Background
Aquaculture in the Nordic countries is dominated by salmonid (Salmonidae spp) species such as rainbow trout and Atlantic salmon. However, high mortality of fish before slaughter is a serious threat to the financial and sustainability of the fish farming industry, and also comprises a significant fish health and welfare challenge. Fish mortalities often occur during stressful handling events like grading, parasite treatment, and transportation, but the specific reasons behind these deaths are unclear. However, a growing body of evidence indicates that cardiac rupture is the cause of death in many fish. Indeed, salmonid fishes can develop a range of cardiovascular diseases that affect both the heart and blood vessels. For instance, farmed salmonids are reported to have a high prevalence of coronary arteriosclerosis1, which may constrain blood flow to the heart, making it less resilient to handle stress. The implications of impaired cardiac function due to arteriosclerosis are supported by data from laboratory studies showing that rainbow trout (Oncorhynchus mykiss) with experimentally occluded coronaries display reduced cardiac and aerobic metabolic capacities as well as warming and hypoxia tolerance2. Unfortunately, there are currently no effective screening tools for monitoring cardiovascular health to early detect heart diseases in salmonids. In human medicine, electrocardiogram (ECG) analyses are widely used for screening and diagnosing various cardiac pathologies, but this has so far not been widely applied in fish. The present study was designed to comprehensively evaluate the suitability of ECG recordings as a screening tool for heart disease in salmonid fish. Specifically, we mapped abnormalities in the ECG following experimentally induced myocardial ischemia in rainbow trout with surgically ligated coronaries.
Material and methods
Fish were anesthetized in freshwater (10°C) containing buffered MS-222 and custom-made ECG electrodes were implanted subcutaneously. One experimental group (coronary ligated) had the main coronary artery occluded permanently by using a silk suture to induce myocardial ischemia, while a second group (sham operated) was treated identically except that the coronary artery was only exposed but not ligated. Experiments were performed on both anaesthetized and unanesthetized fish. Provocation maneuvers were also used to experimentally elevate the fish’s heart rate (i.e., atropine injection and chasing stress) in ordered to increase the sensitivity for detecting any cardiac abnormalities. After the experiments, all fish were euthanized and the heart was removed and histologically examined to link cardiac electrophysiological properties with heart morphology and pathology.
Results and Discussion
At necropsy, the coronary ligated fish had comparatively pale hearts relative to the vivid red hearts of sham operated fish. While all sham operated fish survived, there was only a 55% survival rate among the coronary ligated fish at 10 days post-surgery. Four out of five deceased fish showed signs of blood accumulation in the pericardial sac of the heart (hemopericardium) at necropsy, and this was likely due to atrial/ventricular rupture. Indeed, one of the coronary ligated fish displayed signs of ventricular aneurysm, which may be an underlying cause of cardiac rupture. Coronary ligation significantly affected ECG characteristics in both anesthetized and non-anesthetized fish. Abnormalities in the QRS morphology, which represents ventricle depolarization, included a loss of QRS voltage and prolonged QRS duration as a consequence of myocardial ischemia. The loss of QRS voltage suggests extensive myocardial injury leading to a loss of viable myocardium mass 3, while the prolonged QRS complex duration has been linked to an increased risk of ventricular arrhythmias in humans. In addition, the coronary ligated fish showed atrioventricular (AV) conduction delays, which manifested as 1st and 2nd-degree AV blocks. The 2nd degree AV block typically occured at high heart rates (Fig. 1).
Conclusion
We show that ECG analyses of both anaesthetized and unanaesthetised fish have the potential to be used for screening and diagnosis cardiac diseases in salmonid fish. Our intention is now to apply this technique under farming conditions as a quick and non-lethal screening tool to predict and reduce the risk of mortality from heart disease in farmed salmonids.
References
1. Brijs, J. et al. Prevalence and severity of cardiac abnormalities and arteriosclerosis in farmed rainbow trout (Oncorhynchus mykiss). Aquaculture 526, 735417 (2020).
2. Morgenroth, D. et al. Coronary blood flow influences tolerance to environmental extremes in fish. J. Exp. Biol. jeb.239970 (2021).
3. Zena, L. A. et al. It takes time to heal a broken heart: ventricular plasticity improves heart performance after myocardial infarction in rainbow trout, Oncorhynchus mykiss . J. Exp. Biol. 224, (2021).
4. Wallbom, N. et al. Increased reliance on coronary perfusion for cardiorespiratory performance in seawater-acclimated rainbow trout. J. Exp. Biol. 1–11 (2023).