Summer Sore Time is here!!

Although I have already written a blog concerning summer sores in horses (go to: http://michaelporterdvm.blogspot.com/2013/06/habronemiasis-summer-sore-in-horse.html) , the condition is a recurring one and it deserves another visit!! Below are several images from different horses that suffer from recurring habronemiasis or equine summer sores. The horse in Figure 1 suffers from recurring habronemiasis in all 4 limbs, whereas the horse in Figure 2 recently developed summer sores along his sheath!! These summers sores are medium in size and treatable with topical and systemic medication. However the horse in Figure 3 is suffering from a horrible summer sore of his lower limb that has not been treated but allowed to grow unabated and permanently disfigure the limb. It is frustrating to the horse owner that these lesions tend to return every year with the return of flies however if treated early and aggressively, management of this condition is quite do-able!!

Figure 1

Figure 2

For horses with recurring or first time habronemiasis, I recommend the following:

1: Systemic treatment with a dewormer containing Ivermectin. Treat twice, approximately 3-4 weeks appart.
2: Topical treatment with Dr. Porter's summer sore cream!!
3: If the summer sore is excessively large or in a location that is difficult to treat with topical medication, I recommend systemic therapy with corticosteroids. I prefer to medicate with a tapering dose of oral prednisolone over 30 days.

It is critical that the barn management incorporate a fly control program of their choosing. In addition, horses which suffer from recurring summer sores need to be treated the moment there is any redness or mild irritation present in the old summer sore sites!! If treated early, the lesion will typically respond favorably to topical medication without the need for systemic corticosteroids! 

Figure 3

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

Core Biopsy Please!!

The only way to know for sure would require a core biopsy. This procedure is quite simple to perform in the field and will save significant time with regards to treatment and resolution of clinical signs. The first image was treated unsuccessfully as a "summer sore" with medication that actually worsened the clinical signs. The biopsy results were consistent with Pythiosis and aggressive medial/surgical treatment was instituted. The prognosis for full recovery is not good once there has been significant disruption of the soft tissue structures and in a case that involves the lower pastern/coronary band area. Early diagnosis is key!

The case below was treated as a suspect "summer sore" with minimal improvement. Biopsy results indicated that it in fact it was a case of Habronemiasis that was not responsive to the treatment provided (systemic and topical ivermectin). Habronemiasis or "Summer Sores" is a condition that results from flies depositing larvae within the skin causing a significant allergic reaction. Treatment involves medication to kill any living fly larvae and most importantly medication to suppress the local allergic reaction. Treating with ivermectin systemically and topically is often not enough!! Yet that is how many try to treat this condition! For small lesions, topical treatment with concentrated cortisone can be effective, however, for more aggressive lesions, systemic administration with corticosteroids is necessary. Importantly, systemic treatment with corticosteroids is the treatment you definitely want to avoid if you are dealing with Pythiosis! Hence the importance of a core biopsy.