Understanding Summer Sores and Effective Fly Control

Flies are commonly associated with nuisances, but one condition that often goes unnoticed is habronemiasis, also known as summer sores, granular dermatitis, or jack sores. This ailment arises from a complex interplay between the horse, stomach worms, and their intermediate hosts: house, face, and stable flies. While stomach worms can cause inflammation in the horse’s stomach lining, their most significant threat arises when they invade fresh wounds or moist areas on the horse’s body.

Typical signs of summer sores include non-healing skin lesions, intense itching, and the formation of exuberant granulation tissue, often referred to as “proud flesh.” These sores can appear anywhere a wound occurs, but they are especially prone to developing in moist areas like the prepuce, lower abdomen, corners of the eyes, or margins of the lips—areas where flies commonly feed.

In the typical life cycle of stomach worms, flies pick up the larvae from horse manure, old bedding, or rotten feed and deposit them near the horse’s mouth. The horse ingests the larvae, which then travel to the stomach and mature into adult worms in about two months. These adult worms typically cause little damage to the horse. However, they lay eggs that pass in the horse’s manure, and flies subsequently pick up the hatched larvae, starting the cycle anew.

The issue arises when stomach worm larvae are deposited by flies onto fresh wounds or areas of moisture. In these cases, the larvae cannot mature into adult worms and instead migrate around the horse’s wound, causing local inflammation and severe itching. As a result, the horse often chews on the lesion, leading to the development of proud flesh and non-healing lesions that can persist for years and worsen over time.

Summer sores exhibit a distinctive appearance, often appearing “greasy” with blood-tinged fluid draining from them and sometimes containing yellow or white calcified “rice grain-like” material. These sores primarily occur during the spring and summer, coinciding with peak fly activity. If left untreated, the lesions may appear to improve during the winter but flare up again come spring.

Treating summer sores can be challenging and may require a multifaceted approach. In cases with small lesions, deworming the horse with either ivermectin or moxidectin paste dewormers can effectively kill the worm larvae and aid in healing. Dewormers without these active ingredients will not yield the same results.

For more severe cases with significant proud flesh formation, surgical removal of the flesh may be necessary before treatment can begin. Following this, deworming with one of the aforementioned products should be accompanied by the topical application of a mixture containing glucocorticosteroids and DMSO, which helps reduce inflammation and itching. If the lesion occurs on the horse’s legs, wrapping may be necessary to protect the wound and prevent chewing.

In cases of severe lesions, oral or injected antibiotics and corticosteroids can also be administered. Additionally, cryotherapy, involving the freezing of the lesion with liquid nitrogen, has proven beneficial in some instances.

To prevent further stomach worm larvae from entering the lesions, effective fly control is essential. Here are some tips for successful fly control:

  1. Regularly remove manure, excess feedstuffs, wet straw, and other materials at least twice weekly to eliminate fly breeding sites and halt the hatching of fly larvae (maggots).
  2. Properly manage compost piles to maximize heat production, which helps kill hatching fly maggots.
  3. Employ parasitic wasps as biological control agents to combat house and stable flies.
  4. Utilize insecticides, fly traps and baits, residual fly sprays for premises, fly prevention face masks, and repellents.
  5. Consider feeding insect growth regulators (IGRs) to your horses. These IGRs pass through the horse’s intestinal tract and interfere with the growth and development of fly maggots in their manure.

By understanding the causes and effective treatments for habronemiasis, and implementing robust fly control measures, horse owners can safeguard their equine companions from the discomfort and potential complications associated with summer sores.

By Staff writer