There’s nothing more aggravating than an itchy spot you can’t reach. For a horse, the midline of the belly certainly qualifies. This is what happens when the horse is suffering from summer midline dermatitis.
Midline dermatitis is a seasonal problem that coincides with warm weather and biting insects. The problem usually begins with migrating microfilaria (tiny larvae) of the neck threadworm, Oncocerca cervicalis. Adults of this parasite live inside the nuchal ligament, the strong ligament under the mane which runs from the base of the skull to the withers.
Adults produce microfilaria which travel under the skin until they reach the midline of the belly. This is also a favorite feeding spot for biting flies since the horse cannot get at it easily to chase them off. When the flies feed, they pick up the micorfilaria and carry them to the next horse they bite, which also becomes infected.
Meanwhile, the horse’s belly is being irritated by both the presence of the larvae and the biting insects. The end result is a seasonal skin irritation with itching, swelling, oozing, hair loss and sometimes even open skin.
The Onchocerca microfilaria can be controlled using the deworming medication ivermectin every 3 to 4 weeks. Horses with very heavy burdens may actually be worse for a while after deworming as the body reacts to the dead microfilaria. The adults producing the microfilaria can’t be killed by the ivermectin but it is believed the treatment can eventually suppress production of microfilaria.
The local irritation should be addressed first by cold water hosing for at least 5 to 10 minutes. After it dries, try addressing the itching with Bactine or Zim’s Crack Creme, both human OTC products you can get at any drug store. Use a thick layer of a soothing salve on top of this. It will also form a physical barrier to help block biting insects. Look for ingredients like Calendula, Comfrey, Chamomile, Witch Hazel and Plantain for soothing effects.
You can also help the horse re-establish homeostasis to quiet down skin reactions by supplementing antioxidants. Vitamins C & E, bioflavinoids, lipoic acid combine well with Turmeric, Ginger, Boswellia and other natural plant-based antioxidant to quench free radial stress in irritated tissues.
It takes some time and effort but diligent care can bring summer midline dermatitis under good control.
Eleanor Kellon, VMD