All horse owners have to deal with the nuisance of summer insect pests. It seems to simply be part of life around horses. However, some of these pests aren't just a nuisance, they can actually cause a health hazard to your horses.
Case in point is the common fly. Among other things, flies can carry the larvae of two different types of stomach worm, Habronema muscae and Draschia megastoma, known as Spurids. When these larvae are deposited on skin abrasions, they can cause skin lesions known as summer sores.
These sores can heal up quickly in winter, but often recur in the warmer months of summer, hence the name. The lesions seem to cause intense itching and horses will chew and bite at them, delaying the healing process and in many cases allowing secondary infections to occur.
If you've had to deal with summer sores, you will know that topical applications of antibiotics and salves aren't fully effective. They seem to start responding and then come back again. When the horse bites and chews at them they can quickly take on a bloody and unsightly appearance. In order to control the larvae that cause the infection, it's necessary to apply an anthelmintic directly to the site. Dichlorvos and Trichlorfon have been shown to be effective in treating summer sores.
Prevention is better than cure and the prevention of choice is a strict deworming program. Ivermectin is effective against the Spurid larvae which cause summer sores, so a deworming program which includes at least two treatments of Ivermectin each year will control both summer sores and stomach worms.