Mammary infection with Staphylococcus aureus in cows: progress from inoculation to chronic infection and its detection

The progress of Staphylococcus aureus infection from inoculation to the ear ly chronic stage was examined in 12 Israeli-Holstein cows (four primiparous and eight multiparous) for up to 48 d after inoculation. Before inoculatio n, the primiparous cows were free from any infection and the multiparous co ws were infected by coagulase-negative staphylococci. Two quarters in each cow were inoculated intracisternally following milking with 2000 cfu of a l ocal prevailing Staph. aureus strain: VL-8407. Infection was established in 21 out of 24 quarters. The control quarters remained free from infection d uring the study, with no significant change in function. No statistically s ignificant differences were found between primiparous and multiparous cows in the responses examined. Somatic cell count (SCC) increased within 24 h o f inoculation and remained high for the duration of the study. In the infec ted quarters mean In (XCC) increased within 24 h from 9.9 +/- 0.5 before in oculation to 13.0 +/- 0.2 after inoculation I most of the cells were neutro phils. N-acetyl-beta-glucosaminidase activity, expressed as In(nnmol/min pe r 1), was increased from 0.9 +/- 0.6 to 24 +/- 0.2 by inoculation, and was highly correlated with SCC. The Staph. aureus count fluctuated with no part icular relationship with SCC. The phagocytic activity of neutrophils was si gnificantly lower in the inoculated than in the control quarters and this d ifference increased with time after inoculation. CD8(+) T lymphocytes were the main subpopulation of lymphocytes found in inoculated quarters. after i noculation, maximum but not minimum electrical conductivity (EC) recorded d uring milking increased significantly. The rises in maximum EC varied signi ficantly among cows. The rises in SCC were associated with a persistent inc rease in EC in only one of the eight cows examined. No clinical signs mere observed, and milk yield and composition were not affected during the study period. The results suggest that some strains of Staph. aureus may induce a relatively mild response in mammary glands of cows in mid lactation, and that the concomitant development of such chronic Staph. aureus infections i n two quarters may not be detected by changes in the EC of composite milk a nd in the yield of the cow.