Lymphoma

TL;DR. Lymphoma is a common cancer of the white blood cells that affects solid organs in dogs and cats, most frequently presenting as swollen lymph nodes or chronic digestive issues, but it is highly responsive to modern chemotherapy treatments.

Lymphoma often first presents as painless swelling in the peripheral lymph nodes, which are distributed throughout your pet's body.

What is it?

Lymphoma, also known scientifically as lymphosarcoma, is a cancer of the lymphatic system. Specifically, it is a malignancy of lymphocytes, which are a vital type of white blood cell responsible for protecting the body from infection. Because lymphocytes travel throughout the body via the blood and lymphatic vessels, lymphoma can develop almost anywhere.

It is crucial to distinguish lymphoma from lymphoid leukemia. While both involve abnormal lymphocyte production, they originate in different parts of the body. A leading veterinary internal medicine reference clarifies this distinction:

"Lymphoma (i. e. , malignant lymphoma, lymphosarcoma) is a lymphoid malignancy that originates from solid organs or tissues (e. g. , lymph nodes, liver, spleen, eye); this distinguishes lymphomas from lymphoid leukemias, which originate in the bone marrow..."

When lymphoma develops, it typically presents in one of four primary anatomic forms, depending on which organs are most heavily affected:

• Multicentric: This form primarily targets the external lymph nodes. It is by far the most common form in dogs, accounting for the vast majority of canine cases. Affected pets usually develop firm, swollen, painless lumps under the jaw, in front of the shoulders, or behind the knees.
• Alimentary (Gastrointestinal): This form targets the stomach, intestines, and local abdominal lymph nodes. It is the most common form of lymphoma in cats. It disrupts the body's ability to absorb nutrients, leading to chronic digestive issues.
• Mediastinal: This form develops within the chest cavity, specifically affecting the thymus gland or the lymph nodes located between the lungs. The physical presence of a mass in the chest often compresses the lungs, making it difficult for the pet to breathe.
• Extranodal: This form refers to lymphoma that targets a specific organ outside the primary lymphatic system. Common sites include the skin (cutaneous lymphoma), the eyes (ocular lymphoma), the kidneys (renal lymphoma), or the central nervous system (neurological lymphoma).

While lymphoma can strike pets of any age, it is primarily a disease of middle-aged to older animals. Most dogs diagnosed with the condition are between 6 and 12 years old, though veterinarians occasionally diagnose it in young puppies.

Causes & risk factors

In veterinary medicine, the exact cause of lymphoma is considered multifactorial, meaning it arises from a complex combination of genetic predispositions, environmental influences, and immune system factors.

In cats, viral infections play a historic and significant role. Feline Leukemia Virus (FeLV) and Feline Immunodeficiency Virus (FIV) are strongly associated with the development of lymphoma. Cats infected with FeLV are significantly more likely to develop the mediastinal or multicentric forms of the disease at a younger age.

Researchers have also investigated potential infectious links in both species. For instance, certain bacterial infections have been studied to see if they trigger the chronic inflammation that can lead to cancer. A prominent veterinary textbook notes:

"...may play a role in the development of gastric lymphoma in cats(Bridgeford et al, 2008). Recently, Borrelia spp. infection has been linked to the development of non-Hodgkin lymphoma in people and in a horse (Ferreri et al, 2009). However, to the author's knowledge, the link between Lyme disease and lymphoma has not been investigated in dogs. In dogs the etiology of lymphomas is considered multifact"

Environmental factors such as exposure to secondhand tobacco smoke, pesticides, and strong household chemicals are also suspected of increasing the risk of lymphoma in pets, though definitive causal links remain a subject of ongoing research.

Signs to watch for

Because lymphoma can develop in many different organs, the clinical signs vary widely depending on which anatomic form is present.

Cardinal Signs

• Lymphadenopathy: This is the hallmark sign of multicentric lymphoma. You may feel firm, enlarged, painless lumps under your pet's jaw, in front of their shoulder blades, or in the folds behind their knees. These lumps often feel like firm marbles or golf balls beneath the skin.

