Abstract:
Although West Nile virus (WNV) is endemic to South Africa (RSA), it has only become
recognized as a significant cause of neurological disease in humans and horses locally in the past
2 decades, as it emerged globally. This article describes the epidemiological and clinical presentation
of WNV in horses across RSA during 2016–2017. In total, 54 WNV-positive cases were identified by
passive surveillance in horses with febrile and/or neurological signs at the Centre for Viral Zoonoses,
University of Pretoria. They were followed up and compared to 120 randomly selected WNV-negative
controls with the same case definition and during the same time period. Of the WNV-positive cases,
52% had fever, 92% displayed neurological signs, and 39% experienced mortality. Cases occurred
mostly in WNV-unvaccinated horses <5 years old, during late summer and autumn after heavy rain,
in the temperate to warm eastern parts of RSA. WNV-positive cases that had only neurological signs
without fever were more likely to die. In the multivariable analysis, the odds of WNV infection
were associated with season (late summer), higher altitude, more highly purebred animals, younger
age, and failure to vaccinate against WNV. Vaccination is currently the most effective prophylactic
measure to reduce WNV morbidity and mortality in horses.
