Lymphoma is the most common neoplasm of the hematopoietic system in cats. There are 2 basic types: retroviral and nonretroviral. Feline leukemia virus (FeLV) accounted for as many as 70% of cases of feline lymphoma before the 1980s, at which time testing, elimination, and quarantine programs and vaccination drastically reduced the incidence of FeLV. Nonretroviral-associated lymphomas now constitute the most common type of feline lymphocytic cancer.

This retrospective study examined the patient database at the Veterinary Medicine Teaching Hospital (VMTH) at the University of California, Davis, covering the period 1983 through 2003. The purpose was to gain background information on lymphoma incidence, particularly non-FeLV-related mediastinal lymphoma. Of the 546 cats with lymphoma identified, those positive for FeLV, FIV, or both were excluded from a detailed analysis. The authors made several important findings. First, they noted that only 14.5% of the cases were retrovirus (mainly FeLV)-associated lymphomas, compared with more than 70% of cases that were FeLV-associated before institution of FeLV control measures in the 1970s and 1980s. This confirmed a decrease in the number of cats with FeLV-associated tumors presented to the VMTH. Second, although FeLV infection has decreased, the incidence of viral-negative lymphomas has nearly doubled. Third, a relatively high incidence of mediastinal lymphoma in Siamese-type breeds was found, and the age at diagnosis (2 years) was considerably younger than that of other cats with mediastinal lymphoma (median age, 11 years). Finally, a change in the predominant types of nonretroviral-associated lymphomas was found, with marked increases seen in the abdominal (especially intestinal) and atypical (mainly nasopharyngeal) forms. The authors concluded that lymphosarcoma remains a persistent problem in cats. Because factors other than FeLV (e.g., chronic inflammation) seem to contribute to its development, the authors suggest that the role of inflammatory bowel disease, diet, and environmental influences undergo further study.

COMMENTARY: The incidence of feline lymphoma has increased over the past decade despite an absolute reduction in the incidence of FeLV infection. This 20-year retrospective evaluation of cases presented to UC Davis uncovered 546 cases of lymphoma, 467 of which were negative for both or either FeLV and FIV. The median age of disease remains unchanged at 11 years in all but the cases of mediastinal lymphoma, in which it remains at 2 years. This latter form remains overrepresented by Siamese-type breeds (Siamese, oriental shorthair, colour-pointed Shorthair, and Javanese breeds). With the risk factor of FeLV infection having declined, other causes must be considered. These include genetics (Siamese-type breeds), and triggers, including exposure to cigarette smoke and uncontrolled chronic inflammation. Areas in which chronic inflammation predisposes to lymphoma are the gastrointestinal tract (mouth and intestines), nasal passages, and kidneys. Further work is needed to elucidate the factors involved in development and expression of lymphoma in cats.

Feline lymphoma in the post-feline leukemia virus era. Louwerens M, London CA, Pedersen NC, Lyons LA. J VET INTERN MED 19:329-335, 2005.