Sweet Itch in Horses

Sweet itch is often considered a seasonal problem over the warmer months as the midge is dormant over winter. However, in particularly bad seasons, midge attacks may continue to be seen over autumn. Although biting often stops as the seasons change, in some cases symptoms continue due to self-trauma that leads to secondary infection. 

Sweet Itch in horses is a skin condition caused by an immune response to the saliva of the insect, the biting midge, which causes a histamine release. Although not all horses experience this histamine release, in those that do it leads to extreme swelling and itching. This condition affects thousands of horses throughout the world and although all horses may be affected, Icelandic and cob type ponies appear to suffer from this more than thoroughbred horses.

The most common symptoms seen are thickening of the skin and itching. Other signs may be:

• Alopecia along the mane, back, and tail
• Severe itching which is usually localised to the back, tail, and mane
• Open or broken skin that may become infected or bleed
• Itching may spread to the legs, chest, and stomach
• The horse may cause trauma to themselves in an attempt to relieve themselves of the itching
• May not tolerate tack on their skin due to heightened sensitivity or pain
• May become extremely agitated when flying insects are around 

Types

Sweet itch is one of a number of causes of pruritus in horses. This condition is characterized by itching skin; types of pruritus in horses are:

Parasitic - Caused by parasites such as the midge, Demodex mite, fleas, ticks, and lice 

Infectious – Caused by skin disease of bacterial or fungal infection; the most common causes are staphylococcal infections, Malassezia overgrowth, and dermatophytosis

Allergic – This may be due to environmental factors such as pollen, dust or spores

Sweet itch is caused following a bite from a small insect called a midge. The skin of the horse is one of the most major immune barriers his body has, which protects against foreign invaders. When the midge bites, saliva is left on the skin of the horse. In many horses, the immune system will recognise this as a foreign protein which will then be eliminated in a normal immune response. In horses who suffer from sweet itch, the immune system instead triggers an excessive immune response, known as a type one hypersensitivity reaction. This response then causes a large release of substances, one of these being histamine. The release of the histamine can cause localized itching and swelling. In order to scratch, horses often cause self-mutilation and further damage to the skin.

Your vet will examine your horse’s skin, paying particular attention to the sites that are most affected and the character of the distribution. Your veterinarian will focus on trying to determine what is causing the itching and development of pruritus. This will require them to rule out other possible triggers. Useful diagnosis tools may be:

• Skin scrapings that enable your vet to check for parasites such as horse lice and infestations of other skin parasites, such as mange mites
• A skin or exudate sample to rule out yeast and fungal infections

In the case of secondary infection, a culture and sensitivity test may be required. This is used to test for bacteria present in the affected skin. Your veterinarian will take a sample from the skin using a sterile cotton culture swab. This sample will then be sent to a laboratory for testing. At the laboratory this is spread onto a bacterial growth medium. If growth occurs, it is watched for growth patterns and characteristics that indicate bacterium type. A “sensitivity” is then determined by introducing antibiotics into the bacteria colony, bacteria death indicates effectiveness.

The most effective way to treat this condition is to prevent exposure to the midge. This can be done by: 

• Moving your horse away from environments likely to have high midge populations such as wetlands or near water sources
• Stabling your horse at the most active time of the midge – dawn and dusk
• Using a full body blanket on your horse to prevent the midge from biting
• Regularly applying insect repellent to your horse
• Apply a thick oil to prevent the midge from biting
• Prevent midges entering stables by using fly screens at doors and openings
• Consider placing your horse on a windy bank or installing fans in stables as midges are unable to fly effectively 

If a secondary infection has occurred, your horse may require antibiotic treatment. This may be through topical ointments, or in severe cases, systemic antibiotic therapy.

The prognosis for your horse is fair, and although horses that show increased immune responses to midge bites are unable to be cured, this can be managed. The above steps can be taken to reduce the level of exposure your horse has. If your horse is exposed to midges discuss the use of antihistamines and prednisolone cream with your veterinarian to decrease itching and immune response.