Swollen lymph nodes in dogs signal that the immune system is responding to something, whether that’s a local infection, a bodywide illness, or cancer. The causes range from minor (a skin infection draining into nearby nodes) to serious (lymphoma, which accounts for 7 to 14% of all cancers diagnosed in dogs). Because the list of possibilities is so broad, the location, size, and speed of the swelling all matter when figuring out what’s going on.
What Lymph Nodes Do and Where to Find Them
Lymph nodes are small filtering stations scattered throughout your dog’s body. They trap bacteria, viruses, and abnormal cells, then activate immune cells to mount a defense. When a node is working overtime, it swells. You can feel five sets of superficial lymph nodes on most dogs: under the jaw, in front of the shoulders, in the armpits, in the groin, and behind the knees. In a healthy medium-sized dog, most of these feel like small, soft beans or aren’t noticeable at all.
A single swollen node usually points to a problem in the area that node drains. A swollen node under the jaw, for instance, often reflects a dental infection or mouth wound. When multiple nodes across different parts of the body enlarge at the same time, that’s called generalized lymphadenopathy, and it suggests something systemic is going on.
Infection: The Most Common Cause
Infections are the number-one reason lymph nodes swell in otherwise healthy dogs. The type of infection determines the pattern.
Skin infections: Bacterial skin infections (pyoderma) are extremely common. When a skin infection becomes severe or widespread, possibly secondary to allergies, mange, or another underlying condition, the nodes nearest the affected skin enlarge to help fight off the bacteria. Treating the skin problem typically brings the nodes back to normal size.
Tick-borne diseases: Several tick-transmitted infections cause lymph node swelling alongside fever, lethargy, and joint pain. Ehrlichiosis and anaplasmosis are among the most frequent culprits. Rocky Mountain spotted fever and tularemia also cause swollen nodes, with tularemia often producing a painful node near the tick bite along with a skin ulcer at the bite site. Coinfection with more than one tick-borne organism is common, which can make symptoms more severe or harder to pin down.
Fungal infections: In certain regions of the United States, inhaled fungal organisms cause systemic infections that enlarge lymph nodes throughout the body. Blastomycosis is most common in the Ohio and Mississippi River valleys and the Great Lakes region, while valley fever (coccidioidomycosis) occurs in the desert Southwest. These infections can also cause coughing, eye problems, and skin lesions.
Lymphoma: The Diagnosis Owners Fear Most
Lymphoma is a cancer of white blood cells, and it’s one of the most common cancers in dogs. In 80 to 85% of cases, the first sign is painless, generalized swelling of the peripheral lymph nodes. The nodes can grow to 3 to 10 times their normal size, feel firm, and move freely under the skin. Unlike infection-related swelling, the enlargement is typically symmetrical (the same nodes on both sides of the body) and doesn’t seem to bother the dog when touched.
Early on, many dogs with lymphoma act completely normal. They eat well, play, and show no outward signs of illness. That’s what makes it deceptive. Lethargy, weight loss, and appetite changes tend to appear only as the disease advances. If your dog’s lymph nodes have ballooned seemingly overnight and you can’t find an obvious infection, lymphoma is high on the list of possibilities your vet will investigate.
Veterinarians stage lymphoma on a scale from I to V. Stage I means only a single node is involved, while stage V means cancer has spread to the bone marrow or organs beyond the liver and spleen. Dogs are also classified by whether they’re showing systemic signs like fever or significant weight loss. Most dogs are diagnosed at stage III or higher because the swelling is often the first clue owners notice, and by that point multiple node groups are affected.
Treatment typically involves chemotherapy given every one to two weeks over a four-to-six-month course. Many dogs tolerate chemotherapy far better than humans do, maintaining a good quality of life throughout treatment. Remission rates depend on the cancer subtype, but the goal is always to extend comfortable, active time.
Other Cancers That Spread to Lymph Nodes
Lymphoma starts in the lymph nodes themselves, but other cancers can metastasize to them. Mammary gland tumors, mast cell tumors, oral melanoma, and various carcinomas and sarcomas all commonly spread to the nearest lymph node first. In these cases, the swollen node is a secondary finding rather than the primary problem. Your vet may notice a swollen node and then go looking for the original tumor elsewhere in the body.
How Vets Figure Out the Cause
The fastest first step is a fine-needle aspirate, where a vet inserts a small needle into the swollen node and draws out a sample of cells. This takes seconds, rarely requires sedation, and can often distinguish between reactive (infection or inflammation), cancerous, and metastatic cells within a day or two. If the aspirate isn’t conclusive, a full biopsy, blood work, or imaging may follow.
Your vet will also ask questions that help narrow the possibilities: Has the swelling appeared suddenly or gradually? Is one node affected or several? Has your dog been in a tick-heavy area, or in a region where fungal infections are common? Are there any other symptoms like coughing, skin lesions, or changes in appetite? The pattern of swelling combined with other symptoms often points the investigation in the right direction before any lab results come back.
Signs That Need Urgent Attention
Not every swollen lymph node is an emergency, but certain combinations of symptoms are. If your dog has swollen nodes along with extreme lethargy, pale gums, vomiting, or complete refusal to eat, that warrants an urgent vet visit rather than a wait-and-see approach. Pale gums in particular can signal anemia or internal bleeding, and dehydration from vomiting or not eating compounds the problem quickly. Rapid breathing or difficulty breathing alongside swollen nodes is another red flag, as enlarged internal nodes or fluid buildup can compromise the airways or chest cavity.
If the swelling is mild, your dog is eating and acting normally, and you can get an appointment within a day or two, that timeline is usually reasonable. But don’t wait weeks hoping it resolves on its own. Even benign causes benefit from early treatment, and serious causes benefit from early detection.