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* Department of Health Management, University of Prince Edward Island, Charlottetown, Prince Edward Island, C1A 4P3 Canada
Department of Production Animal Health, Faculty of Veterinary Medicine, University of Calgary, Calgary, Alberta, T2N 1N4 Canada
Department of Population Medicine, University of Guelph, Guelph, Ontario, N1G 2W1 Canada
Faculté de medicine vétérinaire, Université de Montréal, Saint-Hyacinthe, J25 7C6 Canada
1 Corresponding author: r.olderiekerink{at}gddeventer.com
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: incidence rate • clinical mastitis • pathogen-specific mastitis • Canada
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Several studies have been conducted in the past to estimate the incidence rate of clinical mastitis (IRCM) in Europe (Barkema et al., 1998b; Peeler et al., 2000; Barnouin et al., 2005; Nyman et al., 2006; Bradley et al., 2007), North America (Dohoo et al., 1983; Erskine et al., 1988; Bartlett et al., 1992; Sargeant et al., 1998), Australia (Daniel et al., 1982), New Zealand (McDougall, 1999), and Africa (Kivaria et al., 2006). The distribution of pathogens isolated from clinical mastitis samples differs considerably among countries and even among studies within a country. In Norway, for example, Staphylococcus aureus is the most frequently isolated bacteria from clinical mastitis samples, followed by Strep. dysgalactiae (Reksen et al., 2006). In the Midwestern United States, coliforms were the most frequently isolated bacteria (Erskine et al., 1988). In Europe, clinical Klebsiella mastitis occurs less frequently than clinical Escherichia coli mastitis, whereas in the United States, Klebsiella and E. coli mastitis are of equal importance (e.g., Barkema et al., 1998b; Roberson et al., 2004). In New Zealand, coliforms are less important as mastitis-causing pathogens; Strep. uberis is the main concern in both clinical and subclinical mastitis (McDougall, 1998).
Canadian studies of the IRCM are scarce and are limited historically and geographically. A study conducted from 1993 to 1996 in 32 herds in British Columbia found very low lactational IRCM based on farm records, ranging from 5.6 to 10.5% in cows in first lactation and in fifth and greater lactations, respectively (Van Dorp et al., 1999). More recent studies in 48 and 65 selected commercial Ontario dairies estimated the lactational IRCM to be 22 and 20%, respectively (Sargeant et al., 1998; McLaren et al., 2006). The lactational IRCM was reported to be 17% in an earlier study in Ontario as part of a disease cohort study (Dohoo et al., 1983). No bacteriology of milk samples was performed in most Canadian studies, except in the study by Sargeant et al. (1998), who isolated Staphylococcus spp. other than Staph. aureus most often from clinical mastitis samples, followed by coliforms and Streptococcus spp.
Geometric mean BMSCC may differ by geographic region (Norman et al., 2000). An association exists between the prevalence of pathogen-specific IMI in the herd and BMSCC (Roberson et al., 2004). In addition, some studies have reported a difference in pathogen-specific IRCM related to BMSCC (Schukken et al., 1989b; Barkema et al., 1998b). Pathogen-specific IRCM may therefore differ between geographic regions.
Because no nationwide studies of the IRCM have been conducted in Canada, and because the IRCM and distribution of mastitis-causing bacteria may differ geographically, the objective of this study was to determine regional pathogen-specific IRCM on Canadian dairy farms. Additionally, the association of pathogen-specific IRCM with BMSCC and barn type was determined.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Sampling
Participating producers were asked to collect a milk sample aseptically from every quarter that had visible signs of clinical mastitis and to record cow identification; quarter; date; clinical signs such as abnormal milk, abnormal udder (swollen, red, or hard), or fever; whether the cow was off feed; teat injury; and the treatment, if the cow was treated. Every producer received a milk sampling package consisting of sample tubes, alcohol pads, latex gloves, an instruction sheet, a protocol for aseptic collection of milk samples, recording forms, and labels. Milk samples were stored in a freezer on the farm (at approximately –20°C) and collected every 4 to 6 wk by the veterinarian or the Canadian Quality Milk Program coordinator, who sent the frozen milk samples on ice packs by overnight courier to the Atlantic Veterinary College (Charlottetown, Prince Edward Island) for bacterial culture.
