To determine whether subclinical cases, together with clinical cases, of African horse sickness (AHS) occur in immunised horses in field conditions, whole
blood samples were collected and rectal temperatures recorded weekly from 50 Nooitgedacht ponies resident in open camps at the Faculty of Veterinary
Science, University of Pretoria, Onderstepoort, during 2008–2010. The sampleswere tested for the presence of African horse sickness virus (AHSV) RNAby a
recently developed real-time RT-PCR. Itwas shown that 16% of immunised horses in an AHS endemic areawere infected with AHSV over a 2 year period, with
half of these (8%) being subclinically infected. The potential impact of such cases on the epidemiology of AHS warrants further investigation.