The salmonid fish industry faces a significant challenge with high mortality rates, leading to an annual loss of tens of millions of fish and posing a serious threat to the financial and environmental sustainability of the fish farming industry. While no particular event can be linked to the high mortalities, diseases affecting the heart may have a significant impact on fish performance and stress tolerance, as many fish die during stressful handling procedures when cardiac workload increases. Unfortunately, effective screening tools for early diagnosis and timely precautionary measures to achieve better productivity and economic performance, as well as improve fish welfare, are currently limited. In human medicine, an electrocardiogram (ECG) is often the first-line diagnostic tool for evaluating patients with suspected heart conditions. However, the ECG has not been widely applied in fish health thus far. The present study was designed to comprehensively evaluate the suitability of the ECG as a potential screening tool for cardiovascular disease risk in fish. Specifically, we mapped abnormalities in the ECG by inducing myocardial ischemia through coronary artery occlusion (coronary ligation) in anaesthetised rainbow trout, and used provocation maneuvers to manipulate the fish’s heart rate (e.g., by changing artificial ventilation) to increase the sensitivity for detecting any cardiac abnormalities.
Myocardial ischemia significantly affected ECG characteristics. We found loss of QRS voltage and prolonged QRS duration as a consequence of myocardial ischemia, suggesting extensive myocardial injury and delayed ventricular activation. In addition, myocardial ischemia caused heart blocks, typically occurring at high heart rates, which could be alleviated during bradycardia (Fig.1). There was a 45% mortality rate among fish suffering from myocardial ischemic injury. At necropsy, some deceased fish showed hemopericardium (blood accumulation in the pericardial sac), likely due to atrial/ventricular rupture. In conclusion, the present study makes a substantial contribution to the development of new real-time diagnostic tools capable of advancing diagnostic and monitoring methods utilized in aquaculture. It also demonstrates that electrocardiography has the potential to be used for screening and diagnosing cardiac conditions in salmonid fish.