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- Bluetongue has been prevalent amongst sheep since the importation of the first Merinos into South Africa in 1789.
- The Orbivirus genus, that causes bluetongue, has at least 24 virus serotypes.
- Bluetongue is transmitted by blood-sucking midges.
- Bluetongue is a seasonal disease that mainly occurs in summer and autumn, especially after good summer rains.
- Infected animals develop a fever and oedema of the lips and face with the lips, nose and eye mucous membranes possibly bleeding.
Bluetongue can be eradicated in sheep flocks by timeously administering the triple-action bluetongue vaccine, strictly according to instructions. A lot of producers have succeeded in eliminating this problematic animal disease by following this route.
Dr Baltus Erasmus, retired veterinarian who was previously associated with Onderstepoort and Deltamune, says this sheep disease has been prevalent since the importation of the first Merinos into South Africa in 1789. Cattle and almost all indigenous goats are susceptible, but rarely contract the disease. However, they contribute to keeping the bluetongue viruses alive. Non-indigenous goats and animals such as alpacas are highly susceptible and can easily die from the disease.
At least 24 virus serotypes belonging to the genus Orbivirus cause bluetongue. South Africa is home to 21 of these serotypes. This is problematic – if an animal has contracted bluetongue from one virus serotype, it will build up resistance only against that specific virus serotype. If one of the other serotypes infects the animal, it will become ill.
Spread of the virus
Bluetongue is transmitted by blood-sucking midges. If a midge bites an infected animal, the virus multiplies in the midge’s intestinal tract and spreads to the salivary glands; if the midge bites another animal seven to ten days later, the virus-containing saliva is injected into the animal.
This explains why the disease is more likely to occur during times and in regions marked by conditions that are favourable for the hatching of large numbers of insects. Bluetongue is a seasonal disease that occurs mainly in summer and autumn, particularly after good summer rains. In areas that are extremely cold, the disease typically disappears around two weeks after the first frost. In warmer areas the disease can occur all year round, peaking in summer and autumn.
Symptoms of bluetongue
Infected animals develop a fever and oedema of the lips and other parts of the face (sometimes even the ears). The lips, nose and eye mucous membranes may bleed. There is also visible erosion of the superficial tissue on the nose, edges of the nostrils and mucous membranes in the mouth, especially around the incisors and molars.
The tongue may be very swollen and protrude from the mouth. The tongue can be blue purple in colour – hence the term bluetongue – due to poor blood circulation. However, this type of tongue discolouration is only seen in a few sheep.
Another clear sign of infection is inflammation of the hoof coronets (coronitis). A purple-red band forms at the junction of the skin and the hoof. This painful condition is caused by vascular bleeding in the horn and can last from three to seven days. Sheep may have difficulty walking or could prefer to lie down.
Sick animals should be removed from the flock immediately and herded into a shaded camp that has enough water and soft feed. Sick animals normally lose a lot of weight, and wool sheep’s fleece is usually badly damaged. Depending on the conditions, the mortality rate varies from zero to as much as 20% of the flock. Animals can contract pneumonia during sudden cold spells and broad-spectrum antibiotic treatment should be considered.
Control through vaccination
As the damage caused by bluetongue can be severe, early vaccination is the only solution. The first bluetongue virus was successfully cultivated in the fifties. By 1960, different serotypes of the virus were described, and it also became evident that a single vaccine would be ineffective against all the viruses as sheep cannot launch an immune reaction against so many different serotypes. Hence, antibodies to only two or three of the most dominant strains were produced.
In 1976, two more serotypes were isolated and included in the vaccination programme. Three different vaccines were eventually produced, each counteracting five different serotypes of the virus. The vaccines must be administered separately, at least four weeks apart, to ensure broad protective immunity.
It is essential to vaccinate sheep at six months of age using the three agents separately. The vaccine can be administered together with other medications, except another attenuated vaccine. Producers who vaccinate their animals timeously, according to the instructions and repeated annually, will succeed in keeping bluetongue out of their flocks. If done this way, bluetongue is 100% controllable, concludes Dr Erasmus. – Andries Gouws, Stockfarm
For more information, phone Dr Baltus Erasmus on 083 730 6265 or email firstname.lastname@example.org.