Enlarged Lymph Nodes in Pets: Causes, Testing, and Treatment Options
Most people never think about their pet’s lymph nodes until one day, there’s a noticeable lump where there wasn’t one before. A bump under the jaw, a swelling behind the knee, or a vet who pauses mid-exam to say something feels off. Lymph nodes are small, bean-shaped glands found in clusters throughout your pet’s body, quietly doing the unglamorous work of filtering out harmful cells and coordinating immune responses. When they enlarge, a condition called lymphadenopathy, it’s your pet’s body flagging that something needs attention.
As an AAHA-accredited practice, Harbor Pines Veterinary Center follows rigorous diagnostic standards when evaluating lymph node changes. We start with a thorough physical exam and work through testing methodically so we can give you a clear answer about what’s happening and what your pet needs. Request an appointment if you’ve noticed any new lumps or swelling on your dog or cat.
What Lymph Nodes Do and Why They Swell
Lymph nodes are the immune system’s local checkpoints. Distributed throughout the body, they filter lymph fluid draining from surrounding tissue, trap pathogens and foreign material, and coordinate immune responses when something harmful is detected. When a lymph node enlarges, it is responding to a signal from a nearby infection, a systemic illness, or cancer in or near the lymphoid tissue itself.
Knowing where lymph nodes are located makes it easier to understand what you might discover at home. The nodes accessible by gentle palpation include:
- Mandibular nodes under the jaw
- Prescapular nodes in front of the shoulders
- Axillary nodes in the armpits
- Inguinal nodes in the groin
- Popliteal nodes behind the knees
The node itself is not the problem. What is causing it to enlarge is what the workup must determine.
What Can Cause Lymph Nodes to Enlarge?
Lymphadenopathy has many possible causes, and the pattern of enlargement gives meaningful clues before testing even begins.
Infections and Inflammation
Bacterial, viral, fungal, and parasitic infections are among the most common drivers. Lymph nodes that are painful when palpated are more likely dealing with infection or inflammation.
Localized infections swell the nodes draining that specific region: a dental abscess enlarges the mandibular or submandibular nodes; a paw wound may enlarge the popliteal node behind the knee. If only one lymph node is enlarged, it’s most likely related to infection or inflammation in a nearby body part.
Systemic infections can enlarge nodes throughout the body simultaneously. In Southern California’s outdoor environment, tick-borne diseases including Lyme disease are among the regional causes worth considering. Leptospirosis, spread through water and soil contaminated by infected wildlife, is also prevalent in the region and can produce lymphadenopathy alongside its effects on the kidneys and liver. Fungal disease from environmental organisms can also produce significant lymphadenopathy alongside systemic signs.
Cancer
Canine lymphoma originates in lymphocytes, the immune cells that populate lymph nodes, and is one of the most common cancers in dogs. It produces painless, firm, symmetrically enlarged nodes across multiple groups, often before the dog shows any other signs of illness. Many families are taken completely by surprise at a routine wellness exam when their seemingly healthy dog has significantly enlarged nodes.
The diagnosis and subtype of canine lymphoma shape the entire treatment and prognosis conversation, which is why accurate testing matters so much from the start. Feline lymphoma is also common in cats but more often presents with gastrointestinal involvement rather than the generalized peripheral lymph node enlargement typical in dogs.
Several types of cancer in pets beyond lymphoma can also spread to lymph nodes. Any enlarged node near a known mass deserves evaluation to assess regional spread.
Less Common Causes
Drug reactions, vaccine responses, immune-mediated disorders, and lymphedema (impaired lymphatic drainage) can also produce lymph node changes. These require the same systematic approach because they look identical on the surface.
What the Examination and Workup Involves
Reading the Pattern
When we palpate lymph nodes, we assess size, texture, symmetry, mobility, and whether nodes are tender or fixed to surrounding tissue. A single warm, tender node near a recent wound tells a different story than painless, firm, symmetric enlargement across multiple groups. The pattern of involvement narrows the differential significantly before any testing begins.
Fine-Needle Aspiration and Biopsy
Fine-needle aspiration (FNA) is the most efficient first step. A small needle collects cells from inside the node for microscopic examination. The procedure is quick, typically takes only a few minutes, and most pets do not require sedation. Cytology from FNA often provides a clear, rapid answer about whether the cells are reactive (responding to infection), inflammatory, or cancerous.
The decision between cytology vs biopsy depends on what the initial sample shows. When FNA isn’t enough information to make a proper diagnosis, when small-cell lymphoma is suspected, or when tissue architecture is needed for a complete picture, core needle or surgical biopsy provides more detail.
