Characterization of clinical mastitis occurring in cows on 50 large dairy herds in Wisconsin.

In recent years, the US dairy industry has experienced significant demographic changes, with an increase in the number of large herds. The objectives of the present study were to characterize clinical mastitis occurring in cows on large dairy herds in Wisconsin. Participating herds (n=50) were required to have a minimum of 200 lactating animals, participate in monthly DHI testing (including monthly individual cow somatic cell count), use computerized herd records, use a milking routine that included fore-stripping quarters for detection of mastitis, and use antimicrobials to treat affected cows. After study personnel visited the farm, each herd was instructed to enroll the next 17 cows that experienced clinical mastitis, regardless of severity. At detection of clinical mastitis and 14 to 21 d after treatment ended, duplicate quarter milk samples were collected from all affected quarters and used for microbiological analysis. Treatments of affected cows were performed according to existing individual farm protocols. Cow level follow-up data was collected for 90 d after enrollment. Microbiological diagnoses at enrollment included gram-negative (35.6%), no growth (27.3%), gram-positive (27.5%), and other (9.6%). Of the 741 cases, the most prevalent pathogens were Escherichia coli (22.5%), followed by environmental streptococci (12.8%), Klebsiella spp. (6.9%), and coagulase-negative staphylococci (6.1%). Bacteriological cure was 75.0% for cases caused by gram-negative pathogens (n=136), 50.8% for cases caused by gram-positive pathogens (n=128), 47.5% for cases caused by other pathogens (n=40), and 73.2% for cases which did not result in microbial growth (n=123). Of the 583 cases with severity recorded, the distribution of mild, moderate, and severe symptoms was 47.8, 36.9, and 15.3%, respectively. The majority of cases presenting with severe symptoms were caused by gram-negative pathogens. Treatment cure was greater for gram-negative pathogens and cases for which no pathogens were recovered as compared with cases caused by other etiologies. Cows experiencing severe cases were more likely to receive multiple antimicrobial treatments.

Minimum inhibitory concentrations of Staphylococcus aureus recovered from clinical and subclinical cases of bovine mastitis.

Antimicrobials are often used for treatment of bovine mastitis and the possibility of selection for resistant bacteria must be considered. The objectives of this study were to determine the minimum inhibitory concentration of Staphylococcus aureus recovered from cases of clinical and subclinical bovine mastitis, and to determine the prevalence of multidrug resistance in this population. Milk samples were collected from cows on commercial dairy herds (n=13), including quarters (n=1,574) of cows with subclinical mastitis cases, and cows experiencing clinical mastitis cases (n=608). Selected Staph. aureus isolates, obtained from clinical (n=58) and subclinical (n=58) mastitis cases, were used to determine minimum inhibitory concentrations of 12 selected antimicrobials. Of Staph. aureus isolates tested, 87 (75%) did not exhibit resistance to any antimicrobial, 28 (24.1%) exhibited resistance to 1 (n=21) or 2 (n=7) classes of antimicrobials, and 1 (0.9%) exhibited multidrug resistance. All Staph. aureus (clinical and subclinical cases) were inhibited by the range of concentrations tested for ceftiofur and oxacillin. Moreover, no isolates obtained from clinical mastitis cases exhibited resistance to cephalothin, penicillin-novobiocin, or sulfadimethoxine. Of isolates, 3 exhibited resistance to enrofloxacin. Of isolates exhibiting resistance to more than 1 antimicrobial, independent of antimicrobial class, the combination of erythromycin and tetracycline, and ampicillin and penicillin accounted for the majority of resistance. Of isolates tested, 19% were resistant to tetracycline and 14% were resistant to penicillin. Survival curves of Staph. aureus relative to minimum inhibitory concentration demonstrated heterogeneity among case types for ceftiofur, cephalothin, and erythromycin. Multidrug resistance was identified in only 1 isolate obtained from a single farm.

Somatic cell count status across the dry period as a risk factor for the development of clinical mastitis in the subsequent lactation.

