Antibiotic therapy for subclinical mastitis in early lactation : effects on infection, somatic cell counts and milk production : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Animal Science at Massey University
This study was conducted with the main purpose of investigating the effects of antibiotic therapy applied during lactation on the subclinically infected quarters in terms of bacterial infection, somatic cell count (S.C.C.), milk yield and composition (i.e. milkfat and protein percentage). The main objectives involved were:- (1) To identify cows with high somatic cell counts (subclinical mastitis) in early lactation. (2) To identify the individual quarters with high somatic cell counts and bacterial infection. (3) To treat some of these high S.C.C. quarters with an antibiotic, and leave some of them untreated (control quarters). (4) To measure the effects of treatment on S.C.C., bacterial infection, yield and composition of milk. During weeks four to six of lactation 12 cows from a herd of 100 cows at No.3 Dairy Research & Development Unit, Massey University, were identified to have consistently high values for somatic cell counts. Milk samples were aseptically taken from the individual quarters of these cows and subjected to bacteriological tests. Eight subclinically infected quarters with high somatic cell counts were subsequently treated with sodium cloxacillin (Orbenin) formulated in a slow release base. Before antibiotic treatment of any quarters, there was a close association between infection and somatic cell counts in individual quarters. Thus only 12% of the quarters which showed no infection had somatic cell counts higher than 300,000 cells/ml, whereas the corresponding proportion of infected quarters was 74%. Out of the eight infected quarters which were treated with antibiotic, five were cured of infection (62.5% cure rate); in these quarters the average S.C.C. was greatly reduced from 4,207,000 cells/ml before treatment to about 160,000 cells/ml afterwards whereas the three quarters which were treated but not cured showed a slight decrease from 3,991,200 cells/ml before treatment to 2,638,800 cells/ml after treatment. The average somatic cell counts of the five infected control quarters increased slightly from their original value of about 2,061,500 cells/ml up to 2,111,000 cells/ml. There was no significant or consistent effect of antibiotic therapy, successful or otherwise, on milk yield from individual quarters. However adjusted means showed a 10% non-significant difference between the successfully treated quarters and the untreated control quarters in favour of the cured quarters. The effects of treatment on milk composition were small and were significant only for protein percentage. The practical implications of the results were discussed. From the results obtained firm conclusions on the effects of antibiotic therapy of subclinically infected quarters on milk yield, and composition were not possible. The major benefit is likely to be the reduction in the number of infected quarters, and the consequent reduction in the risk of new infection in other quarters. It is however suggested that more work is required to establish the actual effect of antibiotic therapy by obtaining information on the performance of the successfully treated quarters in the following lactation.