Comparison of age- and sex-specific incidence rate patterns of the leukemia complex in the cat and the dog.

Data on cancer cases in cats and dogs were collected systematically by the Animal Neoplasm Registry of Alameda and Contra Costa Counties, Calif., a population-based animal tumor registry. Etiologic relationships were evaluated on the basis of comparisons of age-specific, sex-specific, and age-neutered-specific incidence rate patterns of the leukemias. Age-adjusted annual incidence rates for all leukemias per 100,000 cats or dogs were 224.3 and 30.5, respectively. The cat had 6.1 times more malignant lymphomas and 15.7 times more myeloproliferative disease than the dog. Feline age-specific rates indicated a bimodal age pattern for all leukemias and for malignant lymphoma alone and a single early peak for myeloproliferative disease. In the dog, all age-specific patterns increased with age and peaked later in life. Feline sex-specific, age-adjusted rates showed that the neutered female was at lowest risk, followed by the neutered male, entire female, and entire male. In the dog, the neutered male was at lowest risk while the other three sex categories were clustered. However, the magnitude of expression within each species separately was the same for the neutered male, entire male, and entire female, but not for the neutered female. Neutering decreased the risk of leukemias in the female cat by approximately one-half but did not affect the risk of leukemias in the female dog.