What is a common indicator of infection in lymph nodes of an animal?

Swelling of lymph nodes is a common indicator of infection in animals. When the body detects an infection, the lymphatic system responds by activating lymphocytes, which are white blood cells that help fight infections. This activation leads to an increased number of cells accumulating in the lymph nodes, causing them to become enlarged or swollen. Such swelling indicates that the lymph nodes are working to filter out pathogens and respond to the infection, making it a prominent sign during a physical examination of an animal that may be suffering from an illness.

In contrast, while hardening of lymph nodes may occur in chronic infections or malignancies, it is not an immediate sign of infection like swelling. Discoloration of skin and loss of appetite can be related to various health issues but are not specific indicators of infection in lymph nodes. Therefore, swelling is the most direct and relevant indicator of lymph node involvement in disease processes.