As the heat of summer quickly approaches and the temperature rises, so do the number of flies in and around the barn and stable. The common house and stable fly are intermediate hosts in the disease process of mucocutaneous habronemiasis also known as “summer sores.” These persistent lesions are the direct result of skin, eye, or wound infection due to the stomach parasite Habronema spp or Draschia megastoma. Adult Habronema spp and Draschia megastoma are 6-25mm in length.The eggs and larvae produced by these gastric parasites are passed in the feces of the horse. As stable and house flies selectively lay their eggs in piles of manure, the resultant maggots then eat the larvae or eggs from the adult Habronema spp. When fly pupae mature into adult flies and begin feeding, the Habronema parasite is then deposited on moist regions or wounds found on the horse. Parasites deposited near the horses mouth are swallowed which then complete the cycle in the horse’s stomach.
These ulcerated, non-healing infections often occur at the inside aspect of the eye, around the mouth, on the sheath or distal end of the penis, and in any type of wound. The persistent infections occur during peak fly season hence the term “summer sore.” Summer sores are unique in that they do not heal properly, have visible yellow granules, and are generally bloody with an excessive amount of discharge. The yellow granules are necrotic calcified nematode larvae.
Proper veterinary care is usually warranted for any lesion that is suspected of being a summer sore, due to their persistence. Additionally, certain forms of skin cancer may have a similar appearance.