By Dr Belinda Black BSc BVMS DVSc DACVS-LA
This year has been a particularly bad year for Habronema infestations, otherwise known as Summer Sores, and probably reflects the high fly numbers seen this past summer. These sores typically occur in wounds that are already present, commissures of the lips, the corners of the eyes and prepuce/urethra of stallions or geldings. They are caused by the larvae of Habronema and Draschia worms, and can mimic the appearance of granulation tissue, squamous cell carcinoma (skin cancer), sarcoids or pythiosis (fungal infection seen in the tropics).
Normally these worms live in the stomach of the horse and cause minimal problems. The mature worms lay their eggs and these are passed through the gastrointestinal tract in the horse’s intestinal contents. Once in the manure, the eggs are eaten by stable fly larvae and are normally deposited near the mouth of the horse, where they are swallowed and grow in the stomach, completing the cycle. However, when the Habronema larvae are deposited onto a wound they can not develop properly and migrate through the tissues, causing intense itching. The self-trauma caused by the itching is what makes these sores quickly become bigger.
Your veterinarian can sometimes diagnose habronemiasis by examining the site, although a biopsy may be required to confirm the presence of larvae. Treatment often consists of a multimodal approach including an oral dewormer to kill the larvae, corticosteroids to control the itch, antibiotics to treat secondary infections or, in severe cases, surgery.
Various forms of fly control such as manure collection and disposal, protection of wounds from flies, and personal insect repellents should be implemented for preventing habronemiasis.
