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The only reliable method of preventing bluetongue in sheep is timely and proper vaccination before the start of the bluetongue season.
Bluetongue is a non-contagious viral disease of the Orbivirus genus in the Reoviridae family. It affects ruminants such as sheep, cattle, goats, and game. According to Dr Tom Strydom, an independent veterinary consultant, the name refers to cyanosis (blue or purple discoloration) of the tongue. Morbidity generally ranges from 2 to 30%, but in extreme cases it can rise to as high as 70%.
Lees meer oor bloutong by skape.
Bluetongue is transmitted by biting midges of the Culicoides genus. Of the more than 1 000 identified Culicoides species, fewer than 20 are considered vectors. The spread of bluetongue is linked to the activity of these midges, which peaks during warm, wet seasons and declines during colder periods. Environmental factors such as temperature, humidity, rainfall, wind speed, and the presence of marshy areas influence vector survival and reproduction, and therefore the incidence of the disease.
Vectors’ transmission capacity is further affected by ambient temperature, the extrinsic incubation period of the virus, midge lifespan, feeding frequency, and the number of females feeding on animals. Daily temperature fluctuations may increase transmission rates.
Distribution and variants
The spread of bluetongue is limited by the geographical distribution of its insect vectors, which can survive through winter.
To date, 22 of the 28 recognised serotypes of bluetongue virus (BTV) have been identified in South Africa. These include serotypes BTV-1 to -20, as well as BTV-22 and -24. Among these, BTV-1 to -6, -8, -10, -11 and -24 are more prevalent and frequently associated with severe clinical signs and high pathogenicity, sometimes resulting in large-scale epidemics.
Other serotypes, such as BTV-9, -12, -13, -16 and -19, are less common, while -7, -15 and -18 occur sporadically. Of all the serotypes, BTV-1 to -6 and -10 are most often linked to clinical signs and high pathogenicity.
Immunity following recovery is serotype specific. Antibodies developed against one serotype do not confer complete protection against others. Sheep that recover from bluetongue are therefore immune only to the specific serotype to which they were exposed.
Clinical signs
Bluetongue affects sheep breeds differently. Indigenous sheep and antelope may become infected with the virus but typically show no clinical signs. In contrast, foreign breeds – particularly British meat breeds – are far more susceptible and often develop severe disease. Crossbreeds such as the Dorper are partially resistant and have lower mortality rates.
The disease causes significant direct economic losses due to high morbidity and mortality, stillbirths, abortions, foetal abnormalities, low birthweights, reduced milk production and fertility, weight loss, prolonged gestation, decreased conception rates, and reduced meat and wool production. Indirect losses include the cost of annual vaccination, diagnostic testing, vector control, and treatment.
The incubation period in sheep ranges from two to seven days. Clinical signs include fever, bleeding and ulceration of the oral and nasal mucosa, excessive salivation and nasal discharge, and swelling of the lips and tongue. Inflammation of the coronary band of the hoof may result in lameness. Other signs include weakness, depression, weight loss, severe diarrhoea, vomiting, and pneumonia. Impaired circulation may cause the tongue to turn blue. Pregnant ewes may abort, and animals that recover can experience wool break, leading to partial or complete fleece loss.
In cattle, the presentation and severity of disease signs depend on the specific viral strain. Goats usually exhibit mild or no signs. The virus can ‘overwinter’ in midge populations during periods of low midge activity.
Vaccination is non-negotiable
The most effective treatment for infected sheep is supportive care with sick animals isolated in a shaded pen with fresh water. Provide soft greenfeed, and if oral lesions are severe, offer high-quality pellets that can be easily swallowed after being moistened with saliva. Consider a broad-spectrum antibiotic when the risk of pneumonia is high.
The only reliable method of preventing bluetongue in sheep is timely and proper vaccination before the start of the bluetongue season. Two vaccines are registered in South Africa for this purpose: the Onderstepoort Biological Products (OBP) bluetongue vaccine and Design Biologix’s Blu-Vax.
OBP bluetongue vaccine
The three pentavalent (five-strain) vaccines each contain different bluetongue virus serotypes: bottle A contains types 1, 4, 6, 12 and 14; bottle B contains types 3, 8, 9, 10 and 11; and bottle C contains types 2, 5, 7, 13 and 19.
Vaccines must be administered at least three weeks apart. However, an interval of four weeks or longer is recommended, as studies have shown improved immune response and broader protection against the different serotypes.
