferrets, pigeons, and dogs), however, do not develop disease
despite being persistently infected with BDV (1,8,18,19). The
pathohistologic picture after experimental infections resembles that of
natural infections. Most experimentally infected animals develop
perivascular and parenchymal central nervous system (CNS) infiltrations and
have infectious virus in the brain tissue. Frequently, the clinical picture
in experimentally infected animals does not differ from that of
spontaneously infected natural hosts.

The result of BDV infection in rats depends on the inbred rat strain and
the virus isolate used for infection. Black-hooded rats do not show
clinical signs after BDV infection with BDV isolates despite persistent
virus infection and mononuclear infiltration of the CNS (40). Lewis rats,
however, are highly susceptible and are therefore used extensively for
studies of BD pathogenesis (see below; 5,8,19). When infected with various
BDV variants, Lewis rats exhibited clinical manifestations such as
behavioral disorders, paralytic disease, or obesity, in addition to
fertility problems (8,19), which indicates that BDV can form virus variants
with different biologic properties.

Pathogenesis

BDV is a highly neurotropic agent that gains access to the CNS, probably by
intraaxonal migration through the olfactory nerve or nerve endings in the
oropharyngeal and intestinal regions (18,41). Virus spreads throughout the
CNS by intraaxonal transport and centrifugally into the peripheral nerves.
Antibody titers in naturally infected animals, such as in horses with
clinical BD, are relatively low and are found in sera and cerebrospinal
fluid (CSF) (19,20,28,29). Whether BDV-specific antibodies are neutralizing
is not certain (5,42); the coexistence of BDV-specific antibodies and
infectious virus in the CSF contradicts this assumption (43).

Extensive studies in the Lewis rat showed that the immune response to viral
antigens after BDV infection does not elicit protective immunity but rather
an immunopathologic reaction in which T cells play an important role
(5,44). After adult rats are infected intracerebrally or intranasally,
productive virus replication is found in the entire CNS. Once introduced
into the rat's CNS, BDV usually causes a persistent infection with
continuous productive replication in the brain and spinal cord (5,8). In
immunocompetent animals, no infectivity was found in extraneural tissues at
any stage of infection (5,45). In newborn animals, in contrast, the virus
spreads throughout the whole organism; BDV-specific antigen was found in
parenchymal cells of numerous organs, and infectious virus was found in
excretions (45). The age of rats at the time of infection affects the
capacity of host cells to support virus replication and, therefore, to
control the spread of BDV.

The reason for the variation in virus replication is still unclear, and
differences in the host's immunocompetence cannot be the only factor, since
in adult immunosuppressed rats the virus is strictly neurotropic (45). All
immunocompetent animals develop antibodies to BDV-specific antigens, and
these antibodies coexist with infectious virus in the CNS (5). The outcome
of BDV infections in rats depends on the rat strain and the passage history
of the virus (see above; 19,40).

Despite productive virus replication in the CNS, immunocompromised animals
(immunologically incompetent newborn and athymic rats) and immunosuppressed
(by cyclophosphamide or cyclosporine A) animals do not become ill with BD
or encephalitis, as do immunocompetent animals (5-8,29,44,46,47). The
hypothesis of a virus-induced cell-mediated immunopathologic basis of BD
was confirmed in adoptive transfer experiments in which spleen cells from
infected rats were transferred into immunosuppressed virus carriers (5).
Recipient rats developed BDV-specific antibodies, encephalitis and disease
(5). The critical role of T cells in the pathogenesis of BD was
demonstrated in adoptive transfer experiments: transfer of BDV-specific
major histocompatibility complex class II-restricted T(subscript H)1 cell lines into
drug-tolerized virus carriers resulted in acute meningoencephalitis and
disease (Table 1; 6-8,29,47).

Table 1. Adoptive transfer of Borna disease virus (BDV)
T(subscript H)1 cells before and after BDV infection
---------------------------------------------
                      Transfer of  Clinical
 Group   CY           T(subscript  BD
         treatment    H)1-cells    
 --------------------------------------------
 Control     yes        day +10       no
 BDV         yes           no         no
 BDV          no        day +10       yes
 BDV         yes         day +3       yes
 BDV         yes        day +10       yes

 BDV          no      day -25 and     no
                           -5
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Four-week-old Lewis rats were inoculated intracerebrally with BDV; some
rats were treated with cyclophosphamide (CY) (150 mg/kg) for
immunosuppression 24 hours later. Animals were intravenously injected with
>2x10(superscript 6) BDV-specific T(subscript H)1 cells on the indicated days,
either before (-) or after (+) infection with BDV.
---------------------------------------------------------------------------

