By Greg Keefe, Kimberley MacDonald, and Marguerite Cameron, Maritime Quality Milk, Atlantic Veterinary College, University of Prince Edward Island
Mastitis is the most common and costly infectious disease of dairy cattle worldwide and is most frequently bacterial in origin (Erskine et al., 2003; Halasa et al., 2007). As a result, it is also the most common reason for antibiotic use in the Canadian dairy industry, accounting for more than half of all the antibiotics used by dairy producers (Leger et al., 2003). In a Wisconsin study involving 20 conventional dairy herds, approximately 80% of all antimicrobial drug use was for mastitis (Pol and Ruegg, 2007). In that study, 38% of antimicrobial doses were intramammary for clinical mastitis, 17% were given parenterally for clinical mastitis, and 28% of antimicrobial doses were for dry cow therapy.
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Administration of intramammary antibiotics during lactation results in high drug concentrations in milk, necessitating that milk be discarded for the duration of the treatment regime and for a withdrawal period following the last dose to allow clearance of all residues. In fact, approximately 90% of residue violations in milk can be traced back to mastitis treatments (Erskine et al., 2003). Substantial economic losses result from milk discard, in addition to treatment, extra labor, and culling costs. The risk of development of antibiotic resistance is also a concern from a veterinary and human medical standpoint. Both the type of infectious agents found in bovine mastitis and the classes of antibiotics used for therapy overlap between human and veterinary medicine (Health Canada, 2002). Selective pressures from antimicrobial use, mutations, or acquisition of foreign resistance determinants can mediate antimicrobial resistance (Tikofsky et al., 2003). For these reasons, the judicious use of antibiotics by veterinarians and producers continues to be emphasized throughout the dairy industry.
Despite these concerns, the use of antibiotics in food-producing animals is necessary for the treatment and successful cure of many common bacterial infections. To be used effectively, an appropriate antibiotic must be chosen with a spectrum of activity, which includes the pathogenic agent that is causing the disease. While residue risk and antimicrobial resistance are of concern, treatment should not be withheld if such an action conflicts with the principles of humane care of the animals. This paper examines the role that on-farm culture systems may play in clinical mastitis and dry cow therapy. The effects of these techniques on case outcome, risk of new infections, and overall antibiotic usage are also explored.
Clinical mastitis is caused by a wide range of bacteria with vastly different pathogeneses and natural history of infection. A recent Canadian study reported that 44% of milk samples submitted from more than 3,000 cases of clinical mastitis yielded no bacterial growth (Olde Riekerink et al., 2008). In most of these cases, the assumption can be made that the host’s natural defenses cleared the bacterial infection completely or to a level below the detection limit of the culture method prior to sample collection. Historically, Gram-negative infections have been reported to have high self-cure rates, prompting a recommendation of no antibiotic treatment in uncomplicated cases (Pyörälä, 2009). Mild to moderate cases of coliform mastitis are reported to have high spontaneous cure rates, calling into question the need to use antibiotics (Wilson et al., 1999; Erskine et al., 2003; Hogan and Smith, 2003; Lago et al., 2011a). However, certain coliform strains may contain virulence factors that increase persistence, making antibiotic therapy warranted. Spontaneous cure of some pathogens is also frequently observed, with a large proportion of clinical milk samples culturing negative on standard bacteriology (Olde Riekerink et al., 2008; Lago et al., 2011a). Antibiotic treatment of Gram-positive infections, including staphylococcal and streptococcal species, however, is widely reported as beneficial for increasing the probability of cure and preventing the risk of chronic subclinical mastitis and decreased production for the remainder of the lactation (Van Eenennaam et al., 1995; Wilson et al., 1999; Oliver et al., 2004). When designing a treatment regime, having information on the causative organism in order to choose an antimicrobial with an appropriate spectrum of activity is important (Constable and Morin, 2003). As for other microbial infections, rational mastitis therapy requires the targeting of treatment toward specific pathogens. In a recent publication, a leading group of mastitis researchers concluded that mastitis caused by Gram-positive agents needs different approaches than mastitis caused by Gram-negative bacteria, and that with new diagnostic tools, routine use of broad-spectrum antimicrobials without diagnosis could be considered an outdated practice (Hogeveen et al., 2011).
