Approach to Interpretation of the Leukogram in Ruminants
Most sources report that neutrophils predominate in neonatal calves and lambs, with a reversal of this ratio about a week after birth.29 One study examining reference interval data in Holstein cows longitudinally from 1957 through 2010 suggests that, over time, mean neutrophil counts have increased while other leukocyte counts have dropped.28 A survey of other laboratories suggested that this observation had been made elsewhere but was not universal.
Potential causes hypothesized include genetic selection, changes in management practices, and decreased prevalence of viral pathogens such as bovine viral diarrhea and bovine leukosis virus. Because a predominance of neutrophils compared with lymphocytes regardless of whether absolute cell counts deviate from the reference intervals has been used as an indicator of inflammatory disease in cattle, these findings may be significant to ruminant leukogram interpretation. In light of these findings, clinicians should be especially attentive to the source of reference intervals provided by laboratories.Corticosteroid leukograms are relatively common in ruminants, as opposed to the changes associated with excitement, which are rare. Stress leukograms are commonly observed in cattle, sheep, and goats in the periparturient period. Stress leukogram changes are often superimposed on inflammation. As described previously, cattle have a sluggish marrow response to increased demand for neutrophils, resulting in the frequent observation of neutropenia in the first 24 to 48 hours after an acute inflammatory stimulus. Persistent neutropenia of more than 3 to 4 days indicates marrow suppression or inability of the marrow to respond to excessive tissue demands. As in the horse, rebound neutrophilia may ensue, often accompanied by a significant left shift that may include myelocytes and metamyelocytes in addition to band neutrophils. The left shift will resolve as neutrophil numbers increase with marrow compensation for increased demand. In chronic or low-grade inflammation, there may be no abnormalities in the leukogram, in which case evaluation of acute phase proteins may be helpful. The leukogram responses of sheep to inflammation generally mirror those of the bovine, whereas goats tend to respond more like small animal species with less frequent periods of leukopenia in response to inflammation.