Lactating cows become partially refractory to frequent intramammary endotoxin infusions: recovery of milk yield despite a persistently high somatic cell count.

Midlactation cows were infused with 10 micrograms endotoxin in the same two homolateral quarters after each of several consecutive milkings to study the effect of prolonged, endotoxin-induced mastitis on lactational performance. The initial infusion induced an acute response with systemic involvement. Inflammation developed in infused quarters, and milk production declined and milk composition was altered in all quarters. Subsequent infusions failed to induce systemic responses. Furthermore, milk yield and composition in uninfused quarters returned to pre-treatment levels despite further infusions. In infused quarters, milk yield, protein percentage and serum albumin concentration showed partial recovery during the endotoxin infusion period. In contrast, decreased lactose concentration, and increased cell count, N-acetyl-beta-D-glucosaminidase and lactoferrin levels persisted throughout the infusion period. After infusions were stopped, all measurements returned to near pretreatment levels. These data demonstrate that systemic, but not local, responses become refractory to multiple intramammary endotoxin infusions, and that multiple infusions have continued but little progressive or permanent, inhibitory effects on lactational performance despite a persistent mammary leucocytosis.