Lymphoma in senior cats: symptoms, treatment and prognosis

For most owners, the diagnosis arrives in fragments. Your senior cat has been losing weight for a few months — nothing dramatic, just a slow erosion that you noticed when you picked her up and her ribs felt sharper than they used to. She has been vomiting once or twice a week, and you have been switching foods and adjusting portions, half-convinced this is just an aging stomach. The vet runs bloodwork, then an ultrasound, and the conversation that follows uses a word you were not expecting: lymphoma.

Lymphoma is the most common cancer in cats, accounting for roughly 30 percent of all feline cancer diagnoses, and it is overwhelmingly a disease of middle-aged and senior cats. It is a cancer of the lymphocytes — the white blood cells that patrol the immune system — and because lymphocytes live in nearly every tissue, lymphoma in cats can appear in many different forms with very different behaviors. The most common form in cats is gastrointestinal, which is why the disease so often hides behind ordinary-looking weight loss, vomiting, and reduced appetite.

The harder truth and the gentler one arrive together. Most forms of feline lymphoma cannot be cured. But the most common form — small-cell gastrointestinal lymphoma — is unusually treatable, with many cats achieving remission on a simple oral protocol and living comfortable years after diagnosis. Even more aggressive forms often respond well to modern chemotherapy and can give cats meaningful months of good life. This guide explains what lymphoma actually is in cats, how it is diagnosed and staged, the full range of treatment options, what to realistically expect from each, and how to support a cat through the months and years that follow.

What lymphoma actually is

The lymphatic system is a network of vessels, organs, and tissues that runs throughout the body. Lymph nodes — small bean-shaped filters scattered along that network — sit in clusters under the jaw, in front of the shoulders, behind the knees, and deep inside the chest and abdomen. The system carries a clear fluid called lymph, which is full of white blood cells called lymphocytes. Those lymphocytes come in two main families: B-cells, which produce antibodies, and T-cells, which coordinate immune responses and attack infected cells directly.

Lymphoma is a cancer in which lymphocytes — most often B-cells, sometimes T-cells — begin to multiply uncontrollably and accumulate in the tissues where they live. Because lymphocytes are everywhere, lymphoma is not really a single disease but a family of diseases that share a common cell of origin. A cat with lymphoma in her small intestine behaves very differently from a cat with lymphoma in her chest or her kidneys, even though the underlying cancer cell is similar. The location, the cell type, and the size of the abnormal cells all shape what owners and veterinarians experience.

Why senior cats develop lymphoma

The honest answer is that we usually do not know exactly why an individual cat develops lymphoma. Two known risk factors stand out, however, and both have changed over the past few decades.

Feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) historically accounted for a large fraction of feline lymphomas, particularly in young cats. FeLV-positive cats can develop lymphoma at 60 times the rate of FeLV-negative cats. Thanks to widespread vaccination and indoor lifestyles, FeLV-related lymphoma has become much less common in much of the world, and the typical lymphoma patient today is an older, FeLV-negative indoor cat. Both viruses are still tested for as part of any lymphoma workup.

Chronic intestinal inflammation is now the leading suspect for the most common form, small-cell gastrointestinal lymphoma. Many veterinary oncologists and internists believe that long-standing inflammatory bowel disease (IBD) can transform over time into low-grade lymphoma, which is why distinguishing the two on biopsy is sometimes difficult. Other potential factors include exposure to secondhand smoke, which has been linked to a higher risk of feline lymphoma, and genetics, with Siamese and other Oriental breeds appearing slightly overrepresented in some studies. For most individual cats, the cancer arrives without a single identifiable cause to point to. Owners often wonder what they could have done differently. They almost never could have prevented it.

Types of feline lymphoma

Veterinary oncologists classify feline lymphoma by where it appears in the body and by the size and grade of the cancer cells. The type matters enormously, because it changes the symptoms, the diagnostic approach, and the outlook.

Gastrointestinal lymphoma

Gastrointestinal lymphoma is by far the most common form in cats today, accounting for roughly 60 to 75 percent of feline lymphoma cases. It affects the stomach, small intestine, large intestine, and associated abdominal lymph nodes. Within this group, oncologists draw a critical distinction between two subtypes that look superficially similar but behave very differently.

