Immunization of horses with a polyvalent live-attenuated African horse sickness vaccine: Serological response and disease occurrence under field conditions

African horse sickness (AHS) is a non-contagious, insect-borne disease of equids caused by a RNA virus (AHSV), which belongs to the genus Orbivirus, family Reoviridae. The disease is endemic in sub-Saharan and western Africa, where prevention strictly depends upon vaccination. The present paper aims at evaluating the serological response and the occurrence of AHS in horses bred under field condition and regularly immunized using the commercially available live attenuated vaccine (LAV) produced by Onderstepoort Biological Products. The study was carried out in a farm located in the district of Windhoek (Namibia), where the disease is endemic. A total of 72 cross-breed horses, out of the 150 housed on the farm, were subdivided in six age groups, from 2 to 7 years-old. Each group consisted of 12 heads which were born during the same breed- ing season and had undergone from four to nine vaccination courses. AHSV specific immune response was evaluated by serum-virus neutralization test. Data about the clinical occurrence of the AHS from 2006 to 2011 were made available. The immune response, in terms of number of seropositive horses and serum neutralizing titers, was quite variable among horses and against different serotypes. Neutralizing antibodies against all serotypes were recorded in all the horses only after eight vaccination courses at 6 years of age onwards. Immune response to AHSV-5 and 9, which are not included in the LAV formulation, were also established. A severe AHS epidemic occurred in Namibia in 2011. On the farm under study, a total of 32 animals were clinically affected, 12 died, 11 of them were 2 year-old or younger. Our data confirm that vaccination with LAV is a useful tool to reduce the severity of the disease in ende- mic areas. However, clinical and sometimes fatal AHS can still affect young vaccinated horses, thus high- lighting the necessity to better understand the immune response to AHSV and to dispose of more effective vaccines. 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license