Big Lymph Nodes

What Swollen Lymph Nodes Tell Us About Your Pet’s Health

You are giving your dog a butt rub and feel something that does not belong: a smooth, firm bump behind the knee, or maybe that chin scratch for your cat reveals two matching lumps under the jaw. Swollen lymph nodes are one of the most common findings that bring people in for evaluation, and it makes sense that they cause concern. Lymph nodes are part of the immune system, and when they enlarge (a clinical sign called lymphadenopathy), it means your pet’s body is responding to an infection, inflammation, or in some cases, cancer.

At South Shores Pet Clinic, we take the time to figure out why your pet’s lymph nodes are reacting. Our diagnostic services help us move from “something is swollen” to a clear answer you can act on. Whether the cause turns out to be a treatable infection or something that needs longer-term management, we will explain every step. Book an appointment if you have found a lump or noticed swelling on your pet, or if another vet has recommended further testing.

What Do Lymph Nodes Actually Do?

Lymph nodes are immune checkpoints distributed throughout the body. They filter the lymph fluid draining from surrounding tissues, capture pathogens and foreign particles, and coordinate the immune response when something harmful is detected.

Most of the time, lymph nodes do their work invisibly. You do not feel them, your pet does not notice them, and they simply function as the background surveillance system keeping infections from spreading unchecked. When a node enlarges, it has been activated by a signal from somewhere nearby or from the circulation, and that signal is what we need to identify.

The lymph node locations accessible to gentle palpation at home include the mandibular nodes under the jaw, the prescapular nodes in front of the shoulders, the axillary nodes in the armpits, the inguinal nodes in the groin where that belly rub discovery often happens, and the popliteal nodes behind each knee.

Routine physical examinations, including lymph node palpation, are a standard part of every preventative care visit at South Shores Pet Clinic. Finding an enlarged node during a wellness visit before any other symptoms develop is one of the most valuable outcomes of consistent annual care.

What Causes Lymph Nodes to Swell?

Lymphadenopathy is a clinical finding with many possible causes. Before any testing begins, the pattern of involvement gives us meaningful diagnostic information: which nodes are enlarged, how firm and painful they are, and whether the swelling is isolated or distributed across multiple groups.

The pattern of involvement tells a diagnostic story:

• Single node near an injury or infection: tends to trace back to a local wound, abscess, or infection in the tissue that drains to that node
• Multiple nodes across different areas: points toward a body-wide process, whether infectious or cancerous
• Painful, warm nodes: tend to lean toward infection or active inflammation
• Painless, firm, symmetric nodes: more often associated with cancer, particularly lymphoma

Infections and Reactive Swelling

Bacterial, viral, fungal, and parasitic infections are among the most common drivers of lymph node enlargement.

Local Infections

A single enlarged mandibular node under the jaw often traces back to a dental abscess, a facial wound, or an oral infection. A popliteal node swollen behind the knee typically reflects something happening below: a paw infection, a wound, or a skin problem on that limb. The node closest to the problem is usually the first to respond.

Systemic Infections

Systemic infections enlarge multiple nodes simultaneously.

• Tick-borne diseases: Lyme disease can produce generalized lymphadenopathy alongside fever and joint pain. Ehrlichia and Anaplasma, two tick-borne organisms active in coastal Southern California, can cause lymph node swelling alongside fever, lethargy, and decreased appetite and are worth screening for in dogs and cats with outdoor exposure.
• Bacterial diseases: Leptospirosis causes multi-system involvement and is also transmissible to people. Mycobacteriosis, a bacterial infection from organisms in the Mycobacterium family, can produce chronic lymph node swelling, often with draining skin lesions, in cats with outdoor or wildlife exposure.
• Fungal diseases: Fungal disease from environmental fungi can cause significant lymph node changes in this region, including Valley Fever and Histoplasmosis.
• Viral diseases (cats): Feline leukemia virus (FeLV) suppresses the immune system and can cause lymph node changes through its direct effects and through the secondary infections and lymphoma it predisposes cats to developing. Feline immunodeficiency virus (FIV) commonly causes persistent lymph node enlargement, particularly early in infection, as the immune system mounts its initial response.
• Parasitic diseases: Toxoplasmosis, caused by a parasitic organism shed through infected prey animals and undercooked meat, can produce lymphadenopathy alongside fever, lethargy, and respiratory or neurological signs in some cats. Heavy burdens of intestinal parasites like roundworms or Giardia, or significant external parasite loads from fleas and ticks, can also drive a significant immune response resulting in node enlargement.

