Borna disease (BD) is an infectious, sporadic neurological disease caused by Borna disease viruses 1 (BDV-1) and 2 (BDV-2). BDVs are a member of the family Bornaviridae in the order Mononegavirales. BD was first recognized is a disease of horses in the 18th century in Germany, where it was initially called 'Hitzige Kopfkrankheit der Pferde' which when translated to English is known as 'Heated head disease of horses'. The diseasse was later renamed after the city of Borna in Saxony, located in southern Germany, where the first epidemic disease occurred in cavalry horses in 1885. Borna disease occurs in many different animal species worldwide, including humans; however it is known for it's significance in relation to domesticated animals, including livestock and particularly horses and sheep. It has also less frequently been reported to occur in cats, dogs, cattle, goats, deer, llamas, alpacas, monkeys, lynx, shrews, sloths, rabbits, as well as humans. Incidences of BD in horses and sheep peak in March to June, although cases have been reported to occur year round.
Although BDV has been found in animals worldwide, the highest clinical incidence in animals and the verified classical BD cases are primarily concentrated in central Europe. Borna disease is characterized by slowly progressing, chronic infections of neurons , resulting in meningoencephalomyelitis with associated neurological and behavioral signs in affected horses. Early signs of BD in horses are initially behavioral, such as excitability, aggressiveness, lethargy or depression. As the disease progresses, horses often start to show neurological associated signs such as ataxia, decreased sensory, paralysis, paresis, and circling.
Transmission of BDV in horses is not well understood, but evidence has suggested that it is likely spread through a probable nasal infection route. Prevalence of BDV is higher on farms which do no practice proper rodent control and hygiene. This is likely because BDV is transmitted in rodents, vertically in mice and horizontally in rats via urine.
The natural incubation period of BD in horses is 2-3 months, however experimental tests have found it to be up to 143 days.