Infections that localize to the heart are rare but serious in any animal.
A recent study published in the Canadian Veterinary Journal by Dr. Brianne Henderson and colleagues at the University of Guelph’s Ontario Veterinary College sought to better understand heart infections in horses.
They focused specifically on infections of the heart valves, which is called valvular endocarditis. The endocardium is the tissue that lines the inner chambers of the heart and continues over the valves. By searching Ontario Veterinary College medical records, they identified and summarized 20 cases of this condition between 1993 and 2020.
The fact that there were only 20 cases in this 27-year period confirms that this is a rare condition in horses. There were stallions, geldings and mares affected. Breeds included Thoroughbreds, Standardbreds, Quarter horses and others. Additionally, horses were young, with seven years as the average age.
This can be a tricky disease to diagnose —veterinarians relied on blood tests, ultrasound of the heart (echocardiography), blood culture, and if the horses were euthanized, autopsy examination. Abnormalities in the blood include anemia (low volume of red blood cells needed to carry oxygen) as well as increased white blood cell count and increased inflammatory proteins, both of which can indicate inflammation somewhere in the body.
Clinical signs in horses with valvular endocarditis are vague and non-specific in most instances. These include weight loss, fever, rapid breathing and lameness. Specifically pointing to the cardiovascular system, though, horses may have increased heart rates and heart murmurs that can be heard via a stethoscope.
Heart valves with endocarditis are thickened from the inflammation with clumps projecting off them. They can look a bit like cauliflower florets to make a food analogy.
Thickening from inflammation changes how the valves function, making them unable to open or close properly. Heart murmurs are the noises created by valves that aren’t function properly and the subsequent turbulent blood flow.
The clumps of inflammatory material from the heart valves can break off and block downstream blood vessels that supply other vital organs such as the guts, heart muscle, kidneys and lungs. Blockages in the small blood vessels cut off the normal blood supply and the tissues die from lack of oxygen. These little clumps of material also carry bacteria to distance sites where they can create new pockets of infection. In this way, infection in the heart can affect other major organs and severely compromise the patient.
Of the 17 horses that underwent an autopsy examination in this study, 13 had evidence of these types of lesions elsewhere in the body.
In many cases, the cause of the infection is unknown. Infection anywhere else in the body can localize to the heart valves by spreading in the blood stream.
Horses that have a jugular catheter to deliver medications or intravenous fluids sometimes develop infections and clots where the catheter is placed. This is one example where the infection can then spread from the jugular vein and start infection in the heart valves.
Horses with valvular endocarditis have a poor prognosis, which was confirmed by the study. Of the 20 horses, 17 were euthanized, while three were treated and discharged from hospital.
Information on one of the discharged horses was available; it survived for at least one year, while the outcome of the other two is unknown.
If treatment is attempted, it takes long-term antibiotics and follow-up testing to determine if the treatment is working, which can be very expensive.
This is an important study because it brings a Canadian perspective to a rare disease in horses that can be challenging for veterinarians to diagnose and treat.
Dr. Jamie Rothenburger, DVM, MVetSc,PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine. Twitter: @JRothenburger