Udder health and risk factors for subclinical mastitis in organic dairy farms in Switzerland

A study was performed in 1997 to estimate the prevalence and to investigate the etiology of subclinical mastitis in Swiss dairy herds managed under gu idelines of controlled organic farming. It was planned as a longitudinal st udy over a period of 1 year and included a stratified random sample of 152 certified organic farms and 1907 cows. Two farm visits (the first from June to October when cows were on pasture, the second from January to March whe n cows were confined to barns) were performed on each farm. At each visit, farm management and individual-cow data (with emphasis on milking procedure s and udder sanitation) were recorded. California mastitis tests (CMTs) wer e performed on each udder quarter of all cows in lactation. Milk samples wi th CMT >1+ were submitted for somatic cell counting (SCC), bacteriological examination and to test for antibiotic susceptibility. The SCC and germ-cel l counts of monthly bulk-tank milk samples were available through Dairy Ins pection and Advisory Services and milk production data of 567 herd-book cow s were available from breeding associations. Possible individual and enviro nmental predictors of subclinical mastitis were identified using logistic m odels adjusted for clustering of the data at herd and cow levels. Data were analyzed separately for cows from 7 to 100 and from 101 to 305 days post p artum. Prevalences of subclinical mastitis at the quarter level were 21.2% for lactation period 7-100 days and 34.5% for 101-305 days post partum. The geometric mean SCC in bulk-tank milk samples was 85.6 x 10(3) cells/ml. Sa mples at 7-100 and 101-305 days post partum were positive for Staphylococcu s aureus in 16.0 and 7.4%, for coagulase-negative Staphylococci in 51.5 and 50.6%, for Streptococcus agalactiae in 0.0 and 0.8%, for other Streptococc i in 19.4 and 15.6%, for E. coli in 1.0 and 0.4%, and for Corynebacterium b ovis in 25.7 and 45.1%, respectively. Risks of subclinical mastitis increas ed significantly with increasing days post partum and advancing age of cow. Cows that were sampled when staying in alpine dairies had considerably hig her risks of subclinical mastitis than cows staying in home barns. Signific antly lower risks of subclinicalmastitis were observed in farms where CMT w as performed regularly as a control measure. Bacteria in milk from cows wit h mastitis exhibited antibiotic resistance at a comparable frequency as fou nd previously in conventional farms. (C) 2000 Elsevier Science B.V. All rig hts reserved.