Investigation of equine encephalitis cases during the West Nile virus (WNV) epidemics in Greece

Reasons for performing study:

Clinicopathological and laboratory investigation of horses showing severe neurological signs during the 2010 WNV-lineage 2 human epidemic in Central Macedonia (Northern Greece). Implementation of preventive measures was also addressed.

Objectives:

To describe the clinical signs, supportive treatment and outcome in WNV cases and report the use of an inactivated WNV vaccine used for the protection of horses during the following epidemic periods.

Study design:

Clinical study.

Methods:

Laboratory testing included serology, real-time RT-PCR and histopathology. All affected animals received supportive treatment (furosemide, dexamethasone, B-complex vitamins and NSAIDs). The Equip WNV vaccine was used for immunisation of horses.

Results:

WNV-specific IgM antibodies were detected in 17 horses with neurological signs. The specificity of the detected antibodies was confirmed by seroneutralisation tests. The maximum duration of IgM was determined to be 60 days. Clinical signs included weakness of hindlimbs, ataxia and tremors (17/17), altered mental state (10/17), hypersensitivity (7/17), inability to swallow (3/17), recumbency (3/17) and convulsions (3/17). Supportive treatment was successful in 14/17 horses. The 3 recumbent horses died as a result of the infection, or were subjected to euthanasia. Histopathological findings observed in the CNS were mild perivascular cuffing of mononuclear cells, axonal swelling, glial nodules, microhaemorrhages and neuronal necrosis. None of the vaccinated horses showed clinical signs. Three unvaccinated horses showed clinical signs and developed WNV-specific IgM antibodies.

Conclusions:

The Nea Santa-Greece-lineage 2 strain responsible for the massive human epidemic of 2010 was also highly pathogenic for horses. Timely administration of supportive treatment is important to the prognosis of the cases. WNV infection should be included in the differential diagnosis of horses with encephalitis in Greece. Vaccinations can effectively protect horses, especially in areas of Greece where the virus seems to have become endemic.