Small-cell (low-grade, lymphocytic) gastrointestinal lymphoma is a slowly progressive disease most often caused by malignant T-cells. Cats with small-cell GI lymphoma typically present with chronic vomiting, gradual weight loss, intermittent diarrhea, and a slowly waning appetite — symptoms that closely mimic IBD. The disease is unusually responsive to oral chemotherapy, and many cats achieve remission and live two years or longer with treatment.

Large-cell (high-grade, lymphoblastic) gastrointestinal lymphoma is an aggressive B-cell or T-cell disease that tends to form discrete intestinal masses, sometimes large enough to cause partial obstruction. Affected cats often deteriorate quickly with severe weight loss, persistent vomiting, lethargy, and sometimes acute abdominal pain. Large-cell GI lymphoma is treated with multi-agent intravenous chemotherapy and has a more guarded outlook, though many cats still respond meaningfully.

Mediastinal lymphoma

Mediastinal lymphoma develops in the lymph nodes and thymic tissue inside the chest, between the lungs. It is most common in younger cats, particularly Siamese, and is more often associated with FeLV when it occurs. As the mass grows, it crowds the lungs and major vessels. Affected cats often present with rapid breathing, open-mouth breathing or panting, exercise intolerance, regurgitation, or fluid accumulation around the lungs (pleural effusion). A mediastinal mass on chest x-rays in a sick cat is one of the classic emergency presentations of feline lymphoma.

Multicentric lymphoma

Multicentric lymphoma — the form involving peripheral lymph nodes that is so common in dogs — is relatively uncommon in cats. When it does occur, owners may notice firm enlargement of the lymph nodes under the jaw, in front of the shoulders, or behind the knees. Because peripheral node involvement is unusual in cats, an enlarged peripheral lymph node always warrants prompt evaluation.

Renal (kidney) lymphoma

Renal lymphoma settles in one or both kidneys, causing them to enlarge and impairing their function. Affected cats typically present with the signs of kidney disease — increased thirst, increased urination, reduced appetite, weight loss, and sometimes vomiting. Unlike chronic kidney disease, in renal lymphoma the kidneys are usually noticeably enlarged on examination or ultrasound. Renal lymphoma carries an additional concern: the disease can spread to the central nervous system in some cats.

Nasal lymphoma

Nasal lymphoma is the most common cancer of the feline nasal cavity. Cats present with chronic nasal discharge (often from one nostril at first), sneezing, noisy breathing, facial swelling or asymmetry, sometimes a deformed bridge of the nose, and occasionally a bulging eye if the tumor extends behind it. Nasal lymphoma is often surprisingly responsive to radiation therapy, with many cats achieving long-lasting remissions.

Other forms

Less commonly, lymphoma can develop primarily in the central nervous system (causing seizures, weakness, or behavior change), in the eye (causing inflammation, blood in the eye, or sudden blindness), in the skin (causing patchy hair loss or ulcerated lesions), in the liver, in the spleen, or in the bone marrow. These extranodal forms are uncommon but important to recognize, because their early symptoms often look like other diseases first.

The signs and symptoms

Because cats are masters at hiding illness and because lymphoma can appear in so many tissues, the symptoms vary widely and are often subtle. The most useful framework is to recognize the patterns associated with the most common form — gastrointestinal — while staying alert to the signs of other presentations.

Weight loss

Gradual, unexplained weight loss is the single most common early sign of feline lymphoma, particularly the small-cell GI form. The change is often slow enough that owners do not notice until they pick the cat up and feel her ribs and spine more sharply than before. A senior cat losing weight while still seeming to eat is one of the strongest signals that bloodwork and imaging are needed.

Chronic vomiting

Many cats with GI lymphoma vomit intermittently — sometimes once a week, sometimes several times — for weeks or months before diagnosis. Owners often blame hairballs, food sensitivities, or eating too fast. Persistent vomiting in a senior cat, especially when paired with weight loss, deserves a full diagnostic workup rather than another diet change.

Reduced or changed appetite

Appetite changes can go in either direction. Many cats with lymphoma slowly lose interest in food, leaving more in the bowl each day, picking at favorites and refusing kibble. A few cats — especially those with intestinal disease that interferes with absorption — actually eat more than usual but still lose weight.

Diarrhea or changes in stool

Cats with intestinal lymphoma frequently develop soft stools, intermittent diarrhea, or thinner stools. Some develop constipation when an intestinal mass partially obstructs the colon. Blood in the stool is uncommon but possible.

