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Cutaneous habronemiasis commonly known as “summer sores” or “Florida sores” is a common skin disease of horses primarily caused by three species of parasites: Habronema muscae, Habronema microstoma and Drashia megastoma. The house fly and the stable fly are responsible for carrying the larvae of these parasites. Therefore they are called the intermediate host in the life cycle of these nematodes.
In summary, the fly carries the immature stage of the parasite from the environment into the horse’s skin, especially in open wounds or wet areas. The horse’s immune system mounts a reaction towards the parasite and exuberant granulation tissue is formed as shown in the pictures. This uncontrolled reaction occurs in order to clear the infestation (get rid of the larvae) and is called a hypersensitivity reaction.
Sometimes, the larvae are deposited around the mouth of the horse, the horse accidentally ingests them and they become adults in the stomach of the horse. The adult stages of the nematode then live in the stomach, producing larvae, which are then passed in manure. When ingested by maggots (immature flies) from the horse’s manure, the cycle is repeated.
Habronemiasis can be confused with other diseases such as proud flesh (granulation tissue), skin tumors (ie sarcoids), carcinomas, fungal granulomas, pythiosis or other skin infections. There is no breed or gender predilection, although some horses are more predisposed than others. There have been no good scientific explanation why some horses are more predisposed than others. Although a presumptive and clinical diagnosis can be made by debriding the wound and finding yellow calcified material of the size of rice grains (larvae) surrounded by granulation tissue, the best and most accurate diagnosis is made with a biopsy. Even though while a “summer sore” may appear anywhere in the body of the horse, there are several areas of predilection. These areas are the legs, eyelids, third eyelid, conjunctiva, genital areas of geldings and stallions and any area of trauma to the skin.
Treatment and prevention
Treatment should be directed towards controlling the hypersensitivity reaction and eliminating the parasite. Usually Habronema species are sensitive to ivermectin and moxidectin. Often, common de-worming techniques do not work, therefore there is the need for additional treatments. In the legs, topical therapies (“summer sore ointments”) should be applied. The area should be kept bandaged at all times until the disappearance of any blood remnants or exposition of any live tissue. Laser therapy is a great option for treating lesions that are difficult to bandage.
Fly control is an essential part of the treatment and prevention of this disease. In our experience, the best fly control consists of fly predators. These are creatures that cut the life cycle of the fly by eliminating the fly larvae. Fly sprays can also be used. If fly sprays are used, they need to be used aggressively in order to maintain the adult fly population always under control. In addition, it is essential to keep stalls clean at all times in order to achieve best fly control.
Careful attention needs to be paid to unresponsive lesions or to fast growing lesions. If a lesion does not responds to therapy, if it is growing too fast or if it persists over the winter, it is important to consider further diagnostics. A good veterinary-patient-client relationship will help achieve the right diagnosis and prompt treatment.