Investigation of transmission of vaccine strains of African horse sickness virus in weanling foals kept under field conditions following the use of a commercial attenuated live virus vaccine

Abstract

African horse sickness (AHS) is a World Organisation for Animal Health (OIE) listed disease and is a controlled disease in terms of the Animal Diseases Act (Act No 35 of 1984) in South Africa. It affects equids with horses being the most susceptible and often suffering high mortalities. This makes the disease very important to the South African equine industry and international equine trade. In Southern Africa, control of the disease relies heavily on annual vaccination with the Onderstepoort Biological Products (OBP) polyvalent AHSV live attenuated vaccine (LAV). It is divided into two combinations: AHSV-LAV Combination (Comb) 1 contains a trivalent vaccine including AHSV types 1, 3 and 4 and AHSV-LAV Comb 2 contains a quadrivalent vaccine including AHSV types 2, 6, 7 and 8. The causative agent of AHS is the AHS virus (AHSV). AHSV, bluetongue virus (BTV) and equine encephalosis virus (EEV) belong to the genus Orbivirus in the family Reoviridae. The virus is transmitted between equids by haematophagous midges (Culicoides spp.). According to some field studies, BTV-LAV strains can be transmitted by Culicoides midges. No studies have yet been carried out to investigate whether AHSV-LAV strains can be transmitted in the same manner. The aim of this study was to determine if AHSV can be detected using RT-qPCR in Culicoides midges and unvaccinated weanlings following first vaccination of roughly half the weanlings in a group using a commercial polyvalent AHSV-LAV. This study started in March and ran until June 2014. The AHS controlled area in the Western Cape Province, has historically been free from AHS with only isolated outbreaks occurring periodically. These outbreaks typically occur from February to May (Sinclair, Buhrmann & Gummow 2006, Venter, Koekemoer & Paweska 2006, Grewar et al. 2013). Approximately 130 Thoroughbred weanlings were kept in open camps on two Thoroughbred stud farms in the AHS protection zone of the AHS controlled area where surveillance has shown that field AHSV has not circulated since at least 1996. Approximately half of the foals on each farm were vaccinated with a commercial polyvalent AHSVLAV and the other half were unvaccinated for the duration of the study. Weekly blood samples were collected and subjected to group-specific and type-specific RT-qPCR assays to assess the presence of AHSV nucleic acid for the duration of the study. One Onderstepoort (220V ultraviolet down-draught suction) light trap was set close to where the horses were kept on each farm overnight once a week starting on the day of first vaccination. Insect catches were cleaned so that they contained only Culicoides. The largest catch from each month on each farm was selected for species composition analysis under a stereomicroscope. Culicoides were tested for the presence of AHSV nucleic acid using RT-qPCR assays. Seven recently blood-fed Culicoides were tested to determine the host species on which they had fed using a multiplex PCR This study showed that RNAaemia can be detected with RT-qPCR in weanlings after vaccination with the commercial LAV under field conditions. The age of weanlings at first vaccination seems to affect the detection of RNAaemia due to interference by maternal antibody. The order in which the vaccine combinations are administered affects the AHSV types detected using type-specific RT-qPCR assays. Multiple AHSV types were detected in individual weanlings which increases the chances of reassortment occurring. AHSV type 1 and 3 were detected in weanlings following vaccination with AHSV-LAV Comb 2 only. Possible reasons for this occurrence include mechanical transmission, naturally circulating wild virus or the presence of these AHSV types in the vaccine bottle. The lack of AHSV detection in the midges supports the theory that large numbers of Culicoides need to be present to compensate for the low infection prevalence and possibly transmit vaccine virus from the small proportion of horses with a high enough viraemia (Venter, Koekemoer & Paweska 2006). Although the study did not show the transmission of vaccine virus, it reaffirms the fact that the behaviour of the LAV is not fully understood and needs further investigation and possibly replacement. This is especially important in the light of the spread of BTV into areas where it previously did not occur and the fact that vaccine transmission was suspected to be the cause of the AHSV outbreak in the Western Cape Province in 2014.