The spleen is an important lymphoreticular organ that functions in immune defense. Unlike the spleen of pinnipeds, the spleen in cetaceans is much smaller and rounded, often with several “accessory” spleens nearby. When opening up the abdomen of a porpoise the spleen is usually hidden behind the stomachs.
The thymus is a lobular gland located cranial to the heart and caudal to the hyoid apparatus. The thymus changes in size dramatically with ontogeny, starting off very large in young animals and slowly shrinking with age. The thymus is involved in “training” the T-cells of the immune system early in life.
During a necropsy, one of the most important clues for COD is swollen or inflamed lymphnodes. Lymphnodes are located throughout the body and in general should be rounded, smooth, and uniform in color. When an infection is present however, the lymphnodes nearest the infection will often swell and may become inflamed turning red, purple, or dark in color. Lymphnodes are also great for detective work! If you are looking at an animal with multiple lesions, tumors, or massive infection, the lymphnodes can act as a breadcrumb trail to tell you where the problem started. This is because as the disease process progresses it will often have progressed further in lymphnodes near where the problem started. Sampling all lymphnodes near and far from a lesion can help the pathologist localize the origination of a disease process.
Additional information about the lymphoreticular system in the bottlenose dolphin can be found in Cowan and Smith, 1999.
Detailed annotated images of the lymphoreticular system in a harbor porpoise are shown below.