Consequently, conventional immunization with inactivated virus preparations
or purified virus-specific antigens did not confer protective immunity
against clinical BD, despite detectable levels of virus-specific antibodies
after immunization (25; Richt, unpub. data). However, when viable
BDV-specific T(subscript H)1 cells were passively transferred into naive rats before
virus infection, animals were protected against clinical BD after challenge
(Table 1; 29,47,48, Schmeel et al., unpub. obs.). In contrast, injection of
the same viable BDV-specific T(subscript H)1 cells after BDV-infection induced clinical
disease faster in BDV-uninfected animals than in BDV-infected control
animals (29,47,48, Schmeel et al., unpub. obs.). Thus, in BDV infections of
the CNS with immunopathologic lesions, the same virus-specific immune cells
that cause disease can also prevent disease, depending on the dynamics of
virus replication at the time of the T-cell response.

Several reports of the pathogenesis of BD in BDV-infected animals indicate
that the pathogenesis of BD in naturally and experimentally infected
animals is also immunopathologic since the pathohistologic picture,
including CNS-infiltrating cells in naturally BDV-infected animals,
resembles that of experimentally infected rats (19); and immunosuppression
or splenectomy of experimentally BDV-infected rabbits (49) and splenectomy
of BDV-infected monkeys (Macaca mulatta; 50) affected the severity of
clinical signs or prevented disease.

BDV Isolates from Naturally Infected Horses

To study the genetic variability of BDV, virus isolates were derived from
three clinically ill horses in Germany (Lower Saxony, Rhineland-Palatina,
Bavaria), euthanized between 1989 and 1995. BDV-specific antibodies were
detected in the serum, and infectious virus, viral proteins, and viral RNA
were detected in the CNS of all three horses (Pfeuffer et al., unpub. obs.,
51). Molecular analysis of five different ORFs was performed. The coding
regions of ORFs I, II, III, IV, and x1 were amplified by the reverse
transcriptase-polymerase chain reaction (RT-PCR) technique with total RNA
isolated from the CNS (36); the amplified cDNA was cloned and sequenced.
The sequences of the respective ORFs were aligned and compared with viral
sequences derived from a cell-culture-adapted virus strain, BDV strain V
(14), which was isolated in 1929 from a horse with clinical BD. In the
analyzed clones, two deletions resembling post-transcriptionally spliced
mRNA species were identified (Pfeuffer et al., unpub. obs., 51). Similarly,
two introns were recently identified when cell-culture-adapted BDV isolates
were molecularly analyzed (16,17).

The degree of homology between the analyzed BDV isolates and the reference
BDV strain V isolated more than 60 years ago (14) for the p24 BDV gene is
summarized in Table 2. There is a maximal divergence of 2.0% at the
nucleotide level and 2.5% at the amino acid level (Table 2). These data
agree with recently published observations showing a high de
gree of
homology within the p24 BDV gene of wild-type and cell-adapted viruses
(52,53). When the other four ORFs were similarly analyzed, a homology of
>95% at the nucleotide level and >97% at the amino acid level was found
(Pfeuffer et al., unpub. obs., 51), indicating that in addition to the
previously analyzed p24 (ORF II; 52,53) and p38 (ORF I; 53) genes, the
other BDV-ORFs (ORFs III, IV, x1) also exhibit high genetic stability
within one host species, the horse (Pfeuffer et al., unpub. obs., 51).

Table 2. Percentage of p24 Borna disease virus gene sequence homology

           Strain V    Horse     Horse      Horse
                        #215     #640       #2300
 ----------------------------------------------------
 Strain V               99.0      97.5      99.0
 Horse#215    98.7                98.5      100.0
 Horse#640    98.0      98.2                98.5
 Horse#2300   98.8      98.7      98.2
---------------------------------------------------------------------------
Numbers above diagonal are %homology at the amino acid level; numbers below
diagonal represent the %homology among nucleotides. Strain V: isolated 1929
from a horse with clinical BD from Lower Saxony. Horse #215: isolated 1989
from a horse with clinical BD from Rhineland Palatine. Horse #640: isolated
1993 from a horse with clinical BD from Lower Saxony. Horse #2300: isolated
1995 from a horse with clinical BD from Bavaria
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 Figure 1 A.[Image]
 Figure 1 B. [Image]

Figures Not Available in Ascii


 Figure 1. Immunohistologic analysis of consecutive
 brain sections from the cerebral cortex of horse
 #215 with A) the monoclonal antibody Bo18, specific for
 the p38/p39 BDV-protein and B) a rabbit monospecific
 serum, specific for the p24 BDV-protein. Immunoreactive
 neurons are only detected with the p24-specific
 antiserum. Papa-nicolaou-counterstain,
 x130.

