Heart Murmurs and Valvular Heart Disease in the Horse
Dr. Melissa Mazan
Specific Causes of Valvular Heart Disease
Endocarditis refers to an infection of the inner lining of the heart. Although it is not common, it is an important cause of valvular heart disease in the horse.
Scamp, a 12 year old Quarterhorse gelding, lived in a comfortable barn with 10 other horses. They were all well-loved companions, and did a fair amount of traveling to small local shows, hunter paces, and the like. A respiratory virus had gone through the barn a month and a half ago, and everyone had recovered quickly except Scamp. He had had a fever and a cough for 5 days, and his veterinarian, suspecting that he had a secondary bacterial respiratory infection on top of a virus, put him on antibiotics for 7 days.
He seemed to do well, with the fever going down after just a few days of treatment. After that, Scamp did well for a week or two, but then he seemed to deteriorate. He lost his appetite, and began to lose weight. He didn't cough anymore, but he looked a little ragged.
His owner called his veterinarian, Dr. Green, who decided to do a thorough work-up before putting Scamp back on antibiotics. He found pitting edema under his belly and the jugular veins were distended. He also detected a pulsation in his veins and a high fever of 104.2 degrees Fahrenheit.
Dr. Green also found an abnormally high heart rate and a loud systolic murmur with a regurgitant quality – a grade 5 out of a possible 6. He suspected endocarditis, and recommended bloodworm, a blood culture, and a consult with a cardiologist for an echocardiogram. In the meantime, he started Scamp on some very broad spectrum antibiotics.
The complete blood count (CBC) showed a high white cell count, and a high fibrinogen level. The chemistry profile also showed a high globulin level, indicating that Scamp's immune system was working hard at trying to eradicate an infection. The lab also called to say that the blood culture was already starting to grow bacteria – indicating that Scamp had an infection in the blood.
The cardiologist scanned Scamp's heart, and, just as they suspected, found an infection involving the mitral valve. The infection had already caused some valve deformity, and Scamp's owner could appreciate that the valve, which should be smooth and working with clockwork precision, was thickened and lumpy. This is called a vegetation; the valve no longer works to keep the blood flowing in one direction.
Dr. Green started Scamp on some very powerful antibiotics until he received the results of the blood culture and could prescribe something more specific to the causative organism. Hopefully, several months on antibiotic therapy would eliminate the organism.
Scamp was on antibiotics for 8 weeks. During the first 3 weeks, he stayed in the hospital on intravenous antibiotics, then he was able to receive oral antibiotics at home. Scamp remained thin and depressed for quite some time, but after the first three weeks, he gradually began to seem a little more like himself. His heart rate came into the normal range, and he no longer had any fevers. After 8 weeks, his blood work looked normal, and he held his tail high as he trotted around the pasture. It took another 4 months before Scamp had regained the lost weight, and was able to go back to his usual rounds of hunter paces and showing.
What happened to Scamp?
Scamp probably got influenza, like all the other horses. Unlike the others, he developed a bacterial infection on top of the viral infection. The bacteria were resistant to the antibiotic that Scamp was on, and eventually spread to the blood, and then to the heart. This is not a common sequela to influenza, but it is a common history in a horse that does develop endocarditis. Scamp was lucky that his veterinarian was careful and astute, and found the endocarditis quickly.