Gross clinical signs have not been reported. In internal organs, pathology is seen in lymphoid organ spheroids and stromal matrix cells of tubules, cuticular epithelium and underlying connective tissues of the stomach and of the cephalothoracic exoskeleton, antennal gland tubules, primary and secondary gill filaments, epithelial pillar cells, hepatopancreas connective tissues, the pericardial septum, epicardium and fixed phagocytes within the myocardium, haemocytes within haematopoietic tissues, glial, neurosecretory and giant cells associated with the segmental nerve ganglia, nerve cell bodies.
In haematoxylin and eosin stained sections of cephalothorax tissues, the presence of aggregates of cells with hypertrophied nuclei, known as spheroids, in the lymphoid organ is the most obvious pathology caused by Mourilyan virus. Spheroids numbers, the extent of cytoplasmic vacuolization within spheroid cells, and the amount of necrotic cell debris within spheroids, increase in relation to infection severity. In severe infections, ectopic spheroids may also be detected in gill and in connective tissue associated with various cephalothorax organs.