Nevertheless, antigenic variations of BDV wild-type isolates were found
when brain material from approximately 50 horses with BDV infections were
screened by immunocytochemistry and immunoblot assays with BDV-specific
monoclonal and monospecific antibodies (51). Two isolates (horse #215,
horse #2,300) showed a remarkable difference in antigenicity (Figure 1A,B).
These isolates were not recognized by the p38 specific monoclonal antibody
(mAb) Bo18 (25,51; Figure 1A). Polyclonal reconvalescent (51) and
monospecific p38 antiserum or monospecific p24 antiserum (Figure 1B),
however, reacted with BDV-specific proteins from both isolates in
immunocytochemistry assays (Figure 1B) and immunoblots (51), indicating
that the p38 and p24 BDV-proteins were synthesized. The recognition site
for the mAb Bo18 within the p38 BDV-protein, however, was altered. To
obtain detailed information on the location of these changes, we cloned and
analyzed the p38 BDV gene of two horse isolates. Sequence analysis showed
several nucleotide mutations within the respective p38 BDV genes; only one,
however, led to an amino acid replacement (51). The amino acid replacements
were adjacent to each other within the N-terminal part of the p38 protein,
indicating that they are within the epitope of mAb Bo18 (51).

BDV and Human Neuropsychiatric Disorders

BDV's wide host range and its association with behavioral abnormalities in
many species such as rhesus macaques (1,50), tree shrews (Tupaia glis; 39),
and rats (5,8) suggest that BDV may be involved in human neuropsychiatric
illnesses. Seroepidemiologic studies have demonstrated BDV-specific
antibodies in sera of psychiatric and neurologic patients in Germany,
Japan, and the United States (13,51,54-71). Recent studies with many
patients showed a significantly higher prevalence of BDV serum antibodies
among hospitalized psychiatric patients and a moderately higher
seroprevalence among neurologic patients than among controls (13,56,60,68).
Investigations of CSFs of BDV-seropositive patients acutely ill with
psychiatric disorders (mainly schizophrenia and affective psychoses) showed
intrathecally synthesized BDV-specific immunoglobulin (Ig) G in 25% of the
patients (59).

A slightly higher level of seroprevalence was found among psychiatric
patients in an area with high incidence of natural BD in horses (the
BD-endemic area around Gnzburg in Southern Germany) than among psychiatric
patients in an area without endemic BD in the United States (Philadelphia
area) (6.8% vs. 4.5%). The incidence in healthy controls was considerably
lower in the United States (0% to 1%) than in the Gnzburg area (3% to
3.5%; 55). These studies indicated a high number of clinically
inconspicuous BDV-seropositive persons in the BD-endemic area. Therefore,
as is the case in horses, inapparent BDV infections may be very common in
humans (28,29,55).

An epidemiologic investigation of BDV seroprevalence in a large number (n =
4,673) of surgical (control), neurologic, and psychiatric patients showed
continuous age-dependent increase in seroprevalence in controls and
neurologic patients (Figure 2; 55); the dynamics of seropositivity were
different in psychiatric patients. A twofold higher seroprevalence in young
neurologic patients paralleled a higher prevalence in patients with acute
or chronic meningoencephalitis and psychiatric disorders (56,71).
Seroprevalence among the youngest quartile (aged 17-30 years) of
psychiatric patients was 600% higher than among surgical patients (Figure
2;55), which suggests a role of BDV infections in the pathogenesis of
psychiatric disorders (psychiatric disorders frequently begin in younger
persons). 

Figure 2. [Figure not available in ASCII.]
Borna disease virus seroprevalence among different age groups.
Psych=psychiatric patients; Neuro=neurologic patients; Surg=surgical
(control) patients. Reproduced with permission from Habilitationsschrift
der Fakultt fr Klinische Medizin der Universitt Ulm (55).

