Insect bite hypersensitivity – or IBH, the technical term for sweet itch – makes the summer months a misery for many horses, ponies and donkeys.

The costs of veterinary attention and replacing fence posts that have been damaged trying to satisfy an incessant itch, as well as the repair of fly sheets, makes it an expensive and frustrating condition for owners too.

Strictly speaking IBH is a seasonal, recurrent – that is to say affected animals get it every year – allergic reaction of horses to the saliva of biting midges of the Culicoides family. It is estimated that between 3 and 5% of the UK horse population are afflicted by the condition, with symptoms developing as soon as midges appear in the spring months.

As well as itching, symptoms include hair loss, thickening of the skin and flaky dandruff. The top of the tail and the mane are most commonly affected. Horses with IBH usually show signs between one and five years of age.

There is currently no cure for IBH and efforts have focused in recent years on developing methods to reduce midge numbers and their contact with horses, as well as therapies that provide relief from the condition. Notable advances include midge-busting machines like the Midgeater, rugs such as the Boett blanket and Flybuster, and the novel therapy Cavalesse.

Research has also continued into understanding why only some horses are affected by IBH and designing vaccines that might be used to block the immune response in affected animals. Recent developments in these areas are reported in this article.

Work on the biology of midges has also been given greater impetus because of the role that these insects play in the spread of Bluetongue and their potential as carriers of African Horse Sickness.

It has long been believed that susceptibility to IBH is a genetic trait, inherited from generation to generation.

This is based on the observation that sweet itch is much more common in certain breeds, especially Icelandic horses, and has been confirmed by recent population studies in Shetland ponies.

The inheritance of IBH is not, however, as simple as a single gene being inherited from the sire or dam. Scientists have identified several genes that make individual horses more susceptible to sweet itch and these work in a highly complex manner.

By means of illustration, there are no midges in Iceland but, when transported to Europe, as many as one in four Icelandic horses develop IBH. When the DNA of Icelandic horses was examined, they were found to have similar numbers of these susceptibility genes as other types of horse that are more resistant to sweet itch.

The explanation for this apparent conundrum lies in the fact that Icelandic horses born on mainland Europe – as opposed to being exported from Iceland – are no more likely to get IBH than other breeds. .

Exposure to midges during early years of life appears to be important in helping young horses become tolerant to the proteins present in midge saliva. It is these proteins that, if not recognised by the horse’s immune system as being harmless, trigger the hypersensitivity reaction which we recognise as sweet itch.

These findings have opened the door for researchers to develop ways of kidding the immune system of IBH-affected horses into believing that midges pose no threat. This re-programming of the immune system – known as immunotherapy – has been used in asthma and other human allergies. It involves repeated exposure to small amounts of the proteins that would otherwise trigger the hypersensitivity reaction.

Scientists at Bristol University have already identified the offending proteins present in midge saliva and work is now underway to find the most effective way of administering these in a course of immunotherapy.

One possibility is to immunise foals with these proteins so that their immune systems develop tolerance. Adult horses might be exposed to small but increasing amounts of the proteins through repeated or injections or the incorporation of the proteins in foods that could work by oral medication.

This work should not be confused with the on-going trials of a so-called sweet itch ‘vaccine’. This has been available for several years and is actually based on bacterial cell extracts that are designed to modify the immune response. Initially available only by injection, this product is now available for oral administration as BioEos Sweet Itch Capsules.

It has been reported that some 70% of horses treated with this product show a 50% improvement in clinical signs, with 10% showing substantial reduction in signs and 10% minor improvement. The capsules are given monthly and cost £235 for a year’s treatment. Further information can be obtained from the National Sweet Itch Centre.

Avoiding midge bites has been the mainstay of preventing IBH, with efforts centred on reducing midge numbers, making horses ‘unattractive’ to them, and stopping them actually biting horses.

Midge busting machines have the capacity to trap 2-3000 female midges each day and, with constant use, can significantly reduce midge populations in and around stables and paddocks.

Recent studies have re-examined the behaviour of Culicoides species and confirm that they are most active around sunset. There is also a peak of activity at dawn, but little activity during the afternoon and night. Stabling horses with IBH at period of intense midge activity thus makes sense.

There has been speculation that certain horses might be prone to sweet itch simply because they are more attractive to midges. This has been disproven in several studies, which in fact showed that Culicoides are more attracted to horses without IBH. It is unclear why this is the case.

Insect repellents are essential in the fight against flies and clegs, as well as midges. Research with two common insecticides found that permethrin had little or no effect on the attractiveness of horses to midges, whereas cypermethrin was toxic to midges for up to seven days following application.

Cypermethrin is found in Deosect, which is licensed for use on horses, as well as other products like impregnated ear tags for cattle that might be attached to head collars. The Rambo Flybuster rug is made from fibres infused with permethrin and might offer added protection.

One of the most unlikely sounding midge repellents is Avon’s Skin So Soft, which is available as a spray or lotion. No one foresaw its anti-midge powers when launched in 1961 but it has become a favourite of many including the Royal Marines. It doesn’t appear to work as true repellent, instead it creates an oily barrier that makes it physically impossible for midges to bite.

Veterinary therapies for sweet itch have historically been restricted to antihistamines and corticosteroid anti-inflammatories. A new product called Cavalesse was launched in 2009 and seems to be effective in managing some horses affected by IBH.

Cavalesse is a mix of water soluble vitamins and minerals, including nicotinamide, which works to maintain healthy skin and promote normal immune function. It is given as an oral solution each day combined with a cream that is applied to affected areas of skin. Feedback from our clients, as well as on internet forums, has been mixed but with most agreeing that it is worth a try. The cost is around £120 for a three-month supply.

Dealing with sweet itch can be frustrating, time-consuming and expensive and, undoubtedly a great piece of advice is to just look for a new horse or pony in the summer. Only then will you avoid the horror of your lovely new horse, purchased over the winter, denuding his or her mane and tail the minute spring arrives!