Lethargy and withdrawal

Cats with lymphoma often become quieter, sleep more, hide more, and stop jumping to favorite high spots they used to claim with ease. Because these signs overlap so heavily with normal aging and with arthritis, they are easy to dismiss, and they are often noticed only in retrospect after a diagnosis.

Increased thirst and urination

Cats with renal lymphoma — and some cats with other forms — develop increased thirst and urination, often before any other obvious sign. Because so many senior cat illnesses cause similar changes, increased water intake in an older cat always warrants bloodwork.

Difficulty breathing

Mediastinal lymphoma can cause rapid breathing, open-mouth breathing, exercise intolerance, or refusal to lie flat. Pleural effusion — fluid around the lungs — can develop suddenly and become life-threatening within hours. Any senior cat with new respiratory symptoms needs urgent evaluation.

Nasal signs

Persistent nasal discharge, especially from one nostril, sneezing that does not resolve, noisy breathing, facial swelling, or a deformed bridge of the nose can all be signs of nasal lymphoma. Many cats are treated for chronic upper respiratory infections for months before imaging or biopsy reveals the cancer.

Other signs

Depending on where the cancer settles, cats may show pale gums (from anemia), yellow gums or eyes (jaundice), abdominal distension from fluid or masses, sudden blindness or red painful eyes, seizures, or unusual neurological signs. The diversity of presentations is part of why lymphoma is sometimes called "the great imitator."

How lymphoma is diagnosed

Diagnosis usually proceeds step by step, starting with the basics and moving toward more specific tests as the picture sharpens. The full workup combines confirmation of the cancer with staging tests that map how far the disease has spread and identify factors that affect treatment.

Initial bloodwork and urinalysis

Most workups begin with a complete blood count, chemistry panel, total T4 (to rule out hyperthyroidism, which causes similar weight loss), urinalysis, and FeLV/FIV testing. Bloodwork may show anemia, elevated white blood cell count, abnormal lymphocyte numbers, low albumin (especially in GI lymphoma), elevated liver values, or elevated kidney values. None of these findings is specific for lymphoma, but together they help direct the next steps.

Imaging

Abdominal ultrasound is one of the most important tests for cats with suspected GI lymphoma. It can show thickened intestinal walls, enlarged abdominal lymph nodes, masses in the intestines or kidneys, and fluid in the abdomen. Chest x-rays evaluate the lungs and look for a mediastinal mass or pleural effusion. Skull imaging or CT may be needed for nasal disease.

Fine-needle aspirate and cytology

For accessible lesions — peripheral lymph nodes, masses in the liver or spleen, kidney enlargements, or larger intestinal masses — fine-needle aspirates can sometimes confirm lymphoma without surgery. The procedure uses a small needle to collect cells that are smeared on a slide and reviewed under a microscope. Aspirates are very useful for high-grade (large-cell) lymphoma but often inadequate for diagnosing small-cell GI lymphoma, where the cancerous lymphocytes look very similar to the normal lymphocytes that infiltrate inflamed bowel.

Biopsy

For most cases of small-cell GI lymphoma, a tissue biopsy is needed to make a confident diagnosis and to distinguish lymphoma from inflammatory bowel disease. Biopsies can be obtained by endoscopy (relatively non-invasive but limited to the upper GI tract) or by exploratory surgery (more invasive but allows full-thickness samples and access to the entire intestinal tract). Biopsies are processed by a veterinary pathologist over several days.

Special testing: PARR and immunohistochemistry

When biopsies cannot clearly distinguish IBD from small-cell lymphoma, additional tests can help. PARR (PCR for Antigen Receptor Rearrangement) and immunohistochemistry can identify clonal populations of lymphocytes — a feature of cancer rather than inflammation. These tests have meaningfully improved diagnostic accuracy and have spared some cats from unnecessary chemotherapy and others from unnecessary delay.

Staging

Once the diagnosis is confirmed, oncologists may recommend additional tests to stage the disease — most commonly chest x-rays if not already done, and sometimes a bone marrow aspirate if blood counts are abnormal. For small-cell GI lymphoma, formal staging is often less critical than for high-grade disease, because the treatment plan is similar regardless.

