SYMPTOM
Swollen lymph nodes
Enlarged lymph nodes felt as firm lumps, usually painless but sometimes tender, under the jaw, in front of the shoulders, or behind the knees.
Reactive Lymphadenopathy
Lymph nodes are sentinel stations of the immune system that filter lymphatic fluid and mount immune responses. When infection, inflammation, or foreign material is detected in the tissues drained by a particular lymph node, that node enlarges as immune cells proliferate. This reactive enlargement is a normal immune response and is the most common cause of palpable lymph node swelling. The enlarged node is typically smooth, mobile, and may be mildly tender.
Lymphoma
Lymphoma is one of the more common cancers affecting dogs and cats, and generalised enlargement of multiple lymph nodes is one of its hallmark presentations. In the multicentric form, multiple peripheral lymph nodes enlarge progressively, often to several times their normal size. The enlarged nodes are typically firm, non-painful, and symmetrically distributed. The condition may be accompanied by changes in appetite, energy levels, and body weight as the disease progresses.
Infectious
Systemic infections can produce generalised lymph node enlargement as the immune system responds to the widespread presence of the infectious agent. Tick-borne diseases, fungal infections, and certain viral conditions are well-recognised causes of generalised lymphadenopathy. The lymph node enlargement is typically accompanied by other signs of systemic illness such as fever, lethargy, appetite loss, and signs specific to the particular infectious agent involved.
Metastatic Disease
Lymph nodes that drain regions containing primary tumours may enlarge as cancer cells spread through the lymphatic system and establish within the node. This metastatic involvement typically affects the regional lymph node closest to the primary tumour first. The enlarged node may be firm, fixed to underlying tissues, and irregular in shape, contrasting with the smooth, mobile character of reactively enlarged nodes.
Immune-Mediated
Certain immune-mediated conditions can produce lymph node enlargement as the immune system becomes chronically activated. Systemic lupus erythematosus, immune-mediated polyarthritis, and other autoimmune conditions may include lymphadenopathy as part of their clinical presentation. The lymph node enlargement reflects generalised immune activation and may fluctuate with disease activity.
Why timing matters
Early observation
Early lymph node enlargement may be detected incidentally during routine veterinary examination or noticed by an observant owner who feels lumps beneath the skin during grooming or petting. Mildly enlarged nodes may be soft, mobile, and non-tender, making them easy to overlook. In reactive conditions, early enlargement may correlate with a recent wound, skin infection, dental procedure, or vaccination in the region drained by the affected node. The animal may otherwise appear entirely well.
Later presentation
As lymph node enlargement progresses, the nodes may become more obviously palpable or even visually prominent beneath the skin. In conditions such as lymphoma, the nodes can enlarge to several centimetres in diameter, creating visible swellings under the jaw, in front of the shoulders, behind the knees, or in the groin. Progressive enlargement may be accompanied by the development of other clinical signs such as weight loss, reduced appetite, increased lethargy, or breathing difficulties when enlarged nodes or a mass in the chest press on the lungs or cause fluid to build up around them. Fast or laboured breathing is a time-critical sign that cannot be judged at home.
The trajectory of lymph node enlargement varies with the cause. Reactive enlargement from local infection typically resolves as the triggering condition is addressed, with nodes returning to normal size over days to weeks. In dogs, lymphoma-related enlargement progresses over weeks, and the MSD Veterinary Manual states that untreated dogs die or are euthanised within 4 to 6 weeks. Infectious causes may fluctuate with the activity of the underlying infection. Tracking the size, distribution, and consistency of enlarged nodes over time helps characterise the nature of the lymphadenopathy.
Conditions commonly associated
Lymphoma in Dogs
Lymphoma arises from uncontrolled proliferation of lymphocytes within the lymphatic system, and as abnormal cells accumulate within lymph nodes, the nodes enlarge into the firm, painless swellings owners can feel under the jaw, in front of the shoulders, or behind the knees.
Lymphoma in Cats
In the multicentric form of lymphoma, several peripheral lymph nodes in a cat can become palpably enlarged, often painlessly, although alimentary (gut) lymphoma is now by far the most common presentation of lymphoma in cats.