Identification of causative organisms would allow an appropriate antibiotic to be chosen and would enable selective or targeted treatment strategies to be employed. The need for a user-friendly, rapid diagnostic culture system that could be utilized by the dairy producer has therefore been identified (Sears and McCarthy, 2003; Leslie et al., 2005; McCarron et al., 2009). Two large field trials have been conducted in North America to evaluate the short- and long-term implications of an on-farm, culture-driven, selective clinical mastitis therapy program (Lago et al., 2011a; Lago et al., 2011b; MacDonald, 2011).
A 3M Petrifilm on-farm culture system (POFCS) was developed by McCarron et al. (2009). In laboratory trials, the culture system accurately identified 93.8% of clinical mastitis (CM) cases caused by Gram-positive organisms after 24 hours of incubation. A negative predictive value of 89.7% was reported, giving confidence in the diagnosis of non-treatment cases. Others have also reported good test characteristics with 3M Petrifilms for the diagnosis of CM (Leslie et al., 2005; Silva et al., 2005; Wallace et al., 2011).
To fully evaluate the POFCS, a Canada-wide clinical trial was conducted. A total of 997 clinical cases were enrolled of which 621 cases from 48 farms met all criteria and had complete records. All mild to moderate CM cases were randomly assigned to the POFCS group or a treated control (TC) group. Using the on-farm system, 64% of samples were identified as Gram-positive, leading to a reduction in antibiotic treatment in all cases by 36%. No significant differences in the risk of requiring additional antibiotic treatment (changes from the initial treatment protocol) were detected between POFCS and TC groups. Pre-treatment milk samples were collected and submitted frozen for standard bacteriology. Cases allocated to the TC group were promptly treated with cephapirin sodium, whereas POFCS cases were treated only if a Gram-positive organism was identified as the cause after 24 hours of incubation. Producers recorded the date that the milk returned to normal appearance and the date the milk returned to the bulk tank for sale. Follow-up milk samples were collected 14 to 21 days and 28 to 35 days following the CM event. Standard bacteriology was performed on all pre-treatment and follow-up samples.
Accuracy of the POFCS in the hands of producers was reduced compared to previous reports. The sensitivity and specificity of the POFCS varied widely among farms according to the frequency of use. Many small farms used the system less than once per month, which resulted in an overall sensitivity and specificity of 63.8% and 48.4%, respectively. Within herds using the culture system at least once per month, sensitivity of the culture system reached 81.8%, while specificity remained low at 45.6%. The negative predictive value improved from 57.7% to 82.1% with more frequent use. Despite some inaccuracy in diagnosis, there was no overall effect of treatment group (POFCS versus TC) on the odds of clinical and bacteriologic cure or on the number of days from mastitis onset until the milk returned to normal appearance. However, when the treatment group was further divided into correct, false-positive, and false-negative diagnoses, the Gram-positive cases, which were falsely diagnosed as non-treatment cases, were more likely to fail to cure clinically and bacteriologically. The impact of false-negative Gram-positive diagnosis (which resulted in longer days to clinical cure) counteracted the beneficial effect of early return to salable milk for correctly diagnosed non-antibiotic treated animals. Cows were followed for a minimum of 4 months following enrollment into the clinical trial to monitor for CM recurrence. No differences in either the probability of recurrence of mastitis due to the causative organism or a different pathogen, or in the days to that recurrence, were detectable between treatment groups. Similarly, no significant differences in the risk of cows being culled were observed between POFCS and TC groups.
