Abstract:
West Nile virus (WNV) has gained international attention in recent years as a
globally emerging disease, particularly after large epidemics occurred in North
America in the past 20 years. Although endemic to South Africa, it has only been
recognised as a significant cause of neurological disease in either humans or horses
since 2008. This retrospective study provides an epidemiological and clinical
description of WNV disease in horses in South Africa during 2016–2017, when 54
cases, most of which occurred during 2017, were diagnosed by passive surveillance
at the Centre for Viral Zoonoses (CVZ), University of Pretoria. Cases were followed
up and then statistically compared to a randomly selected set of 120 WNV-negative
controls from the CVZ database of the same time period, which complied with similar
case descriptions. Clinical presentation of WNV cases was found to be remarkably
similar to international trends, with 89% neuroinvasive disease and 39% case fatality
rate, mostly displaying typical, significant neurological signs: ataxia (74%), hindleg
paralysis (35%), paresis (30%), total paralysis (28%), tremors / muscle fasciculations
(19%), foreleg paralysis (17%) and laminitic stance (9%). Approximately half of the cases exhibited pyrexia. Cases that had only neurological signs were more likely to
die while cases with pyrexia, with or without neurological signs, were more likely to
recover.
Most of the cases were in Thoroughbred, Warmblood or Arabian horses, while local
or mixed breed horses were the least represented. Cases occurred mostly in WNVunvaccinated
horses less than 5 years old, specifically in the late summer and
autumn months after heavy rain in the temperate to warm Eastern parts of South
Africa. Cases were located mainly in Gauteng, KwaZulu-Natal Midlands and the
Northern Cape with fewer cases in the Free State and Western Cape provinces. In
the multivariable logistic regression analysis, the odds of WNV infection was
associated with season (higher during March-April vs. all other times), altitude
(higher at 1293–1466 m vs. other categories), breed (lowest in mixed and local
breeds), younger age and failure to vaccinate against WNV. Based on these
findings, risk-based recommendations may be made to horse owners; in particular,
vaccination against WNV, which is currently the most effective prophylactic measure
available to reduce disease, severity of clinical signs and mortality.