Blood Work and Imaging
A complete blood count and chemistry panel screen for systemic infection, organ involvement, and metabolic abnormalities. Chest radiographs and abdominal ultrasound assess internal lymph node involvement and organ changes that are not palpable from the outside. These tools are part of staging when lymphoma is a serious consideration.
AAHA accreditation at Harbor Pines means the diagnostic standards, protocols, and equipment we use meet the highest benchmarks in veterinary medicine. Our team brings that rigor to every case regardless of complexity.
Treatment Is Always Matched to Diagnosis
| Cause | Approach |
| Bacterial infection | Targeted antibiotics; source control if applicable (dental care, wound treatment) |
| Tick-borne disease | Specific antimicrobials based on organism identified |
| Fungal infection | Extended antifungal therapy |
| Lymphoma | Chemotherapy protocols tailored to B-cell or T-cell subtype |
| Metastatic cancer | Depends on primary tumor; surgical, oncologic, or palliative options |
| Reactive lymphadenopathy | Treat the underlying cause; monitor for resolution |
For lymphoma, the realistic goal for most patients is remission rather than cure. Many dogs achieve months to a year or more of good quality life during treatment, and that time and quality matter. We have direct, honest conversations with every family about what the realistic range of outcomes looks like for their specific pet.
Monitoring Lymph Nodes at Home
If your pet is being monitored, a monthly check at home takes only a few minutes:
- Feel beneath the jaw on both sides for the mandibular nodes
- Check in front of each shoulder for the prescapular nodes
- Feel softly in the armpits and groin
- Check behind each knee for the popliteal nodes
You do not need to know exact sizes to notice change. If something feels different from the last time you checked, larger, firmer, or newly apparent, request an appointment rather than waiting for a scheduled visit. Earlier evaluation matters for most conditions driving lymph node enlargement.
When to Come In Urgently
Not all lymphadenopathy carries the same urgency, but some presentations warrant same-day evaluation rather than a scheduled appointment in coming days.
Come in promptly:
- Rapid swelling that developed over hours to a day, particularly across multiple areas
- Any lymph node swelling accompanied by pale or yellowish gums
- Swelling in the throat or neck area that appears to affect swallowing or breathing
- Significant lethargy or collapse alongside new swelling
- Lymph node changes in a pet already diagnosed with cancer
Schedule soon:
- A new lump discovered at home that was not present at the last exam
- Gradual swelling without other acute symptoms
- A node that a previous veterinarian asked you to monitor and that has grown
If you are unsure which category applies, call us. Reaching out with questions is always the right approach when something is new or has changed.
Frequently Asked Questions
Can a swollen lymph node resolve on its own?
Yes, reactive nodes from resolving infections often return to normal size as the infection clears. Nodes that remain enlarged beyond two to three weeks, that are growing, or that come with other symptoms should not be watched indefinitely at home.
Are swollen lymph nodes painful?
Reactive nodes from active infection are often tender. Lymphoma nodes are typically painless, which is one reason pets may seem completely fine despite significant enlargement. Absence of pain does not mean the cause is benign.
Do I need to come in even if my pet seems healthy?
Yes, if you have found a new lump or noticed enlargement. Lymphoma specifically can produce large, dramatic node swelling before any other signs of illness develop, and earlier evaluation means more options remain available.
How long does the workup take?
FNA cytology typically returns results within a few days from our reference laboratory. Blood work from our in-house laboratory is available the same day. If biopsy is needed, histopathology results generally take one to two weeks.
Is there anything I can do to monitor lymph nodes between visits?
A monthly home check of accessible nodes takes only a few minutes. What matters most is noticing change: if something feels larger, firmer, or new compared to last month, that is the signal to call. Photos taken over time can also help document progression for the veterinary conversation. A lymph node locations chart illustrates the full distribution for dogs, and cat lymph nodes follow a similar general pattern.
From First Discovery to a Clear Plan
Discovering a lump on your pet is worrying, and waiting to understand what it means is often the hardest part of the process. At Harbor Pines Veterinary Center, we approach every finding with the rigorous evaluation it deserves: thorough physical assessment, the right diagnostic tests in a logical sequence, and clear communication about what we find and what it means. Through our mobile services, we can bring that same level of care to your pet at home when a clinic visit is difficult.
If you’ve noticed anything unusual, request an appointment and we will help you get to answers as efficiently as possible. Our AAHA-accredited services and rigorous diagnostic standards mean every step of that process meets the highest benchmarks in veterinary medicine. Your pet deserves that level of care, and your family deserves that level of clarity.