Insects
Surely one of the main triggers is a biting insect in the genus cullicoides that is found worldwide.
Common names are biting midges and no-see-ums. Other biting insects like sand flies, stable flies, and black flies may also cause allergic reactions and the horse owner will notice that his horse becomes very sensitive towards flying insects and gets greatly agitated.

The breeding sites of the biting midges are commonly found in wet marshy areas; however, running streams constitute the main breeding sites. Until recently, people were of the opinion that mainly standing water was more of a culprit. There are over 1,000 species in the genus Cullicoides. Breeding sites vary for the different species. The primary breeding areas will depend on the species found in the particular geographic location.

What you need to know about cullicoides
   The biting midge is one of the essential triggers of Summer Eczema, so it is important to know as much as possible about its habits. It is interesting to know that only the females bite.
Soon after hatching the females mate and require the blood of a host animal for their eggs to mature. A single female biting midge can produce up to 400 eggs per batch. The larvae cannot develop without moisture. With the warmer sun in the early spring the biting midge start to fly. This is the beginning of “Eczema Time”.
   The blood-sucking female bites and injects saliva to prevent the blood from clotting and some horses experience allergic reactions to the saliva resulting in itching. From March to late October, sometimes even longer, depending on the geographical region, the biting midges are prevalent, therefore the name Summer Eczema.
In higher elevations, or right along the coast, horses find more relief from the midges since the midges are poor fliers and don’t fly in windy conditions. Because of that factor horses are more likely to develop Summer Eczema in regions where the climate provides excellent conditions to reproduce, e.g., wetlands, marshy areas, lakes. The feeding time of the midges is at its peak during early morning and evening; they even fly during the day if conditions are right. It is said that midges can cover between 2 to 8 kilometers during the night hence it is advisable to pasture horses 10 kilometers away from standing or flowing water.
   Recent research has shown us that moving streams have posed a significantly greater risk than standing water. Unfortunately, we don’t always have a choice.