We explore the defect distribution and the degradation in electron beam pumped Zn1-xCdxSe/ZnSe laser structures by combining cathodoluminescence measurements in a scanning electron microscope with transmission electron microscopy. We found that degradation occurs via the formation of [100]-oriented dark line defects, and that it involves the formation of a characteristic type of defect, namely dislocation loops within the ZnCdSe quantum well, which grow to form a characteristic network as degradation processes.