What are ‘summer sores’?
Summer sores aka cutaneous habronemiasis is a skin disease in horses that is caused by parasites – specifically stomach worms from the Habronema species. These ‘summer sores’ can present in many different forms but most often appears as non-healing lesions that have ‘proud flesh’ aka inflamed / infected granulation tissue. These ulcerated lesions often contain small, gritty, yellow nodules called ‘sulphur nodules’ that look like tiny grains of rice. They typically occur when flies are abundant (spring/summer) hence the name. These parasites do not affect humans.
Where do summer sores occur?
The most common sites include the penis/sheath, corners of the eyes, lip folds, and in existing wound sites. As these lesions are painful and irritating, there is often secondary trauma from rubbing or biting the area. For penile lesions there may be swollen sheaths with difficulty/pain during urination or an abnormal urine stream; and for eye lesions there may be secondary corneal ulceration.
Why do summer sores form?
Typically, in the normal lifecycle, Habronema parasites exist in the stomach and rarely cause any significant problems. Normally the adults in the stomach pass eggs in the manure which are then ingested by flies, which then deposit the larvae around the horse’s mouth where they are swallowed and develop into adults in the stomach, and so the cycle continues. Summer sores develop when these flies deposit the larvae in wound beds or different mucosal membranes (such as the penis or eyes) where the lifecycle cannot be continued and they cause a local inflammatory reaction in these regions instead. These stomach worms are different to stomach ‘bots’ aka Gasterophilus species.
How are summer sores diagnosed?
These can be tricky to diagnose as these lesions can easily resemble other conditions such as cancerous lesions (squamous cell carcinoma), sarcoids, or Buruli ulcers. These parasites eggs are so small they will also not show up on a faecal egg count. Adult parasites can sometimes be seen as an incidental finding on gastroscopy. If you suspect a summer sore, it is important to get a veterinarian to examine – additional diagnostics such as biopsies may be indicated if the history and clinical signs are not definitive.
How do you treat summer sores?
Treatment of summer sores can be difficult depending on the severity of the lesions. Typically, the lesions must be manually debrided by a veterinarian under sedation to remove the sulphur granules to encourage healing, alongside a combination treatment of oral ivermectin (Equimec) anthelmintic, systemic anti-inflammatories such as dexapent/bute, and topical anti-inflammatory/helminthic creams. Severe lesions may need surgical remove of lesions particularly those of the urethral process of the penis.
How do you prevent summer sores
Prevention of summer sores primarily involves disrupting the normal lifecycle of the parasite by effective fly control – such as regular removal of manure, use of fly masks/rugs/boots, fly repellent. Consulting with your veterinarian about deworming protocols – strategic use must be implemented as blanket treatment with dewormers will only drive resistance in this parasite but also in all the other parasites present in the gut.