Initial investigations indicated a correlation between BDV-specific
antibodies and affective psychoses (54,67). In a random group of
psychiatric patients, however, we found a similar distribution of
psychiatric disorders among BDV-seropositive and BDV-seronegative patients
(multiple psychiatric disorders in one patient and brain atrophy were
higher among the BDV-seropositive patients [71]). An unequivocal
classification to a definite psychiatric disease is not possible
at the moment.

If a causal relationship exists between BDV infections in naturally
infected animals and BDV seropositivity in humans, identifying patients
with symptoms and disorder progression similar to those of BDV-infected
animals (paresis, weakness of legs, excitability, or depression) would be
important. Recently, we identified a BDV-seropositive neurologic patient
who exhibited abnormal body postures, depressive and apathetic behavior,
and paresis (57). The patient worked at his father's farm, where one of
four horses and four of 10 sheep had antibodies specific for BDV (57). The
patient initially had a cerebral seizure and later neurologic and
psychiatric symptoms resembling those of BD in animals and not typical of
other known human encephalitic disorders, possibly implicating an acute,
mild BDV encephalitis. Although isolation and molecular analysis of the
respective viruses did not show an infectious chain, BDV transmission from
animals to humans seems possible.

In addition, to study possible horizontal transmission of BDV within
families, we reviewed the medical history of approximately 200
BDV-seropositive neuropsychiatric patients for coincident onset of mental
or neurologic disorders during 1 year within each family. Of four families
meeting the criteria, two were studied in detail; both had neuropsychiatric
illness and BDV seropositivity in the primary patient and also in a second
family member in the household (58). In two additional families, only the
primary patients, not the other psychiatrically ill family members, showed
BDV seropositivity (58). In summary, humans can be infected by BDV or a
similar agent; a correlation between BDV infection in animals and humans
exists; and BDV infections seem to contribute to or initiate various
psychiatric syndromes.

To verify the etiologic role of BDV in human neuropsychiatric disorders, we
tested specimens from the CSF of more than 40 BDV-seropositive
neuropsychiatric patients for isolates of BDV or a related agent. The CSFs
were spread onto cell cultures or injected intracerebrally into rabbits.
BDV or a related agent was detected in the CSF of three neuropsychiatric
patients (two patients with acute lymphocytic meningoencephalitis, one
patient with acute schizophrenic psychosis; 67). The CSF of the
schizophrenic patient showed high amounts of BDV-specific IgG, which became
normal 5 years later when the CSF of this patient was again analyzed during
myelography before lumbar surgery; at that time the patient was mentally
healthy (60). Injection of the respective CSFs onto cell cultures showed
expression of BDV-specific antigens (by BDV-specific staining pattern in an
indirect immunofluorescence assay). Expression of BDV antigen, however, was
lost, when these cells were passaged (13,67). In addition, all three
cerebrospinal fluids induced BDV-specific antibodies in inoculated animals;
however, neither BDV replication nor BDV-specific antigen was detected in
the CNS of any of these animals (13,67).

Most recently, de la Torre and coworkers (72) detected both BDV-specific
antigen and BDV-RNA in four autopsied human brains with hippocampal
sclerosis and astrocytosis, which confirms previous studies that BDV can
infect human brain tissue; whether BDV infection contributes to the
pathophysiology of human neuropsychiatric disorders, however, warrants
further investigation.

Before this study, BDV-specific proteins (73), BDV-specific RNA
(63,64,69,74,75), and infectious BDV (76) were found in peripheral blood
mononuclear cells of psychiatric patients. In addition, the presence of
BDV-RNA in these cells in 4% to 5% of Japanese blood donors was also
reported (63). Since these results have important implications for BD
pathogenesis, the intra vitam diagnosis, and the safety of blood and blood
products, we conducted a double-blind study of BDV-specific markers in the
blood of psychiatric patients. Samples were aliquoted before distribution
to two different laboratories and tested independently. Forty-two
seropositive psychiatric patients and four seronegative controls from a
BD-endemic area in Germany were analyzed. Consecutive samples obtained over
a 9-month period were included in the study. The nested RT-PCR method was
used with primers specific for the p24 BDV-gene, the internal primers
identical to the one used by Kishi and coworkers (63,64). Positive results
for BDV-specific RNA were obtained only in the laboratory where BDV is
routinely studied. Simultaneous analysis of the samples in another
laboratory yielded negative results (62). In addition, cocultivation of
peripheral blood mononuclear cells from 47 samples of seropositive patients
over two passages and from eight samples over 8 to 10 passages with rabbit
embryonic brain cells did not provide evidence of BDV or BDV-specific
antigens (62). We thus concluded that contamination can have a serious
influence on the outcome of studies using nested RT-PCR methods to probe
for BDV-specific sequences in the peripheral blood mononuclear cells of
psychiatric patients.

