Prevalence and antimicrobial resistance of Campylobacter in US dairy cattle

Transcription

1 Journal of Applied Microbiology ISSN ORIGINAL ARTICLE Prevalence and antimicrobial resistance of Campylobacter in US dairy cattle M.D. Englen 1, A.E. Hill 2, D.A. Dargatz 3, S.R. Ladely 1 and P.J. Fedorka-Cray 1 1 USDA-ARS, Bacterial Epidemiology and Antimicrobial Resistance Research Unit, Richard B. Russell Agricultural Research Center, Athens, GA, USA 2 Animal Population Health Institute, College of Veterinary Medicine and Biomedical Sciences, Colorado State University, Fort Collins, CO, USA 3 USDA-APHIS, Centers for Epidemiology and Animal Health, Fort Collins, CO, USA Keywords antimicrobials, Campylobacter coli, Campylobacter jejuni, dairy cattle, resistance. Correspondence Paula J. Fedorka-Cray, USDA-ARS, Bacterial Epidemiology and Antimicrobial Resistance Research Unit, Richard B. Russell Agricultural Research Center, 950 College Station Road, Athens, GA , USA. pcray@saa.ars.usda.gov Note: The mention of trade names or commercial products in this manuscript is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the US Department of Agriculture. 2006/0362: received 14 March 2006, revised 23 June 2006 and accepted 4 September 2006 doi: /j x Abstract Aims: To obtain an overview of the prevalence and antimicrobial resistance of Campylobacter in faeces of US dairy cows in Methods and Results: Faeces from 1435 cows, representing 96 dairy operations in 21 US states, were collected for the culture of Campylobacter. A total of 735 Campylobacter strains were isolated (51Æ2% positive samples) with 94 operations positive (97Æ9%) for Campylobacter. From this collection, 532 isolates (473 Campylobacter jejuni and 59 Campylobacter coli) were randomly selected for susceptibility testing to eight antimicrobials: azithromycin, chloramphenicol, ciprofloxacin, clindamycin, erythromycin, gentamicin, nalidixic acid and tetracycline. The C. jejuni isolates exhibited resistance to tetracycline (47Æ4%), nalidixic acid (4Æ0%) and ciprofloxacin (2Æ5%), while the C. coli strains exhibited some resistance to all antimicrobials except chloramphenicol and ciprofloxacin. Only 3Æ6% of the C. jejuni isolates were resistant to two or more antimicrobials but 20Æ3% of the C. coli strains were multiresistant. Conclusions: On most operations, at least one cow was positive for Campylobacter and more than half of the cows sampled were shedding Campylobacter. The C. coli isolates had significantly higher levels of resistance to macrolides and to tetracycline compared with the C. jejuni strains, but were susceptible to ciprofloxacin. Significance and Impact of the Study: This study demonstrated a high prevalence of Campylobacter on US dairy operations; however, US dairy cattle have not been recognized as a major source of human infection compared with poultry. Campylobacter coli appears to develop antimicrobial resistance more readily than C. jejuni from the same environment. Introduction Campylobacter is recognized as a major cause of acute bacterial gastroenteritis in humans worldwide, comparable with or even surpassing Salmonella (Friedman et al. 2000). In the US food-borne Campylobacter infections are estimated to be responsible for more than 2Æ0 million cases, more than hopitalizations, and 99 deaths annually (Mead et al. 1999). Typical signs of infection include abdominal cramping, fever and diarrhoea lasting several days to more than a week (Skirrow and Blaser 2000). The majority of infections are self-limiting and most individuals recover in less than a week, but up to 20% may relapse or experience prolonged or severe illness requiring antimicrobial treatment. Campylobacter jejuni and Campylobacter coli are the most frequently isolated species from cases of human infection (Engberg et al. 2000). Campylobacter is commonly found in the intestinal tract of food animals, and poultry and poultry products 1570 No claim to original US government works