The objective of this study was to determine the risk of clinical mastitis in the first 120 d in lactation based on previous somatic cell count (SCC) history in a herd with a very low prevalence of contagious pathogens. A total of 218 cows from a university herd were enrolled at dry-off. Duplicate quarter milk samples were collected from all quarters at dry-off, postcalving (2 to 9 d in milk), and before treatment of all first cases of clinical mastitis that occurred during the first 120 d of the subsequent lactation. Quarter SCC statuses across the dry period were defined based on comparison of quarter SCC between the date of dry-off and the postcalving sampling periods. The relationship between the probability of developing clinical mastitis in the first 120 d of lactation and SCC status across the dry period and other explanatory variables was assessed using logistic regression. In the first 120 d postcalving, 68 first cases of clinical mastitis occurred in 47 cows. Of quarters that experienced a microbiologically positive clinical case, the same microorganism was never isolated from milk samples obtained at dry-off or consistently isolated from milk samples collected at all sampling periods. Coagulase negative staphylococci were the most prevalent pathogens isolated from subclinical intramammary infection, whereas gram-negative pathogens were the most common pathogen associated with clinical cases. Quarters that had at least 1 case of mastitis during the previous lactation were 4.2 times more likely to have a first case of clinical mastitis in the current lactation than quarters that did not have clinical mastitis in the previous lactation [odds ratio (OR) = 4.2 (1.8, 10.0)]. Quarters of cows of greater than fourth parity were 4.2 times more likely to have a first case of clinical mastitis than quarters of cows of second parity [OR = 4.2 (1.4, 10.0)]. Quarters with SCC > or =200,000 cells/mL at dry-off and postcalving were 2.7 times more likely to experience a first case of mastitis than quarters with SCC <200,000 cells/mL at both periods [OR = 2.7 (0.97, 7.67)].

Dynamics of somatic cell counts and intramammary infections across the dry period.

The objectives of this research were to study the relationship between somatic cell count (SCC) and intramammary infection (IMI) across the dry period and the risk of subclinical mastitis at the first dairy herd improvement (DHI) test of the subsequent lactation. A secondary objective was to determine SCC test characteristics for diagnosis of IMI at both the cow and quarter levels. A total of 218 cows from a university herd were enrolled at dry-off. Duplicate quarter milk samples were collected from all quarters at dry-off, calving and on the day of the first DHI test. Somatic cell count status across the dry period was defined based on the comparison of quarter SCC from dry-off and the post-calving sampling periods and comparison of composite SCC from DHI samples from the last test and first test of the following lactation. Of new IMI detected from post-calving milk samples (n=45), 46.7, 26.7 and 11% were caused by CNS, Streptococci and Gram-negative bacteria, respectively. Of cured IMI at post-calving (n=91), 61.5, 23.1 and 9.9% had CNS, Streptococci and Coryneforms isolated from dry-off milk samples. The most frequent microorganisms related to cured IMI were CNS (33%). Of chronically infected quarters across the dry period (n=10), only one had the same species of pathogen isolated from dry-off and post-calving samples. The sensitivity of a SCC threshold of 200,000 cells/mL for detection of subclinical IMI was 0.64, 0.69 and 0.65 for milk samples obtained at dry-off, post-calving and first DHI test, respectively. The specificity was 0.66, 0.84 and 0.93 for milk samples obtained at dry-off, post-calving and first DHI test, respectively. Quarters with SCC> or =200,000 cells/mL at both dry-off and post-calving sampling periods were 20.4 times more likely to be subclinically infected by a major pathogen (rather than being uninfected) and 5.6 times more likely to be subclinically infected by a minor pathogen (rather than being uninfected) at the first DHI test than quarters with SCC<200,000 cells/mL at both periods. Cows with SCC greater than 200,000 cells/mL at both the last and the first DHI test between lactations produced 9.1 kg less milk on the first DHI test day than the average milk production of cows with SCC less than 200,000 cells/mL at both periods.