Insect bite hypersensitivity (IBH) is most often caused by Culicoides spp., although other insect species are also thought to play a role in causing this disease. The disease has been reported to occur in most countries, with a prevalence of up to 28% in some countries in some breeds. The only truly effective therapy currently available is to prevent horses from coming into contact with Culicoides midges, which can be achieved by stabling horses during certain times of the day or by covering them with insect blankets. The effect of a new dietary supplement containing vitamins, polypeptides, and amino acids on IBH severity was tested at the start of the Culicoides season (April 2009). Fifty horses participated in this placebo-controlled, double-blinded study: 25 horses received the supplement and 25 received placebo for 30 days; thereafter all horses received the supplement for a further 30 days. The investigators scored the severity of IBH in all horses, based on clinical evaluation and digital photographs taken before and after the first 30 days of the trial. The owners were asked to assess the severity of the signs prior to the investigation and after the first and second 30-day periods. For each period, IBH clinical symptoms were classified as increased, decreased, or the same. Results showed that investigator-assessed symptoms became worse in more horses receiving placebo than in horses receiving supplement, indicating a positive effect of the supplement. However, there were no treatment-group differences when symptom severity was scored by the horse owners. No side-effects were observed.