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.