African horse sickness (AHS) is a non-contagious viral disease transmitted by arthropod vectors namely Culicoides (Avaritia) imicola Kieffer and Culicoides (Avaritia) bolitinos Meiswinkel endemic to sub-Saharan Africa. The disease affects all equine species, where its severity increases in horses foreign to Africa. Currently, vaccination is the only means of controlling the disease.
African horse sickness poses a great risk to South African equines, not only due to the high mortality rate, but also due to the large scale restrictions implemented on the movement of horses for breeding or competition and on the international exportation of horses by the Department of Agriculture, Forestry and Fisheries (DAFF) and the World Organisation for Animal Health (OIE).
A prospective study was undertaken between 2013 and 2014 by the Department of Veterinary Tropical Diseases and the Equine Research Centre (ERC), Faculty of Veterinary Science (FVS), University of Pretoria to determine the presence of Culicoides midges, the vector of the African horse sickness virus (AHSV) and the prevalence of disease at two equine establishments on the East Rand, Gauteng Province, South Africa.
The two establishments differed extremely, when looking at infrastructure, management and vaccination protocols, this being the primary reason for their inclusion into the study.
In the study, which started in December 2013, EDTA blood samples were collected and rectal temperatures recorded every 14 days over six months, from 28 Friesian / Lusitano and Appaloosa horses both resident in stables and open camps at the two establishments. The horses ranged in age from yearlings to four years. The EDTA samples were tested for the presence of AHSV and equine encephalosis virus (EEV) dsRNA by RT-qPCR (Quan et al. 2010).
The clinical picture of the horses was recorded and rectal temperatures monitored for presentation of clinical cases caused by both viruses. It was shown that a total of nine (32%) cases of AHSV and five (18%) cases of EEV were identified in the 28 horses included in this study, where 89% of the horses had been vaccinated against AHS.
As part of the risk assessment at each establishment it was essential to monitor the presence of the known vectors of AHSV. Therefore the conventional down-draught Onderstepoort black-light trap was operated overnight at various intervals throughout the study. The infection rate using RT-qPCR of the collected Culicoides midges was lower than the previous assumptions made by the owner and consulting veterinarians based on the mortality rate during the previous AHS season. Both AHSV and EEV were detected in separate single pools of collected midges. The low number of positive midges found in this study during 2014 could be explained by the occurrence of both diseases followed by the very active midge season of 2013. It is hypothesized that the prevalence of these diseases is dependent on seasonal patterns where a build-up of virus must reach a critical level after which spilling over will occur into associated equine populations (Venter et al. 2014).
The present study also investigated the relationship between prevention strategies; primarily vaccination with a registered vaccine and the incidence of both diseases, where it shows that the prevalence of disease is dependent on the various prevention strategies implemented at each establishment.
The presence of subclinical infection as seen in this study requires further investigation as it has a major impact on the movement of equines and the possible introduction of disease into naïve populations. The analysis of EE in the study, which is more prevalent than AHS, however does not cause severe disease, assists in the evaluation of wild-type virus transmission, as there is no commercial vaccine is available for EE. The presence of the virus assists in the study of the virus/host dynamics, natural maintenance cycles and the transmission of orbiviruses amongst South African horses. (Venter et al. 1999).