Problem #1: Colic
Colic just means a horse has pain in its abdomen. There are many causes of this pain and different worms can cause colic. Round worms can cause the pain when a young horse, such as a weanling or yearling, is heavily infected with adult round worms. These young horses seem to become heavily infected at an early age because their immune system does not prevent the larvae from developing into adults. Worms can become very large and completely block the small intestine, in rare cases; they can cause a rupture of the small intestine. This may occur shortly after dewormer is given and all the worms die and the horse tries to pass this entangled mass all at once. This occurs less frequently now because most owners are aware of this problem and deworm their young horses early and often to avoid the problem. Fecal egg counts on young horses over 3-4 months of age can assist an owner in evaluating what the likelihood of having an infection of adult round worm may be.
Any age horse can also show signs of colic from worms. When I graduated from Veterinary school in 1970, large strongyles were the main cause of colic in older horses. Strongylus vulgaris was the one most often responsible. The larvae migrated to the cranial mesenteric artery where they caused severe irritation to the lining of the artery. Blood clots would form in these areas of irritation. These clots would break off and travel to smaller arteries that supplied blood to the areas of the digestive tract. The size of the blood clot would determine how large an area of the intestine would be affected. The size of the affected area would determine generally how severe the colic was. If a large enough area was affected it would prevent the normal peristaltic movement and an impaction of food would occur. Fortunately, most of the time new blood supply would develop and the peristaltic movement resumed before any permanent damage had occurred. However, occasionally larger areas were affected which led to necrosis (death of the intestinal tissue) of the intestine and rupture of the intestine. The only treatment is surgery to remove the section of dead intestine. Fortunately the use of Ivermectin has nearly eliminated Strongyle vulgaris and this cause of colic.
Small strongyles can also cause colic even though they do not migrate through the body as large strongyles do. They only burrow into the intestine wall where they become encysted as part of the normal life cycle. If there are large numbers of these encysted larvae and many of them leave their cysts all at the same time this process may cause enough inflammation to cause some mild signs of colic such as occasionally rolling, restlessness, getting up and down often, not eating, depression listlessness, or slow to no GI sounds. What causes the larvae to stay encysted and then leave the cysts at one time is not well understood. One theory is that there are a large number of adults in the intestine so the larvae wait until there are fewer adults to compete with. Sometimes it appears to be seasonal, in the early spring or late fall when there are many adults in the digestive tract to lay eggs when horses go on pasture in the spring so the contamination is at its highest rate. Another time seems to be when a horse is dewormed and the adults are all killed off.
In the last 7-10 years tapeworms have become more suspected of causing colic. When I graduated from Veterinary School in 1970, when a horse died and a necropsy was done if tapeworms were found in the intestine it was generally noted but it was rare that tapeworms were blamed for causing colic. However, now it is widely accepted that if large numbers of tapeworms are found in the area of the intestine where the small intestine empties into the large intestine the cause of death will be listed as colic. It appears that large numbers of tapeworms that attach to the lining of the intestine can cause enough inflammation and swelling in the area to prevent the normal progression of digestive material and gas through the area. This restriction of movement causes distention of the intestinal wall, which can become great enough to cause colic.