Ask Our Vet

How do I counter bluetongue?

Dear Prof McCrindle,

It’s hard to get the live bluetongue vaccine. I’ve heard there is a new killed vaccine, but how does it work? Kind regards,

Luke

Dear Luke,

The recent heavy rains have meant that midges will be flying soon and transmission of bluetongue is becoming likely.

Onderstepoort Biological Products (OBP) is not yet producing sufficient quantities of live vaccine.

A new formalised vaccine, produced by Design Biologix, was registered under Act No. 36 in 2023 for use in South Africa. At this stage, the vaccine is only registered for use in sheep, not goats. It is available in 20ml and 100ml bottles and the instructions are included. It is not recommended for use in adult ewes and breeding rams in the breeding season, as further research is needed on the effects of the vaccine on semen and embryos.

Onderstepoort Biological Products’ live vaccine is given in three doses, three weeks apart, outside of the breeding season from June to August.

Bluetongue virus affects sheep, goats, cattle, camels and several wildlife species. Altogether, 28 strains have been identified internationally, and of these, 22 strains have been found in South Africa. The BTV-1–6, -8, -10, -11 and -24 serotypes are more often associated with epidemics of clinical bluetongue disease in sheep, and have high epidemic potential in South Africa. The most lethal strains have been identified as BTV-1–6 and -10 serotypes.

The disease is carried by flying midges called Culicoides. They fly at dawn and dusk and lay their eggs in soil with a high nitrate content, typically due to a layer of animal manure left in grazing camps and housing. In Africa, sheep are often housed above ground in pens with a slotted floor. This means that the manure falls through the slots and can be harvested and composted. It also keeps predators out at night.

PREVENTING BLUETONGUE

My personal advice to prevent bluetongue is to keep your sheep in a camp from dusk to dawn, as well as on misty, rainy days when the midges are flying.

Midges seldom fly higher than 2m.

The walls of your sheep kraal should be more than 2,2m high and made of green shade cloth. I looked online and 50m costs about R4 000, which is less than the slaughter price of a single sheep.

Sprinkling diatomite over the grazing pastures after cutting the grass short, in January, makes the soil too alkaline for the midge eggs to hatch. It also decreases the number of flies.

Removal and composting of sheep manure with diatomite means that this manure can be used as a nitrogen source in planted pastures, particularly lucerne. Kind regards,

Prof Cheryl McCrindle

ȊȲSource: Vet Q. 2020 Dec;40(1):258-321. Saminathan M, Singh KP, Khorajiya JH, et al. ‘An updated review on bluetongue virus: epidemiology, pathobiology, and advances in diagnosis and control with special reference to India’. Visit pubmed. ncbi.nlm.nih.gov/33003985.

Cheryl McCrindle is an emeritus full professor in the Faculty of Veterinary Science at the University of Pretoria, and has extensive experience in animal health. Email her at farmersweekly@caxton.co.za. Subject line: Ask our vet.