African horse sickness vaccination status correlated with disease outcome in South Africa

African horse sickness (AHS) is one of the economically most important equid diseases in southern Africa, contributing significantly to equine morbidity and mortality. Annual vaccination with the Onderstepoort Biological Products polyvalent live attenuated vaccine has been the mainstay of prevention in South Africa. The study objectives were to determine if there is a significant relationship between multiple variables (vaccination status, number of AHSV [African horse sickness virus] serotypes contracted, clinical presentation, order of vaccine administration, age, sex and mean Ct value) and case outcome. The study population consisted of samples of AHS cases from South Africa submitted to the Veterinary Genetics Laboratory, University of Pretoria, that were confirmed positive by real-time RT-qPCR from 1 September 2017 to 30 June 2019 with a definitive disease outcome. At a univariable level, unvaccinated horses were 8.7 times more likely to die compared with horses that were vaccinated annually. Vaccination status was not statistically significant at a multivariable level, possibly due to insufficient sample size. Annual vaccination was shown to be protective. The pulmonary form of the disease and a lower Ct value had an increased likelihood of non-survival. Vaccination order was significant at a multivariable level (AHS2 vaccine administered first had a higher likelihood of survival). The study confirmed that increased case fatality was not due to vaccine failure but instead due to multiple variables, with an increased population of unvaccinated horses being one of these.