The economics of the POFCS were driven by two major factors: pathogen species distribution and accuracy of farmer diagnosis. For herds that used the system more than once per month and were able to improve accuracy, there was a small net economic benefit, but for the average herd in the study, with a high number of Gram-positive infections and inaccurate diagnoses, net return was negative. Overall, herds in the study had a high rate of POFGS Gram-positive infections (64%). By comparison, Lago found only 41% Gram-positive infections (Lago et al., 2011a). Modeling the use of the POFCS under these conditions of high no growth and Gram-negative cases yielded between $20 and $60 per case depending on the accuracy of diagnosis. The greatest potential for economic return in using a POFCS for selective treatment strategies is in herds with accurate diagnoses and where there is a high incidence of non-treatment (Gram-negative and no growth) cases.
Minnesota Easy Culture System
The Minnesota Easy Culture System II Bi-plate (MECS), developed by the University of Minnesota Laboratory for Udder Health, is a culture plate with one half consisting of proprietary factor medium that is selective for Gram-positive bacteria and the other half consisting of MacConkey medium for the identification of Gram-negative bacteria. A multi-state clinical trial was conducted in eight commercial dairy farms to evaluate the ability of the MECS to guide clinical mastitis treatment decisions (Lago et al., 2011a). In that study, 449 mild to moderate clinical mastitis quarters from 422 cows were assigned to MECS-based treatment or blanket treatment (positive control) with cephapirin sodium. Quarters with Gram-negative or no growth did not receive intramammary therapy. In total, the number of cows treated with antibiotics was reduced by 49%. Fifty-six percent of cases had Gram-negative or no growth on initial culture, and a small number of these cases required a secondary antibiotic treatment. No statistically significant differences were observed in days to clinical cure or bacteriological cure risk between cases assigned to the positive-control and cases assigned to the MECS-based treatment program. Treatment failure risk (presence of infection, secondary treatment, clinical mastitis recurrence, or removal from herd within 21 d after enrollment) was not different between the two groups. A trend toward decreased days of milk discard (5.2 vs. 5.9) in favor of the MECS-based treatment program was observed.
In a separate publication, the researchers reported on long-term outcomes of MECS-based treatment (Lago et al., 2011b) Milk production, linear score somatic cell count, and risk of mastitis recurrence or removal by culling or death were not different between cows with mastitis cases assigned to the positive-control group versus the MECS-based treatment group (Lago et al., 2011b).
Selective Dry Cow Therapy
Since the development of the Five Point Mastitis Control Plan during the 1960s, it has become common practice to treat all cows at dry-off with an intramammary antibiotic. The purpose of dry cow therapy (DCT) is to clear up any existing infections (treatment) and to prevent new infections from being acquired (prophylaxis) during the dry period. Dry cow intramammary antibiotics decrease the risk of new infections for a short period after administration, but their activity diminishes over time and they may not be protective in the late dry period, just before calving (Oliver et al., 1990). For treating existing infections, the role of DCT is well established (Dingwell et al., 2003). The prophylactic role of DCT has been evaluated in two recent meta-analyses. In these evaluations, blanket dry cow therapy showed significant protection against new intramammary infection (IMI), particularly against streptococci, but variable impacts on staphylococci, and no observed effects against coliforms (Halasa et al., 2009; Robert et al., 2006).
Blanket dry cow therapy for healthy cows is not practiced in all countries. It may have its disadvantages and certainly adds to antibiotic consumption. The use of selective dry cow therapy has been advocated (and successfully implemented) in some countries (Osterås et al., 1999). In a recent study in Pennsylvania, low somatic cell count cows (low-SCC cows) were randomly assigned to DCT or no treatment. Milk yield of untreated and treated low-SCC cows at dry-off did not differ significantly during the following lactation. However, the treated low-SCC cows had 35,000 lower SCC (16% lower) in the subsequent lactation than the untreated cows. There was a herd effect on the impact of selective treatment on both SCC and milk yield (Rajala-Schultz et al., 2011).