The vaccine may be administered concurrently with any inactivated vaccine but not together with another live vaccine. Vaccination of pregnant ewes is mainly a concern when they are immunised against bluetongue for the first time. Young ewes that have received the three vaccines prior to breeding may be safely vaccinated in subsequent years, even during pregnancy. However, vaccination during the first half of pregnancy is not recommended.
Lambs born to vaccinated ewes should be immunised at six months of age. If vaccinated earlier, a repeat dose should be administered at age six months. All animals must be revaccinated annually. Vaccination of rams should be avoided for two months prior to and during the breeding season.
Design Biologix’s Blu-Vax vaccine
Blu-Vax is an inactivated, polyvalent oil-emulsion vaccine that provides protection against 11 serotypes: 1, 2, 3, 4, 5, 7, 12, 13, 16, 17, and 24.
The vaccine may be administered to lambs from seven days of age, as well as weaned lambs born to vaccinated ewes. A booster dose is required three to four weeks later. Blu-Vax is safe for use in pregnant ewes and breeding rams. An annual booster vaccination is recommended.
Bluetongue versus FMD
Regardless of the vaccine used, vaccination must be carried out in spring (August to October) to ensure adequate immunity against bluetongue before midges become more active. Early clinical signs of bluetongue may be confused with that of foot-and-mouth disease (FMD). Bluetongue is characterised by inflammation of the tissue, bleeding, and oedema; FMD causes small vesicular (fluid-filled blister) lesions. The seasonal and non-contagious nature of bluetongue and the role of Culicoides vectors are key distinguishing features. Vesicular stomatitis should also be considered in the differential diagnosis. Vesicular stomatitis, which mainly affects cattle, pigs, horses and even humans, should also be considered as part of the differential diagnosis.
Vaccination and treatment
Dr Johan van Rooyen of Steynsburg Animal Hospital recommends the following measures:
Vaccination
- Handle live vaccines with care and always maintain the correct temperature. Avoid direct sunlight. Store mixed vaccines in a cooler and protect syringes, tubes, and bottles from sun exposure.
- Use vaccines within two hours of mixing.
- Do not administer any other live vaccine simultaneously.
- Inactivated vaccines (e.g., against pulpy kidney) may be given with the bluetongue vaccine but only using separate syringes and needles.
- Lambs should be vaccinated at 70 days with A, 91 days with B, and 112 days with C. A booster of all three vaccines is required at one year of age, before the next summer.
- Administer vaccines in the correct order: A → B → C.
- Ideally, allow 21 days between vaccinations; in emergencies, this may be shortened to 14 days.
- Sheep must receive three separate vaccines at least twice. These three vaccines can later be combined for rams (post-mating) and ewes (post-weaning).
- Expect a fever reaction on day ten, which may reduce milk production in ewes and lower fertility in rams (semen quality may be affected for up to two months; permanent infertility is rare).
- Do not vaccinate pregnant animals, as the vaccine may harm the foetus.
- Wool breakage may occur, especially in animals weakened by parasites or malnutrition.
- Sterilise needles and syringes with steam or boiling water only – never chemicals.
- Always use a new, clean needle, as contamination can destroy the vaccine.
- Avoid vaccinating animals in poor condition, under nutritional stress, or infested with parasites.
- Revaccination is advised if animals have been infected with wireworm.
- Bluetongue-infected animals often die from Pasteurella pneumonia; vaccination improves survival rates.
- Mature animals generally do not require vaccination if initial doses were completed correctly and other health measures are in place.
Treatment
- Rinse the mouth and nose with a strong saline solution.
- Keep sick sheep indoors, out of sunlight, with clean bedding and good ventilation.
- Treat dehydration with a solution of 2ℓ water, one teaspoon salt, and one tablespoon sugar. Provide liquid greenfeed (smoothie).
- Administer antibiotics effective against pasteurellosis (e.g., Advocin, Disulfox, Sulfatrim). Use Terramycin only as a last resort.
- For pain, inflammation, or fever, inject an anti-inflammatory such as Metacam or Ketofen, or insert 3-7-10 Disprin rectally.
- Kyroligo may help improve condition. – Carin Venter, Stockfarm
For more information, contact Dr Tom Strydom at strydomtom@gmail.com or 083 261 5891, or Dr Johan van Rooyen at veterin.sdh@nokwi.co.za or 082 463 3087.
