HORSES

A case of eastern equine encephalitis  

Murray Hazlett, Jaimee Gardner, Davor Ojkic

A mature mare located southeast of Owen Sound and recently vaccinated for rabies was found in the field in lateral recumbency, unable to rise, and paddling. The day prior, the owner reported the horse was normal and did some mild lunge work. The horse would rise to sternal recumbency, but only briefly before falling to the right again. She was bright and alert, but would grind her teeth and was unable to drink. She was unresponsive after 6 h to IV flunixin and IM dexamethasone therapy and was euthanized. The brain was removed the next day and sent to the AHL for testing.

The day following euthanasia of this animal, another horse on the property was found with almost identical neurologic signs - down in lateral/sternal recumbency. The second horse was more mentally stuporous, unable to swallow, and was euthanized after 24 h of no response to therapy. This horse was disposed of without testing.

In the tested mare, histologic examination of the brain revealed severe, neutrophil-rich encephalitis compatible with either bacterial encephalitis or eastern equine encephalitis virus (EEEV) infection (Fig. 1a). Tests for rabies virus, West Nile virus (WNV), and equine herpesvirus 1 (EHV-1) infection were negative. The rtPCR test for EEEV was strongly positive on both fresh brain tissue (inactivated by TriPure. Ct 18.09), and on scrolls prepared from formalin-fixed paraffin-embedded tissue (Ct 18.68). Immunohistochemistry was also strongly positive for EEEV antigen (Fig. 1b).

From Aug 1 to Oct 31 of this year, 5 horses (or their brains) with clinical signs of encephalitis have been submitted to the AHL for postmortem examination. Two of these were nonspecific nonsuppurative encephalitis, one was suspicious for Sarcocystis neurona, and there were single cases of WNV and of EEEV infection.

Although rare, we tend to think of EEEV as less common than WNV infection of horses, however, since 2008 we have confirmed more cases of EEEV than WNV or rabies virus (Table 1) infection in horses.

EEEV is an arbovirus with seasonal incidence, transmitted by mosquitoes from wild birds. Of the 8 pathology cases seen at the AHL since 2008, 1 occurred in 

August, 6 in September, and 1 in early October.   AHL

Figure 1. Cerebral cortex of EEEV-infected horse.

a) Widespread neutrophil-rich encephalitis. H&E. (Close-up in insert).

b) Immunohistochemistry of EEEV demonstrating large amounts of brown-staining viral antigen in neurons (arrows).

Table 1. AHL pathology cases of diagnosed viral encephalitis in horses.