Effect of prolonged duration therapy of subclinical mastitis in lactating dairy cows using penethamate hydriodide

AIM: To evaluate the efficacy of nil, three or six treatments of penethamate hydriodide on successive days for treatment of subclinical mastitis on the bacteriological cure proportion and subsequent somatic cell count (SCC).METHODS: Milk samples were collected from all glands of 102 cows with SCC of >200,000 cells/mL, and at least one gland with a rapid mastitis test (RMT) score >0. Cows from which a bacterial pathogen was isolated were treated (Day 0) with either three (3xPH; n=31 cows), six (6xPH; n=30) or no (Control; n=31) daily I/M injections of 5g of penethamate hydriodide. Milk samples were collected on Days 21 and 28 for bacteriology. Bacteriological cure was defined as having occurred when the pathogen isolated pre-treatment was isolated from neither of the post-treatment samples. Post-treatment, cow-level composite SCC data were collated. The identity of Staphylococcus aureus isolates was confirmed by PCR and antimicrobial resistance patterns for these isolates were determined by zone diffusion testing.RESULTS: Corynebacterium spp. (40%) and S. aureus (32%) were the most common isolates pre-treatment. The proportion of glands cured increased from 0.16 (SE 0.04) for the Control, to 0.32 (SE 0.06) for the 3xPH, and 0.56 (SE 0.02) for the 6xPH groups (p<0.001); decreased with increasing age (p=0.02); and was lower for S. aureus than for other isolates (p<0.001). Of the 60 isolates defined as S. aureus by phenotypic bacteriology, 55 were confirmed as S. aureus by PCR. Of these, 24 (45%) were resistant to penicillin. One of 24 (4%) glands with S. aureus resistant to penicillin was cured compared with 7/29 (24%) sensitive isolates (p=0.04). The geometric mean SCC at the subsequent herd test declined from 481 (95% CI=350-659)x10(3) cells/mL in Control, to 352 (95% CI=255-486)x10(3) cells/mL in 3xPH, and 276 (95% CI=199-383)x10(3) cells/mL in 6x PH cows (p=0.05).CONCLUSIONS: Increasing the duration of treatment of subclinical mastitis using penethamate hydriodide in dairy cows resulted in a higher bacteriological cure proportion, a lower proportion of glands infected post-treatment, and a reduced SCC. Cure proportion was reduced in older cows, and for glands infected with S. aureus, especially when resistant to penicillin.CLINICAL RELEVANCE: This study demonstrated that bacteriological cure of subclinical intramammary infections can be increased by increasing the duration of therapy, but a number of cow and pathogen factors also affected the probability of cure.