Classification

Virion Properties

Virus Replication

Clinical Features and Epidemiology

Natural infection with classical Borna disease virus has been described predominantly in horses and sheep in areas of central Europe, but occasional cases have also been reported in other species. Experimentally, the host range is wide, from chickens to primates. For many years, Borna disease was considered sporadically enzootic only in certain areas of central Europe. However, seroepidemiologic investigations have shown that infection in horses is more widespread. Antibodies have been found in approximately 12% of healthy horses in Germany, and a greater prevalence is found in enzootic areas and in stables with affected horses. Although bornavirus-specific antibodies have also been found in horses in Asia, Australia, the Middle East, and the United States, clinically manifest classical Borna disease has not been unambiguously identified outside of central Europe. The great majority of bornavirus infections of mammals are clinically inapparent so that the incidence of disease typically is low, even in enzootic areas. Importantly, however, only genetic sequence data has been described to date for the majority of novel bornaviruses and, with the notable exception of the avian bornaviruses (see below), the clinical impact and epidemiology of these infections remains poorly defined.

The route of natural Borna disease virus infection of horses is uncertain, but oronasal transmission from direct contact has been proposed on the basis of results from experimental infections of laboratory rats. The incubation period is highly variable, ranging from weeks to months, and the course of clinical Borna disease can be acute or protracted. Mortality is high in horses that develop neurological manifestations. Affected horses may initially present with fever and behavioral changes that become increasingly worse, including depression, altered eating behavior, and head pressing. More advanced cases are characterized by profound neurological disturbances, including proprioceptive deficits and abnormal stance, and dysfunction of individual cranial nerves because of involvement of their respective nuclei. Early neurological signs are attributable to dysfunction of the limbic system, whereas, during later stages of the disease, dysfunction of motor systems causing paralysis may predominate. In terminal stages there is anorexia, compulsive circular walking, head tremor and pressing, and convulsions. Ophthalmologic disorders including nystagmus, pupillary reflex dysfunction, and blindness may occur in advanced stages. The course of the disease is variable, 3–20 days approximately, and usually ends in death.

There has been considerable debate, based on the findings of serological and virological investigations, as to whether classical Borna disease virus infection is zoonotic and responsible for human psychiatric disorders. Failure to independently confirm putative instances of zoonotic transmission, coupled with the possibility of cross-contamination, make it exceedingly unlikely that classical Borna disease virus is associated with human disease. Although Borna virus disease-specific antibodies can be present in sera of patients with various psychiatric diseases, they can also been found in clinically healthy people. Recently, however, a novel bornavirus that is genetically distinct from the classical Borna disease virus was detected in variegated squirrels (Sciurus variegatoides) and in humans raising these squirrels who developed fatal encephalitis.

The distinctly seasonal occurrence of Borna disease cases in spring and early summer, as well as its geographically-restricted range, suggest an essential natural reservoir of infection. There is now compelling evidence that small mammals are the natural reservoir host of Borna disease virus as the virus has been detected in the tissues of white-toothed shrews (Crocidura leucodon) in endemic areas of Switzerland and Germany. Moreover, recent data confirm that infected shrews shed infectious virus via various secretions and excretions.

Pathogenesis and Pathology

The laboratory rat has provided the most thoroughly investigated experimental model of Borna disease virus infection. The intranasal route appears most likely to be responsible for natural infections, with virus passing intra-axonally to the olfactory bulbs of the brain from olfactory nerve endings. The virus is then disseminated throughout the central nervous system. In rats, and perhaps other animals, there are substantial differences in pathogenesis, depending on when infection is initiated. Infection of immunotolerant newborn rats results in a persistent infection of the central nervous system, as well as other organs, with minimal tissue injury or neurologic alteration but deficits in learning and memory. In contrast, infection of adult rats of certain strains leads to severe lesions in the central nervous system, and marked behavioral changes. The virus replicates in both neurons and glial cells, and persistent infection is characteristic of Borna disease virus infection.

Borna disease virus infection does not elicit a protective immune response; rather, infection stimulates a T-cell-mediated immunopathologic reaction (a delayed-type hypersensitivity response) that contributes to disease progression. Histologically, this manifests as non suppurative (mononuclear inflammatory cell) meningoencephalitis accompanied by astroglial and microglial activation. Infiltrating cells include CD4⁺ and CD8⁺ T cells, macrophages, and B-cells. Experimentally, whereas infection of adult immunocompetent animals regularly results in disease, infection of neonatal or immunocompromised animals leads neither to encephalitis nor to disease, despite the persistence of the virus. Antibodies produced in response to Borna disease virus infection are nonneutralizing, and most likely do not contribute to disease pathogenesis. Adoptive transfer of immunoglobulins from infected animals to immunocompromised recipients does not induce pathological changes or disease whereas adoptive transfer of virus-specific T cells does reproduce the disease. Clinical manifestations of Borna disease likely reflect virus-mediated alterations in cellular signaling pathways and modulation of protein functions, eg, cellular enzymes.

Infection with Borna disease virus in horses and sheep induces a severe encephalomyelitis that principally affects the gray matter (polioencephalomyelitis); histologically, there typically is extensive perivascular cuffing with lymphocytes, macrophages, and plasma cells. Neuronal necrosis is not a feature, but distinctive eosinophilic intranuclear inclusions in neurons, called “Joest-Degen” bodies, are characteristic, even pathognomonic of Borna disease, although they cannot be demonstrated in all cases. Lesions are especially prominent in the gray matter of the olfactory bulb, basal cortex, caudate nucleus, and hippocampus. The retina typically also is involved in experimentally infected laboratory animals.

