The leukocytes.
Dogs and cats respond to many diseases by changes in leukocyte numbers. Infectious diseases often cause leukocytosis due to neutrophilia. Left shift may accompany the leukocytosis, indicating that the marrow is mounting a response to the disease. Left shift also indicates that the marrow has fallen somewhat behind the needs of the animal. Degenerative left shift is considered a poor sign. Lymphopenia and eosinopenia also are found in infectious diseases. Lymphocytosis may occur during the recovery or if the disease becomes chronic. The nature and duration of the infection determine the magnitude of the monocyte response. It is erroneous to consider a disease chronic based only upon monocytosis. Trauma, autoimmunity, or any disease with significant tissue destruction can evoke a monocyte response. Leukopenias are relatively common in cats and are found with moderate frequency in dogs. Drugs, viral diseases (such as FeLV, feline enteritis, parvovirus), ehrlichiosis, and hereditary conditions may cause panleukopenias or single leukopenias. Occasionally leukocyte examination provides evidence of a specific etiology (such as with ehrlichiosis). Sometimes changes occur which, although not specific for a disease, may provide a strong evidence of a particular disease (such as in salmon poisoning). Leukocyte evaluation should include not only total count and differential count (with calculation of absolute numbers of the different cells) but also morphologic examination of the cells by qualified people. In many practices the only qualified person is the veterinarian.
D000818:Animals; D002371:Cat Diseases / Q000097:blood*; D002415:Cats; D002634:Chemotaxis, Leukocyte; D004283:Dog Diseases / Q000097:blood*; D004285:Dogs; D004804:Eosinophils / Q000502:physiology; D006402:Hematologic Diseases / Q000662:veterinary*; D006410:Hematopoiesis; D007962:Leukocytes / Q000502:physiology*; D007964:Leukocytosis / Q000097:blood / Q000662:veterinary; D007970:Leukopenia / Q000097:blood / Q000662:veterinary; D008214:Lymphocytes / Q000502:physiology; D008264:Macrophages / Q000502:physiology; D009503:Neutropenia / Q000097:blood / Q000662:veterinary; D009504:Neutrophils / Q000502:physiology; D010587:Phagocytosis; D012288:Rickettsiaceae Infections / Q000097:blood / Q000662:veterinary