2 have been well documented as a major source of Campylobacter infections in humans (Corry and Atabay 2001). Beef and dairy cattle are also carriers of Campylobacter (Stanley and Jones 2003). Although red meat is infrequently contaminated with Campylobacter, dairy products, especially raw milk, are recognized as potential sources of human Campylobacter infection (Altekruse et al. 1999; Jacob-Reitsma, 2000). Studies on the prevalence of Campylobacter in dairy cattle have been reported from Denmark (Nielsen 2002) and the UK (Atabay and Corry 1998; Stanley et al. 1998; Brown et al. 2004; Leatherbarrow et al. 2004). In the USA, Wesley et al. (2000) reported results for both Campylobacter and Arcobacter from the 1996 National Animal Health Monitoring System (NAHMS) survey. The recent study by Harvey et al. (2004) included data on the prevalence of Campylobacter in lactating dairy cows from operations in the north-east, desert south-west and Pacific west regions of the USA.These authors also tested Campylobacter isolates for resistance to 11 antimicrobials. Few other reports have included data on antimicrobial resistance in Campylobacter isolated from dairy cattle. Recent studies from the USA have included antimicrobial resistance data, although these were focused on individual states or regions (Sato et al. 2004; Bae et al. 2005; Dodson and LeJeune 2005). In this study, the prevalence and antimicrobial resistance of Campylobacter in faeces of cows from operations participating in the 2002 NAHMS survey of the health and management of US dairy cattle (USDA 2002; USDA 2005) was examined. Materials and methods Study design and bacterial isolation The NAHMS Dairy 2002 study involved 21 major US dairy states representing 82Æ8% of US dairy operations and 85Æ5% of US dairy cattle population. The participating states include: California, Colorado, Florida, Idaho, Illinois, Indiana, Iowa, Kentucky, Michigan, Minnesota, Missouri, New Mexico, New York, Ohio, Pennsylvania, Tennessee, Texas, Vermont, Virginia, Washington and Wisconsin. Samples were collected between March and September of 2002 from a total of 96 operations, each having 30 or more dairy cows. For each operation, samples were collected by rectal retrieval from approx. 40 cows in their second or higher lactation. Samples were shipped for overnight delivery to the USDA-ARS-BEAR laboratory in Athens, GA for the isolation of Campylobacter. Approximately 15 of the 40 samples from each of the 96 sample sets were tested for the presence of Campylobacter. From each sample with Campylobacter growth, a single colony was selected for resistance testing. The isolation and identification methods used for Campylobacter were the same as those previously described (Englen et al. 2005). Ninety-four of the 96 sample sets were positive for Campylobacter. Bacterial identification and antimicrobial susceptibility testing For 26 of 94 positive sample sets, all available isolates were tested for antimicrobial susceptibility. For cost reasons, five isolates (or as many as were available for sets with fewer than five isolates) were randomly chosen for antimicrobial susceptibility testing from the remaining 68 of 94 sample sets that were positive for Campylobacter. The isolates were identified to the species level using the Campylobacter BAXÒ PCR (DuPont Qualicon, Wilmington, DE, USA), a multiplex assay specific for C. coli and C. jejuni (Englen and Fedorka-Cray 2002). A total of 532 of the 735 isolates, including 473 C. jejuni and 59 C. coli, were selected for susceptibility testing to eight antimicrobials. The EtestÒ method (AB-Biodisk, Piscataway, NJ, USA) was used according to the manufacturer s directions as previously described (Englen et al. 2005). This protocol has been adopted for use by the National Antimicrobial Resistance Monitoring System (NARMS, gov/cvm/narms_pg.html). Briefly, 150 mm Mueller Hinton plates containing 5% lysed horse blood (B-D Biosciences, Sparks, MD, USA) were inoculated with 100 ll of a cell suspension equal to a 1Æ0 McFarland standard. The inoculum was swabbed evenly across the entire plate surface, and four EtestÒ strips were laid at right angles onto each plate. The plates were put into zip-top bags and incubated in a microaerobic atmosphere (5% O 2, 10% CO 2 and 85% N 2 ) for 48 h at 42 C. Following incubation, the point at which the zone of growth inhibition intersected the strip was read as the minimum inhibitory concentration (MIC) of the antimicrobial in lg ml )1. Quality control ATCC strains C. jejuni , Escherichia coli and Staphylococcus aureus were tested biweekly to confirm susceptibility to all eight antimicrobials. The antimicrobial resistance break points (MICs) used were those established by NARMS in accordance with Clinical and Laboratory Standards Institute guidelines: azithromycin, 2 lg ml )1 ; chloramphenicol, 32 lg ml )1 ; ciprofloxacin, 4 lg ml )1 ; clindamycin, 4 lg ml )1 ; erythromycin, 8 lg ml )1 ; gentamicin, 16 lg ml )1 ; nalidixic acid, 32 lg ml )1 ; tetracycline, 16 lg ml )1. Data analysis To account for potential clustering by herd and for differences in number of isolates per herd, descriptive statistics were calculated using PROC CROSSTAB in SUDAAN No claim to original US government works 1571

