Blood Clots in Horses

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

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.