Cancer: When Lymphoma Is Involved

Canine lymphoma is one of the most frequent causes of sudden, dramatic lymph node swelling in dogs. It originates in the lymphocytes that populate lymph nodes and produces painless, firm, symmetrically enlarged nodes across multiple groups. What makes this discovery so often shocking is that many dogs appear entirely well: eating normally, happy on walks, playing with toys. The swelling arrives before anything else changes.

The diagnosis and subtype of canine lymphoma shape the entire treatment and prognosis conversation, which is why accurate testing matters from the start. Feline lymphoma is also common in cats, though it more frequently presents with gastrointestinal involvement than the peripheral node swelling typical in dogs.

Multiple types of cancer in pets beyond lymphoma can also spread to lymph nodes. Any enlarged node near a known mass warrants evaluation to determine whether cancer has moved regionally.

Immune-Mediated Conditions, Allergies, and Other Causes

Several non-infectious, non-cancerous conditions can also produce lymph node swelling.

• Immune-mediated hemolytic anemia: the immune system attacks red blood cells, causing reactive lymph node enlargement as the immune response intensifies
• Allergies: particularly atopic dermatitis with secondary skin infections, which can produce localized node swelling in regions draining the affected skin
• Vaccination reactions: occasionally produce transient local node swelling at or near the injection site, typically self-resolving within a few days
• Lymphedema: impaired lymphatic drainage produces node changes that require the same systematic workup as more common causes

What Does the Evaluation Involve?

The diagnostic process moves systematically from physical findings to targeted testing, and each step informs the next.

Reading the Physical Exam

Careful palpation of all accessible lymph node groups gives us meaningful information before any test is run. Size, texture, consistency, symmetry, whether nodes are tender to touch, and whether they move freely or feel fixed to surrounding tissue all contribute to narrowing the differential.

Painless, firm, symmetric nodes across multiple groups in a dog who otherwise seems well points strongly toward a systemic process. A single warm, tender node near a recent wound or skin problem points toward reactive local inflammation. One pattern requires urgent investigation; the other supports monitoring.

Fine-Needle Aspiration

Fine-needle aspiration (FNA) is typically the first diagnostic step when the exam points toward investigation. A small needle placed into the node collects cells for microscopic evaluation. The procedure is brief, minimally invasive, and most dogs and cats do not require sedation. Cytology from FNA answers the central question in many cases: are these cells consistent with reactive inflammation, immune activation, or cancer?

When FNA isn’t enough information or when specific subtype confirmation is needed for treatment planning, surgical biopsy provides tissue from greater depth. The decision between cytology vs biopsy is made based on what the initial sample shows. Our comprehensive surgical care allows for safe lymph node biopsy, and we can send the sample to our laboratory partner for full assessment.

Blood Work and Imaging

A complete blood count and chemistry panel screen for systemic infection, organ disease, and metabolic changes. Tick-borne disease testing and testing for certain bacteria, viruses, and parasites help to narrow down the cause. Chest radiographs and abdominal ultrasound assess internal lymph node involvement and organ changes not detectable on the surface exam. These tools are important when lymphoma is under serious consideration, as staging guides treatment recommendations and prognosis.

Our diagnostics provide rapid in-house results, and our team explains what each finding means in plain language as results come in.

How Is Treatment Matched to the Cause?

Treatment follows diagnosis. Starting treatment before the cause is confirmed is how pets end up on the wrong medications for months.

For lymphoma, the realistic goal is remission rather than cure for most patients. Many dogs achieve meaningful additional comfortable time with treatment, and what that looks like for your specific pet is a conversation we have directly, honestly, and without pressure.