Treatment options

Feline lymphoma is one of the most treatable cancers in veterinary medicine, particularly the small-cell gastrointestinal form. The goal of treatment is rarely cure — it is to put the disease into remission, restore quality of life, and extend comfortable time. Modern feline protocols are designed around the realities of cat ownership: minimizing visits, minimizing stress, and choosing drugs that cats tolerate well.

Oral chemotherapy for small-cell GI lymphoma

The standard treatment for small-cell GI lymphoma is a simple, well-tolerated oral protocol: chlorambucil (a low-dose chemotherapy drug) given every two to three weeks, combined with daily prednisolone (a steroid). Both medications are given at home. Roughly 70 to 95 percent of cats with small-cell GI lymphoma achieve remission on this protocol, with median survival of 1.5 to 2.5 years and many cats living 3 years or longer. Side effects are usually mild, and most cats continue to behave entirely like themselves.

CHOP multi-agent chemotherapy for large-cell lymphoma

For high-grade large-cell lymphoma — gastrointestinal, mediastinal, or multicentric — CHOP is the most effective protocol. The name is an acronym for the four drugs used in rotation: Cyclophosphamide, Hydroxydaunorubicin (doxorubicin), Oncovin (vincristine), and Prednisolone. The drugs are given at the clinic on a weekly to biweekly schedule over a 19- to 25-week course. Around 50 to 65 percent of cats with high-grade lymphoma reach remission on a CHOP protocol, with median survival of approximately 6 to 9 months and a meaningful minority of cats alive at one to two years. Cats tend to tolerate CHOP somewhat better than dogs, with milder side effects on average.

Single-agent chemotherapy

For families who cannot pursue full CHOP — because of cost, distance from a specialty hospital, or stress on the cat — single-agent protocols using doxorubicin alone, lomustine, or other drugs are reasonable alternatives. Response rates and survival times tend to be shorter than full CHOP but still meaningful for many cats.

Radiation therapy

Radiation has a particularly important role in nasal lymphoma, where it can produce remissions of 1.5 to 3 years or longer. It can also be useful for mediastinal lymphoma in some cases and for localized residual disease after chemotherapy. Radiation requires daily anesthesia for a series of treatments at a specialty hospital and is therefore not feasible everywhere.

Prednisolone alone

Prednisolone is an inexpensive oral steroid that often shrinks tumors and improves appetite, energy, and comfort for a period of time. It is not a true cancer treatment — it does not target lymphoma cells the way chemotherapy does — and cats treated with prednisolone alone usually have median survival of only 1 to 3 months for high-grade disease. Importantly, cats who have received steroids alone often respond less well to subsequent chemotherapy, because the disease has had time to develop resistance. If chemotherapy is on the table, talk to your veterinarian before starting steroids.

Palliative care

For families who choose not to pursue chemotherapy — because of the cat's age, comorbidities, stress with handling, cost, or personal values — supportive palliative care is a respectable and compassionate path. This typically includes prednisolone, antinausea medication (often mirtazapine and maropitant), appetite support, fluids when needed, and careful attention to comfort. A thoughtful palliative plan can make weeks to months meaningful and peaceful.

What to expect during treatment

Veterinary chemotherapy is consciously designed to preserve quality of life. Doses are calibrated to slow or stop the cancer while keeping side effects mild. Most cats continue eating, grooming, and behaving like themselves throughout treatment.

Side effects

For oral chlorambucil-based protocols, side effects are usually minimal — occasional mild GI upset, transient drops in white blood cell or platelet counts, and rarely a hepatitis-like reaction. For injectable CHOP, roughly 75 to 85 percent of cats experience minimal or no significant side effects. The most common issues are mild nausea, soft stools, transient appetite loss, and temporary fatigue lasting a few days after a treatment. Cats almost never lose their fur from chemotherapy, although whiskers may thin and the coat may feel slightly different. Severe reactions requiring hospitalization occur in a small minority of cats.

Vet visit schedule

Small-cell protocols typically require visits every 2 to 4 weeks during the early months for bloodwork, then every 1 to 3 months once the cat is in stable remission. CHOP protocols require weekly to biweekly visits during the active treatment phase, then less frequent rechecks afterward. For cats who find vet visits stressful, oncologists can often coordinate with primary veterinarians, use Fear Free handling techniques, and pre-medicate with gabapentin to make visits easier.