A questionnaire was administered on every farm during the study period. The questionnaire was designed with closed questions and semiclosed questions only. Questions were tested on 3 farms and by 3 technicians at the Atlantic Veterinary College to test whether they were understood easily and interpreted correctly. The questionnaires were changed and improved where necessary. After a final version was decided on, the questionnaire was translated into French, but no further testing was conducted on this version. Upon receiving the questionnaire, all answers were coded and checked, entered twice with data-entry software (EpiData Entry; Lauritsen and Bruus, 2006), and both entries were compared, to check for errors. Specific cow and lactation data, such as calving dates, parity, and culling dates, and specific herd data, such as BMSCC and herd size, were obtained from the regional DHI organizations.
Laboratory Analysis
Bacteriological culture of milk samples was performed according to National Mastitis Council standards (Hogan et al., 1999). Staphylococcus aureus was identified by Gram stain, a positive catalase test, 
-and β-hemolysis on blood-esculin agar, and a positive tube coagulase test. Escherichia coli and Klebsiella spp. were identified by typical appearance on blood agar and were subsequently inoculated on MacConkey agar. In addition, a triple-sugar iron agar, a urea, a citrate, and a spot indole test were set up for identification. One modification was made when using highly selective media for identifying Streptococcus spp. as suggested by Zadoks et al. (2005). Streptococcus spp. not splitting esculin on a blood agar plate with 0.1% esculin were considered to be Strep. dysgalactiae; the remaining streptococci were plated on an Enterococcosel agar (BD Diagnostic Systems, Sparks, MD) and incubated for 24 h at 37°C. Streptococci that were not splitting esculin on the Enterococcosel agar were considered to be Strep. uberis; the remaining organisms on Enterococcosel were considered to be group D Streptococcus or Enterococcus spp. Streptococcus agalactiae was identified by typical appearance on either modified Edward’s medium or blood esculin agar, gram-positive staining, a negative catalase test, a positive Christie, Atkins, Munch-Petersen (CAMP) test, and a positive latex agglutination test (Remel PathoDx, Remel Europe Ltd., Dartford, Kent, UK).
Ten microliters of milk was cultured and the number of colony-forming units of each of the bacterial species was counted. The contagious pathogens Staph. aureus and Strep. agalactiae were considered to cause an IMI if 1 colony (100 cfu/mL) was isolated (Hogan et al., 1999). Isolation of 
200 cfu/mL of environmental mastitis pathogens (E. coli, streptococci other than Strep. agalactiae, Enterococcus spp., coagulase-positive staphylococci other than Staph. aureus, Klebsiella spp., Arcanobacterium pyogenes, Serratia spp., Pseudomonas spp., and Pasteurella spp.) or 1,000 cfu/mL of Corynebacterium bovis, CNS, yeasts, molds, fungi, or Bacillus spp. was considered significant. Milk samples with 3 or more different species were considered to be contaminated unless Staph. aureus or Strep. agalactiae was isolated.
Statistical Analysis
Data were examined for unlikely values; no data were excluded for this reason. All cases of mastitis recorded by the producers were initially used in the analysis. A second or third case of clinical mastitis in the same lactation, regardless of the culture result, was considered a new case if at least 14 d had passed between the previous and the current case of clinical mastitis (e.g., Barkema et al., 1998b).
