The current available Horse Sickness vaccine is a polyvalent vaccine containing attenuated strains in two components: one trivalent (containing serotypes 1, 3 and 4), and one quadrivalent (containing serotypes 2, 6, 7 and 8). Serotypes 5 and 9 are not included in the vaccine because serotypes 6 and 8 are reportedly providing adequate cross-protection. The two vaccines should be administered at least three weeks apart but the period in between can be extended.
The vaccine is a modified live vaccine consisting of attenuated microorganisms which, after administration, replicate, resulting in an immune response similar to that induced by natural infection. The effectiveness of the African Horse Sickness vaccine is largely dependent on humoral immunity (otherwise known as antibody-mediated immunity). Therefore, the administration of several serotypes included in the vaccine results in the production of antibodies against each serotype. The response, however, of individual horses to vaccine administration may vary and in some individuals, antibodies against one or more of the serotypes might not even be detectable. Despite these limitations, the Onderstepoort Biological Products’ (OBP) AHS vaccine is the only vaccine registered for use in South Africa in terms of the Fertilisers, Farm Feeds, Agricultural Remedies and Stock Remedies Act, 1947 (Act No. 36 of 1947) and remains the only currently approved AHS vaccine in South Africa. Inactivated or recombinant vaccines may prove to be viable alternatives to the current available vaccine, but are not commercially available at the time.
New vaccine regulations
New regulations regarding the period during which horses should be vaccinated have come into place but currently only pertain to horses residing within the controlled AHS area and the Thoroughbreds registered under the NHRA. New regulations stipulate that horses should be vaccinated between 1 June and 31 October each year. The main reason for this is to try and vaccinate all horses during a time when the midges are less active and minimise the risk of a vaccine-related outbreak of African Horse Sickness.
Vaccination during high vector periods increases the risk of possible antigen transmission between equines by Culicoides vectors, and may present a serious threat to maintaining free zone status according to the OIE guidelines to implementing any form of equine export. From an ethical perspective, vaccinating during high risk periods of the year may present a greater risk to naive equines residing in the AHS controlled area where vaccination is only allowed with permission.
Frequently asked questions
Can I vaccinate my horses with another vaccine, such as equine flu, at the same time?
Yes, a horse may receive the equine flu vaccination at the same time and it should not interfere with the African Horse Sickness vaccine.
What are the requirements for moving registered equines into the controlled area of the Western Cape?
No equine may be moved into or between zones of the AHS controlled area without a movement permit issued by the state veterinarian, no more than two weeks before the movement. Permits can be cancelled in the event of an AHS outbreak. Horses can currently move within 40 days of their last vaccination (previously 60), but not longer than 24 months from the last vaccination.
Are there new ways of handling a case of African Horse Sickness?
Unfortunately there is still no treatment for this disease and supportive therapy remains the only means of treatment at the time. Keeping horses calm and avoiding any form of stress is very important. Should a horse be close to a referral facility, hospitalisation could prove useful, however, the risk of transportation as a form of stress should be considered. Supportive therapy such as anti-inflammatories or corticosteroids remains part of the treatment.
Apart from vaccinating and midge control, is there any other way to boost my horse’s immune system?
The immune system is one of the most complex systems in the horse. Like humans, horses should consume nutrients to support all body systems, including the immune system. Supplementation of vitamins and minerals may certainly aid the immune system. There is no specific way of boosting the immunity, and feeding a balanced diet remains one of the most important aspects of equine nutrition.
Text: Dr Adrienne Viljoen
Dr Adrienne Viljoen is the internal medicine specialist at Fourways Equine Clinic in Blue Hills, Kyalami.