IMMUNE SYSTEM
Lymph nodes
The lymph nodes are oval (or bean shaped) red-brown nodules located within the lymphatic vessels. They are covered by a smooth transparent capsule, and blood vessels, nerves and lymphatic vessels enter at a tiny depression called the hilus.
The mandibular lymph nodes consist of 2-4 nodes lying along the ventral border of the mandible. The cervical lymph nodes are around 5-8 mm diameter and located in adipose tissue cranial to the scapula. The deep nodes lie adjacent to the trachea between the internal and external jugular veins. In young animals it is covered by the thymus.
CLINICAL NOTE
Harsh abrasive foods can pierce the oral mucosa and enable Streptococcus Zooepidemicus, a normal oral pathogen, to access the cervical lymph nodes and cause abscessation. Guinea pigs present with pus-filled ventral cervical lymph nodes which need lancing or, in severe cases, surgical removal (Huerkamp et al. 1996).
Thymus
The cervical thymus is so readily accessible that the guinea pig has been used extensively for immunology research. In immature animals it is located in the cranial mediastinum and subcutaneously in the neck where it surrounds the trachea ventrally and laterally. It is composed of two yellowbrown, oval lobes extending from the angle of the mandible to approximately half way to the thoracic inlet. In the adult it becomes mainly replaced by fat (Breazile & Brown 1976; Harkness & Wagner 1995).
CLINICAL NOTE
Although it is possible to perform endotracheal intubation in the guinea pig, take care to access the palatal ostium lying centrally; a lateral slip could cause damage to the vascular velopharyngeal folds.
Spleen
The spleen is relatively large compared to that found in most rodents and rabbits. It lies on the left side, lateral to the greater curvature of the stomach and attached by the gastrosplenic ligament (Breazile & Brown 1976).