Diagnosis

Premortem diagnosis of Borna disease is difficult because several diseases can induce similar clinical signs in horses, including rabies, tetanus, equine herpesvirus 1-induced encephalomyelitis, protozoal encephalomyelitis, West Nile and other flaviviral diseases, and the equine alphavirus encephalitides. Diagnosis is usually confirmed by the demonstration of antibodies in the serum and/or, preferably, in the cerebrospinal fluid. This can be done using indirect immunofluorescence staining of persistently infected Madin–Darby canine kidney cells as substrate. Alternative serological tests include western immunoblotting and ELISA. Virus isolation in susceptible cultured cells can be used to confirm bornavirus infection, although RT-PCR and sequence analysis are increasingly used. Immunohistochemical staining of viral antigen is used to confirm postmortem diagnosis of Borna disease in sections of brain with typical histologic lesions. Alternatively, in situ hybridization can be used to detect the presence of bornavirus RNA in tissue sections.

Immunity, Prevention, and Control

Although antiviral therapy has shown some promise in experimental studies, there is currently no specific treatment for animals with classical Borna disease. Similarly, although a number of inactivated, live-attenuated, and recombinant vaccines have been developed and tested for the potential prevention of Borna disease virus infection, there are concerns regarding their safety and efficacy and none are yet licensed or commercially available. Although direct (horse–horse) contact transmission of Borna disease virus has not been clearly demonstrated, the identification and quarantine of carrier animals is potentially important to limit spread of the virus. Given the recent identification of novel bornaviruses in a wide variety of animal species, there is a considerable need for further studies to better characterize the epidemiology of these infections and their potential clinical importance.

Clinical Features and Epidemiology

Avian bornavirus is the apparent causative agent of proventricular dilation disease, a progressive and devastating neurological and gastrointestinal disease of birds that was first recognized in the 1970s among macaws exported from Bolivia (syn. macaw wasting disease). Although the overwhelming majority of cases of proventricular dilation disease occur in psittacine birds (parrots), the disease has now been identified in approximately 80 species of birds including nonpsittacine species. Affected birds present with signs of progressive neurological and/or gastrointestinal tract dysfunction, including weight loss, dysphagia, regurgitation, ataxia, and proprioceptive deficits. The disease is usually fatal but subclinical infections potentially result in birds that continuously or intermittently shed the virus. Sudden death may also occur. At least 14 different genotypes of avian bornavirus have now been detected in pet and wild birds, including both psittacine and nonpsittacine species such as songbirds [canaries (genera Serinus and Crithagra), finches (family Fringillidae)] and waterfowl, ie, swans (genus Cygnus), geese, and ducks. Avian bornavirus infection apparently occurs in birds worldwide, and particular genotypes of avian bornavirus are more common than others.

The epidemiology of avian bornavirus infections is poorly characterized, including the natural reservoir(s) of infection and routes of virus transmission. The incubation period in experimentally infected birds has ranged from 20 to 200 days after inoculation. Bornavirus RNA can be detected in the feces and cloacal and crop contents of naturally infected birds, suggesting that direct horizontal transmission of the virus might be important. However, co-housing of infected and susceptible birds does not consistently lead to virus transmission and efforts to experimentally infect susceptible birds by oronasal inoculation of virus have been unsuccessful to date. Similarly, the role of vertical transmission of virus is uncertain.

Pathogenesis and Pathology

Experimental avian bornavirus infections of cockatiels and canaries have provided much information on the virus and its pathogenesis, although canaries do not typically develop the proventricular dilation disease syndrome that occurs in cockatiels. To date, most experimental studies have been done using a single virus genotype (avian bornavirus 4). Depending on the route of infection (intracerebral, intramuscular, intravenous), inoculated birds develop virus-specific antibodies between approximately 7 and 60 days after infection, and bornavirus RNA is present within the feces of inoculated birds between 20 and 70 days after infection. Naturally infected birds with high viral loads and with high bornavirus-specific antibody titers are more likely to develop clinical proventricular dilation disease. As occurs in bornavirus-infected mammals, avian bornavirus-specific antibodies are not protective in birds. The correlation of clinical signs, gross and histological lesions, and detection of viral RNA and antigen vary between individuals so that subclinically infected birds can have typical histological lesions and either restricted (eg, central nervous system only) or widespread distribution of bornavirus antigens.

Typical necropsy findings of birds with proventricular dilation disease include emaciation with atrophy of pectoral muscles and dilatation of the crop, proventriculus, ventriculus, and small intestine. Histologic changes include mononuclear (lymphocyte, macrophages and plasma cells) inflammation within both the central and peripheral nervous systems (ie, lymphocytic encephalitis, myelitis, ganglionitis, and neuritis and combinations thereof). Involvement of the gastrointestinal autonomic nervous system is responsible for signs of gastrointestinal dysfunction, including paralytic dilatation of the esophagus and proventriculus. Inflammation also may involve the myocardium and cardiac conduction system, the adrenal glands, and eyes.

Like mammalian bornaviruses, avian bornaviruses are neurotropic but also show affinity to the gastrointestinal tract and peripheral organs. The role of immunopathologic mechanisms of cellular injury in mediating disease expression in bornavirus-infected birds remains to be characterized. Clearly, occurrence of disease varies markedly between bird species and between individuals.

Diagnosis