Herd management practices associated with bovine leukemia virus incidence rate in Michigan dairy farms.

The objective of this study was to identify associations between herd management practices and the incidence rate of bovine leukemia virus (BLV) infections in Michigan dairy herds. Previous management risk factor studies were of antibody prevalence rather than the rate of recent infections. Milk samples were collected from cohorts of cows on 112 Michigan dairy herds and tested for BLV using an antibody capture ELISA (n = 3849 cows). Cows were subsequently followed for an average of 21 months. Cows negative for anti-BLV antibodies and still present in their respective herds were retested by the same antibody capture ELISA to estimate within-herd incidence rates. The overall crude incidence rate was 1.46 infections per 100 cow-months at risk for the 1314 retested cows in 107 herds. The average within-herd incidence rate was 2.28 infections per 100 cow-months (range: 0 to 9.76 infections per 100 cow-months). A negative binomial regression model was used to identify herd management practices associated with the within-herd incidence rate. Results of the final multivariable model identified higher herd prevalence, milking frequency, needle reuse, as well as housing post-parturient cows separately, to be associated with increased incidence rate. Utilization of sand bedding for the lactating herd was found to be associated with decreased incidence rates. Results of this study suggest potential routes of BLV transmission which should be further investigated as disease control targets in ongoing control programs.

Controlling bovine leukemia virus in dairy herds by identifying and removing cows with the highest proviral load and lymphocyte counts.

The objective of this field trial was to reduce bovine leukemia virus (BLV) transmission and prevalence in commercial dairy herds using proviral load (PVL) and lymphocyte count (LC) measurements as indicators of the most infectious animals for culling or segregation. Bovine leukemia virus causes lymphoma in <5% of infected cattle, and increased lymphocyte counts (lymphocytosis) in about one-third. Recent research has shown that dairy cows infected with BLV have altered immune function associated with decreases in milk production and lifespan. Recent findings show that a minority of infected cattle have PVL concentrations in blood and other body fluids of over 1,000 times that of other infected cattle. In combination with a high LC, these animals are thought to be responsible for most transmission of BLV in a herd. Milk or blood samples from adult cows in our 3 Midwestern dairy farm field trials were tested semiannually with ELISA for BLV antibodies, and ELISA-positive cattle were then retested using a blood LC and a quantitative PCR test for PVL to identify the animals presumed to be most infectious. Herd managers were encouraged to consider PVL and LC status when making cull decisions, and to segregate cows with the highest PVL and LC from their BLV ELISA-negative herd mates where possible. After 2 to 2.5 yr of this intervention, the incidence risk of new infections decreased in all 3 herds combined, from 13.8 to 2.2, and the overall herd prevalence decreased in all 3 herds combined from 62.0 to 20.7%, suggesting that this approach can efficiently reduce BLV transmission as well as prevalence. This is encouraging, because a very low prevalence of BLV infection would make it economically feasible to cull the remaining ELISA-positive cattle, as was achieved in national eradication programs in other countries decades ago.

Short communication: Herd-level variables associated with overmilking in Michigan dairy herds.

The objective of this study was to determine the herd-level variables that are associated with overmilking in 64 Michigan dairy herds with a mean herd size of 451 cows (range: 59-2,771 cows). Participating producers completed surveys to indicate their mastitis management practices and attitudes. Additionally, milking protocols were observed and milk flow dynamics for 3,824 cows were estimated using digital vacuum recorders. The median duration of overmilking was 47 s (95% confidence interval, CI: 38.6 to 55.9 s), with a mean of 55% (95% CI: 49.5 to 61.1%) of cows within each herd overmilked by at least 30 s. Median milking time for all herds was 324 s (95% CI: 302 to 346 s) and was found to be positively correlated with median duration of overmilking (r = 0.670). Backward multivariate analysis was used to determine which of 45 herd-level milking and management variables were associated with median duration of overmilking. Median duration of overmilking was negatively associated with the duration of time needed to complete 1 milking for the entire herd (adjusted R2 = 0.13). Herds that operate milking facilities below maximum daily capacity may be prone to overmilking. Given the low coefficient of determination, variables unaccounted for in this study, such as equipment function or manual detachment by milking operators, are likely the most important risk factors for overmilking.

