Veterinary Pathology`s Role In Horse Care

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Pathology is commonly associated with abnormal diseases and findings, although it’s technically the study of the nature, cause and progress of disease in general. Pathologists study tissue and cell samples and also perform autopsies or necropsies, as they’re called when done on animals, to learn the likely cause of death.

Dr. Beth Valentine is a widely respected veterinary pathologist and researcher who was first associated with Horse Journal when she contacted us in 1996 about her work with EPSM (equine polysaccharide storage myopathy). We return to her now to help us understand probable causes for unexplained death in horses and the pathologist’s role in furthering diagnosis and treatment of disease.

Is finding out the cause of a horse’s death the most common reason a horse would have a necropsy performed'

This is certainly the case when a horse has died unexpectedly. But we also perform necropsy examinations to confirm clinical suspicions, to learn more about particular disease processes — for example, to try to determine how often a tumor in the skin or mouth spreads to other parts of the body — and to document the extent and severity of problems for the clinicians, owners, and, sometimes, insurance companies.

Do you usually find the cause'

Yes, especially if the horse died unexpectedly. Sometimes, though, we have some answers but still some unanswered questions. This is particularly the case in horses with colitis (inflammation of the colon), in which there are a variety of causes and many are difficult to detect. So, we can determine the extent and severity of the colonic injury to explain death or justify euthanasia, but we don’t always have a cause for the inflammation.

What are the most common causes of horses ”dropping dead” suddenly'

If the horse is observed to suddenly drop dead, this is usually due to sudden cardiovascular dysfunction or rapid onset pulmonary (lung) edema. A heart with a fibrous scar for whatever reason can suddenly develop an arrhythmia that can be fatal. Toxins such as those in yew plants (Taxus species) cause heart failure and sudden death. Rupture of large blood vessels such as the aorta is also a cause of sudden death. HYPP-positive horses are also prone to sudden death, perhaps due to the effect that too much potassium in the blood can have on the heart, but we’re still not exactly sure of the cause when an HYPP-positive horse drops dead. Pulmonary edema is sometimes the only lesion found at postmortem of sudden-death horses. This has been particularly true in racing horses. No one really knows the exact cause of this yet, either. 

More often a seemingly normal horse is found dead in the stall or pasture, with no one observing for several hours prior to death. Here we can find other issues, most often intestinal twists or ruptures that cause rapid onset of shock and death.

How often do necropsies produce unexpected findings'

It’s still so difficult to see inside a horse with current imaging procedures that we certainly find unexpected things in horses more often than small animals.

For example, I examined a horse that had a severe progressive respiratory disease that was unresponsive to any treatment and found that a large blackberry branch had been inhaled and had worked its way through the lung into the chest cavity.

I also had a horse with unexplained illness and evidence of abdominal infection (peritonitis) that was found to have a wire that migrated out of the stomach into the spleen, causing a large abscess that eventually ruptured.

Can you give our readers some examples of how the science of pathology has led to better understanding of diseases'

Ah, you’ve given me a great opening here! Muscle disease, and in particular tying-up, is a great example of how pathology has led to better understanding of disease in horses. For many years it was said that there was no reason to biopsy muscle from a horse that tied up because the pathologist would not be able to tell you anything useful. That was actually true at the time, when muscle biopsy samples were examined only by routine stains and were looked at only for evidence of muscle damage and not for a cause of muscle damage.

Now that we know we need special procedures, including special stains, to evaluate muscle from horses, and we know that we need to look for things other than damaged fibers, we are able to detect the abnormal glycogen and glycogen-related polysaccharide that builds up in horses with polysaccharide storage myopathy, the most common cause of tying up and other muscle issues in horses.

By recognizing that buildup of polysaccharides indicates something is abnormal in carbohydrate metabolism in these horses, we were able to design diets high in fat and fiber and low in starch and sugar, and control the problem well in most cases.

It seems that biopsies aren’t done as often as they could or should be, even when it would be a simple thing to do, like a skin biopsy. Do you think biopsies are underutilized'

Yes. I think that biopsies from horses are becoming more common, though, with better sedatives, anesthetic agents, biopsy instruments, and more referral clinics for horses. I absolutely agree that when there is a skin problem that can’t be immediately diagnosed and treated, or if there is a skin mass that doesn’t quickly go away, biopsy is an important part of the work up. A lot of skin lumps in the horse look alike until you get them under the microscope, and the pattern of change within cases is important for determining the cause.