Hematological characteristics of rats spontaneously developing eosinophilia

Hematological and genetic characteristics of newly found eosinophilic rats were studied. Hematologically, high blood eosinophil counts started at 6 we eks of age. Almost all 10-week-old rats had eosinophilia with individual co unts above 500/mu l and 5 to 100 times the normal level. Proliferating eosi nophils had normal morphology. An increase in lymphocyte counts was observe d at 5 weeks of age, one week earlier than the onset of eosinophilosis. In bone marrow, proliferation of eosinophils was also observed at 8 weeks of a ge and thereafter progressed, suggesting a role in the pathogenesis of eosi nophilia in this rat. The results of genetic cross experiments revealed the disease to be hereditary. The spontaneously eosinophilic rat therefore war rants attention as a model for studying the underlying mechanisms of human and animal eosinophilia.