Habronemiasis (Summer Sore) in a Horse!

The images in Figure 1-3 are from a horse that presented for a large, painful swelling along the mare's ventral abdomen. The mare had been treated by the referring veterinarian with anti-inflammatory medication and oral antibiotics with minimal improvement. A core biopsy revealed chronic inflammation with no specific cause identified and an ultrasound exam merely revealed diffuse edema. Close examination of the swelling (Figures 2-3) noted yellow granules which were consistent with sulfur granules. This finding is consistent with 2 diseases in horses which are cutaneous pythiosis and cutaneous habronemiasis. Identifying which pathogen is causing the lesions can be tricky and involves skin biopsies with special staining for pythiosis.

Figure 1

Figure 2

Figure 3

According to the Merck Veterinary Manual, "cutaneous habronemiasis or summer sores are a result of fly larvae which enter existing wounds or moist skin and migrate into the tissue causing irritation and a chronic granulomatous reaction".  As such, management of this condition involves fly control, medication for encysted larvae, AND medication for inflammatory reaction. Fly control is an ongoing issue at most farms and becomes increasingly important as the warm, summer months approach. The most common method for treating for the encysted larvae is a simple dose of oral ivermectin. Unfortunately, this is were most horse owners stop with treating summer sores. There are a multitude of topical therapies which include varying amounts of ivermectin and antibiotics. Unfortunately, these topical therapies are NOT appropriate for treating the chronic inflammatory reaction. As a result, many summer sores continue to increase in size and result in horrible wounds as imaged in Figures 4 and 5. These represent the MOST common location for summer sores in horses that being the lower limbs.

Figure 5

Figure 4

The large swollen plaque across the ventral abdomen is a uncommon location for a summer sore as is the upper lip noted in Figure 5. However, these lesions can develop on any part of the horse's body that is accessible by a fly!! It is critical that the horse owner understands that the chronic, inflammatory reaction is a result of a hypersensitivity or allergic reaction to the encysted larvae, before and AFTER it has been effectively killed with systemic ivermectin. As such, the horse must be treated for the hypersensitivity with products that contain immune suppressive doses of corticosteroids. In small, focal cases, the lesions are effectively treated with topical treatment however large summer sores will need systemic corticosteroid administration!! In my experience, this is the ONLY way to effectively rid the horse of this condition. Unfortunately, once a horse develops summer sores, they will be predisposed the following fly season to re-develop sores in the same regions. I have developed a highly effective topical cream with the help of a pharmacy and would be happy to dispense this product to owners of horses that are dealing with this highly annoying condition!!

Figure 6