Cytologic features of bone marrow in normal and mastitic cows.

Rib bone marrow was aspirated from normal cows and from cows with experimentally induced or naturally acquired mastitis. Nucleated cell counts in marrow films were made on the basis of differentiation of 500 cells. The myeloid/erythroid ratio was less than 1.0 in normal cows and generally greater than 1.0 in mastitic cows. The marrow reserve of mature neutrophils was nearly depleted in response to experimentally induced Streptococcus agalactiae mastitis (4th postinoculation day) or in an acute flare-up of naturally acquired Str agalactiae mastitis. A dose of 0.05 mg of Escherichia coli endotoxin, introduced into a single, normal, lactating quarter produced similar results within 8 hours. Concurrently, with depletion of marrow reserves of mature neutrophils, a neutropenia with left shift developed in the blood, and immature granulocytes increased in the marrow. The intensification of granulopoiesis returned the marrow reserves to normal or above normal within a period of 4 to 5 days, thereby reestablishing homeostasis.