As the name suggests ‘myocarditis’ describes inflammation of the heart muscle (myocardium). Myocarditis can occur with or following bacterial or viral infection, but in many horses a specific cause cannot be determined. One common cause is due to toxicity from ingestion of growth-promoting antibiotics, such as monensin or lasalocid. This occurs from contamination of horse feeds at feed mills, or by accidently feeding cattle feeds to horses that have been supplemented with monensin. Another important cause of myocarditis is snake bite.
Signs
Clinical signs range from none, through reduced athletic performance, intolerance to exercise, collapse, and sudden unexpected cardiac death. Many affected horses have changes in heart rate (normal to fast), changes in heart rhythm (arrhythmia), and cardiac murmur.
Diagnosis
The diagnosis of myocarditis is usually made by measuring cardiac biomarkers in serum. Some of these are also used to detect myocarditis and myocardial infarction in people. The most common of these is cardiac troponin I (cTnI). This is a protein that is released into the blood when there is disruption of a cardiac muscle cell (myocyte). Highly sensitive human assays are available, some of which have been validated in horses. Other less sensitive assays are alpha-hydroxybutyrate dehydrogenase (α-HBD) or calculating the relative proportion creatine kinase isoenzymes.
Ultrasound of the heart is important for assessing heart muscle contractility, the size of the left ventricle, and the width of the interventricular septum and left ventricular free wall. Essentially, the heart enlargers, the walls thin, and the heart beat is weak.
Treatment
The treatment of myocarditis involves elimination of any known underlying diseases, rest, and anti-inflammatory drugs.
Prognosis
Prognosis is very much dependent on the underlying cause, the degree and duration of heart muscle inflammation, and the amount of fibrosis that develops after inflammation. Some horses will succumb to fatal ventricular arrhythmias, others will develop pump failure and congestive heart disease. If the inflammation is relatively short-lived and mild, and the condition recognised early, the prognosis can be very good. It is recommended to rest horses if they have fever or active viral disease.
Tags: Cardiology