Research has revealed that sweet itch (Dutch: SME) has a hereditary factor. Further European research must be conducted to reveal which genes are responsible for this condition. When this knowledge can be applied to breeding it will become possible to reduce the number of horses that are suffering from sweet itch. Dr.ir. Anouk Schurink (B Sc) explains.
Why is this research necessary?
‘To relieve the itching the horses damage their mane and tail by rubbing. There is no cure for this condition and little research has been done to investigate the effectiveness of supplements and ointments or gels. If we manage to gain more insight into the genetic background of sweet itch (SME) we can hopefully diminish the number of affected horses in the future.’
What kind of disorder is sweet itch?
‘It is an allergic reaction which is caused by tiny flies called midges. The females need blood to produce eggs. They bite the horses and their saliva contains specific proteins that can cause an allergic reaction.’
How did you investigate the hereditary characteristics?
‘We have based our research on the data and lineages of approximately 6,000 Shetland ponies and over 3,500 Friesian horses. Our research has revealed a 20% hereditary factor that influences sensitivity to sweet itch. There is no other factor except exposure to midges that is known to have such a major influence on the occurrence of the condition.’
What is the benefit of this information?
‘Knowing the degree of heredity helps to assess the breeding value. If the breeding values of stallions and mares are known it becomes possible to assess the sensitivity of offspring.’
Does this mean that a mare and stallion who are both free from sweet itch will produce offspring who are unaffected by it?
‘If an illness is caused by one gene it is possible to work out the chances whether the offspring will be affected or not. In the case of sweet itch a combined number of genes determines the occurrence. And environmental factors play a role as well.’
Is it fair to assume that sweet itch can be eradicated by selective breeding?
‘It will be a lengthy process, but the number of affected horses can be drastically reduced. If the breeding value for sweet itch can be established by introducing progeny testing of breeding stallions then the number of affected horses can be reduced by 3% per generation. This is how the breeding programme can decrease the sensitivity to sweet itch in the population.’
It would be even better if sweet itch could be wiped out at a faster rate.
‘This can only be achieved if we can pinpoint exactly which genes are responsible for sweet itch. This will be the subject of further research of the European-based project Horsegene.’
How will Friesian horse owners benefit from the results of this research?
As soon as the responsible genes for the development of sweet itch have been identified, breeders can start to use this information when choosing a partner and in doing so help to speed up the process of elimination. Moreover, DNA information can give a reliable genetic indication for the sensitivity of individual horses. Then even young horses can be diagnosed for their proneness to sweet itch.’
Wouldn´t it be a better option to stop breeding with Friesians who are suffering from sweet itch?
‘That would be a little too drastic, especially since there are quite a number of Friesians who are affected by sweet itch and the population has a narrow genetic basis. Besides, such a specific individual Friesian horse might have other positive characteristics that can help to improve the population. Not breeding with sweet itch-affected Friesian horses will probably result in a faster reduction of affected horses. It is something to keep in mind when choosing a breeding partner.’
As posted on the KFPS Newsletter June 9, 2014
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