The association of BMSCC with IRCM was assessed by using a negative binomial regression analysis of IRCM separately on the natural logarithm of BMSCC and BMSCC category. The association of IRCM with either region, province, or barn type corrected for region was also analyzed by negative binomial regression. Regions were defined as western provinces, Ontario, Québec, and Atlantic provinces to have sufficient herd numbers per geographic region. The western provinces consisted of British Columbia, Alberta, Saskatchewan, and Manitoba. The Atlantic provinces consisted of New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland. On the basis of the geometric mean of monthly BMSCC during the study period, herds were assigned to either a low, medium, or high BMSCC category: <150,000, 150,000 to 250,000, or >250,000 cells/mL, respectively.
Cows were at risk during the time the herd was enrolled in the study. Per lactation, the time at risk, in days, started at the calving date, when the cow entered the herd, when the herd entered the study, or 14 d after the last mastitis diagnosis date. The time at risk ended when the cow had mastitis, the cow died or was culled, the farm stopped participating in the study, or the cow started a new lactation. The incidence rate was calculated as the number of mastitis cases per 36,500 d at risk (100 cow-years) in a herd. Incidence rate, time at risk, and overdispersion of the models were assessed as described by Dohoo et al. (2003). All statistical analyses were performed by using Intercooled Stata 8.2 (Intercooled Stata for Windows, version 8.2, Stata Corporation, College Station, TX).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). The Atlantic provinces and Ontario had approximately equal proportions of tie-stalls and free-stalls (Figure 1). The average herd size was 106 cows and ranged between 23 and 649 cows (dry and lactating cows). Because a proportional representation of farms across the Canadian provinces was attempted, Québec had the largest proportion of participating herds (26; 24.5%) whereas only 1 Newfoundland herd (0.9%) participated (Table 1). The geometric mean BMSCC was the lowest on the participating Prince Edward Island farms (146,000 cells/mL; Table 1), whereas the participating Manitoba farms had the highest geometric mean BMSCC (262,000 cells/mL).
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). No bacteria were isolated in 1,330 (43.9%) samples, and 262 (8.6%) samples were considered contaminated. Streptococcus agalactiae was found in 4 (0.1%) clinical mastitis cases, all of which were retrieved from 1 farm in Québec. Severity was recorded in 2,311 (73%) of the recorded clinical mastitis cases. In 230 (10%) cases, the cow was off feed or had a fever.
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and 3, and Table 1
). Incidence rates were different by province (P < 0.01; Table 1). Ontario had the highest IRCM, 31.2 cases per 100 cow-years, compared with Manitoba, where the IRCM was 7.6 cases per 100 cow-years. Herds in Ontario and Québec had the highest mean herd IRCM compared with other regions, and the herds in the Western provinces had the lowest IRCM (Table 1). The IRCM was highest in the first week after calving, declined considerably in the second week, and showed a declining trend toward late lactation and a slight upward trend from wk 45 onward (Figure 4). Heifers had a higher IRCM than older cows in the first 2 wk of lactation, between 2 wk and 45 wk they had a lower IRCM than older cows, and late to very late in lactation heifers and older cows tended to have similar IRCM (Figure 4).
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). The low, medium, and high BMSCC categories consisted of 30 (28%), 52 (49%), and 24 (23%) herds, respectively, after categorization. Low and medium BMSCC herds had higher IRCM than did high BMSCC herds (Table 3). Staphylococcus aureus IRCM was higher in the high BMSCC herds than in the medium BMSCC herds, but not significantly, whereas the Staph. aureus IRCM in low BMSCC herds was significantly higher than that in medium BMSCC herds (Table 3). The E. coli IRCM was higher in low and medium BMSCC herds than in high BMSCC herds. The culture-negative IRCM was higher in low and medium BMSCC herds than in high BMSCC herds (Table 3).
