Maturation arrest of neutrophils-a possible reason for the leucopenia in sodium selenite induced sub-chronic selenosis in cow calves

The effect of long-term administration of sodium selenite on leucocyte indices of peripheral blood of calves was determined. Nine calves, 9-12 months old, with an average body weight of 104 kg were divided into three groups. Calves of groups 2 and 3 were administered with sodium selenite at 0.1 and 0.25 mg/kg body weight for 98 consecutive days. The clinical signs characteristic of selenosis viz. alopecia, cracking of hooves, intradigital lesions and discoloration of hard palate, started appearing from 45 to 60 days onwards with high dose, whereas only subtle indications of toxicosis were observed in the low-dose group. The prolonged administration of sodium selenite produced a progressive and dose-dependent decline in the circulating leucocyte count with concomitant decline in the circulating neutrophil count. There was a high negative correlation (0.94) between blood selenium levels and neutrophils. Granulocyte/agranulocyte ratio was also significantly reduced in the treated animals. Evaluation of bone marrow smears revealed a decline in the myeloid to erythroid ratio. In addition, there was also maturation arrest of neutrophils at promyelocyte or myelocyte level as shown by differential granulocyte count in the bone marrow. The results indicated that host's immune response may be adversely affected.