June 16, 2026 — Decades ago, a young cat diagnosed with lymphoma would have been infected with feline leukemia virus. Lymphoma was a disease of young cats, striking at 3 to 5 years of age, and the feline leukemia virus was almost always to blame.
Then came the feline leukemia vaccine.
What followed was one of veterinary medicine's underappreciated success stories. As vaccination became widespread, feline leukemia virus infections declined dramatically, and so did the lymphomas linked to it. Cats that once died young from virus-driven cancer were now living well into their senior years.
But as cats began living longer, a different form of lymphoma took hold: one with no viral driver and no clear cause. Gastrointestinal (GI) lymphoma emerged as the dominant cancer pattern in cats. Today, the average age at diagnosis is 10 to 12 years. The disease is now one of the most commonly reported cancers in cats, and researchers are still working to understand why it develops and how to treat it more effectively.
The feline leukemia vaccine saved a generation of cats. Morris Animal Foundation is working to protect the next one.
What Is GI Lymphoma in Cats?
Lymphoma develops when a single lymphocyte, a type of white blood cell that plays a central role in immune function, acquires genetic mutations that allow it to divide without the normal controls that keep cell growth in check. In cats, this process most commonly unfolds in the gastrointestinal tract, particularly the small intestine, which is home to large numbers of immune cells.
The disease causes chronic digestive disruption. Cats with GI lymphoma typically experience weight loss, vomiting and diarrhea. These symptoms can persist for months before a diagnosis is reached, and because they are common to many conditions, lymphoma is not always the first diagnosis considered.
Two Diseases in One Name
Feline GI lymphoma is not a single disease. It exists in two main forms, small-cell lymphoma and large-cell lymphoma, and the difference between them is clinically significant.
Small-cell lymphoma (also called low-grade lymphoma) is the more common form. It is slow-growing, and cats diagnosed with it generally respond well to treatment. An oral chemotherapy drug called chlorambucil, combined with an oral steroid such as prednisone, produces strong response rates. Many cats treated for small-cell lymphoma live for several years after diagnosis.
Large-cell lymphoma (also called high-grade lymphoma) carries a much more serious prognosis. Despite treatment with multi-drug chemotherapy protocols, most cats survive only a few months. It is an aggressive disease that remains difficult to manage.
The Diagnostic Challenge
Several tools exist to diagnose GI lymphoma. Cytology, which involves examining cells collected by aspiration, works well for large-cell forms but can miss small-cell disease. Histology, which involves examining stained tissue sections under a microscope, is the most common diagnostic method. Flow cytometry can identify whether the cancerous cells are B cells or T cells, which can refine prognosis. Clonality assays help determine whether a population of lymphocytes is cancerous or inflammatory.
That last distinction matters because small-cell lymphoma can look nearly identical to inflammatory bowel disease under the microscope. Even experienced pathologists can find it difficult to differentiate the two. For cat owners and their veterinarians, a missed or delayed diagnosis means lost time.
Current diagnostic tools were not designed with the full complexity of feline GI lymphoma in mind. That is the gap a new Morris Animal Foundation-funded study is working to close.
New Technology, New Possibilities
Researchers at the University of Florida are taking a different approach to understanding feline GI lymphoma. Rather than examining cells in isolation, the team is mapping gene activity across intact tissue, identifying which genes are turned "on" or "off" in different regions of the intestine while preserving the tissue's structure.
This spatial approach creates a detailed picture of how cancer cells interact with the immune cells and supportive cells around them. By comparing these maps from healthy cats with those from cats with lymphoma, the team can trace how the disease transforms normal tissue, how it evades immune detection, and which biological pathways might eventually be targeted for treatment.
The research team has already collected tissue samples and validated that the technology works in feline tissue. Their next step is to expand the number of cats studied and build the first complete reference atlas of feline intestinal lymphoma. This atlas will serve as a resource for veterinary and human medicine alike, since cats naturally develop lymphomas that share important features with human disease, meaning discoveries made in cats can inform cancer research in people.
Why This Research Matters
The history of feline lymphoma is a story of progress creating new problems. The feline leukemia vaccine did exactly what it was designed to do, and in doing so gave rise to a generation of older cats more vulnerable to a different, more complex disease.
Closing that gap requires better diagnostic tools, a clearer picture of how GI lymphoma develops, and treatments that can distinguish between a disease that responds well and one that does not. Morris Animal Foundation is funding the science that makes that possible.
For the cat owners who watch their companions lose weight and energy, and for the veterinarians who need more than a microscope slide to make the right call, this research represents a meaningful step forward.
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Cancer is a leading cause of death in cats, and veterinarians have limited treatment options due to a lack of funding for research focused on the many cancers that take our cats' lives too soon.
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