Does your horse appear to have a summer cold or summer sores? If so, the cause may be a parasite infestation of either ascarids, also known as large roundworms, or stomach worms, also known as habronema.
Palomino horses in summer pasture
What appears to be a summer cold or summer sores in your horse may be an infestation of worms.
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Ascarids, also called large roundworms, are often found in young horses. They can grow to be twelve inches long and can be found in the hundreds in the small intestine, leading to poor nutrition and causing coughing, colic, and diarrhea. If they reach the lungs, they may cause pneumonia.
These parasites have developed strategies to survive adverse conditions in the environment. Warm, wet conditions are necessary for the development of larvae that can infect horses. Hot, dry conditions were previously thought to be the enemy of parasites, but summer weather has been shown to maintain populations of parasites that have adapted to all types of weather conditions.
Research conducted by Dr. Michael Sukhdeo of Rutgers University, has shown that small strongyles can actually survive very hot dry conditions by “dehydrating.” In this state the worms are brittle, use very little energy, and can survive long periods in the pasture. They cannot infect a horse in the dehydrated stage but will rehydrate when rain returns.
When a drought is broken, large numbers of infectious stage parasites may become viable in pastures. Dragging pastures in summer will just serve to spread these dehydrated parasites.
Parasites can survive in the infectious stage at temperatures of 45 to 85 degrees. Eggs can hatch and develop into infectious stage larvae in 3-5 days at temperatures in the upper 70’s when moisture is adequate. At cooler temperatures, this may take weeks or months.
Large roundworms can cause stunted growth, digestive problems, ruptured gut, and death.
What appears to be a summer cold can be caused by larval stages in the lungs. Usually, foals and young horses are affected by large roundworms, and immunity to these worms develops at about 18 months.
Stomach worms or habronema live in colonies in the wall of the horse's stomach. Eggs that pass in the feces are picked up by flies, which serve as intermediate hosts. When the fly feeds on the wounds or around moist areas of the horse's body, the larvae escape from the mouth parts of the flies and may be swallowed by the horse.
A large number of habronema can produce severe gastritis. Tumor-like enlargements may occur in the wall of the stomach and, if these tumor-like enlargements rupture, peritonitis usually ensues.
In addition to internal problems, habronema cause summer sores and conjunctiva or inflamed, sore eyelids.
Diagnosis of both large roundworms and stomach worms is usually by a fecal egg count.
An on-going parasite control program is the best way to keep equines from suffering the consequences of parasite infestations. If you don't already have a scheduled program, your veterinarian can help you establish one that will work best for your horse.