Abstract:
African horse sickness (AHS) is one of the economically most important equid diseases in Southern Africa, contributing significantly to equine morbidity and mortality. This makes the incidence, distribution and control of AHS a matter of importance to affected and at-risk countries. AHS is caused by African horse sickness virus (AHSV), of the genus Orbivirus, family Reoviridae. Disease occurrence and spread are mainly determined by the spread of its insect vector, midges of the Culicoides spp.
Annual vaccination with the Onderstepoort Biological Products polyvalent live attenuated AHS vaccine is currently the mainstay of prevention and protection in South Africa. The vaccine consists of two components given at least 3 weeks apart. Component one (AHS1) consists of types 1,3 and 4; component two (AHS2) consists of types 2, 6, 7 and 8. Types 6 and 8 provide cross-protection to types 9 and 5, respectively. Most horses only develop neutralizing antibodies against all types after multiple vaccination courses.
Disease diagnosis is performed with real-time reverse-transcriptase quantitative polymerase chain reaction (RT-qPCR) which is World Organization for Animal Health (OIE) approved and used in South Africa for disease outbreak and surveillance purposes.
This study retrospectively reviewed the history of suspected AHS cases from South Africa submitted to the Veterinary Genetics Laboratory, University of Pretoria, that were confirmed positive by real-time RT-qPCR during the period 1 September 2017 - 30 June 2019 to assess the effect of AHS vaccination on case outcome. For this period 233 AHSV RT-qPCR positive samples were obtained. The study confirmed that increased case fatality was not due to vaccine failure, but rather due to multiple variables, with an increased population of unvaccinated horses being one of these. Of the 115 cases that died 90 were unvaccinated or had a questionable vaccination history. With the case fatality rate for unvaccinated cases (0.80) being increased when compared to vaccinated cases (0.57). A significant association was shown between vaccination and case outcome (p=0.001), with unvaccinated animals being more likely to die. Other variables that were shown to influence disease outcome included the number of days since last AHS vaccination, age and sex.
