J. Dairy Sci. 90:249���261 ��� American Dairy Science Association, 2007. Treatment Practices and Quantification of Antimicrobial Drug Usage in Conventional and Organic Dairy Farms in Wisconsin M. Pol and P. L. Ruegg1 Department of Dairy Science, University of Wisconsin, Madison 53706 ABSTRACT The objective of this study was to develop a method to quantify antimicrobial drug usage and treatment practices on conventional and organic dairy farms that had been recruited to represent a broad spectrum of potential exposure to antimicrobial drugs. Data on dis- ease prevalence and treatment practices of organic (n = 20) and conventional (n = 20) farms were obtained dur- ing a farm visit using a survey instrument. A standard- ized estimate of antimicrobial drug usage was devel- oped using a defined daily dose (DDD) of selected com- pounds. Density of antimicrobial drug usage was expressed as the number of DDD per adult cow per year. Differences in prevalence and management of selected diseases between conventional and organic farms were identified. The overall estimated prevalence of selected diseases was greater for conventional farms compared with organic farms. Organic farmers reported use of a variety of nonantimicrobial compounds for treatment and prevention of disease. Conventional farmers re- ported that penicillin was the compound most com- monly used for dry cow therapy and cephapirin was most commonly used for treatment of clinical mastitis. On conventional farms, the estimated overall exposure to antimicrobial drugs was 5.43 DDD per cow per year composed of 3.58 and 1.85 DDD of intramammary and parenteral antimicrobial drugs, respectively. Of total intramammary antimicrobial drug usage, treatment of clinical mastitis contributed 2.02 DDD compared with 1.56 DDD attributed to the use of dry cow therapy. Of total parenteral treatments, the distribution of expo- sure was 0.52 (dry cow therapy), 1.43 (clinical mastitis treatment), 0.39 (treatment of foot disease), 0.14 (treat- ment of respiratory disease), and 0.32 (treatment of metritis) DDD. For treatments of foot infections (0.33 DDD), respiratory infections (0.07 DDD), and metritis (0.19 DDD), the mean density of ceftiofur usage was significantly greater compared with other compounds. Received May 10, 2006. Accepted August 3, 2006. 1Corresponding author: plruegg@wisc.edu 249 Key words: dairy, antimicrobial drug, mastitis, treatment INTRODUCTION In modern dairy cattle operations, antimicrobial drugs are administered for both therapeutic and pro- phylactic purposes. Most antimicrobial drugs are used therapeutically for treatment of bacterial infections. Some antimicrobial drugs are used prophylactically to prevent disease in healthy animals during periods of increased susceptibility. The use of intramammary an- tibiotics at dry off is common in US dairy herds. Dry cow therapy (DCT) is typically administered as a treat- ment for existing subclinical mastitis infections and for prevention during the nonlactating period (McEwen and Fedorka-Cray, 2002 Aarestrup, 2004 Phillips et al., 2004). In the United States, a limited number of antimicro- bial drugs are marketed for intramammary treatment of mastitis. Antimicrobial classes include ��-lactams (penicillin, cephapirin, ceftiofur, amoxicillin, hetacillin, and cloxacillin), macrolides (erythromycin), coumarines (novobiocin), and lincosamides (pirlimycin) (FDA���Cen- ter for Veterinary Medicine, 2005). A national survey of dairy herds (USDA/APHIS/VS/CEAH, 2005) reported that 75% of farms used intramammary DCT in all cows. The same survey reported that cephapirin was the most used drug (42% of the cows), followed by peni- cillin/dihydrostreptomycin (32%), and cloxacillin (13%). Other drugs used for DCT included penicillin/novobio- cin, novobiocin, penicillin, and erythromycin. A recent study of Wisconsin dairy herds participating in a milk quality improvement program reported that only 8% of farms were not using any form of DCT (Rodrigues et al., 2005). Antimicrobial drugs are also used to treat other infec- tious diseases of dairy cows, including respiratory and uterine infections and infectious foot disease. Com- pounds commonly used to treat foot infections include sulfonamides, ��-lactams, tetracyclines, and lincomycin (Merck Veterinary Manual, 2005). Various drugs are used for treatment of respiratory disease or metritis, including ceftiofur and other ��-lactams, tylosin, tilmi-

