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June 4, 2018

Nothing as annoying as itching. That’s why life for horses suffering from sweet itch can be downright hell. It is actually an allergy, but this one is embedded ‘in the bar code’, DNA research has shown up.
‘Sweet itch is a seasonal hypersensitivity. Complaints usually disappear with the advance of winter. It is a reaction to the saliva of the female Culicoides midge. Their level of activity is dependent on the location, seasons and weather conditions. In blustery weather for example, they are blown away because they are so tiny. Neither are they very keen on direct sunlight’, says Anouk Schurink who works for Wageningen University & Research.
‘Severe itching is the most noticeable symptom. Horses with sweet itch always rub themselves.’ There are simple and effective solutions to alleviate the stress. Schurink: ‘Don’t expose horses to these Culicoides midges, for instance by using fly- or eczema rugs. Keeping horses indoors during the times when the midges are active – especially at dusk – is another option.’ Up till now there is no medicine against sweet itch.

Why does one horse in the herd contract sweet itch while others are unaffected? The Horsegene Project is a form of research that zooms in on the DNA and genetic predisposition of horses and provides part of the answer. Anouk Schurink: ‘Several factors are found to be of influence. The phenotype equals genotype plus environment. The phenotype is some sort of observation, like height at withers or sport performances. It can also be a particular disease. The phenotype can be detected and is partly influenced by genetics. Age and breed are other factors of influence. The environment is another additional factor that defines the phenotype. In the case of sweet itch that could be bugs. But it may also be the feed or the location where the horse is kept, or stable management. The phenotype is influenced by all these factors.’
‘In case a horse is not genetically predisposed it will not be vulnerable for sweet itch or other illnesses. If there are no bugs the horse will not develop sweet itch. Specific characteristics of the horse determine whether the horse develops sweet itch and how bad the symptoms will be. At the same time specific environmental factors determine whether the horse develops the above symptoms and to what extent.’

Twenty percent
How can be established if a horse is genetically predisposed to develop sweet itch? Anouk: ‘We have analysed a lot of sweet itch- related data for the Horsegene Project. At foal inspections inspectors checked the dams for symptoms of sweet itch. These phenotypes have been analysed as well as the lineages of these horses. On that basis we have been able to establish that susceptibility for sweet itch is influenced by genetic factors/heredity by about twenty percent. Our genetic research has enabled us to establish that this illness is polygene, which means that a multitude of genes play a role. Moreover, the heredity aspects of sweet itch are complex.’

As posted in the June 2018 KFPS newsletter