Herd-level variables associated with premilking stimulation time in Michigan dairy herds.

The objective of this study was to determine the herd-level variables that were associated with total stimulation time during the premilking routine in 64 Michigan dairy herds. The mean herd size was 452 cows (range = 59 to 2,771 cows). For each herd, surveys were administered to producers to gather mastitis management practices and attitudes. Additionally, milking protocols were observed and milk flow dynamics were measured by use of digital vacuum recorders. Backward multivariate regression analysis was used to determine which of 47 herd-level milking and management variables were associated with mean total stimulation time. Mean total stimulation time was 14.2 s (range = 2.4-40.8 s) and was positively associated with increasing latency period (time interval between first stimulation and cluster attachment). Total stimulation time was negatively associated with greater herd size and number of visits to each cow in the premilking routine. In summary, increased stimulation time is more likely in herds that foster a lower sense of urgency of cow throughput during milking, as evidenced by a positive association with longer latency periods and fewer preparation visits per cow. Tactile stimulation is critical for efficient milk ejection; if inadequate, cows are at greater risk of delayed milk ejection and bimodal milk flow, which in turn has been associated with teat congestion and reduced milk flow. This study offers insight as to some of the herd factors that may be limiting adequate tactile stimulation.

Herd-level variables associated with delayed milk ejection in Michigan dairy herds.

The objective of this study was to determine which herd-level variables were associated with delayed milk ejection (bimodal milk let-down) in 64 Michigan dairy herds. Median herd size was 294 cows (range 59 to 2,771 cows). For each herd, milking protocols were observed and milk flow dynamics were estimated by use of digital vacuum recorders. Surveys were also administered to the producers to measure mastitis management practices and attitudes. Milk flow dynamics were recorded for a total of 3,824 cow milkings, with a mean of 60 milkings per herd (range of 11 to 154). Backward multivariable analysis was used to determine which of the 47 herd-level milking and management variables were associated with delayed milk ejection (cows with milk let-down periods between milking cluster attachment and the incline phase of milk flow of >30 s). Delayed milk ejection occurred in an average of 25% of the cows in each herd (range 0 to 75%). A multivariable model found that the proportion of cows in a herd with delayed milk ejection was negatively associated with mean total time of tactile stimulation during premilking routines and positively associated with herd size.

Effect of infection with bovine leukemia virus on milk production in Michigan dairy cows.

The objective of this study was to determine the association between individual cow-level milk production and bovine leukemia virus (BLV) infection as measured by milk BLV-ELISA. Dairy Herd Improvement technicians collected milk samples from 10 cows from each of first, second, third, and 4+ parity cows in 105 Holstein herds with ≥ 120 milking cows. Milk samples were tested for the presence of anti-BLV antibodies by ELISA. Additional data regarding the cows and the herds were collected by farm survey and Dairy Herd Improvement records. A set of mixed-effect models using all cows and only 2+ parity cows were used to investigate the association between BLV ELISA-corrected optical density and 305-d mature equivalents of individual cows. The BLV milk positivity was associated with decreased 305-d mature-equivalent yields, especially among the older cows. Additionally, increasing milk ELISA-corrected optical density was associated with increasing loss of milk production at the cow level. In summary, our results provide evidence that BLV infection is associated with decreased milk production in Michigan dairy cows.

Bovine leukemia virus and cow longevity in Michigan dairy herds.

To determine the association between infection with bovine leukemia virus (BLV) and cow longevity, a stratified random sample of 3,849 Holsteins in 112 Michigan dairy herds was followed for an average of 597 d following testing for BLV antibodies with an ELISA milk test. The hazard ratio of 1.23 indicates that BLV-positive cows were 23% more likely than their BLV-negative herd mates to die or be culled during the monitoring period. This result is adjusted for lactation number, which is also positively associated with an increased risk of leaving the herd. Because herd was included in models, the effect of BLV ELISA on cow longevity was a within-herd comparison in which BLV-infected cattle were compared with their uninfected herd mates. The analysis of 4 ELISA optical density (OD) groups demonstrated a dose response such that cows with higher OD values had decreased survival compared with cows with lower OD values. Cows with OD values above 0.5 were at 40% greater risk of dying or being culled than were their uninfected herd mates. These results support the contention that the association of BLV with cow longevity, when added to other economic impacts, may warrant the control of BLV in our US dairy cow population.

Isolation and molecular characterization of vancomycin-resistant Enterococcus faecium from swine in Michigan, USA.

In 2008, we identified vancomycin-resistant enterococci (VRE) in Michigan swine, which was the first report of VRE in livestock from North America. Continued sampling in 2009 and 2010 was conducted to determine whether VRE persisted in Michigan. In 2009, swine faecal and feed samples (n=56), county fair pig barn manure samples (n=9) and pooled Michigan State Fair pig barn manure samples (n=18) were screened for VRE. In 2010, swine faecal samples were collected from 26 county fairs (n=73) and nine commercial swine farms in six states (n=28). Recovered VRE isolates were molecularly evaluated by polymerase chain reaction, restriction fragment length polymorphism, pulsed-field gel electrophoresis (PFGE), S1 nuclease digestion and multilocus sequence typing (MLST). Six VRE isolates were identified in 2009 from the State Fair, and another six (8.2%) were recovered from the five county fairs in 2010. All 12 isolates were highly related to the first-reported VRE from Michigan swine: all were confirmed to be vancomycin-resistant Enterococcus faecium (VREf) carrying vanA gene on Tn1546 (Type D), were negative for IS1251, hyl and esp gene, carried a 150-160 kb megaplasmid, and have closely similar PFGE patterns with >80% similarity. Classified as ST5, ST6 or ST185 by MLST, all belong to the clonal complex 5, a strain recognized to be circulating among European pigs. This study reveals that VREf are widespread in Michigan swine and persist in the historical absence of the use of agricultural glycopeptides.

Performance of a commercial serum ELISA for the detection of antibodies to Neospora caninum in whole and skim milk samples.

Control of Neospora caninum infection in cattle depends on specific, ante-mortem detection of infected animals and limiting their use as breeding stock or by culling. The objective of the present study was to determine appropriate cut-off values and diagnostic performance of a milk ELISA test using whole and skim milk in a commercial serum ELISA test (IDEXX Neospora Ab). Serum and milk samples were obtained from a total of 475 lactating cows from two herds with and two herds without a previous history of N. caninum-associated abortion. Overall seroprevalence determined by the ELISA was 18.3%. Compared to serum ELISA values, correlation and overall performance assessed by receiver operating characteristic analysis was higher when either whole or skim milk samples were diluted 1:2 compared to undiluted or 1:5 diluted samples. Diagnostic performance for analysis of whole and skim milk was compared at cut-off values that achieved a desired operating characteristic of at least 95% specificity. For whole milk diluted 1:2 and a cut-off of 0.14 (S/P ratio), sensitivity and kappa values were 74.7% (95% CI 64.3-83.4) and 0.70 (95% CI 0.61-0.78), respectively. For skim milk diluted 1:2 and a cut-off of 0.30, sensitivity and kappa values were 77.0% (95% CI 66.8-85.4) and 0.72 (95% CI 0.64-0.80), respectively. Using the selected cut-offs, the IDEXX Neospora Ab Test is equally suited for the analysis of whole and skim milk as a screening tool in neosporosis control programs.

Association between bovine leukemia virus, production, and population age in Michigan dairy herds.

The objective of this study was to determine the herd-level effect of bovine leukemia virus (BLV) infection on dairy production, culling, and cow longevity. During routine herd testing, Dairy Herd Improvement Association technicians collected milk samples from about 40 cows from each of 104 randomly selected Michigan dairy herds averaging ≥120 milking cows and 11,686 kg of milk/yr. Milk samples were analyzed for the presence of anti-BLV antibodies by ELISA, and herd- and lactation-specific estimates of BLV prevalence were computed to determine which were the most predictive of herd milk production, culling rate, and cow longevity (proportion of cows in their third or greater lactation). On this basis, the herd BLV index (an unweighted mean BLV prevalence rate for lactation number 1, 2, 3, and ≥4) was selected as the measure of BLV prevalence that was the most highly associated with BLV economic impact. Step-down multivariate analysis was used to determine the extent to which any of 19 herd-level management variables may have confounded the association of BLV index and measures of herd economic impact (milk production and cow longevity). The BLV index was not associated with the 12-mo culling rate, but was negatively associated in the final multivariable model with the proportion of cows that were ≥third lactation, and was negatively associated with herd milk production. In summary, increased prevalence of BLV within Michigan dairy herds was found to be associated with decreased herd milk production and decreased cow longevity. Our results provide evidence that BLV infection is associated with herd-level economic impacts in high-performing dairy herds.

Carriage of Clostridium difficile and other enteric pathogens among a 4-H avocational cohort.

Clostridium difficile (CD), Salmonella, Campylobacter and enterohemorrhagic Escherichia coli (EHEC) are major causes of morbidity in a variety of enteric diseases in humans and animals, but subclinical carriage in both is probably more common than are clinical cases. Little is known regarding the prevalence of these pathogens in animals raised for exhibit at Michigan county fairs or the frequency with which Michigan citizens raising these animals may have been subclinically colonized. To address these issues, 361 fecal specimens from 158 humans and 203 of their farm animals were cultured for CD, Salmonella and Campylobacter. Additionally, 50 people and their cattle were tested for EHEC. No EHEC, Salmonella or Campylobacter were detected. However, 16 specimens (4.4%) were positive for CD: 13 humans, two horses and one pig. None of the farm animal specimens submitted by any of the 13 CD-positive humans were positive for CD. Strain characterization [toxinotype, pulsed-field gel electrophoresis (PFGE)] demonstrated that the human CD isolates were similar to what has been reported previously in the general US population. We conclude that horses and farm animals (cattle, sheep, goats and swine) at 1-2 months before market weight showed no evidence of wide-spread carriage of the common enteric pathogens, including the recently reported CD toxinotype V. These results provide no support to the hypothesis that 4-H members or others visiting county fair animal husbandry projects in these counties may be at increased risk for acquisition of CD, Salmonella, Campylobacter or EHEC from animals.

Veterinarian injuries associated with bovine TB testing livestock in Michigan, 2001.

Testing all the cattle in an entire state with a uniform procedure for each animal affords an opportunity to relate human injury data to a known number of animals handled while carrying out a standardized procedure. Our objective was to describe the type and incidence density of injuries associated with TB-testing a large number of cattle herds, and to delineate the various factors associated with the risk of injury. A survey was mailed to all veterinarians (N = 259) who had completed at least five official bovine TB (bTB) herd tests in Michigan in 2001. We collected data regarding basic demographics and health status, work experience, veterinary specialty, and practice information. Each veterinarian was also requested to complete a separate injury questionnaire for each injury received while TB testing livestock in 2001. Accurate addresses were found for 247 eligible veterinarians, 175 (71%) of whom returned the survey. Thirty-six veterinarians reported a total of 53 injuries (10 major, 12 minor and 31 self-treated). Hands (29%) and legs (21%) were the anatomic locations most frequently injured, with sprains/strains (30%) and abrasion/contusion (30%) the most common types of injuries sustained. The overall incidence density of injuries was 1.9 per 10,000 animals tested. Female gender (RR = 3.3), being employed by the government (RR = 4.5), and smoking (RR = 6.0) were significantly associated with a higher rate of injury. Significant colliniearities were found between some risk factors associated with an increased rate of injury and participants thought 81% of their injuries could have been prevented. These results are explained by the administrative structure of the bTB testing program in Michigan, and the changing demographics of the veterinary workforce.

Absence of Mycobacterium bovis infection in dogs and cats residing on infected cattle farms: Michigan, 2002.

A cross-sectional field study was performed to evaluate infection in dogs and cats living on farms with Mycobacterium bovis-infected cattle. The purpose was to determine pet infection status and assess their risk to farm families and/or tuberculosis-free livestock. Data and specimens were collected from 18 cats and five dogs from nine participating farms. ELISA testing for M. bovis and M. avium was conducted. Fifty-one biological samples were cultured; all were negative for M. bovis, although other Mycobacterium species were recovered. No radiographic, serological or skin test evidence of mycobacterial infection was found. These negative results may be due to the low level of M. bovis infection in the cattle and the limited duration of exposure of pets to infected cattle residing on the same farm. No evidence was found to indicate that pets residing on M. bovis-infected Michigan cattle farms pose a risk to humans or M. bovis-free livestock; however, precautionary advice for farm owners was provided.

Comparison of antimicrobial susceptibility of Staphylococcus aureus isolated from bulk tank milk in organic and conventional dairy herds in the midwestern United States and Denmark.

An observational study was conducted to compare the antimicrobial susceptibility patterns of Staphylococcus aureus isolated from bulk tank milk in organic and conventional dairy farms in Wisconsin, United States, and southern Jutland, Denmark. Bulk tank milk samples and data regarding management and production were collected from 30 organic and 30 conventional dairy farms in Wisconsin and 20 organic and 20 conventional dairy farms in Denmark. S. aureus isolates were tested for resistance against 15 antimicrobial agents by semiautomatic microbroth dilution methods in each country. Of the 118 bulk tank milk samples in Wisconsin, 71 samples (60%) yielded at least one S. aureus isolate, and a total of 331 isolates were collected. Of the 40 bulk tank milk samples from Denmark, 27 samples (55%) yielded at least one S. aureus isolate, and a total of 152 isolates were collected. Significant differences between organic and conventional dairies were detected only to ciprofloxacin in Wisconsin and avilamycin in Denmark. Significant differences (P < 0.05) between the two countries were detected in nine antimicrobials. Denmark had a higher probability of having reduced susceptibility to ciprofloxacin and streptomycin (P = 0.015 and 0.003, respectively). Wisconsin isolates had a higher probability of having reduced susceptibility to seven other antimicrobial agents (bacitracin, gentamicin, kanamycin, penicillin, sulphamethoxazole, tetracycline, and trimethoprim). We found small differences between organic and conventional farm types in each country and larger differences between the two national agricultural systems.

Comparison of prevalence and antimicrobial susceptibilities of Campylobacter spp. isolates from organic and conventional dairy herds in Wisconsin.

The prevalence and antimicrobial susceptibilities of Campylobacter spp. isolates from bovine feces were compared between organic and conventional dairy herds. Thirty organic dairy herds, where antimicrobials are rarely used for calves and never used for cows, were compared with 30 neighboring conventional dairy farms, where antimicrobials were routinely used for animals for all ages. Fecal specimens from 10 cows and 10 calves on 120 farm visits yielded 332 Campylobacter isolates. The prevalence of Campylobacter spp. in organic and conventional farms was 26.7 and 29.1%, and the prevalence was not statistically different between the two types of farms. Campylobacter prevalence was significantly higher in March than in September, higher in calves than in cows, and higher in smaller farms than in large farms. The rates of retained placenta, pneumonia, mastitis, and abortion were associated with the proportion of Campylobacter isolation from fecal samples. The gradient disk diffusion MIC method (Etest) was used for testing susceptibility to four antimicrobial agents: ciprofloxacin, gentamicin, erythromycin, and tetracycline. Two isolates were resistant to ciprofloxacin, and none of isolates was resistant to gentamicin or erythromycin. Resistance to tetracycline was 45% (148 of 332 isolates). Tetracycline resistance was found more frequently in calves than in cows (P = 0.042), but no difference was observed between organic and conventional farms. When we used Campylobacter spp. as indicator bacteria, we saw no evidence that restriction of antimicrobial use on dairy farms was associated with prevalence of resistance to ciprofloxacin, gentamicin, erythromycin, and tetracycline.

Effect of anionic salt and highly fermentable carbohydrate supplementations on urine pH and on experimentally induced hypocalcaemia in cows.

The objective of this experiment was to determine the effect of dietary grain on calcium homeostasis. Six rumen-fistulated dairy cows with 3 or more previous lactations and no history of parturient paresis were randomly assigned to a sequence of diets in a crossover study with 4 periods of 10 days each. Dietary treatments were: A control ration consisting of wrap grass silage alone (1), the control ration supplemented with ammonium chloride and ammonium sulphate salt solution (2), control ration following a period with supplementation (3) and control ration supplemented with increasing amounts of barley from 4 to 10 kg/cow per day, expected to produce subclinical rumen acidosis (4). Daily intake of the diets was adjusted to 14 kg DM/cow per day. On day 11, the calcium-regulating mechanisms in cows were challenged until recumbency by a standardized intravenous EDTA infusion and cows were left to recover spontaneously. Anion supplementation and the feeding of highly fermentable carbohydrate lowered urine pH below 7.0 due to subclinical acidosis. During spontaneous recovery from EDTA induced hypocalcaemia, the cows more quickly regained a whole blood free calcium concentration of 1.00 mmol/L if they had most recently been supplemented with either anionic salts or with increasing amounts of barley, as compared to the basic ration. It is concluded that so-called slug-feeding or 'steaming up' with highly fermentable carbohydrates before parturition in milk fever susceptible cows enhanced calcium homeostasis similar to the effect seen in cows on anionic diets.

Mycobacterium bovis (bovine TB) exposure as a recreational risk for hunters: results of a Michigan Hunter Survey, 2001.

SETTING: Tuberculosis caused by Mycobacterium bovis (bovine TB) is endemic in the white-tailed deer population of north-eastern Michigan. Hunters may be exposed to M. bovis via cutaneous inoculation while field dressing deer or by ingestion of undercooked venison. Michigan hunters have received inconsistent messages about their risk of acquiring tuberculosis from recreational exposure to deer. The most common health advice offered has been to wear gloves while field dressing deer and to cook venison products thoroughly. OBJECTIVE: Data were collected to quantify these self-protective activities and to characterize hunters practicing these activities. DESIGN: In 2001, we surveyed 1833 hunters who had successfully harvested deer in or near Michigan's bovine TB endemic area in 2000. RESULTS: The survey response rate was 78%. Most hunters (89%) reported field dressing deer, 43% of whom wore gloves. Most hunters (95%) reported eating venison, 55% of whom reported their venison was always cooked thoroughly. Several hunter characteristics, including older age, female sex, higher awareness level, and area of residence, were significantly associated with the practice of these self-protective activities. CONCLUSION: The survey results suggest that hunters should receive consistent advice encouraging glove use while field dressing deer and the thorough cooking of venison products before consumption.

Efficacy of systemic ceftiofur as a therapy for severe clinical mastitis in dairy cattle.

The objectives of this study were to determine the efficacy of intramuscular administration of ceftiofur to reduce the incidence of case-related death and culling following severe clinical mastitis in lactating dairy cattle. A total of 104 cows with severe clinical mastitis (systemic signs) were enrolled in the study and randomly assigned to one of two treatment groups. Immediately after detection of the case, one group was administered 2.2 mg/kg of ceftiofur intramuscularly, and the dose repeated at 24-h intervals for a total of five doses. The second group of cows did not receive systemic antibacterial therapy. Additionally, all cows in both treatment groups received intramammary pirlimycin (Pirsue) in the affected quarter every 24 h for a total of up to three doses. Also at the onset of the case, all cows on the trial were administered a supportive therapeutic regimen of fluids and anti-inflammatory agents that varied from farm to farm, but was standard within each herd at the discretion of the herd manager and veterinarian. Of all cases 14/104 (13.5%) resulted in a lost cow (died or culled). The proportion of cases that resulted in a lost cow and were treated with ceftiofur (4/51; 7.8%) did not statistically differ from cows that were not treated with ceftiofur (10/53; 18.9%). However, the proportion of cases that resulted in lost cows was higher for those cases that yielded a coliform organism on culture (14/56; 25.0%) than cases that did not yield coliforms (0/48; 0.0%; P < 0.001). Thus, among coliform cases, cows that were not treated with ceftiofur were more likely to be culled or die (10/27, 37.0%; P < 0.05) than cows treated with ceftiofur (4/29, 13.8%). We conclude that intramuscular administration of ceftiofur did not affect the outcome of severe clinical mastitis when all etiologic agents are included in the analysis. However, for severe clinical mastitis cases caused by coliform organisms, ceftiofur therapy reduced the proportion of cases that resulted in cow death or culling. This benefit may be realized because of the amelioration of bacteremic-related pathogenesis.

Trends in antibacterial susceptibility of mastitis pathogens during a seven-year period.

Milk samples collected from dairy cattle suspected of having mastitis were submitted to the Microbiology Laboratory of the Animal Health Diagnostic Laboratory, Michigan State University, for bacteriologic culture. A total of 2778 isolates, from the years 1994 to 2000, were isolated, identified, and subjected to in vitro antimicrobial susceptibility testing using the disk diffusion method, in accordance with National Committee on Clinical Laboratory Standards (NCCLS) standards. Isolates included in this study were Streptococcus uberis, Streptococcus dysgalactiae, Streptococcus agalactiae, Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, Serratia marcesens, and Pseudomonas aeruginosa. The proportion of bacterial isolates determined to be susceptible did not change during the 7-yr period for the majority of bacterial-antibacterial interactions tested. However, analysis for linear trend in proportions determined that there were increases in the proportion of S. aureus isolates that were susceptible to ampicillin, penicillin, and erythromycin. For Strep. uberis, increases in the proportion of susceptible isolates occurred for oxacillin, sulfa-trimethoprim, gentamicin, and pirlimycin, and a decrease in the proportion of susceptible isolates occurred with penicillin. For Strep. dysgalactiae, increases in the proportion of susceptible isolates occurred with erythromycin, gentamicin, sulfa-trimethoprim, and tetracycline. For Strep. agalactiae, increases in the proportion of susceptible isolates occurred with sulfa-trimethoprim. Among E. coli isolates, there was an increase in the proportion that were susceptible to ampicillin and cephalothin. Among K pneumoniae isolates, there was an increase in the proportion that were susceptible to ceftiofur. Overall, there was no indication of increased resistance of mastitis isolates to antibacterials that are commonly used in dairy cattle.

The effect of ascorbic acid and L-histidine therapy on acute mammary inflammation in dairy cattle.

Ascorbic acid and L-histidine were investigated as antioxidant therapies for acute mammary inflammation. Mastitis was induced in eight nonpregnant Holstein cows by intramammary infusion of endotoxin. Treatments were administered in a 4 x 4 Latin square crossover design with 1-wk periods between challenges with endotoxin. Four individual treatments, control, ascorbic acid only, L-histidine only, and ascorbic acid plus L-histidine, were applied. Two doses of 25 g of ascorbic acid administered intravenously at 3- and 5-h postendotoxin challenge increased milk production recovery (9% higher, P < 0.02) and tended to reduce the extent of rumen stasis. Two doses of 25 g of L-histidine similarly administered decreased plasma antioxidant activities 5.5% (P < 0.05). However, ascorbic acid and L-histidine had no effects on rectal temperature, heart rate, respiratory rate, and dry matter intake. The data suggested that ascorbic acid provided some potential benefit for recovery from acute mammary inflammation in dairy cattle.