How Can You Reduce the Risk of Lymph Node Problems?

Not all causes of lymphadenopathy are preventable, but many of the most common infectious triggers are- and consistent preventive care is the most reliable strategy for catching everything else early.

Stay current on preventive care and vaccinations. Regular preventative care visits give our team the chance to palpate lymph nodes at every exam, catch changes before symptoms develop, and keep vaccines up to date against the viral diseases that can drive lymph node enlargement in cats and dogs. This includes protecting cats against FeLV and keeping dogs current against Lyme disease and leptospirosis where exposure is a concern.

Maintain consistent dental care. Dental disease is one of the most common sources of the local bacterial infections that enlarge mandibular and submandibular lymph nodes. Professional dental cleanings at appropriate intervals, combined with home care, reduce the chronic bacterial load that keeps the nodes in the jaw region persistently activated.

Keep up with parasite prevention and regular grooming. Fleas, ticks, and intestinal parasites are direct triggers for immune activation and lymph node swelling in sensitized pets, and tick-borne diseases can cause systemic lymphadenopathy that is easy to mistake for cancer on initial discovery. Regular parasite prevention removes these triggers before they become a clinical finding.

Our grooming team provides an additional layer of monitoring that most people do not think about. During every grooming appointment, our groomers are in close physical contact with your pet’s entire body- including the neck, armpits, groin, and behind the knees where lymph nodes sit. When a groomer notices something new or unusual, they communicate directly with our veterinary team so the finding is followed up promptly. This kind of hands-on, whole-body familiarity with your pet is one of the less-discussed benefits of keeping grooming and veterinary care under the same roof.

How Quickly Should You Come In?

Not all lymph node swelling carries the same urgency, but some presentations warrant evaluation sooner rather than later.

Come In Within 24 to 48 Hours

• Newly discovered firm lump that was not present at a previous exam
• Swelling that appeared rapidly over hours
• Node near a known mass that has grown

Same-Day Walk-In

We accept walk-ins for:

• Swelling accompanied by pale gums, labored breathing, or collapse
• Throat area swelling that appears to affect swallowing
• Acute lethargy or significant behavior change alongside new swelling

Schedule Within the Week

• Gradual enlargement without other acute symptoms in an otherwise well pet

If you are uncertain which category applies, call us. We would rather help you triage over the phone than have you wait when urgency is appropriate.

How Do I Check My Pet’s Lymph Nodes at Home?

A brief monthly home check adds meaningful monitoring between veterinary visits. Gently feel beneath the jaw on both sides, in front of each shoulder, in the armpits, in the groin, and behind each knee. You are not assessing exact sizes; you are noticing change. If something feels larger or firmer than it did last month, that is the signal to call us.

Frequently Asked Questions

Can a swollen lymph node go away without treatment?

Yes. Reactive nodes from resolving infections often normalize as the infection clears. Nodes that stay enlarged beyond two to three weeks, that are growing, or that come with other symptoms should not be watched at home indefinitely.

My pet seems completely healthy. Does swollen still mean serious?

It can. Lymphoma in dogs produces significant node swelling before any other signs develop. A pet who feels fine can still have an evaluation-worthy finding. Earlier investigation always means more options.

How long does it take to get an answer?

FNA cytology results return from our reference laboratory within a few days. Blood work from our in-house lab is available the same day. If biopsy is needed, histopathology typically takes one to two weeks.

Is the procedure painful?

FNA is typically well-tolerated without sedation. The needle is smaller than what is used for blood draws. Most pets remain calm throughout. Our approach prioritizes minimizing stress at every visit.

An Answer You Can Work With

The most anxious part of finding a lump is not knowing what it means. At South Shores Pet Clinic, we move from discovery to diagnosis as efficiently as possible and explain every step along the way. Whether what you found turns out to be reactive, infectious, or something more serious, you will leave understanding what is happening and what comes next.

Book an appointment, walk in during our open hours, or contact us if you have questions before coming in. We’re here to help.