3 (Research Triangle Institute, Research Triangle, NC, USA), which accurately computes point estimates and standard errors for clustered data. Within PROC CROS- STAB, chi-squared tests were used to compare the proportion of resistant isolates between Campylobacter species. Differences were considered statistically significant at P <0Æ05. Resistance patterns in isolates resistant to more than one antimicrobial were identified using PROC FREQ (SAS Institute, Cary, NC, USA). Isolates were categorized as being resistant to 0 1, 2 or 3 4 antimicrobials. The distribution of C. coli and C. jejuni isolates resistant to 2 or 3 4 antimicrobials vs 0 1 antimicrobials was compared using chi-squared tests within PROC CROSSTAB in SUDAAN, with differences between species considered statistically significant at P < 0Æ05. Results Campylobacter prevalence A total of 1435 faecal samples, representing 96 dairy operations in the 21 USA states, were tested for Campylobacter. Ninety-four of the 96 operations (97Æ9%) had at least one dairy cow shedding Campylobacter. Overall, 51Æ2% (735/1435) samples were positive for Campylobacter. Campylobacter jejuni and C. coli antimicrobial resistance Of the 532 Campylobacter isolates tested, 52Æ6% (n ¼ 280) were resistant to one or more antimicrobial. When separated by species, 69Æ5% (n ¼ 41) of 59 C. coli isolates and 50Æ5% (n ¼ 239) of 473 C. jejuni isolates were resistant to one or more antimicrobial. Resistance to the individual antimicrobials by Campylobacter species is shown in Table 1. Regardless of species, resistance to tetracycline was the highest for any of the antimicrobials at 49Æ4% (n ¼ 263). The next highest resistance was to nalidixic acid at 4Æ5% (n ¼ 24), followed by ciprofloxacin at 2Æ3% (n ¼ 12) and azithromycin at 2Æ1% (n ¼ 11). Several significant differences in antimicrobial resistance between C. jejuni and C. coli were identified (Table 1). In particular, C. coli macrolide and lincosamide resistance was significantly greater than that of C. jejuni; it was 17 times higher for erythromycin (8Æ5%, n ¼ 5 vs 0Æ4%, n ¼ 2, P <0Æ05), 10 times higher for clindamycin (8Æ5%, n ¼ 5 vs 0Æ8%, n ¼ 4, P <0Æ05) and 15 times greater for azithromycin (10Æ2%, n ¼ 6 vs 1Æ1%, n ¼ 5, P <0Æ05). Significantly higher resistance for C. coli compared with C. jejuni was also observed for tetracycline (66Æ1%, n ¼ 39 vs 47Æ4%, n ¼ 224, P <0Æ05). Interestingly, C. jejuni strains had a low level of resistance to ciprofloxacin (2Æ5%, n ¼ 12) while none of the C. coli isolates were resistant to this antimicrobial. Table 1 Campylobacter isolates resistant to antimicrobials by species Antimicrobial* Campylobacter jejuni (n ¼ 473) Campylobacter coli (n ¼ 59) Total (n ¼ 532) AZ 5 (1Æ1) 6 (10Æ2) 11 (2Æ1) CL CI 12 (2Æ5) 0 12 (2Æ3) CM 4 (0Æ8) 5 (8Æ5) 9(1Æ7) EM 2 (0Æ4) 5 (8Æ5) 7(1Æ3) GM 1 (0Æ2) 2 (3Æ4) 3 (0Æ6) NA 19 (4Æ0) 5 (8Æ5) 24 (4Æ5) TC 224 (47Æ4) 39 (66Æ1) 263 (49Æ4) Values in parentheses are given in percentage. *AZ, azithromycin; CI, ciprofloxacin; CL, chloramphenicol; CM, clindamycin; EM, erythromycin; GM, gentamicin; NA, nalidixic acid; TC, tetracycline. Significantly different resistance (P < 0Æ05) compared with C. jejuni. Of the 532 isolates tested, 5Æ5% (n ¼ 29) were resistant to two or more antimicrobials and none were resistant to more than four antimicrobials (Table 2). Thirteen of these 29 strains (44Æ8%) were resistant to two antimicrobials while the majority (55Æ2%; n ¼ 16) were resistant to three or four antimicrobials. However, isolates resistant to three or four antimicrobials accounted for only 3Æ0% (n ¼ 16) of the isolates tested. A significantly higher percentage of C. coli compared with C. jejuni were resistant to two antimicrobials (10Æ2%, 6/59 vs 1Æ5%, 7/473; P ¼ 0Æ03) and to three or four antimicrobials (10Æ2%, 6/59 vs 2Æ1%, 10/473; P ¼ 0Æ04) vs 0 or 1 antimicrobial. Resistance to two antimicrobials for C. jejuni strains was observed for ciprofloxacin and nalidixic acid (n ¼ 6), and azithromycin and clindamicin (n ¼ 1). Two C. coli isolates were resistant to gentamicin and tetracycline, three Table 2 Resistance patterns of Campylobacter isolates resistant to two or more antimicrobials (n ¼ 29) No. antimicrobials Resistance pattern* No. isolates with resistance pattern Campylobacter jejuni (n ¼ 17) 2 CI NA 6 ) 2 GM TC ) 2 2 NA TC ) 3 2 CM TC ) 1 2 AZ CM 1 ) 3 AZ CM NA ) 1 3 CI NA TC 6 ) 3 AZ CM TC 2 ) 3 AZ EM TC AZ CM EM NA AZ CM EM TC ) 2 Campylobacter coli (n ¼ 12) *AZ, azithromycin; CI, ciprofloxacin; CM, clindamycin; EM, erythromycin; GM, gentamicin; NA, nalidixic acid; TC, tetracycline No claim to original US government works