POL AND RUEGG 250 cosin, florfenicol, tetracyclines, and sulfadimethoxine (Merck Veterinary Manual, 2005). The use of antimicrobial drugs in dairy farming is not without controversy and there is increasing interest in the use of organic production practices (Dhar and Foltz, 2003). In the United States, animals used for production of organic products may not receive hor- mones to promote growth or antibiotics for any reason. National organic standards prohibit the withholding of necessary treatment from a sick animal but the animal or its products may not be sold as organic if the animal ever received a prohibited compound, including antimi- crobial drugs (USDA National Organic Program, 2002). One study conducted before implementation of the current national organic standards demonstrated that a few antimicrobial drugs had been used by some or- ganic farmers to treat selected diseases but not mastitis (Zwald et al., 2004). A more recent study confirmed that antimicrobial drugs are not used to treat mastitis that occurs on organic farms (Sato et al., 2005a). The objec- tive of this study was to quantify antimicrobial drug usage and treatment practices on conventional and or- ganic dairy farms that were recruited to represent a broad spectrum of potential exposure to antimicrobial drugs. MATERIALS AND METHODS Herd Selection There is no central registry of organic farms (ORG) therefore, ORG farms were contacted using a list (n = 77) of herds that had participated in a previous study (Zwald et al., 2004) or because they were known by extension agents to be certified ORG farms. All ORG farms were invited to participate in the study and all farms that met the inclusion criteria and were available for a farm visit during the study period were enrolled. Conventional farms (CON, n = 20) were recruited by extension agents familiar with the inclusion criteria. No attempt was made to randomly select the herds. A separate part of this study (Pol and Ruegg, 2007) included collection of milk samples for microbiological analysis for determination of antimicrobial susceptibil- ity. To increase the likelihood of recovering mastitis pathogens, enrollment criteria required herds to have a 6-mo average bulk tank somatic cell count (BTSCC) equal to or greater than 250,000 cells/mL. Additionally, to ensure at least one known exposure to antimicrobial drugs, CON farms were required to have used compre- hensive antimicrobial DCT for at least 5 yr. Organic farms were required to be certified organic for at least 3 yr. Journal of Dairy Science Vol. 90 No. 1, 2007 Questionnaire Data on treatment practices were obtained using an 84-question survey instrument (available on request) administered by a single individual (MP) during a farm visit. The questions included inventory and expansion status (3 questions), production (4 questions), feed addi- tives (3 questions), health records (5 questions), and prevalence and treatment practices for udder health and DCT (28 questions), clinical mastitis treatment (19 questions), respiratory disease in adult cows (7 ques- tions), metritis (7 questions), and foot infections in cows (8 questions). The questionnaire was adapted from a previously published survey (Zwald et al., 2004). To aid in identification of antimicrobial drugs, the subject answering the survey was shown 8 laminated pages containing full-color pictures of commercially available veterinary antimicrobial drugs. The questionnaire took approximately 45 to 120 min to complete and was ad- ministered during the farm visit. During the interview, the subjects were asked to esti- mate prevalence of selected diseases, the proportion of diseased animals that received treatment, and the disease-specific mortality. Case definitions for disease were farm specific and represented the farmers��� percep- tion of disease. Depending upon the question, the period for recall was either 2-mo or 1-yr periods before the visit. Estimates of disease prevalence (such as clinical mastitis, respiratory disease, metritis, and infectious foot disease) were based on recall in all farms except for 2 large farms in which estimates of prevalence included information obtained from computerized records. Estimation of Antimicrobial Drug Usage Exposure to Intramammary Dry Cow Products. Subjects reported the number of months and the propor- tion of animals treated with each intramammary prod- uct used for DCT during the previous 5 yr. The following formula was used to calculate the estimated antimicro- bial drug exposure at farm level of each compound used for DCT: AMDCTA = (4 �� DCTA) �� CDMA �� MA where AMDCTA is the total antimicrobial drug ���A��� used for DCT per farm per year, DCTA is the dose (mg or IU) contained in one syringe of compound ���A���, CDMA is the estimated number of cows dried per month with compound ���A���, and MA is the estimated number of months that the compound ���A��� was used. For the analy- sis, the 5-yr average usage of each DCT antimicrobial drug was used and the assumption was made that DCT was administered to all 4 quarters of each cow once per year.