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). Contagious pathogens (Staph. aureus and Strep. dysgalactiae) had the highest IRCM in Québec. Cows in tie-stalls had a higher Staph. aureus, Strep. uberis, CNS, and other streptococcal IRCM compared with those in free-stalls, whereas cows in free stalls had higher Klebsiella spp. IRCM than those in tie-stall barns (Table 5).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Studies such as this one, in which producers selected and sampled cows with clinical mastitis, have some drawbacks. First, the herds were selected for convenience. This method was chosen because producers were asked to take samples and keep records of all clinical mastitis cases. It is likely that this resulted in an overrepresentation of compliant, cooperative producers or producers with mastitis problems who saw this project as an opportunity to get free culturing of milk samples. In this study, many farms had a lower BMSCC compared with the average provincial BMSCC as recorded by the Canadian Dairy Commission (Table 1). These farms represent a different type of management than high BMSCC herds (Barkema et al., 1998a). Producers that were willing to participate were also likely to be more motivated to reduce IRCM on their farms. This convenience selection could have caused an underestimation of the true IRCM in Canadian dairy herds. On the other hand, the herd selection method provided an opportunity for the participating veterinarians to include farms with mastitis problems in the project. Second, detection bias or misclassification bias might have caused underestimation of the IRCM because definitions of clinical mastitis might differ between producers. Each herd was provided with the project definition of clinical mastitis; however, because of the study design, the authors were not able to validate the producers’ definition of clinical mastitis. Third, particularly for this study, there was no direct contact between the researcher and the producers, or between the researchers and the veterinarians or Canadian Quality Milk coordinators, which might have curbed the motivation for both coordinators and producers to take samples and might have caused an underestimation of IRCM. In comparison, Barkema et al. (1998b) personally visited every farm every 6 to 8 wk, and this might have been a reason that the IRCM in his study was higher than in the present study. Although these reasons are major drawbacks in estimating the IRCM, alternatively, visiting every farm to diagnose every case of clinical mastitis would require an enormous financial and human workforce effort. Additionally, the researcher’s estimation of IRCM might not reflect the producer’s perceived IRCM. Therefore, the data collection methods used in this study were the most feasible and pragmatic approaches under the given circumstances.
Detection bias could also have occurred because of the severity of clinical mastitis, which is related to the pathogens isolated (Gröhn et al., 2005). Producers could be more likely to sample cows with severe clinical mastitis than cows with less severe symptoms. Detection bias could have differed among farms. Producers who scrutinized foremilk more carefully than other producers (Barkema et al., 1999) could have detected more clinical mastitis cases. However, Lam et al. (1993) concluded, in a study performed in the Netherlands, that the diagnostic capability of producers did not have a negative influence on the validity of producer-observed clinical mastitis incidence in low BMSCC herds.
Because veterinarians in Sweden are required to initiate every treatment involving antibiotics, a recent study reported that producers with high veterinary-treated IRCM were more motivated to treat clinical mastitis than producers that had low IRCM (Nyman et al., 2006). Another method, used in a study in British Columbia, Canada, relied on farm records only (Van Dorp et al., 1999) and possibly resulted in a relatively low IRCM, because producers might not record every case of mastitis. They might have chosen to record only cases of mastitis that were treated, contrary to our study, in which producers were instructed to take milk samples of every case of clinical mastitis, regardless of treatment. Just under half of the cases of clinical mastitis were not treated in our study (results not shown); we hypothesize that these cases were mild cases of clinical mastitis and that producers normally record fewer of these cases. This is possibly reflected in the higher IRCM we found in our study compared with the report of Van Dorp et al. (1999), because we emphasized to the producers to take milk samples of every case of clinical mastitis.
The IRCM reported in this study was higher than the IRCM reported in the most recent study in Ontario (Sargeant et al., 1998). Sargeant et al. (1998) calculated the IRCM by using exclusively full 305-d lactations. However, an underestimation of true IRCM could occur here because cows with clinical mastitis are more likely to be culled before the end of lactation (Seegers et al., 2003).
Herds in Ontario and Québec had a higher IRCM than herds in other regions. More than half of the barns in these regions were tie-stall barns. The difference in IRCM could be explained by the different management styles directly related to the barn type and different IMI risks associated with barn type. In tie-stall barns, it is easier to milk cows with clinical mastitis last or with a separate unit. In free-stall barns, wood shavings are used more often as stall bedding material than in tie-stall barns. Cows kept in tie-stall barns also had proportionally more clinical Staph. aureus and Strep. uberis mastitis compared with those in free-stall barns. More udders are infected with Staph. aureus in Scandinavian countries, specifically Norway and Sweden, which have more tie-stall barns (Østerås et al., 1999). Staphylococcus aureus is considered a contagious pathogen (Fox and Gay, 1993), and the spread and prevalence of this pathogen could be attributed to udder preparation procedures in tie-stall barns. Streptococcus dysgalactiae had the highest IRCM in Québec, where most herds are kept in tie-stall barns, although Strep. dysgalactiae IRCM was not significantly associated with barn type in our study.
The distribution of barn types in this study was similar to that in previous research in a random sample of herds (Olde Riekerink et al., 2006a). Tie-stall barns were more common in Ontario, Québec, and the eastern provinces (Olde Riekerink et al., 2006a) and were positively associated with higher IRCM. Western provinces, such as British Columbia and Alberta, had few or no tie-stall barns compared with other provinces. Region could therefore be a confounder for differences in IRCM between housing systems. When starting a dairy farm in a certain region, a regional common barn type is most likely installed, whereas certain regional styles of farming might have a direct effect on IRCM. The difference in Staph. aureus IRCM was therefore most likely the result of the prevailing barn types by region. By contrast, Strep. dysgalactiae IRCM differed significantly by region and seemed not to be associated with either free-stall or tie-stall barns. An explanation for these regional differences could be sought, for example, in differences in management style, tradition, and herd size. Regional differences in IRCM could therefore be explained only partly by the regional appearance of certain barn types.
The diagnostic tests for mastitis that are described in the Laboratory Handbook on Bovine Mastitis of the National Mastitis Council (Hogan et al., 1999) to differentiate the most frequently isolated Streptococcus spp. are the CAMP, inulin, hippurate, esculin, and NaCl tests. A table is provided in this book showing how Strep. agalactiae, Strep. uberis, and Strep. dysgalactiae should react. The NaCl test is used to differentiate between streptococci and enterococci (Brown et al., 1983). However, in our experience, this test is unreliable. Of a set of 10 Enterococcus spp. based on a positive NaCl test, further diagnostics using PCR techniques (Zadoks et al., 2005) identified 7 isolates as Strep. uberis (results not published). The tests that are recommended by the National Mastitis Council are difficult to interpret, because test combinations do not always match and often the diagnoses are "other" Streptococcus or Enterococcus spp. The authors decided to use Enterococcosel agar to differentiate Enterococcus spp. from non-group D Streptococcus spp. in addition to the tests recommended by the National Mastitis Council. The proportions of clinical mastitis caused by Strep. uberis, Strep. dysgalactiae, and Enterococcus spp. better reflected the proportions that were to be expected, similar to other studies (Barkema et al., 1998b) using Enterococcosel agar.