4 were resistant to nalidixic acid and tetracycline, and one was resistant to clindamycin and tetracycline. Of the 10 C. jejuni isolates resistant to three or four antimicrobials, six (60Æ0%) were resistant to ciprofloxacin, nalidixic acid and tetracycline. The most frequently observed combinations of resistance for the six C. coli isolates in this category were to azithromycin, clindamycin and tetracycline (n ¼ 2), and azithromycin, clindamycin, erythromycin and tetracycline (n ¼ 2). Significantly (P ¼ 0Æ04), five times more C. coli were resistant to three or four antimicrobials compared to the C. jejuni strains (10Æ2%, 6/59 vs 2Æ1%, 10/473). Discussion In this study, the prevalence and antimicrobial resistance in Campylobacter from the US NAHMS Dairy 2002 survey (USDA 2002) was examined. Although a number of studies on the prevalence of Campylobacter in cattle have been reported (reviewed in Stanley and Jones 2003), much less attention has been given to antimicrobial resistance in faecal Campylobacter from dairy cattle. Campylobacter was isolated from faeces from 97Æ9% (n ¼ 94) of the 96 operations visited, and 51Æ2% (735/1435) of the cows sampled were positive for Campylobacter. Wesley et al. (2000) reported a lower animal-level prevalence of 37Æ7% for C. jejuni and 1Æ8% for C. coli for the 1996 NA- HMS survey of US dairy cows, and these values are similar to the 31Æ2% for C. jejuni and 5Æ8% C. coli reported in dairy cattle from Washington state (Bae et al. 2005). In contrast, a recent study of Campylobacter in healthy US lactating dairy cows found mean prevalence values of only 5Æ2% for farms in the desert south-west, 2Æ9% for the north-east and 5Æ0% for the Pacific west (Harvey et al. 2004). This is similar to an overall prevalence of 7Æ3% for C. jejuni from cull dairy cows in the state of Ohio (Dodson and LeJeune 2005). Differences in study designs, sampling methods, sample handling and isolation methods used probably contribute to the variability among prevalence studies on Campylobacter in dairy cattle.these differences may also reflect seasonal differences in the carriage of Campylobacter in dairy herds. For example, in the state of Wisconsin, Sato et al. (2004) found 36Æ8% of cows positive for Campylobacter in March while only 18Æ9% were positive in September. This seasonal periodicity in the shedding rate of Campylobacter in cattle and calves has been demonstrated in the UK, with peaks occurring in the spring and autumn (Stanley et al. 1998). Although the results from this study were higher than those previously reported, seasonal periodicity likely contributed to the prevalence observed in our study. Additional factors contributing to the prevalence of Campylobacter may also include the geographical distribution and production practices of the dairy operations. The age of the animals sampled has also been shown to be a factor in prevalence of Campylobacter from cattle. Newborn calves become colonized within 4 days, with maximal Campylobacter shedding occurring at 1 2 months of age, then declining until animals are 7 months or older (Stanley and Jones 2003). Among all antimicrobials tested, resistance to tetracycline was the highest (49%). Tetracyclines are often included in milk replacers for young calves and used when treating individual animals with reproductive disorders or lameness on US dairy operations (USDA 2005), and high levels of tetracycline resistance are often seen in Campylobacter from food animal production (Avrain et al. 2003; Ge et al. 2003; Ishihara et al. 2004). Among Campylobacter species, tetracycline resistance in C. coli was significantly higher compared with C. jejuni (66Æ1% vs 47Æ4%). These numbers are quite similar to those we recently reported for tetracycline resistance in Campylobacter from feedlot cattle: 51Æ6% overall resistance, 65Æ7% for C. coli and 49Æ1% for C. jejuni (Englen et al. 2005). Other recent US studies found somewhat lower overall levels