HORSES

Myocardial fibrosis in horses  

Ashley Lamond, Bruce Robinson, Murray Hazlett

When white streaking is seen on gross examination of horse hearts, rule-outs include fibrosis, myocarditis, and neoplasia (lymphoma). Myocardial fibrosis is sometimes regarded as an incidental finding, and sometimes as the cause of death depending on the amount of myocardium affected, clinical history and, in the case of unexpected death, the absence of other causes of death.

Recently, a 15 year-old Percheron gelding was presented with a 1-month history of weight loss, increased recumbency, tachycardia, and an abnormal gait. He was euthanized and a field postmortem revealed pericardial streaking with increased pericardial fluid and some streaking in the small intestine (Fig 1). Histology samples were submitted and the only lesion of significance was found in the sections of heart where there was severe patchy interstitial myocardial atrophy and fibrosis in several of the sections (Fig 2).  This fit with the clinical signs seen, except perhaps for the abnormal gait.

The myocardial changes seen were of some duration. Rule-outs in horses with myocardial fibrosis include sub-lethal ionophore toxicosis with residual scarring or vitamin E selenium deficiency. This particular horse did not have access to cattle or other livestock feed. Box elder toxicosis was also considered as a possibility that could probably produce similar lesions if the horse survived the initial intoxication. Multifocal myocardial infarction with residual scarring could also be involved. Strongylus vulgaris infection has also been associated with this myocardial fibrosis. In most cases, a cause is not determined.

Myocardial fibrosis is sporadically diagnosed by pathologists, and a record search found 23 coded equine pathology cases since 2008. In 13 of these cases, the pathologist felt the fibrosis was likely associated with death or the cause of euthanasia, perhaps being associated with an arrhythmia. A specific cause was not found in any of these 13 cases (Table 1).   

Breed Sex Age y History Histologic diagnosis
Standardbred G 4 Dropped dead after jogging. Subacute localized myocardial degeneration and fibrosis
Equine G 10 Collapse on trail Multifocal myocardial fibrosis with myofiber atrophy
Hanoverian M 16 Sudden death Myocardial degeneration and severe fibrosis
Standardbred M 8 Poor performance

Multifocal fibrosis and epicardial fibrosis
Thoroughbred F 4 Ventricular tachycardia Myxomatous and fibrous connective tissue surrounding the sinoatrial node
Dutch Warmblood G 22 Found dead, AV dissociation previously Myocardial necrosis and fibrosis
Donkey G 22 Died within 15 min looked like he had a seizure Mild multifocal interstitial myocardial fibrosis
Quarter Horse G 15 Sudden death Localized myocardial fibrosis
Equine G 7 Died under exercise Focal myocardial fibrosis - probable acute arrhythmic heart failure
Clydesdale F 5 Died under exercise Cardiomyopathy with multifocal interstitial myocardial fibrosis
Donkey G 8 General weakness Severe myocardial fibrosis and myocarditis
Oldenburg G 16 Heart perivascular inflammation and fibrosis Chronic mild to moderate lymphoplasmacytic myocarditis and perivasculitis with fibrosis
Percheron G 15 1-month weight loss Myocardial fibrosis

Figure 1. Pale streaking (arrows) in the heart of a15 year-old Percheron.

Figure 2. Myocardial fibrosis (arrows) in the heart in Fig. 1.