All too often, the cause of an encephalitis remains unknown and leaves clinicians, patients and owners of pets with considerable uncertainty about its origin, treatment options and, hence, prognosis. A substantial proportion of these cases remains unsolved using conventional diagnostic methods.

One of the controversial encephalitides of possibly infectious origin is the so-called ‘staggering disease’ of domestic cats (Felis catus). It has been described first in the 1970s in the region between Stockholm and Uppsala, which remains a hotspot of ‘staggering disease’ to the present.

The most prototypic clinical sign of ‘staggering disease’ is hind leg ataxia with a generally increased muscle tone resulting in a staggering gait. In addition, a broad range of other neurologic signs may occur, including the inability to retract the claws, hyperaesthesia and occasionally tremors and seizures. Behavioural alterations include enhanced vocalization, depression, becoming more affectionate, and rarely aggression. The disease progression usually lasts a few days to a few weeks, but may also continue for more than a year, and it generally results in deterioration requiring euthanasia for animal welfare reasons.

For a long time, Borna disease virus 1 (BoDV-1), which causes neurologic disorders in various mammals, has spearheaded the panel of aetiological candidates. However, results suggesting natural BoDV-1 infections in cats with ‘staggering disease’ remained inconclusive. Rustrela virus (RusV), a relative of rubella virus, has recently been shown to be associated with encephalitis in a broad range of mammalian hosts.

The authors of this study report the detection of RusV RNA and antigen in brain tissues of 27 out of 29 cats with non-suppurative meningoencephalomyelitis and clinical signs compatible with ’staggering disease’ from Sweden, Austria, and Germany, but not in non-affected control cats.

They were not able to detect bornavirus RNA or BoDV-1 nucleoprotein in any of the 29 tested cats with clinicopathological features consistent with ‘staggering disease’. The authors believe their work indicates that RusV is the long-sought cause of feline ‘staggering disease’. Given its reported broad host spectrum and considerable geographic range, RusV may be the aetiological agent of neuropathologies in further mammals, possibly even including humans.

The route of RusV transmission within its presumed reservoir as well as from there to other hosts remains unknown. The possibility of RusV shedding by infected cats remains to be elucidated. However, the apparently spatially restricted occurrence of the phylogenetic clusters argues in favour of a continuous viral spread only within a locally bound reservoir, such as small mammals, whereas more mobile hosts, including domestic animals that may be transported over long distances, serve predominantly as erroneous dead-end hosts. The sporadic occurrence of ‘staggering disease’ in domestic cat populations, the apparent lack of outbreak series within cat holdings, as well as the almost exclusive restriction to cats with outdoor access, often originating from rural areas, further support this assumption.