of tetracycline resistance in Campylobacter, including 44% from lactating dairy cows (Harvey et al. 2004), 45% from organic and conventional dairy herds (Sato et al. 2004), 42Æ3% from cattle farms (Bae et al. 2005) and 36% from cull dairy cows (Dodson and LeJeune 2005). A recent European study found much lower levels of tetracycline resistance in C. coli (41Æ2%) and C. jejuni (19Æ9%) isolated from cattle (Bywater et al. 2004). This may be a consequence of more limited use of tetracycline in the participating countries (Germany, Italy and the UK) and the susceptibility test method used (agar dilution). However, the Etest has been shown to produce results for Campylobacter comparable with other standard methods including agar dilution (Luber et al. 2003; Oncul et al. 2003). Macrolides/lincosamides are used infrequently in the US dairy cattle to treat respiratory disease and mastitis (USDA 2005). Overall resistance to any of the three macrolide/lincosamide drugs included in this study (azithromycin, clindamycin and erythromycin) did not exceed 2Æ1% (11/532). However, the C. coli isolates were times more likely to be resistant to macrolides compared with the C. jejuni strains. In US feedlot cattle, the macrolides tilmicosin and tylosin are frequently used therapeutics, and tylosin is also commonly added to feed (USDA 2000). We recently reported that the overall macrolide resistance in Campylobacter from the US feedlot cattle was 1Æ1% or less and no higher than 3Æ0% for erythromycin resistance in C. coli strains, despite much greater usage of these agents in feedlot cattle (Englen et al. 2005). Other recent US studies found no macrolide No claim to original US government works 1573

5 resistance in Campylobacter from dairy cattle (Sato et al. 2004; Dodson and LeJeune 2005). In contrast, Bae et al. (2005) reported 8Æ6% overall resistance to erythromycin in Campylobacter from cattle in Washington state, and 2Æ9% and 31Æ8% erythromycin resistance for C. jejuni and C. coli respectively. Regional differences in the use of macrolides in cattle in the US may account for some of the variation in the results among recent reports, as well as differences in susceptibility test methods used. Nonetheless, several studies have also reported high levels of macrolide resistance in C. coli from swine (Aarestrup et al. 1997; Saenz et al. 2000; Van Looveren et al. 2001; Bywater et al. 2004), supporting the interpretation that C. coli acquires antimicrobial resistance more readily than C. jejuni. Fluoroquinolones are not approved for use in dairy cattle in the USA. However, a low level of resistance to ciprofloxacin was observed with the C. jejuni strains (2Æ5%, n ¼ 12), although no ciprofloxacin resistance was observed for C. coli. Interestingly, resistance to nalidixic acid was almost twice that of ciprofloxacin overall (4Æ5% vs 2Æ3%), and resistance to this quinolone was twofold greater in C. coli (8Æ5%, n ¼ 5) compared with C. jejuni (4Æ0%, n ¼ 19). We also recently reported a twofold difference in resistance to nalidixic acid in C. coli (22Æ4%) compared with C. jejuni (10Æ2%) from the US feedlot cattle (Englen et al. 2005). Fluoroquinolones are approved for use in the US beef cattle which may account for the higher levels of nalidixic acid resistance reported in Campylobacter from this livestock source. In that study (Englen et al. 2005), we also found greater resistance to ciprofloxacin in C. coli compared with C. jejuni (9Æ0% vs 1Æ8%). The levels of ciprofloxacin resistance in C. jejuni in the present study and from feedlot cattle are comparable (2Æ5% and 1Æ8% respectively), and very similar to the reported 3Æ0% fluoroquinolone resistance in C. jejuni from Danish cattle (Aarestrup et al. 1997). The study by Bywater et al. (2004) found higher resistance to ciprofloxacin in C. coli and C. jejuni from cattle (23Æ5% and 13Æ5% respectively), again perhaps owing in part to differences in drug use practices, the susceptibility test method or other factors. Resistance to quinolones typically involves discrete point mutations in the gyra gene, which encodes DNA gyrase (a type II topoisomerase) (Drlica and Zhao 1997). The Thr86-Ile substitution in Campylobacter gyra confers cross-resistance to both ciprofloxacin and nalidixic acid (Wang et al. 1993; Gibreel et al. 1998), and all 12 of the ciprofloxacin resistant C. jejuni isolates were also resistant to nalidixic acid. Twelve isolates (seven C. jejuni and five