Lymph nodes (or glands), are small masses of tissue that can be found throughout the body. They play an integral part in the functioning of the dog's immune system, acting as filters for the blood, and as storage places for white blood cells. Consequently, they are often the first indicators of disease in the tissues.
When tissues become inflamed, the regional lymph nodes that these tissues drain into will also become inflamed and swollen in response. This swelling is due to a reactive increase in white blood cells (hyperplasia) due to the localized presence of an infectious agent. This is medically defined as reactive hyperplasia: when white blood cells and plasma cells (antibody secreting cells) multiply in response to a substance that stimulates their production (antigenic stimulation), causing the lymph node to enlarge.
Lymphadenitis is a condition in which the lymphatic glands have become inflamed due to infection. Neutrophils (the most abundant type of white blood cell, and the first to act against infection), activated macrophages (cells which eat bacteria and other infectious agents), and eosinophils (cells which fight parasites and allergy causing agents) will migrate into the lymph node during an episode of lymphadenitis. This convergence of cells results in the swollen feel and appearance of the nodes.
Cancerous cells may also be found in a lymph node biopsy (tissue sample). Cancer cells may be primary, originating in the lymph node (malignantlymphoma), or may be there as a result of the spread of cancer from another location in the body (metastasis).
Symptoms and Types
Lymph nodes can usually be detected by touch, but sometimes there will be no clinical symptoms. Swelling can be felt in the area beneath the jaw (submandibular), or around the shoulder. Swelling in one of the legs is also possible as a result of swollen lymph nodes at the back of the leg (popliteal), or near the joint of the leg (axillary – correlating with the armpit). Swollen nodes in the area near the groin (inguinal) may make defecation difficult for your dog. Your dog may also lose its appetite due to nausea, and have an urge to regurgitate when it does eat. You can also expect your dog to feel a general malaise as its body fights off the infection. If your dog has severely enlarged lymph nodes it may have trouble eating, or have difficulty with breathing.
Lymphoid hyperplasia: when lymph nodes react to an infectious agent by producing an excess of white blood cells, but are not themselves infected
Lymphadenitis: when the lymph nodes themselves are infected either primarily or secondarily
Sporotrichosis: fungal infection of the skin, acquired from soil, hay, plants (most notably, garden roses); affects the skin, lungs, bones, and brain
Rickettsia: transmitted by ticks and fleas
Bartonella spp: transmitted by biting flies
Brucella canis: sexually transmitted; acquired during breeding
Pasteurella: transmitted through the respiratory system
Yersinia pestis : transmitted by fleas and possibly rodents; also known as the plague
Fusobacterium: infection of the mouth, chest, throat, lungs
Francisella tularensis: tularemia; transmitted by ticks, deer flies, and by the dispersion of gases from an infected animal carcass (frequently occurring during lawn-mowing)
Mycobacterial: transmitted by infected water supply
Allergens: lymph glands respond to an allergic reaction in the body by producing more cells – usually occurs in the lymph nodes near the site of the reaction
Immune-mediated disease: the body’s immune system over-reacts to an invasion, or reacts inappropriately
Eosinophilic infiltration: multiplication of white blood cells responsible for controlling allergy response, or for fighting parasitic agents
Canine hypereosinophilic syndrome: excessive eosinophils, may be associated with leukemia, blood marrow infection, asthma, or allergy