SSRD is an allergy due to insects' bites (culicoides) but appears ONLY when the outside temperature is above 12°C (53°F).

Some horses have an insects' bite sensitivity caused by reactions to bites of culicoides. Their immune system is over reacting to bites (or the response is inappropriate) and more specifically to a foreign substance called allergens. These allergens are considered to be dangerous by the body even though they should be tolerated.

SSRD is the consequence of insects' bites on the skin.

Technical information
The insects we are talking about look like mosquitoes and their salivery gland proteins are responsible for the allergy. They are 4000 species in the world divided in 78 different categories but only 4 of them are registered under "culicoides". These 4 categories still represent 1000 species!

SSRD appearance
SSRD usually appears when the horse is around 3 or 4 years old that is to say when it is no longer protected by the immune system transmitted by its mother. The 3 or 4-year-old horse begins to have its own immune system but it is still weak. All horses experience insects' bites but only the weakest horses will react to them by developing skin infections and SSRD. That is why SSRD can appear anytime in the year , anytime in the day and no matter what the environment may be.

Applications
Applying Derfen will not cure your horse because it is an allergy and not a disease. On the other hand, it will definitely have a soothing and repellent effect. in fact, the only thing we can do is to reinforce the immune system of the horse to make it stronger against parasitic attacks. 

What needs to be done
Helping your horse to be stronger may be simple and usually lies in feed; adding flax to its ration, food supplements or organic regimen will make the immune system stronger but always keep in mind that they are long-term solutions...Results cannot be immediate.

In the meantime, the use of a product ,which repels insects, hydrates the skin and at the same time heals the sores, like Derfen enables to tackle the skin infection.

                WHAT YOU NEED TO REMEMBER

1-SSRD is an allergy caused by insects' bites (culicoides, when you see these swarms of insects in the forests...) and is not contagious.

2-When a horse contracts the allergy, it will be allergic for the rest of its life. The allergy is made possible all the more easily as its immune system has deficiencies.

3-Midges responsible for the infection are almost everywhere but 2 factors will keep them away:
  -windy conditions (not strong enough to fly)
  -cold (below 12°C or 53°F)

4-These midges are to be found in environments such as:
  -woods or  forest nearby
  -damp areas protected from wind

Now, if your horse is rubbing its tail or mane , it does not necessarily mean it is SSRD. Other allergies are possible, such as those to dust mites and symptoms will just be similar. Also, too much or too rich food, as well as sweat can cause itching.
Check with your vet