Common Signs

• Lethargy: A noticeable drop in energy levels or reluctance to play.
• Dyspnea (difficulty breathing) and coughing: Often caused by a mediastinal mass in the chest compressing the lungs.
• Weight loss and anorexia (loss of appetite): Especially common in the alimentary form where the intestines cannot absorb nutrients properly.
• Vomiting and diarrhea: Classic signs of gastrointestinal involvement.

Occasional Signs

• Increased drinking (polydipsia) and increased urination (polyuria): Often triggered by high calcium levels in the blood (hypercalcemia), which is a common metabolic complication of certain lymphomas.
• Eye changes: Including blindness, redness (anterior uveitis), blood in the eye (hyphema), pus in the eye (hypopyon), bulging eyes (exophthalmos), squinting (blepharospasm), or watery eyes (epiphora).
• Skin changes: Including hair loss (alopecia), redness (erythema), scaling (desquamation), and intense itching (pruritus), which are common in the cutaneous form.
• Nasal discharge, sneezing, and noisy breathing (stertor): Occurs when the disease affects the nasal passages.
• Neurological signs: Such as seizures, wobbliness, or behavior changes if the brain or spinal cord is involved.
• Enlarged kidneys (renomegaly) or elevated kidney values (azotemia).
• Facial deformity or swelling.

Rare Signs

• Head and neck swelling (edema): This can occur if a large mass in the chest blocks the major blood vessels returning blood from the head to the heart.

Ocular changes, such as cloudiness or redness from anterior uveitis, can occasionally occur as a secondary sign of lymphoma.

How vets diagnose it

If your vet suspects lymphoma, they will begin with a thorough physical examination, paying close attention to the size and consistency of all external lymph nodes. However, a definitive diagnosis requires laboratory testing.

Your vet will likely start with a fine-needle aspiration (FNA) of an enlarged lymph node or organ. This simple, minimally invasive procedure involves using a small needle to collect cells, which are then stained and examined under a microscope (cytology). In many cases, cytology is highly accurate for diagnosing lymphoma.

However, the gold standard for diagnosis remains histopathologic evaluation (biopsy). A biopsy involves removing a small piece of tissue (or an entire lymph node) and sending it to a veterinary pathologist. This allows the pathologist to evaluate the architecture of the tissue, confirming the diagnosis and identifying the specific grade of the cancer.

For pets with the intestinal form, ultrasound is an invaluable tool. A leading internal medicine reference highlights how ultrasound helps differentiate lymphoma from other chronic gut issues:

"Cats with lymphoma of the small intestine are more likely to have thickening of the muscularis when compared with cats with inflammatory bowel disease (Zwingenberger et al, 2010). Fine-needle aspiration (FNA) and needle biopsy can also be easily performed using this technique to guide the placement of the needle."

Ultrasound imaging helps veterinarians identify the characteristic intestinal wall thickening associated with alimentary lymphoma.

To fully understand the extent of the disease and tailor a treatment plan, your vet will recommend a staging panel. This typically includes:

• Complete blood count (CBC), chemistry profile, and urinalysis: To evaluate overall organ function and check for hypercalcemia.
• FeLV and FIV testing: Essential for all cats diagnosed with lymphoma.
• Abdominal ultrasonography and chest radiographs: To look for internal lymph node involvement or organ enlargement.
• Immunophenotyping (Flow Cytometry or PARR): These advanced tests determine whether the lymphoma involves B-cells or T-cells. This distinction is highly important for predicting how the disease will behave. As noted in veterinary literature:

"In dogs, a T-cell phenotype is quite likely if the patient is a Boxer, has hypercalcemia or a mediastinal mass, or has cutaneous or CNS involvement."

Treatment options

Because lymphoma is a systemic disease (meaning it is carried throughout the body), localized treatments like surgery or radiation are rarely used alone. Instead, chemotherapy is the primary treatment of choice.

