Idiopathic CNS inflammatory disease in horses in Ontario   

Murray Hazlett

Equine central nervous system cases are seen on a fairly regular basis at the AHL. From mid-2007 until December of 2015, pathologists from the AHL and Department of Pathobiology have logged 195 cases. Many of these are degenerative diseases such as equine degenerative myelopathy (EDM) or spinal cord malacia associated with vertebral stenosis (wobbler syndrome) (Table 1). There were 54 cases with idiopathic degenerative or inflammatory CNS lesions documented, 24 of these being inflammatory. Many of the inflammatory changes were very mild lymphocytic cuffs in a few sections that the pathologist felt was not significant (n=13). From the reports, 10 cases had significant inflammation with no etiology identified and that the report indicated may be viral. Depending on the time of year and duration of clinical signs, testing was done for EEEV, EHV, WNV, and Rabies virus without an etiologic diagnosis being established.

The age of these horses with idiopathic encephalitis ranged from 6 d to 18 y, with clinical signs ranging from 24 h or less to “months”. Three were identified as Thoroughbreds, 2 Standardbreds, and 2 Quarter Horses. No geographic predisposition was seen.

The clinical presentation and lesions seen in these horses varies. The mild “incidental” perivascular lymphocytic inflammation seen in some horses may be a residual lesion from a previous bacterial embolus or a resolving viral lesion. It is a recognized incidental lesion in horses.1 In reviewing AHL records, perivascular cuffs were not considered incidental if they were present in association with clinical neurologic disease or in relative abundance.

Routine testing on neurologic cases at the AHL is done for Rabies virus (CFIA), Eastern equine encephalitis virus, Equid herpesvirus-1, and West Nile virus. Other causes of viral encephalitis are possible and are usually not tested for, including other arboviruses (Powassan virus, St. Louis encephalitis virus, Snowshoe hare virus, WEEV) as well as viruses not identified in Canada such as VEEV (Fig. 1).

Table 1. Causes of central nervous system disease in equine submissions to the AHL, 2007-2015.



EDM - equine degenerative myelopathy


EPM - equine protozoal myeloencephalopathy


Wobbler syndrome


Fracture/trauma (skull or vertebral


Eastern equine encephalitis virus




Hepatic encephalopathy


Neoplasia (including cholesteatoma)


CNS disease, no lesions


Equid herpesvirus


West Nile virus


EMN - Equine motor neuron disease


Rabies virus


Parasitic (Halicephalobus)


Undiagnosed degenerative or  inflammatory




Reference  1. Jahns H, et al. Age-related and non-age-related changes in 100 surveyed horse brains. Vet Pathol 2006;43:740–750.


Severe meningoencephalitis in a 4-year-old Thoroughbred.

Figure 1. Severe meningoencephalitis in a 4-year-old Thoroughbred.