Cost

Cost varies widely by region and protocol. An oral chlorambucil-and-prednisolone protocol for small-cell GI lymphoma is one of the most affordable cancer treatments in veterinary medicine, often running $1,500 to $3,500 over the first year including medications and rechecks. A full CHOP course at a specialty hospital typically runs $5,000 to $8,000 for cats. Single-agent protocols run $2,000 to $4,000. Prednisolone-only palliative care is inexpensive — usually under a few hundred dollars over the course of treatment. Pet insurance, payment plans, and nonprofit assistance funds can sometimes help.

Prognosis: what to realistically expect

Prognosis depends heavily on the type of lymphoma, the cell grade, the location, and how the cat responds to the first weeks of treatment. The numbers below are medians, which means half of cats do better and half do worse.

• Small-cell GI lymphoma with chlorambucil + prednisolone: remission in 70 to 95 percent of cats; median survival 1.5 to 2.5 years; many cats live 3 years or longer with continued treatment.
• Large-cell GI lymphoma with CHOP: remission in approximately 50 to 65 percent; median survival 6 to 9 months; a meaningful minority of cats live 1 to 2 years.
• Mediastinal lymphoma with CHOP: highly variable; younger Siamese cats sometimes have surprisingly long remissions, while older cats often do less well; median survival 3 to 9 months in most studies.
• Nasal lymphoma with radiation therapy: remission rates above 75 percent; median survival 1.5 to 3 years, sometimes longer.
• Renal lymphoma with chemotherapy: median survival 3 to 6 months; risk of central nervous system spread.
• Prednisolone alone (high-grade disease): median survival 1 to 3 months; quality of life often improves temporarily.
• Untreated high-grade lymphoma: median survival 4 to 6 weeks from diagnosis.

These numbers are generalizations across many cats. They are not predictions for your cat. Some cats sail through treatment with deep remissions and live well past the medians; others relapse early. A board-certified veterinary oncologist or internist can offer a more individualized estimate based on biopsy results, immunophenotype, and how the cancer responds to the first few weeks of treatment.

Supporting your cat at home

Daily care during and after treatment focuses on nutrition, comfort, monitoring, and protecting against complications. None of it is complicated, but consistency and gentle attention to detail matter.

Nutrition

Maintaining caloric intake is one of the most important things owners can do. Many cats with GI lymphoma eat less because of nausea, and weight loss can quickly become its own problem. Warming food, hand-feeding, offering several small meals a day, and rotating among palatable options often helps. For cats with GI lymphoma, a highly digestible novel-protein or hydrolyzed diet is often recommended, particularly when there is overlap with food-responsive enteropathy. Mirtazapine, an appetite stimulant, is widely used and well tolerated. Maropitant or ondansetron can control nausea. Weight should be tracked at home with a baby scale or at every vet visit.

Monitoring at home

Watch for new vomiting episodes, persistent diarrhea, refusal to eat for more than 24 hours, hiding, lethargy beyond what is normal for that cat, fever, breathing changes, or pale gums. For cats on injectable chemotherapy, the days following each dose are when problems are most likely. Keep a simple notebook or app log; trends are usually more useful than single observations.

Avoiding infection

Chemotherapy temporarily lowers white blood cell counts in the days after each dose, leaving cats slightly more vulnerable to infection. Keep your cat indoors, avoid boarding during low-count windows, and call your veterinarian for a fever, lethargy, or vomiting that persists more than 24 hours.

Handling chemotherapy drugs safely

For 48 to 72 hours after a chemotherapy dose, small amounts of drug may be excreted in urine, stool, and saliva. Wear gloves when scooping the litter box, change litter daily, wash bedding separately, and avoid letting your cat be handled by pregnant family members, young children, or immunocompromised individuals during this window. Pill-form drugs given at home (like chlorambucil) require similar gloved handling. Your veterinary team will give you a specific handout. The risk is small, but reasonable precautions are easy.

Comfort and routine

Cats are creatures of routine, and a stable home rhythm helps during treatment. Familiar resting spots, gentle play, predictable mealtimes, and minimal household upheaval all support a cat's well-being. Heated beds, low-edge litter boxes, and ramps to favorite high places can compensate for reduced energy or weight loss. The cat who was the same cat before the diagnosis is still the same cat after it; treatment is something you do alongside everyday life, not instead of it.

