Sweet itch

Summer seasonal recurrent dermatitis (SSRD) is a chronic inflammatory skin or dermal complaint. It is a seasonal disease : it appears in Spring, intensifies through the summer, and disappears gradually during autumn for the majotrity of animals affected.  Clinical signs will then reappear every year and often get worse over the years.
It is due to the hypersensitivity of certain horses to allergens present in the saliva of  Culicoïde midges. The female culicoides bite the horses and suck blood. It is the most common allergic dermatitis in equines. The prevalence of the condition (ratio of horses affected to the total number of the equine population) is of 1 horse out of 10 in France.

When the condition first appears, bumps called papules can be observed  in the areas where the insect bites occur. They are however difficult to see in the mane and tail and disappear as soon as the horse starts to scatch. SSRD is a highly pruriginous affliction, and following scratching, the mae and tail hars break, scabs appear, as well as partial alopecia (loss of hair). When scratching is intense wounds which can become infected may also appear.

Dermatitis wounds are usually located in the dorsal area (base of the tail, rump, neck and withers, more rarely around the ears) and more rarely on the belly (along the white line). As a genral rule the wounds abate gradually through autumn, and disappear in winter. Over the years the damage becomes chronic and no longer disappears in the winter : the skin thickens and wrinkles, hair no longer grows.

In the most serious cases, horses can lose weight due to the bother caused by the midges, which interrupts a regular eating pattern. The use of the horse can become delicate due to nervousness induced by itching. It can therefore decrease the economic value of the animal.

The first clinical signs appear between 2 to 6 years old : the horses have to be sensitized to the allergens before they express hypersensitivity. Clinical expression is insidious to begin with but often worsens over the years.

Any breed can be affectedA 50 % prevalence (1 out of 2 horses) has been observed in adult Icelandic horses imported to France as adults. This high prevalence could be due to the fact that there is no early exposure to the allergens contained in the culicoides saliva, as there are none in Iceland.

Genetic predisposition

Some lines of horse are more affected than others. This indicates a genetic predisposition of polygenic origin (several genes are implicated) but those genes have not yet been identified. There is also likely to be a link between SSRD and respiratory allergic complaints.

Environmental factors

Les culicoïdes adultes sont présents entre mars et novembre (quand les températures sont supérieures à 10-15°C).

Adult culicoides are presetn from March through to November (in temperatures above 10-15°C)
They are most active at dusk (between 7:00 p.m and 10:00 p.m). For some species there may also be an activityat night and in the morning (2 or 3 hours after sunrise).
The development and breeding cycles of culicoides are linked to favorable climatic conditions :
•    They develop in a temperate environment between 15-35°C ;
•    They don’t like rain or strong winds ;
•    They are exophile : they live outside and do not come into stables .
The females lay their eggs in damp areas. Larvae are therefore found in ponds, along the banks of streams, marsh lands, ruts, silty and muddy areas, and also in tree hiollows, and manure heaps….
Consequently there are areas where culicoides are more abundant.

Diagnosis is mainly based on observation of the clinical signs and their location, as well as the seasonal character of the affliction. When in doubt, complementary examinations can be carried out :
•    A skin biopsy will show up the allergic character of the wounds ;
•    Intra-dermoreaction skin tests, by injecting allergens into the horse’s derma (very small quantities extracted from the insects) – With what is known today, these tests are not very specific, and are therefore difficult to use in the field.

Prevention remains the best option to fight SSRD since there is to date no really effective treatment. Preventive actions should be implemented very early on in the season (at the beginning of spring when temperatures go over 10°C)
The first thing to do is to prevent the culicoides from biting the horses. It is therefore recommended to associate several of the following measures:
•    Bring the horses into closed buildings before the end of the day, and turn them out after daybreak. Mosquito nets soaked in insect repellents can be hung on the entrance to the building ;
•    Avoid fields with damp zones or less than 500m away from the manure heap ;
•    Regularly clean the drinking trough;
•    Use a combo fly sheet with fine mesh- Sheets specifically adapted for sweet itch are available on the market. The sheet should cover the horse from head to tail including the belly. It should be worn continually from March through to November, or at least during hazardous periods. .
•    Vaseline can be spread on the areas likely to be affected, this creates a mechanical barrier against the midges. Avoid using this method when it is hot and sunny (risk of sunburn) ;
•    Insecticidal sprays (particularly containing pyrethroid) can be used, either by means of sprays or shampoos. Repellents (citronella, benzyl benzoate…) can also be used , but their effectiveness is restricted. It is important to use this type of product on a very regular basis for it to be of any use. The effect of a spray is very quickly eliminated due to the horse’s sweat.

A research team has recently developed a vaccine which has shown some signs being effective in alleviating the severity of the symptoms in horses with SSRD. Additional studies are however necessary to approve it. For the moment this vaccine is not available on the market.
Supplementing the horse with essential fatty acids with a high omega 3 and 6 content, such as linseed oil, could also have an effect on decreasing the severity of the clinical signs. Studies are however contradictory.

There is no specific treatment. Numerous studies have tested the efficiency of immunotherapy (repeatedly injecting culicoide extracts) with contradictory results. There is for the moment no standardised protocol to be used in the field.
Medication can be used to help fight prurit :
•    Steroid anti inflammatories  (or cortisone) can be used to temporarily relieve the more severe cases;
•    Antihistamine treatment : variable efficiency depending on the horse, the molecule and dosage used.
Numerous lotions combining repellent, soothing and healing actions are available on the market. They are not veterinary medicines and their efficiency has not been demonstrated. They can be used in addition to other preventive measures.