Lameness in a horse is often easy to diagnose and it's not hard to pinpoint the problem. But occasionally the cause is elusive and the horse owner turns to his veterinarian or farrier for help. Sometimes the clues are subtle and the person observing the horse has a better chance of solving the mystery if he or she knows what to be looking for. Hind leg lameness caused by blood clots is one of those puzzling cases that can be a challenge the first time you see it. The blood clots that impair the hind leg circulation are due to damage from bloodworms.

Strongyles, also called bloodworms or red worms, are the most common internal parasites of horses and also the most damaging. There are more than 50 species of strongyles that parasitize horses, including three species of large strongyles-of which S. vulgaris does the most damage.

When mature, this bloodworm is usually less than an inch long and lives in the intestine where it lays eggs that pass out with manure. The eggs hatch into larvae that crawl to nearby plants during warm weather to be eaten by a horse, which starts the cycle over again. Ingested larvae travel through the stomach and penetrate the intestinal wall to begin extensive migrations through body and bloodstream before reaching maturity and migrating to the large intestine to attach and feed on blood in the intestinal lining. While traveling through the bloodstream, most of the larvae lodge in the mesenteric arteries that supply the intestine with blood, and in the walls of the large arteries leading away from the heart. Too many bloodworm larvae in an artery can cause rupture or blockage. The attached larvae cause inflammation, scarring and roughening of the artery wall and lining, making weak spots in the wall which may then balloon outward (aneurysm). This damage can also lead to formation of blood clots.

Because of the damage, the artery wall is no longer smooth on the inside, and clots can build up on the uneven surfaces. If a blood clot breaks loose and travels on down the artery, it eventually comes to a place too small to pass, and blocks the artery, shutting off the blood flow to the tissues supplied by it.

Although any artery in the horse's body may be damaged by bloodworm larvae, the mesenteric arteries which supply intestines with blood are the most often affected. If a section of intestine suffers from decreased blood supply, there is abdominal pain and colic. The colic may be temporary if a small artery is involved, or fatal if the artery is one that supplies a lot of intestinal tissue. A severe blockage results in death of a section of intestine, and subsequent death of the horse. A small blockage may be survived, since blood may be rerouted to the damaged part through alternate vessels; within a few hours or days the horse may develop new routes for circulation, preventing the death of that piece of intestine. The episode usually leaves some damage and scar tissue, and reduction in function-which may make the horse more susceptible to other types of colic.

Another common site of artery damage is where the aorta (the large artery that leaves the heart and runs under the backbone) branches into the two iliac arteries which supply the hind legs with blood. If blockage occurs here, the horse will be lame in one or both hind legs, and the affected leg or legs will be cold to the touch, due to impaired circulation. If blockage is complete, the leg tissues will die from lack of blood, producing gangrene and death of the horse.

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The lameness usually appears with exercise and disappears with rest. If both hind legs are affected, the horse may at times have trouble supporting his hindquarters and will stagger and wobble. The veins on the affected leg may seem collapsed while the veins on the normal leg will stand out prominently-since they are providing backflow to the heart for an adequate blood supply that came through unimpaired arteries.

The problem is often confused with azoturia (the tying up of large muscles in the hindquarters) because the lameness starts soon after exercise begins, and the horse starts sweating and showing pain and anxiety if forced to continue. The horse may exhibit signs of discomfort, distress and colic.

The main characteristic of this kind of lameness due to artery blockage is that the lameness or discomfort appears with exercise and disappears with rest. A short while after the horse stops moving, he seems fine again. By contrast, a case of azoturia may take several days of rest for full recovery. Also, the horse suffering from a blood clot in the artery will have a cold leg or legs while experiencing an episode. A quick feel of the hind limbs can give an important clue. The horse may also have poor muscle tone in the hindquarters.

Another clue to this condition is that the affected leg will not sweat. The blood circulation is too impaired for fluid to be easily pulled out of the bloodstream to create sweat. And in cold weather, the horse may suffer frostbite or even freezing of the affected leg, due to the lack of proper blood circulation.