What to avoid

• Starting prednisolone before talking to your veterinarian about chemotherapy. A few weeks of steroid alone can reduce how well subsequent chemotherapy works.
• Assuming chronic vomiting is "just hairballs." A senior cat vomiting weekly is showing a medical problem until proven otherwise.
• Skipping the biopsy when small-cell lymphoma is suspected. Distinguishing IBD from lymphoma changes treatment and outlook, and PARR or immunohistochemistry can be the deciding evidence.
• Comparing your cat to humans on chemotherapy. Veterinary chemotherapy is dosed for quality of life, and most cats do not experience the severity of side effects that human patients sometimes do.
• Waiting to see if the symptoms resolve on their own. They will not. Time matters; earlier treatment usually means deeper, longer remissions.
• Unproven alternative therapies as a substitute for treatment. Many supplements and herbal protocols are reasonable as additions to evidence-based care after a conversation with the oncologist, but very few have been shown to slow lymphoma on their own.
• Skipping FeLV/FIV testing. A positive result changes prognosis and treatment planning, and rare cases still occur even in indoor cats.

When to see the vet

Beyond the regular treatment schedule, several changes warrant calling your veterinary team between appointments.

Call soon if your cat shows:

• Refusal to eat for more than 24 hours, or eating noticeably less for several days
• Persistent vomiting or diarrhea lasting more than a day
• Lethargy beyond the typical post-treatment dip
• Pale gums, yellow gums, or unusual bruising
• Weight loss visible on the scale (more than a few ounces in a small cat is significant)
• New lumps, swellings, or enlarged lymph nodes
• Increased thirst or urination, or signs of straining in the litter box

Seek emergency care if your cat shows:

• A fever (rectal temperature above 103.5°F) within 7 to 10 days of a chemotherapy dose
• Open-mouth breathing, panting, or labored breathing at rest
• Severe weakness, collapse, or pale-to-white gums
• Severe vomiting or diarrhea, especially with blood
• Sudden seizures, disorientation, or sudden blindness
• A distended, painful abdomen

Talking about end of life

Most cats with lymphoma eventually relapse — some after a year or more, some sooner. Second-line protocols (rescue chemotherapy with different drugs) can produce additional remissions, but those remissions are usually shorter than the first. There comes a point, for most families, where the right next step is no longer another round of treatment but a focus on comfort and on letting the cat go peacefully when her quality of life can no longer be maintained.

Quality of life scales — the HHHHHMM scale, the Feline Grimace Scale, simple good-day/bad-day calendars — give you a structured, compassionate way to track how your cat is really doing. Honest conversations with your veterinarian or oncologist can help you recognize when more days of treatment are no longer adding more good days of life. Cats are private about pain, and the kindness owners often regret, in retrospect, is waiting too long. There is no "right" moment, only the moment that feels least wrong, made with love and with eyes wide open.

Compassion and hope

A diagnosis of lymphoma is one of the harder phone calls an owner ever takes. The disease is serious, the language is unfamiliar, and the calendar of treatments and rechecks suddenly fills the weeks ahead. It is reasonable to feel afraid. It is also reasonable to feel hopeful.

For many cats — particularly those with the most common small-cell GI form — lymphoma becomes a chronic, manageable condition rather than an immediate ending. Appetite returns. Vomiting stops. Weight stabilizes and sometimes climbs back. The cat who was hiding under the bed comes back to the windowsill, the food bowl, the lap. Treatment is not painless and not free, but it buys real, meaningful time — often years of the cat you have always known. And during those years, the work of love is unchanged: you fill the bowl, you brush the coat, you scratch under the chin, you sit together at the window, and you remind a very good cat, one ordinary day at a time, that she is not facing this alone.

This article provides general educational information about lymphoma in cats. It is not a substitute for individualized veterinary advice. Every cat and every cancer is different, and decisions about diagnosis, staging, and treatment should always be made in partnership with a veterinarian — and ideally, for ongoing oncology care, a board-certified veterinary oncologist or internist — who can examine your cat and interpret her specific bloodwork, imaging, and biopsy results. Never start prednisolone, supplements, or any new medication for a suspected cancer without veterinary guidance, and never delay evaluation of persistent vomiting, weight loss, or breathing changes in a senior cat. If your cat develops a fever during chemotherapy, severe weakness, pale gums, open-mouth breathing, or sudden collapse, seek emergency veterinary care immediately rather than waiting for a scheduled appointment.