Feline Immunodeficiency Virus (FIV)
Generalised lymphadenopathy is a common finding in FIV-positive cats, reflecting persistent immune stimulation as the body responds to ongoing viral replication and associated antigenic challenge.
Feline Leukaemia Virus (FeLV)
Lymph node enlargement in FeLV-positive cats may reflect reactive lymphoid hyperplasia from chronic immune stimulation or the development of lymphoma, one of the most significant neoplastic complications of FeLV infection.
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Multicentric Lymphoma in Dogs
This is the classic presentation, in which lymph nodes throughout the body enlarge as they fill with malignant lymphocytes, often felt as firm, painless swellings under the jaw or behind the knees.
Atypical Mycobacterial Infection
Regional lymph nodes draining an infected area often enlarge as they respond to the infection.
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Brucellosis in Dogs
Brucella canis infection can cause generalised lymph node enlargement that can be felt under the jaw, in the neck or behind the knees, although many infected dogs show no signs.
Cryptococcosis
The immune response to fungal infection can enlarge the lymph nodes draining the affected area.
Ehrlichiosis
Widespread immune activation during ehrlichiosis commonly enlarges the lymph nodes under the jaw and behind the knees.
Juvenile Cellulitis (Puppy Strangles)
The lymph nodes under the jaw often enlarge markedly as part of the generalised inflammatory response seen with this condition.
Leishmaniasis
Generalised lymph node enlargement is a frequent early finding as the immune system responds to this infection.
Oral Melanoma
Oral melanoma commonly spreads first to the local lymph nodes, which can become enlarged and firm as a result.
Anal Sac Adenocarcinoma
Anal sac adenocarcinoma often spreads to the sublumbar lymph nodes inside the abdomen: in a UK series of 81 dogs, 62 already had spread to lymph nodes or further at presentation. These internal nodes cannot be felt from outside the body; they were felt on rectal examination in 30 of the 81 dogs, and imaging found more. Enlargement of peripheral nodes may occasionally be detected if the cancer has spread more widely.
Histiocytic Sarcoma in Bernese Mountain Dogs
Histiocytic sarcoma can involve lymph nodes either as a primary site or through spread from other affected tissues such as the lungs or spleen. Intrathoracic lymph nodes are commonly enlarged in the systemic form, whilst peripheral nodes may be palpable if they are directly infiltrated by cancerous histiocytes.
When to explore further
Enlargement of multiple lymph nodes simultaneously, particularly in a symmetrical pattern affecting nodes in several body regions, may suggest a systemic process such as lymphoma, systemic infection, or immune-mediated disease rather than a localised reactive response. In dogs, this generalised, painless pattern is the presentation in 80 to 85% of lymphoma cases.
Lymph node enlargement without an identifiable local cause such as a nearby wound, skin infection, or recent vaccination, or enlargement that keeps growing, may warrant further investigation to determine the nature of the enlargement.
Enlarged nodes that feel unusually firm, fixed to underlying tissues, or irregular in shape may suggest infiltration by abnormal cells rather than normal reactive enlargement. The feel of a node gives clues but cannot tell reactive, infectious and cancerous enlargement apart; a fine needle sample of the node, examined under the microscope, is needed for that.
Lymph node swelling accompanied by systemic signs such as weight loss, reduced appetite, progressive lethargy, or fever may suggest the lymphadenopathy is part of a broader disease process affecting the animal's overall health.
Rapid enlargement of a single lymph node in the region draining a known or suspected mass elsewhere in the body may suggest metastatic spread and can provide important information about the extent of disease.
Learning to feel the location of the major superficial lymph nodes (under the jaw, in front of the shoulders, behind the knees, and in the groin) during normal grooming or petting sessions can help detect changes early. Noting the size, consistency, and symmetry of any palpable nodes, and whether they are tender to gentle pressure, provides useful baseline information. Photographing or measuring visibly enlarged nodes provides a record that can track changes over time. Observing the animal for any accompanying systemic signs such as appetite changes, weight loss, or altered energy levels alongside the lymph node changes helps build a more complete picture.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS