H.W. Barkema

Invited review: Mastitis in dairy heifers: nature of the disease, potential impact, prevention, and control.

Heifer mastitis is a disease that potentially threatens production and udder health in the first and subsequent lactations. In general, coagulase-negative staphylococci (CNS) are the predominant cause of intramammary infection and subclinical mastitis in heifers around parturition, whereas Staphylococcus aureus and environmental pathogens cause a minority of the cases. Clinical heifer mastitis is typically caused by the major pathogens. The variation in proportions of causative pathogens between studies, herds, and countries is considerable. The magnitude of the effect of heifer mastitis on an individual animal is influenced by the form of mastitis (clinical versus subclinical), the virulence of the causative pathogen(s) (major versus minor pathogens), the time of onset of infection relative to calving, cure or persistence of the infection when milk production has started, and the hosts immunity. Intramammary infection in early lactation caused by CNS does not generally have a negative effect on subsequent productivity. At the herd level, the impact will depend on the prevalence and incidence of the disease, the nature of the problem (clinical, subclinical, nonfunctional quarters), the causative pathogens involved (major versus minor pathogens), the ability of the animals to cope with the disease, and the response of the dairy manager to control the disease through management changes. Specific recommendations to prevent and control mastitis in late gestation in periparturient heifers are not part of the current National Mastitis Council mastitis and prevention program. Control and prevention is currently based on avoidance of inter-sucking among young stock, fly control, optimal nutrition, and implementation of hygiene control and comfort measures, especially around calving. More risk factors for subclinical and clinical heifer mastitis have been identified (e.g., season, location of herd, stage of pregnancy) although they do not lend themselves to the development of specific intervention strategies designed to prevent the disease. Pathogen-specific risk factors and associated control measures need to be identified due to the pathogen-related variation in epidemiology and effect on future performance. Prepartum intramammary treatment with antibiotics has been proposed as a simple and effective way of controlling heifer mastitis but positive long-lasting effects on somatic cell count and milk yield do not always occur, ruling out universal recommendation of this practice. Moreover, use of antibiotics in this manner is off-label and results in an increased risk of antibiotic residues in milk. Prepartum treatment can be implemented only as a short-term measure to assist in the control of a significant heifer mastitis problem under supervision of the herd veterinarian. When CNS are the major cause of intramammary infection in heifers, productivity is not affected, making prepartum treatment redundant and even unwanted. In conclusion, heifer mastitis can affect the profitability of dairy farming because of a potential long-term negative effect on udder health and milk production and an associated culling risk, specifically when major pathogens are involved. Prevention and control is not easy but is possible through changes in young stock and heifer management. However, the pathogenesis and epidemiology of the disease remain largely unknown and more pathogen-specific risk factors should be identified to optimize current prevention programs.

Analytical specificity and sensitivity of a real-time polymerase chain reaction assay for identification of bovine mastitis pathogens

Intramammary infection (IMI), also known as mastitis, is the most frequently occurring and economically the most important infectious disease in dairy cattle. This study provides a validation of the analytical specificity and sensitivity of a real-time PCR-based assay that identifies 11 major pathogen species or species groups responsible for IMI, and a gene coding for staphylococcal beta-lactamase production (penicillin resistance). Altogether, 643 culture isolates originating from clinical bovine mastitis, human, and companion animal samples were analyzed using the assay. The isolates represented 83 different species, groups, or families, and originated from 6 countries in Europe and North America. The analytical specificity and sensitivity of the assay was 100% in bacterial and beta-lactamase identification across all isolates originating from bovine mastitis (n = 454). When considering the entire culture collection (including also the isolates originating from human and companion animal samples), 4 Streptococcus pyogenes, 1 Streptococcus salivarius, and 1 Streptococcus sanguis strain of human origin were identified as Streptococcus uberis, and 3 Shigella spp. strains were identified as Escherichia coli, decreasing specificity to 99% in Strep. uberis and to 99.5% in E. coli. These false-positive results were confirmed by sequencing of the 16S rRNA gene. Specificity and sensitivity remained at 100% for all other bacterial targets across the entire culture collection. In conclusion, the real-time PCR assay shows excellent analytical accuracy and holds much promise for use in routine bovine IMI testing programs. This study provides the basis for evaluating the assays diagnostic performance against the conventional bacterial culture method in clinical field trials using mastitis milk samples.

Prevalence and herd-level risk factors for intramammary infection with coagulase-negative staphylococci in Dutch dairy herds.

In this study, the prevalence of intramammary infection (IMI) with coagulase-negative staphylococci (CNS) in The Netherlands was estimated on 49 randomly selected herds with at least 40 lactating cows. In total, 4220 quarter milk samples were collected. The prevalence of CNS IMI in The Netherlands was estimated at 10.8% at quarter level and 34.4% at cow level, making it the most frequently isolated group of pathogens. Fourteen species of CNS were identified; the most frequently isolated species was Staphylococcus chromogenes (30.3%) followed by Staphylococcus epidermidis (12.9%) and Staphylococcus capitis (11.0%). Prevalence of CNS IMI was higher in heifers compared to older cows. Geometric mean quarter SCC of CNS-positive quarters was 109,000 cells/ml, which was approximately twice as high as culture-negative quarters. Quarters infected with S. chromogenes, S. capitis and Staphylococcus xylosus had a higher SCC (P<0.05) than culture-negative quarters, while quarters that were culture-positive for S. epidermidis and Staphylococcus hyicus tended to have a higher SCC than culture-negative quarters. An increased prevalence of CNS IMI was associated with the herd-level variables source of drinking water not being tap water, housing of dry cows in one group instead of multiple groups, measurement of cow SCC every month, udder health monitoring by the veterinarian, pasturing during outdoor season, percentage of stalls contaminated with milk, and BMSCC>250,000 cells/ml. Although a causal relation between these factors and prevalence of CNS is not proven and for some factors not even likely, knowledge of the associations found may be helpful when approaching CNS problems on dairy farms.

The Influence of Cow Factors on the Incidence of Clinical Mastitis in Dairy Cows

Many cow-specific risk factors for clinical mastitis (CM) are known. Other studies have analyzed these risk factors separately or only analyzed a limited number of risk factors simultaneously. The goal of this study was to determine the influence of cow factors on the incidence rate of CM (IRCM) with all cow factors in one multivariate model. Also, using a similar approach, the probability of whether a CM case is caused by gram-positive or gram-negative pathogens was calculated. Data were used from 274 Dutch dairy herds that recorded CM over an 18-mo period. The final dataset contained information on 28,137 lactations of 22,860 cows of different parities. In total 5,363 CM cases were recorded, but only 2,525 CM cases could be classified as gram-positive or gram-negative. The cow factors parity, lactation stage, season of the year, information on SCC from monthly test-day records, and CM history were included in the logistic regression analysis. Separate analyses were performed for heifers and multiparous cows in both the first month of lactation and from the second month of lactation onward. For investigating whether CM was caused by gram-positive or gram-negative pathogens, quarter position was included in the logistic regression analysis as well. The IRCM differed considerably among cows, ranging between 0.0002 and 0.0074 per cow-day at risk for specific cows depending on cow factors. In particular, previous CM cases, SCC in the previous month, and mean SCC in the previous lactation increased the IRCM in the current month of lactation. Results indicate that it is difficult to distinguish between gram-positive and gram-negative CM cases based on cow factors alone.

Environment and the Inflammatory Bowel Diseases

Inflammatory bowel diseases (IBD), which consists of Crohn disease and ulcerative colitis, are chronic inflammatory conditions of the gastrointestinal tract. In genetically susceptible individuals, the interaction between environmental factors and normal intestinal commensal flora is believed to lead to an inappropriate immune response that results in chronic inflammation. The incidence of IBD have increased in the past century in developed and developing countries. The purpose of the present review is to summarize the current knowledge of the association between environmental risk factors and IBD. A number of environmental risk factors were investigated including smoking, hygiene, microorganisms, oral contraceptives, antibiotics, diet, breastfeeding, geographical factors, pollution and stress. Inconsistent findings among the studies highlight the complex pathogenesis of IBD. Additional studies are necessary to identify and elucidate the role of environmental factors in IBD etiology.

Prevalence of intramammary infection in Dutch dairy herds.

A survey was carried out in 2003 in 49 dairy herds to determine the overall and pathogen-specific prevalence of intramammary infection (IMI) in Dutch dairy herds, and to compare the distribution with four studies performed from 1973 to 1985 in The Netherlands. Herds were randomly selected stratified over the 12 Dutch provinces, had at least 40 lactating cows and participated in the Dutch milk recording system. Quarter milk samples were collected from all 408 cows with a somatic cell count (SCC) >or=250,000 cells/ml and 145 heifers with SCC >or=150,000 cells/ml at the last milk test before the farm visit. Additionally, samples were collected from 519 (approximately 25%) of the remaining low-SCC cows and heifers with a SCC at the last milk test before the farm visit of <250 000 and <150 000 cells/ml, respectively. Bacterial growth occurred in 37.3% of milk samples of high-SCC cows and in 21.1% of low-SCC cows. Coagulase-negative staphylococci (CNS) were the most frequently isolated group of bacteria (10.8% of quarters) and were found in all herds. Prevalence of Staphylococcus aureus IMI was lower in 2003 than in 1973, respectively 1.8% and 6.2% of quarters. Prevalence of Streptococcus uberis and Str. dysgalactiae IMI was almost the same in the five samplings during the 30-year period, at 1.1-1.7 and 0.9-1.5%, respectively. Str. agalactiae was not found in this study. Prevalence of CNS IMI was higher in lactating heifers, while prevalence of Str. uberis, Str. dysgalactiae and penicillin-resistant Staph. aureus IMI was higher in older cows. Because distribution of pathogens changes over time, herd-level samples for bacteriological culturing must be taken regularly to monitor udder health. Additionally, national mastitis prevalence studies give important information through monitoring the national udder health status.

Heifers infected with coagulase-negative staphylococci in early lactation have fewer cases of clinical mastitis and higher milk production in their first lactation than noninfected heifers.

Intramammary infections (IMI) in recently calved dairy heifers are more common than was formerly believed but their relevance for future performance has been studied only rarely. In the present study, the association between the IMI status of fresh heifers and their subsequent udder health, milk production, and culling in first lactation was explored. Quarter milk samples were collected between 1 and 4 d in milk (DIM) and between 5 and 8 DIM from 191 dairy heifers in 20 dairy herds for bacteriological culturing and somatic cell count (SCC) analysis. Monthly milk recording data including composite milk SCC and test-day milk yield (MY) were obtained for the first 285 DIM or until culling. Farmer-recorded clinical mastitis cases were available. Data were analyzed using mixed models and survival analysis. Approximately 80% of the fresh heifers (79.8%) had at least one culture-positive quarter. Coagulase-negative staphylococci (CNS) were the most frequently isolated pathogens (72%), followed by esculin-positive streptococci (4.6%) and Staphylococcus aureus (3.5%). Overall geometric mean SCC at quarter level decreased between the first and second samplings from 348,000 to 116,000 cells/mL. Heifers infected with CNS had an intermediate average test-day SCC (84,000 cells/mL) during the first 285 DIM compared with noninfected heifers (53,000 cells/mL) and heifers infected with major pathogens (195,000 cells/mL). Heifers infected with major pathogens had a much lower average daily MY (18.3kg) during first lactation compared with noninfected animals (21.3kg). That CNS-infected heifers out-produced their noninfected counterparts could be at least partially explained by their significantly lower incidence of clinical mastitis (incidence risk 3.6 vs. 21.0%) during first lactation compared with noninfected heifers. We conclude that although CNS cause the majority of IMI in heifers around calving, they should not be a reason for serious concern.

Meta-analysis of the effect of oral selenium supplementation on milk selenium concentration in cattle

Soils in many regions of the world have a low Se content. Consequently, forages and crops grown on these soils may provide inadequate dietary Se for humans and grazing animals. Selenium supplementation has been used to enhance Se status and milk Se concentration, but results conflict. Milk Se concentration appears to be a useful indicator of animal and herd Se status, and reflects the responsiveness to supplementation. A systematic review and meta-analysis were carried out to summarize all available scientific evidence for the effect of oral Se supplementation on milk Se concentration in cattle. The literature search was based on electronic and nonelectronic databases. Fixed- and random-effects models of meta-analysis were used, and a meta-regression was carried out to evaluate heterogeneity among studies. Random-effects meta-analysis was performed on 42 studies published between 1977 and 2007. Oral Se supplementation resulted in an average increase in milk Se content of 0.16 (95% confidence interval: 0.117, 0.207) micromol/L, with a significant heterogeneity among studies. Weak publication bias was evident, but it did not change the average effect. The continent where the study was performed, Se source, Se dose, and the interaction between source and dose explained 71% of the between-study variance. On average, American cows supplemented with Se yeast (e.g., 6 mg/h per day) had greater milk Se concentrations (approximately 0.37 micromol/L) 75 d after the beginning of supplementation when compared with those supplemented with inorganic forms of Se. This information provides a basis for tailoring daily animal requirements and for enhancing the Se intake of consumers of dairy products.

Antimicrobial use on Canadian dairy farms

Antimicrobial use (AMU) data are critical for formulating policies for containing antimicrobial resistance. The present study determined AMU on Canadian dairy farms and characterized variation in AMU based on herd-level factors such as milk production, somatic cell count, herd size, geographic region and housing type. Drug use data were collected on 89 dairy herds in 4 regions of Canada, Alberta, Ontario, Québec, and the Maritime provinces (Prince Edward Island, New Brunswick, and Nova Scotia) for an average of 540 d per herd. Dairy producers and farm personnel were asked to deposit empty drug containers into specially provided receptacles. Antimicrobial use was measured as antimicrobial drug use rate (ADUR), with the unit being number of animal defined-daily doses (ADD)/1,000 cow-days. Antimicrobial drug use rates were determined at farm, region, and national level. Combined ADUR of all antimicrobial classes was 14.35 ADD/1,000 cow-days nationally. National level ADUR of the 6 most commonly used antimicrobial drug classes, cephalosporins, penicillins, penicillin combinations, tetracyclines, trimethoprim-sulfonamide combinations, and lincosamides were 3.05, 2.56, 2.20, 1.83, 0.87, and 0.84 ADD/1,000 cow-days, respectively. Dairy herds in Ontario were higher users of third-generation cephalosporins (ceftiofur) than in Québec. Alberta dairy herds were higher users of tetracyclines in comparison to Maritimes. Antimicrobial drug use rate was higher via systemic route as compared with intramammary and other routes of administration (topical, oral, and intrauterine). The ADUR of antimicrobials used intramammarily was higher for clinical mastitis treatment than dry cow therapy. For dry cow therapy, penicillin ADUR was greater than ADUR of first-generation cephalosporins. For clinical mastitis treatment, ADUR of intramammary penicillin combinations was greater than ADUR of cephapirin. Herd-level milk production was positively associated with overall ADUR, ADUR of systemically administered ceftiofur, cephapirin administered for dry cow therapy, and pirlimycin administered for clinical mastitis treatment. Herd size and ADUR of systemically administered ceftiofur were also positively associated. In conclusion, β-lactams were most commonly used on Canadian dairy farms. Among antimicrobials of very high importance in human medicine, the use of fluoroquinolones was rare, whereas third-generation cephalosporins and penicillin combinations containing colistin were used very frequently on Canadian dairy farms.

Population dynamics of bovine herpesvirus 1 infection in a dairy herd

An induced outbreak of a bovine herpesvirus 1 (BHV1) infection in a dairy herd is described. The outbreak was induced by injecting three BHV1 seropositive cows with dexamethasone. Within 7 weeks all seronegative cows had seroconverted. Also some seropositive animals showed a significant increase in serum antibody titre. Using these data, parameters of population dynamics such as R0, the basic reproduction ratio, could be estimated. The basic reproduction ratio is a threshold value describing infection dynamics in a population. This parameter is defined as the average number of secondary cases generated by one primary case in a wholly susceptible population of defined density. In this population R0 was estimated to be at least 7. The importance of these findings, and implications for eradication of BHV1 are discussed.