The normal stomach worm (Habronema muscae, Habronema microstoma and Draschia megastoma) life cycle sees these small usually harmless nematodes living within the stomach of the horse. They rarely cause any clinical signs and are maintained by seasonal transmission between yearlings and foals. However, if larval migration goes awry then the condition known as summer sores (or cutaneous habronemiasis or habronematidosis) can occur.
Flies (Musca domestica or Stomoxys calcitrans) carry these microscopic larvae and are able to transmit them to horses either by bite or ingestion. When the mouth of the fly lands on the lips of the horse it deposits eggs which, when ingested, hatch and migrate to the stomach where they mature into adult worms. The worms pass out with manure where they infect pasture and, when picked up by a passing horse, they again migrate to the mouth where they are swallowed. The normal stomach worm life cycle then continues, with the adult worms laying eggs which are passed in the manure where they hatch and the cycle repeats itself.
When aberrant larval migration occurs the stomach worms can move to other parts of the body where they cause an itchy, non-healing granulomatous lesion that is called summer sores (figure below). The sores are found on the legs, prepuce and penis, and around the eyes, nostrils and anal openings. The lesions are itchy and can lead to behavioural changes such as foot stamping, tail switching and a refusal to be ridden. Histopathology of the skin reveals a granuloma with caseated or calcified particles and evidence of eosinophils, mast cells and neutrophils.
If the summer sores do not heal they are treated with a broad spectrum dewormer such as Ivermectin. This kills any worms still living in the body but does nothing to address the itchy wound. A topical ectopical application of Ivermectin or a combination of Ivermectin and a wound dressing is then used to treat the problem.
The summer sores are a severe but manageable clinical condition caused by the abnormal migration of the normal stomach worm larvae to areas other than the stomach. It can be prevented by using the correct dewormers as recommended by your vet, avoiding feeding haylage and picking up the horse’s manure every day. This will ensure the normal larval population is kept to a minimum. If a problem does arise it is important to diagnose it early and start treatment quickly with Ivermectin to prevent the development of more itchy, non-healing ulcerative summer sores. It is also essential to have a good vaccination programme in place so that the correct drench can be given at the right time of the year. Vaccinations against Large Strongyles can be particularly effective in controlling summer sores.