Viral infections of the CNS are often difficult to diagnose because
conventional viral and serologic laboratory methods are still
unsatisfactory. Existing standard techniques are either insensitive and
slow (virus culture) or highly invasive and rare (brain biopsy) and,
therefore, are unsatisfactory as a measure for the accuracy of a new
technique such as PCR or RT-PCR (77). PCR techniques (PCR, RT-PCR, nested
PCR), with their speed and high molecular sensitivity possess great
potential for the diagnosis of viral infections of the CNS.

To overcome the discrepancies of recent RT-PCR studies about whether BDV or
BDV-RNA can be detected in the peripheral blood of humans and animals, a
multicenter study needs to be conducted with an independent center
responsible for collecting and distributing samples to prevent potential
contamination by BDV-RNA. These laboratories need to use similar RNA
isolation and RT-PCR techniques. Furthermore, the methods used by the
laboratories conducting sera or CSF analysis for BDV-specific antibodies in
animals and humans urgently need further standardization.

Conclusion

Studies of the epizootiology and epidemiology of BDV infections in the last
decade underscore the importance of BD as an emerging zoonosis. Data
clearly indicate that BDV or an analogous virus can also infect humans. In
addition to the seroepidemiologic results, data from preliminary studies on
the isolation of BDV from the CSF and the detection of BDV-specific antigen
and BDV-RNA in human brain confirm this conclusion. Therefore, humans
belong to the wide spectrum of animal species, from birds to mammals, that
are susceptible to BDV infections.

The question of whether BDV infections in humans contribute to human
neuropsychiatric disorders remains unsolved. A convincing argument might be
the significantly increased BDV-seroprevalence in psychiatric patients
compared with healthy controls. That some mentally or neurologically
healthy persons are BDV seropositive does not contradict this hypothesis,
since the natural infection in horses remains subclinical in most cases and
experimental intracerebral application of BDV into highly susceptible
animals does not always result in clinical disease (Rott, pers. comm.). In
addition, epizootiologic studies suggest that BDV is not highly contagious
since only individual horses within a stable succumb to clinical disease.
Similarly, analysis of a possible horizontal transmission of BDV within
family members and spouses of approximately 200 BDV-seropositive
neuropsychiatric patients showed only few cases of coincident onset of
mental or neurologic disorders within 1 year. Whether BDV or BDV-RNA is
present in human peripheral blood mononuclear cells remains questionable.
Depending on the virus strain and the host, BDV can induce in animals a
variety of clinical manifestations, from inapparent infections, behavioral
abnormalities, obesity, fertility problems to fatal neurologic disease.
Animals frequently show behavioral abnormalities during the course of the
disease. This was very obvious when tree shrews (Tupaia glis), classified
phylogenetically at the root of primates, were infected with BDV. Here,
mainly disturbances in social behavior and severe apathy were observed. In
contrast, an unequivocal classification of a definite psychiatric disease
with BDV infections in humans is at present not possible.

To establish the etiologic role of BDV infections in human psychiatric
symptoms, the following findings must be considered: symptomatology of BD
in animals is highly variable depending upon many factors such as age,
immune status, and strain of the animal used for infection, in addition to
the genetic makeup of the virus; psychiatric symptomatology is nonspecific
(78) even for known genetic causes, which has represented a major problem
for research investigating known neuropsychiatric disorders caused by
infectious agents. Despite the similarities in BDV infections of animals
and humans, additional future molecular, seroepidemiologic, and clinical
studies will be required to substantiate the possible contribution of BDV
to the pathophysiology of neuropsychiatric disorders.

This paper is dedicated to Prof. Hermann Becht on the occasion of his 65th
birthday.

About the Author

Dr. Richt is a research scientist and group leader at the Institute of
Virology, Justus-Liebig-Universitt Giessen, Germany. His current research
topic is molecular pathogenesis of Borna disease virus infections. His
interests include the epidemiology/epizootiology and immunopathology of
Borna disease virus infections and the replication strate
gy of Borna
disease virus.

Address for correspondence: Jrgen A. Richt, Institut fr Virologie,
Frankfurterstrasse 107, D-35392 Giessen, Germany; fax: 49-641-99-38359;
e-mail: juergen.a.richt@vetmed.uni-giessen.de.

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