Elimination of Methicillin-Resistant Staphylococcus aureus from Mammary Glands of Dairy Cows by an Additional Antibiotic Treatment Prior to Dry Cow Treatment

Abstract

Methicillin-resistant Staphylococcus aureus (MRSA) have been isolated from quarter milk samples of dairy cows, raising concerns over transmission to consumers of raw milk. This study investigates whether pre-treatment before dry-off can increase the success rate of dry cow treatment against MRSA. MRSA positive cows were assigned to two treatment groups. Both groups received dry cow treatment with a licensed product. The test group was additionally treated intramammarily with pirlimycin over seven days prior to the dry-off treatment. The use of pirlimycin increased the elimination of MRSA from previously MRSA positive udder quarters significantly (96.0 vs. 53.3%). However, MRSA were still present in noses and udder clefts of cows in MRSA negative quarter milk samples. New infections were observed in some quarters in both groups. Quarters that remained positive carried the same strain as prior to treatment. All MRSA isolates were associated with clonal complex CC398. Resistance to pirlimycin associated with the genes erm(C) or lnu(B) was observed in one isolate each from new infections after calving. Pretreatment supported the elimination of MRSA from the udder but did not eliminate MRSA from other body sites. Using the treatment will not eliminate the bacteria from the herd.

Keywords: MRSA; dairy sector; dry cow treatment; pirlimycin; skin colonization.

Figure 1

Status of udder quarters before dry-off and their respective diagnosis after parturition. More than half of the udder quarters were negative for MRSA prior to dry-off and after calving. Therefore, only the percentage range above 50% is displayed. neg-neg, negative prior to treatment and after calving; neg-pos, negative prior to treatment and positive after calving; pos-neg, positive prior to treatment and negative after calving; pos-pos, positive prior to treatment and after calving.

Figure 2

Genomic comparison (SNP analysis) of MRSA obtained from quarter milk samples of different cows (C), quarters (Q) and time points (S1, S2, S3, S4). S1 and S2 represent sampling before dry-off. Collection of S3 and S4 was carried out after calving. Cow C15 was affected by mastitis during the trial.