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.