A veterinarian can often diagnose the problem by rectal examination of the horse, feeling the aorta and iliac arteries through the rectal wall. If there is a noticeable decrease in pulsation in the iliac arteries on the affected side, or a vibration in these arteries, a blood clot should be suspected. In some cases the clot can actually be felt as a large bulge (sometimes as big as a grapefruit or larger). A clot in the aorta (the body's largest blood vessel, which travels along the backbone to then branch into the iliac arteries that supply the hind legs) is called an aortic thrombosis.

Over time, the vessels may establish side routes around the clot. If this happens, circulation to the leg or legs improves. The lameness episodes may cease. But if no side routes can be established, the horse becomes progressively worse and eventually must be destroyed.

Diagnosis can often be aided by using ultrasound. The ultrasonic probe is inserted into the rectum to check the arteries going to the hind legs. This can often detect clots that might be missed by manual palpation. If a clot is suspected, ultrasound imaging can help pinpoint it and can also show how much the clot is affecting the blood flow to the hind legs or leg.

In a mature horse, the aorta is about two inches in diameter. A clot may constrict it to just a portion of its normal size, able to carry only a fraction of the blood needed for the muscles during exertion. The blood vessel may be enlarged and bulging from the clot, but the actual space left for the blood to go through may look like a river trying to squeeze through a small gorge, when viewed on the ultrasound's computer screen.

If the ultrasound scanning is done immediately after the affected horse is exercised, looking at the veins that normally collect the "used" blood after it has flowed through the muscles to bring them oxygen and nutrients, the circulation impairment is obvious. In a normal horse, or normal leg, the major vein fills with "used" blood almost immediately, whereas the vein in a leg impaired by a clot in its artery will take several seconds to collect the blood returning to the heart from the muscles.

A horse with impaired hind leg circulation due to a blood clot should not be allowed to exert strenuously. A blood vessel enlarged by a clot may be weakened and more apt to rupture-which could cause the horse to rapidly bleed to death.

Bloodworm damage to the arteries can also cause aneurysms, which are dilated sacs bulging out from the artery wall. When an artery is weakened, it may balloon outward at the weak spot. If the aneurysm becomes large, it may suddenly rupture, causing swift death of the horse. The arteries (especially the aorta and iliac arteries) have a great deal of pressure within them, and if an artery is weakened by a clot or an aneurysm, or brittle with scar tissue from bloodworm damage, it may rupture, usually when the horse's blood pressure is elevated from exertion or excitement.

Some horses with blood clots in the aorta or iliac arteries can be helped by daily doses of aspirin to combat the clotting and thin the blood. If the clot is discovered soon enough, while the blood components that form it are still active and able to respond to aspirin, the clot may partially or completely resolve, eliminating the circulation impairment; the horse recovers to normal. But many clots are not diagnosed soon enough, or are not suspected until the horse has a serious problem, and by then the clots have become fibrous. Once a clot becomes "hard," it won't respond to aspirin.

All too often the mysterious hind leg lameness is not properly diagnosed. The problem is mistaken for episodes of azoturia, or an elusive back problem, or mysteriously weak hindquarters. By the time the true nature of the problem is discovered, it may be too late for the horse, due to tissue death in the hind leg or legs from impaired circulation.

If a horse has a puzzling hind leg lameness or weakness, don't forget to do a simple "feel check" to see if the leg or legs are warm or cold. If a hind leg ever has a lower temperature than the other legs or muscles of the horse, have a veterinarian examine the horse rectally and/or with ultrasound to determine the possibility of a blood clot. Bloodworm damage to the hind leg arteries can be prevented by a diligent deworming program throughout the horse's life, to keep him free of migrating bloodworm larvae. Ivermectin will kill the larvae in the blood vessels as well as the adult worms in the gut. Prevention is the best policy, since treatment of hind leg blood clots is not always successful.

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