Non-antibiotic alternatives to DCT are available. Internal teat sealants act as a physical barrier in the teat canal and prevent bacteria from entering. While intramammary antibiotics lose their efficacy over the length of the dry period, internal teat sealants have been shown to reduce new mastitis cases throughout the entire dry period. The use of internal teat sealants prevented IMI at calving, reduced clinical mastitis during the dry period, and resulted in less clinical mastitis in the first 100 days post calving, when compared to no treatment (Berry and Hillerton, 2002). A study by our group at the Atlantic Veterinary College has shown that teat sealants are just as good at preventing new intramammary infections during the dry period as DCT in cows that do not have infections at dry-off (Sanford et al., 2006). In the previously cited meta-analysis, the authors concluded that internal teat sealants showed significant protection against new IMI during the dry period (Halasa et al., 2009; Robert et al., 2006). Robert cautioned that sealants should be used as a primary prophylactic only in a subpopulation of uninfected cows (Robert et al., 2006).
Recently, a group of leading mastitis researchers concluded that due to established efficacy and societal pressures to reduce antibiotic use, an internal teat sealant-based non-antibiotic approach for dry cow therapy is a good alternative to antibiotics in herds with a high risk for environmental mastitis during the dry period and at calving (Hogeveen et al., 2011).
While internal teat sealers are a practical alternative to DCT for the prevention of new IMI, they have no antimicrobial capacity and should not be used alone in animals with preexisting infections. If producers were able to determine which cows had an IMI at the time of dry-off and which cows did not, they could make selective dry cow treatment decisions (sealant only versus antibiotic) for each individual animal. This, in turn, would reduce the financial costs associated with dry cow treatment (for those using antibiotic and sealants), as well as address concerns about overuse of antibiotics and antimicrobial resistance without compromising the long-term health of the cow.
On-Farm Culture and Selective Dry Cow Therapy
In an effort to provide a firm scientific basis for making selective dry cow therapy decisions, our research group conducted a study to determine the usefulness of an on-farm culture system to make selective treatment decisions on low-SCC cows at dry-off. Cows (n=720) originating from low bulk tank SCC herds (n=16) from Quebec and Prince Edward Island were randomly allocated to treatment groups on the day prior to dry-off. Cows assigned to the control group (n=362) received blanket treatment with dry cow antibiotics and an internal teat sealant. Cows in the study group (n=358) had composite milk samples cultured on-farm using a Petrifilm-based test system and were selectively treated based on the results. Cows positive on Petrifilm received antibiotics and teat sealant at dry-off, whereas cows negative on Petrifilm received an internal teat sealant as a sole treatment. Of the cows assigned to the study group, 47% cultured negative on-farm and did not receive antibiotic at dry-off. When compared to standard bacteriological culture, the Petrifilm-based test system had a sensitivity of 84% and a negative predictive value of 88%, signifying that few infected cows were missed by the system. When comparing IMI rates around calving between the two treatment groups, we found no difference in the prevalence of infection based on quarter milk samples collected at 3 to 4 days post calving. When correctly applied on low-SCC cows, the on-farm selective dry cow therapy system will enable producers to be selective in their approach to the treatment of cows at dry-off without compromise to the health, welfare, and future milk production of their herds. The resulting reduction in antibiotic use may help address consumer concerns regarding antimicrobial use in dairy production and has the potential to reduce the costs associated with dry cow therapy.
Selective treatment of clinical mastitis cases and dry cows based on established pathogen profiles will dramatically reduce antibiotic use in the dairy industry and can be done without short- or long-term negative consequences for milk quality and animal health. On-farm culture can play a key role in supporting evidence-based treatment decisions. Current technologies are less than ideal because they require a time lag between detection of clinical mastitis and therapy decisions. This affects both the utility of the technology in the view of the producer and the economics of the program because delays in onset of treatment for Gram-positive cases influence the overall cost of a culture program. With increasing concern regarding antibiotic use in the dairy industry, developing technologies to aid in these treatment decisions should be an industry priority.