Between 2011 and 2013, three American variegated squirrel (Sciurus variegatoides) breeders died of encephalitis in Germany, leading to the discovery of a new strain of variegated squirrel bornavirus 1 (VSBV-1), which was detectable in both animals and humans.

The strain is genetically close to mammalian bornavirus 1, which infects several mammalian species. The mode of transmission is still unknown, but direct transmission (scratching, biting) is most likely.

The fourth death that occurred in 2013 (brain and spinal meningitis) was linked to VSBV-1 infection from a zoo keeper's Prevost's squirrel (Callosciurus prevostii) by a retrospective research in 2018.

In a study, conducted in 2017 in Germany and the Netherlands, out of 468 squirrels (14 squirrel species), 11 cases showed VSBV-1 positivity, of which 6 were variant and 5 were Prevost squirrels. The virus did not cause symptoms in any of the animals.

Also in 2017, a screening study found 16 cases of the virus in 328 animals, 15 of which were linked to Germany and 1 to Croatia. The positive cases came from 4 squirrel species: Callosciurus prevostii (Prevost squirrel), Callosciurus finlaysonii (Finlayson squirrel), Tamiops swinhoei (Chinese striped squirrel or tree squirrel) and Sciurus granatensis (red-tailed squirrel).

Histological, immunopathological studies have also been performed on the virus in recent years (see, for example, Petzold et al., 2019 and Tappe et al., 2019).

Among other measures, the German Federal Research Institute for Animal Health recommends to have all squirrel holdings tested for VSVB-1.

Public Health England's risk assessment material recommends that in addition to monitoring the literature, veterinarians working with exotic animals should be informed.

Although there are few proven cases and, according to current knowledge, the disease does not spread from person to person, the fatal outcome makes it necessary to consider further measures and to inform relevant organisations in Hungary.