- Wesselsbron virus disease and Middelburg sheep disease. Part 2
|Last update: April 10, 2012 08:28:30 AM|
WESSELSBRON VIRUS DISEASE AND MIDDELBURG SHEEP DISEASE
C. W. A. BELONJE,
A NEW type of mosquito has definitely been proved to be capable of spreading the disease. This insect breeds in shallow recently formed pools, and apparently feeds only for a short period before sunset. The possibility of other ways of transmission cannot be excluded and it is possible that other means may exist. The Wesselsbron virus appears to have considerable powers of resistance and it is not yet known how long it survives in a carcass, and if it can infect sheep by being swallowed or inhaled.
Two distinct viruses have been isolated from the outbreak in the Middelburg area, one of which has been identified as the Wesselsbron virus and other one as a totally new and distinct virus. Whether the one causes only, the abortive form and the other the liver damage, or if both are required together to cause this amazing disease, is not yet clear at present. The fact to be faced is that here again we have an insect vector and the appearance of disease-producing viruses in the Karoo Midlands.
A vaccine has already been prepared and is undergoing tests at the Onderstepoort Laboratories. Preventative inoculation of stock is a powerful means of controlling epidemic diseases, particularly those of a bacterial nature. Viruses unfortunately are very unstable and change characteristics readily. This is particularly the case on the African continent where the changing or mutability of viruses causes considerable difficulties in vaccine production. Bluetongue and Horse sickness for instance, are caused by several varieties of virus, which have to be included in the vaccine to offer full protection. But every now and again the vaccine appears to fail. Investigation then reveals the presence of a new strain, against which all previous known ones included in the vaccine do not immunise.
Preventative inoculation of stock can therefore be regarded as only a partial measure of virus control and much more stress must be laid on the importance of reducing or eradicating insect vectors. If this aspect is ignored, it will only be a matter of time before the next virus makes it disastrous appearance.
To control the spread of the disease regular spraying of small stock with the latest insecticidal solutions was undertaken on one farm. The sheep were collected weekly and sprayed by means of a pressure pump, paying particular attention to the non-woolled parts of the body. Drinking troughs were cleaned out and reservoirs treated to prevent mosquito breeding. Very little improvement was seen at first because the latest insecticides are contact-poisons and mosquitoes can pass on infection before dying; Nevertheless the method was persevered with, the effect became cumulative and the insect vectors became adequately controlled, which was proved by the death rate slowing down considerably.
In other cases removal of sheep from river and vlei camps where mosquitoes breed to high mountain veld was sufficient to stop the disease. All mortality ceased within ten days after such removal.
The presence of this disease has been reported from Colesberg, Graaff-Reinet and Barkly East.
There is ample evidence to show that the Wesselsbron virus can infect cattle, pigs and perhaps horses. It has definitely been proved to be the cause of an influenza-like disease in humans. The possibility exists that in cattle it may be the cause of abortions and of heifers returning to the bull. In the case of adult cows the history is of a decreased milk production for a few days, a temporary and partial lack of appetite and loss of calf. Heifers and dry cows, when settled to the bull, are invariably put to graze in a vlei and are brought in just before calving, when it is discovered that they are not in calf at all.
At the moment the extent of the disease in South Africa is unknown. Furthermore it is not known if the immunity is short or lifelong in the various species of animals and in human beings.
Merino Breeders Journal 19 (3)