Heart Murmurs and Valvular Heart Disease in the Horse
Dr. Melissa Mazan
Oilcan Harry was a 2-year old Standardbred who was not performing as well as his owner would have liked. He had a lifetime mark of 1:56 – good, but not good enough. Because his breeding was good enough that he had the potential to be a faster horse, his owner brought him into the Sports Medicine Clinic at the nearby veterinary school. What sort of housing does Harry have?
At the clinic, Harry had an extremely thorough work-up. The first step, as always, was a thorough history and physical examination. The clinician, resident, and students peppered the trainer with questions.
What is his feeding schedule?
When was he last vaccinated?
Does he get dewormed regularly?
Does he ever cough, or have difficulty breathing?
It turned out that about six months ago Harry had had a case of the flu, as had most of the other young horses in the barn. Harry had seemed to recover well, except for a cough that had never really gone away. There were two other youngsters in the barn who were still coughing as well.
Harry didn't seemed sick other than that, and after the initial episode of influenza, never had any fevers. His trainer was concerned, however, that Harry never seemed to have a very good appetite. He was fed 8 lbs of a sweet feed mix twice daily, and two flakes of timothy/alfalfa hay in the morning and at night – and he seldom finished it all.
On physical examination, Harry seemed to be in good general health, although on the thin side. One of the senior students noticed a systolic murmur with a regurgitant quality – a grade two out of a possible six. Dr. Smith ordered an echocardiograph, which showed that the cause of the murmur was tricuspid regurgitation, meaning that every time Harry's right heart contracted, a little of the blood washed back into the right atrium, instead of progressing all the way into the right ventricle.
The echo suggested that the heart was doing just fine – the dimensions were normal, and the ejection fraction, a measure of the strength of the hearts contractions, was normal.
Concerned about the cough that Harry had had once in a while, Dr. Smith returned to the history. They performed a few more tests – CBC and chemistry profile looked absolutely normal. However, when they listened to Harry's lungs with a rebreathing bag, Harry coughed and coughed. At this time the lungs didn't sound normal. Rather, they could hear a few wheezes when Harry breathed out.
Dr. Smith decided to go a little further, and performed tests to look for small airway inflammatory disease (SAID) – and they found that Harry indeed had all the findings to confirm SAID.
As it turned out, the tricuspid regurgitation was just an incidental finding and not the cause of Harry's poor performance. Many young horses have a small amount of tricuspid regurgitation; in fact, it is more common in young racehorses – probably because their hearts are pumping so much blood.
Harry got the flu just like all the other youngsters in the barn, but his airways remained reactive to stimuli such as dust and molds, and he developed a non-infectious inflammation in the small airways, which limited his ability to get oxygen when he was working at very high speeds.
Dr. Smith decided to treat Harry for SAID. After two months of treatment, Harry was no longer coughing. After 4 months, the trainer called to say that Harry was faster than he had ever been. He probably would never be world class, but the trainer was happy that Harry was performing at the level he was bred for.