First-Line Therapy: Multi-Agent Chemotherapy (CHOP or CEOP)

The gold standard treatment for multicentric lymphoma in dogs and high-grade lymphoma in cats is a multi-agent chemotherapy protocol, most commonly referred to as CHOP (or CEOP). This protocol rotates several different cancer-fighting drugs—typically including cyclophosphamide, doxorubicin (or epirubicin), vincristine, and prednisone—over several months. By combining drugs that attack cancer cells in different ways, this protocol maximizes the chances of achieving complete remission while minimizing the risk of the cancer developing drug resistance.

Second-Line Therapy: COP Protocol

For owners seeking a less intensive or more budget-friendly option, the COP protocol is a common alternative. This combination uses cyclophosphamide, vincristine, and prednisone. While it is generally well-tolerated and can successfully induce remission, the remission times are typically shorter than those achieved with the CHOP protocol.

Rescue Therapies & Specific Forms

If a pet does not respond to initial therapy, or if the lymphoma returns after a period of remission, vets will use "rescue" protocols. These may include drugs like Lomustine (an alkylating agent) or Cytarabine (an antineoplastic drug particularly useful for lymphoma affecting the brain or spinal cord).

It is important to understand that veterinary chemotherapy is very different from human chemotherapy. The primary goal is to extend your pet's life while maintaining an excellent quality of life. Doses are lower, and side effects like severe nausea or hair loss are rare in animals.

Prognosis

Without treatment, lymphoma progresses rapidly. The average survival time for an untreated dog or cat with high-grade lymphoma is only 4 to 8 weeks from diagnosis.

With treatment, the outlook improves significantly:

• Dogs on CHOP chemotherapy: Approximately 80% to 90% of dogs will go into complete remission. The average survival time with this protocol is 12 to 16 months.
• Cats on CHOP chemotherapy: Approximately 65% to 75% of cats achieve remission, with an average survival time of 6 to 9 months.
• Cats with small cell intestinal lymphoma: This is a low-grade, slow-moving form of the disease. Cats with this specific type of lymphoma typically live in excess of 2 years and are often managed successfully at home with oral medications.

Remission means that all clinical signs of the cancer have temporarily disappeared, and your pet feels completely normal. While lymphoma is rarely cured permanently, chemotherapy can buy you many months of high-quality, happy time with your pet.

Prevention

Because the exact causes of lymphoma are multifactorial, there is no guaranteed way to prevent it. However, you can take steps to minimize known risk factors:

• For cats: Keep your cat indoors to prevent exposure to Feline Leukemia Virus (FeLV) and Feline Immunodeficiency Virus (FIV). Ensure your cat is vaccinated against FeLV if they have any outdoor access or live with other cats of unknown viral status.
• Environmental hygiene: Minimize your pet's exposure to secondhand tobacco smoke, lawn pesticides, and harsh household chemicals.
• Routine veterinary care: Regular physical exams, especially in aging pets, allow your vet to feel for early changes in lymph node size, which is key to catching the disease early.

When to call your vet

If your pet has been diagnosed with lymphoma or is undergoing treatment, you must monitor them closely. Contact your veterinarian immediately if you observe any of the following red flags:

• Difficulty breathing, rapid breathing, or persistent coughing
• Sudden blindness, squinting, or severe redness in one or both eyes
• Extreme lethargy, weakness, or collapse
• Persistent vomiting, severe diarrhea, or a complete refusal to eat for more than 24 hours
• Sudden, severe swelling of the face, head, or neck
• A fever or a sudden drop in body temperature

For specific breeds

Certain breeds carry a higher genetic risk for developing lymphoma. If you own one of these breeds, you should be especially vigilant about checking their lymph nodes during routine grooming.

• Highly Predisposed Dogs: Golden Retrievers, Boxers, Rottweilers, Basset Hounds, Airedale Terriers, [English Cocker Spaniels](/p/breeds/englishcockerspaniel_dog), Irish Wolfhounds, Shih Tzus, Siberian Huskies, and Yorkshire Terriers.
• Highly Predisposed Cats: Siamese cats and Oriental Longhairs.

As noted previously, Boxers are highly predisposed to the T-cell phenotype of lymphoma, which tends to be more aggressive and requires prompt, targeted veterinary intervention.

Sources

• Small Animal Internal Medicine, 5th Edition, pages 1192, 1194, 1198, 1200.

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