Staphylococcus aureus was the most frequently isolated pathogen in clinical mastitis, followed by E. coli and Strep. uberis. In an earlier study on herd-level prevalences of contagious mastitis pathogens in Canadian dairy herds, Staph. aureus was isolated from bulk milk in 74% of a random selection of 291 herds (Olde Riekerink et al., 2006a). Finding Staph. aureus most often in milk samples of clinical mastitis was not surprising. Coliforms were most often isolated from cases of clinical mastitis in a study in Ontario (Sargeant et al., 1998), although further differentiation was not performed. Klebsiella spp. were the fifth most frequently isolated pathogens. Recently, researchers have suggested that the incidence of Klebsiella is higher in North America than in Europe (Roberson et al., 2004) and that it is an emerging pathogen in North America (Zadoks and Munoz, 2007). However, husbandry on Canadian dairy farms, particularly in Québec, Ontario, and Atlantic Canada, differs from that on US farms and is more similar to the Western European situation. As a result, the pathogen distribution of western Canadian dairy farms is similar to that of herds with a low BMSCC in the United States (Table 4; Erskine et al., 1988). In the other regions of Canada, the distribution is similar to what was found in European studies (Barkema et al., 1998b). Proportion of CNS was similar to that in a recent clinical mastitis incidence study in the United Kingdom (Bradley et al., 2007). It is difficult to judge whether the cultured CNS were the causative agent of the clinical mastitis or teat canal inhabitants. However, CNS can cause mild to severe clinical mastitis (Jarp, 1991), and some have reported an increase in the CNS IRCM (Taponen and Pyörälä, 2007).
Culture-negative milk samples represented a large part of the milk sample culture results. One of the reasons for a milk sample of a clinical mastitis case to be culture negative in this study is that a considerable proportion of samples were frozen for a few weeks between sampling and culture in the laboratory. Another reason could be that those mastitis cases might have been caused by Mycoplasma. Samples were not tested for Mycoplasma spp. because this requires special growth media. This seems unlikely, however, because the incidence of Mycoplasma mastitis is generally not so high that it could explain most of the culture-negative samples, and the clinical appearance of the culture-negative mastitis cases did not suggest Mycoplasma mastitis. It would be fair to state, based on a recent study in Prince Edward Island, that Mycoplasma prevalence is most likely low in Canada (Olde Riekerink et al., 2006b). Culture-negative results are often attributed to either E. coli (Smith and Hogan, 1993) or Staph. aureus (Sears et al., 1990). The distribution of culture-negative IRCM was strikingly similar to the E. coli IRCM across BMSCC groups (Table 3), regions (Table 4), and barn types (Table 5), whereas the Staph. aureus IRCM had different distributions. This suggests that a great proportion of the culture-negative clinical mastitis cases were caused by E. coli, or at least a pathogen with similar properties, and that this pathogen was not present or viable in the milk sample collected (Zorah et al., 1993) or did not survive the frozen storage before culture (Schukken et al., 1989a).
No linear relationship was found between BMSCC and overall IRCM in this study. This is consistent with some previous research (Barkema et al., 1998b), although other authors have reported an association (Erskine et al., 1988). An association between BMSCC and pathogen-specific IRCM was observed, consistent with the findings of Barkema et al. (1998b). Barkema et al. (1998b) reported that herds with low BMSCC had a higher E. coli and Strep. dysgalactiae IRCM and herds with a high BMSCC had a higher IRCM with contagious mastitis pathogens, such as Staph. aureus. Similar to Barkema et al. (1998b), there was a higher Staph. aureus IRCM in the high BMSCC herds compared with other BMSCC categories and a higher E. coli and culture-negative IRCM in the medium and low BMSCC herds compared with the high BMSCC category, indicating a pathogen-specific difference in IRCM between the BMSCC categories. These findings could suggest that mainly contagious mastitis pathogens contributed to a high BMSCC. If the number of herds in this study had been larger and if the estimates are correct, the additional statistical power might have led to more significant differences in pathogen-specific IRCM between BMSCC categories. The higher BMSCC is most likely caused by an increased Staph. aureus IRCM. Herds with a high Staph. aureus IRCM possibly have more subclinical Staph. aureus infections than herds with a low Staph. aureus IRCM. A higher prevalence of Staph. aureus in the herd is likely associated with a higher frequency of Staph. aureus isolation from consecutive bulk milk samples, which in turn is associated with a higher BMSCC (Jayarao et al., 2004; Olde Riekerink et al., 2006b).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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FOOTNOTES |
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Received for publication October 7, 2007. Accepted for publication December 19, 2007.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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).