Testing for Eastern equine encephalitis virus and West Nile virus encephalopathies

Tim Pasma, Alison Moore                    

Animal Health Laboratory, University of Guelph, Guelph, ON (Pasma); Ontario Ministry of Agriculture, Food and Rural Affairs, Guelph, ON (Moore)

 AHL Newsletter 2023;27(3):23.

Summer brings mosquitoes and unfortunately, they may be infected with West Nile virus (WNV) and Eastern equine encephalitis virus (EEEV).  Depending on where you live in the province, EEEV or WNV may be endemic in your area. To see where previous cases of these diseases have occurred in Ontario and Canada, please refer to the Canadian Animal Health Surveillance System (CAHSS) equine dashboard: https://app.powerbi.com/view?r=eyJrIjoiOGFiMTAwYjAtZDkzZC00ZmZhLTk4ZTgtNGJjYmNjZjU3MmE4IiwidCI6IjE4YjVhNWVkLTFkODYtNDFkMy05NGEwLWJjMjdkYWUzMmFiMiJ9.

Although horses with either disease present with neurologic signs, the clinical signs of EEE are typically more acute and severe with death occurring within 24-48 hours.  Reported clinical signs include depression, ataxia, circling, and/or head pressing, with or without fever, progressing to recumbency and frequently followed by seizure activity.  WNV is typically less severe with a much lower mortality rate. Clinical signs may include ataxia, muscle fasciculations, and mild depression, with or without fever.   However, in rare cases, horses progress rapidly to recumbency and require euthanasia within 48 hours of clinical onset.

The most reliable antemortem test which confirms these encephalitic infections is the serum EEEV IgM ELISA (code xeeevme) and WNV IgM ELISA (code xwnveq), respectively.  However, vaccination for either virus within the previous month will interfere with interpretation of the test result, as recently vaccinated horses will also test positive.  For postmortem testing, the brain can be submitted for EEEV or WNV PCR testing.

Since both of these diseases usually occur in unvaccinated horses, rabies should also be on the differential list.  If rabies is suspected, contact the local public health unit if there is human contact with the saliva of the horse.  If there is animal involvement only, contact the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) for assistance with risk assessment, rabies sample submission and management of rabies exposure cases.  The OMAFRA Rabies Response Request form is available at https://survey123.arcgis.com/share/f24efaeca671431a8f8555f0400644d1 or by phone at 1-877-424-1300.

If the brain is collected for rabies testing, one can ask the technician or pathologist to retain a piece of brain for EEEV/WNV PCR testing, should the rabies test be negative.  Depending on the clinical signs, EHV-1 should also be on the differential list for acutely neurologic horses.  The preferred samples for EHV-1 PCR testing are nasal swab and whole blood (live), or brain tissue (dead).  AHL