Blood Clots in Horses

The formation of blood clots can be considered beneficial and quite natural when they occur in response to cuts or injuries. They provide the natural plug to stop the unintended flow of blood from the body. But, not all blood clots are good. If the clot forms within a blood vessel and it’s not as a result of injury or cut or for any other good reason, and if it doesn’t dissolve through natural means, then problems can occur which will require medical attention -- sometimes emergently.

Blood clots are described as clumps of blood that form with a gel-like consistency. These clots or clumps of blood can form in any blood vessel anywhere and in any organ of the body, whether human or equine.

The symptoms of a blood clot will vary depending on its size and location:

• Sudden onset of breathing difficulties
• Heat, swelling and pain of affected area 
• Thickening of jugular vein if it is affected
• Accumulation of fluid if both jugulars are affected can cause swelling of head and neck
• Clots of aorta and iliac arteries can cause weakness of hind limbs with lameness on either or both sides after exercise but are normal at rest
• Exercise intolerance 
• Poor performance
• Muscle tremors 
• Sweating
• Chronic colic, constipation, diarrhea

Types

The types of blood clots are classified by their location and the syndrome which may be responsible.

Thrombus is basically a blood clot that forms when the flow of blood in an artery or vein is restricted. Often it will cause an obstruction in the blood vessels at its original site.

Embolus is a piece of a clot that has broken off and has travelled through the bloodstream and lodges itself at a point of narrowing away from the initial clot site

Aneurysm is a blood vessel that has become enlarged due to a weakness in the middle layer of the vessel. There is a disruption in the tissue lining the inside of the vessel that can be associated with the aneurysm that can result in a clot forming which will block the blood vessel.

Blood clots of all types will result in starving blood from the surrounding tissues of all types wherever they happen. When tissues are starved of adequate blood supply, damage and death to the tissue can occur.

The causes of blood clots in horses are as varied as the types and locations where they are found. A list is provided below that is not exhaustive but will give you an idea of the types of things your veterinary professional will be assessing in the diagnostic process:

• Blood clot in a vein can result from the insertion of a catheter for a lengthy period of time
• Foreign material, like bacteria, air or fat, that is carried through the bloodstream
• Disruption of the tissue layer that lines a blood vessel that is associated with an aneurysm can lead to a clot forming
• Larvae of the worm Strongylus travels through the arteries of the horse, causing inflammation of the arterial walls which can develop into clots and aneurysms as well
• Acquired and inherited clotting diseases are basically related to the abnormal production of platelets in the blood - acquired defects in the clotting mechanism are more prevalent than inherited
• Inflammatory diseases like colic (digestive disease), laminitis (inflammatory disease of the hoof), equine ehrlichial colitis (colon infection), catheters inserted into jugular for lengthy periods of time and treatments that require various drugs that inflame (irritate) the blood vessels
• Colic with endotoxemia (the presence of bacterial poisons ((toxins)) in the blood) results in higher death rates and a greater risk of blood clots to form
• Laminitis involves several system disorders - tiny clots form in the vessels early in the disease development stages while bacterial toxins in the blood stream are thought to have an effect on vessels that initiates the clotting process

Diagnosis of blood clots in horses will require physical examination of the horse by your veterinary professional to locate areas that are suspect for clots. Blood tests will likely be required to determine if there is a bacterial involvement in an infection. Ultrasound examinations can help to evaluate the blood flow in the aortic and iliac arteries. A good history from the owner or handler will help a great deal in differentiating the possible diagnoses because not all of the above symptoms will present in every situation. The symptoms noted that you pass along to your veterinarian will help him identify the possible initial site of the blockage and the possible cause. A treatment plan will be developed from the incorporation of all of the information available.

As one might expect, the treatment will depend on the size, location, and cause of the blood clot. For example, embolic pneumonia that is caused by endocarditis would be treated with a long term regimen of antibiotics lasting several weeks. Additionally, anti-inflammatory medications as well as those to help reduce fever might also be incorporated in the treatment regimen. Or, if the blood clots are located in the veins, supportive care that includes hydrotherapy of accessible veins, administration of anti-inflammatory medications and injectable antibiotics to try to treat and control the infection may be the suggested therapy. Generally, surgery to remove the clots from jugular veins can be done successfully but is not necessary as the treatments for the inflammation will usually resolve the clot with the proper medication administration.

In the case of aneurysms that occur as a result of Strongylus vulgaris, these have been found rarely to rupture so the major concern here is the formation of blood clots that occur in the intestines, causing colic. Removal of the clot in this case is impractical because another would likely form anyway. The best resolutions to this type of blood clot is to treat with antibiotics and medications that are commonly used to kill the larvae that are traveling through the bloodstream. Additionally, the prevention and control of strongyles for the best future health of your horse.

Horses suffering from embolic pneumonia that is caused by endocarditis have an outlook that is guarded at best and, if the animal survives, the performance of those horses is decreased. For horses suffering from thrombosis of the cranial or caudal vena cava, the prognosis is poor because this type of blood clot doesn’t generally respond well to treatment. The best recommendation for a plan for the management of by cranial mesenteric and aortic iliac thrombosis is simply prevention and control of the strongylus vulgaris parasite.