Bornaviridae family contains only one genus of virus known as Bornavirus. The genus Bornavirus contain only one member species of virus known as Borna disease virus (BDV). BDV is the causative agent of Borna disease (a slow virus infection). Borna disease is clinically defined as a fatal neurologic disease of mammals. However, the disease can also occur in birds as a proventricular dilatation disease. BDV is mainly restricted to central Europe, and the disease is not widely distributed. The natural hosts of viruses in the Bornaviridae family include humans, birds, sheep, horses, reptiles, and rodents. Borna disease virus (BDV) is the only viral species in the genus Bornavirus, and they have a ss(-)RNA genome.
The ss(-)RNA genome of BDV is converted to a ss(+)RNA genome with the virus’ RNA-dependent RNA Polymerase or replicase that carries out this function. BDV measure about 90 nm in diameter; and BDV are generally enveloped viruses. BDV like the Orthomyxoviruses replicate in the nucleus of their host cell, and they are released through a budding process through the cytoplasmic membrane of the infected host cell. The nucleocapsid of BDV is spherical; and the virus is sensitive to UV light, organic solvents and detergents. The natural hosts of BDV are horses and sheep. Aside sheep and horses, BDV can also cause infection in a wide variety of other mammals including deer’s, donkeys, shrews, pigs, mules, rats, cattle’s and rabbits and birds. BDV causes lassitude or lethargy, spasm and paralysis in animals especially in horses. However, BDV causes a neuropsychiatric disorder in humans.
Characteristically, BDV causes infected horses to walk in a staggering manner. Abnormalities in the movement and behavioural patterns of animals infected with BDV are typical characteristics of Borna disease. Infections by BDV in animals and humans are generally asymptomatic in nature. Borna disease (BD) occurs mostly in Europe where the first case of the infection was reported (precisely in a town in Saxony, Germany), but the disease now occur in some parts of the world and have been reported in Africa and Asia. BDV is highly neurotropic in nature; and this implies that the virus has a high affinity for the nerve cells of the central nervous system (CNS).
The affinity of BDV for the nerve cells of its host explains the neuropsychiatric disorder of BD in its hosts including humans and animals. BDV has the ability to downregulate its replication in vivo or in its host, and this leads to a decrease in the amount of virions that will be produced during an infection. Though the immune system mounts a putative attack against the virus, the low production of infectious virions by BDV allows the pathogen to establish a persistent or chronic infection in its host. Amantadine, an antiviral drug that inhibits the uncoating of viruses, is usually used for the treatment of BD in horses, other mammals and humans. The transmission route of BDV still remains largely obscure, and vaccination against BDV infection is available in some economies where the disease is endemic particularly in horses.
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