C. coli), comprising 50% of the 24 quinolone-resistant strains found in this study, were resistant to nalidixic acid but did not show cross-resistance to ciprofloxacin. Bachoual et al. (2001) noted that the Thr86-Ala gyra substitution in C. jejuni resulted in resistance to nalidixic acid (MIC 64 lg ml )1 ) but not to ciprofloxacin (MIC 2 lg ml )1 ). Differences in gyra mutations would explain the observed differences for ciprofloxacin and nalidixic acid resistance. However, as previously noted, the dairy cows from which the Campylobacter were isolated in this study were presumably not exposed to these drugs. The observed low levels of quinolone resistance may therefore be the result of naturally occuring resistance in these Campylobacter populations, which has been reported in nonselected soil bacteria due to sequence variation in gyra (Waters and Davies 1997). A recent report found only 0Æ6% ciprofloxacin resistance in Campylobacter from dairy herds in the state of Wisconsin, and these authors also speculated that the two resistant isolates identified in their study arose by point mutations (Sato et al. 2004). Chloramphenicol use in the US livestock production has been banned for many years and no resistance to this drug was observed in this study. Conversely, we found very low levels of resistance to this antimicrobial in Campylobacter from feedlot cattle (Englen et al. 2005). Similarly, aminoglycosides are not generally used to treat beef or dairy cattle in the USA (USDA 2000, 2005), owing in part to toxicity issues and the long withdrawal period. Only 0Æ6% (n ¼ 3) of the Campylobacter strains included in this study were resistant to gentamicin, comparable with the 0Æ2% (n ¼ 1) observed in Campylobacter from feedlot cattle (Englen et al. 2005). Multiple resistance (isolates resistant to two or more antimicrobials) was observed more than five times as often in C. coli strains (20Æ3%; 12/59) compared with C. jejuni (3Æ6%; 17/473). Resistance to three or more antimicrobials was found in 3Æ0% (16/532) of all isolates (Table 2). Again, resistance in this category was five times more common in C. coli (10Æ2%; 6/59) compared with C. jejuni (2Æ1%; 10/473). Bae et al. (2005) found 10 times more multiple drug resistant C. coli (51Æ5%) than C. jejuni (5Æ1%) isolates from cattle. Several patterns were apparent among the multiresistant isolates. Six of the seven C. jejuni strains resistant to two antimicrobials had the pattern ciprofloxacin and nalidixic acid (Table 2). In addition, six of 10 C. jejuni isolates resistant to three or more antimicrobials combined ciprofloxacin, nalidixic acid and tetracycline. Interestingly, none of the C. coli strains in this study were resistant to ciprofloxacin. Thus, the apparent association between ciprofloxacin and tetracycline resistance in Campylobacter observed in previous studies, including our recent report on feedlot cattle (Piddock 1995; Gaunt and Piddock 1996; Englen et al. 2005), was not seen with Campylobacter from dairy cows in this study. Six C. coli isolates were resistant to three or more antimicrobials and all resistance patterns of these strains 1574 No claim to original US government works

6 included at least two macrolides (Table 2). Four C. jejuni isolates also shared this characteristic pattern. All 10 strains were resistant to azithromycin, suggesting that resistance to either clindamycin or erythromycin confers cross-resistance to azithromycin. However, resistance to erythromycin did not confer cross-resistance to the lincosamide clindamycin and vice versa. Point mutations in the 23S rdna is the primary mechanism conferring macrolide resistance in Campylobacter (Engberg et al. 2001; Gibreel et al. 2005; Vacher et al. 2005), and specific combinations of these mutations likely determine the observed macrolide resistance phenotypes. Antimicrobial susceptibility monitoring programs such as NARMS in the USA ( pg.html) and DANMAP in Denmark (Bager 2000) have provided valuable information on antimicrobial resistance patterns among food-borne bacterial pathogens. These data have been used to assess risks associated with antimicrobial use in agriculture, with the ultimate goal of prolonging the useful lifespan of approved antimicrobials and identifying areas requiring more detailed studies. As we found with Campylobacter from feedlot cattle (Englen et al. 2005), generally low levels of antimicrobial resistance were observed in Campylobacter isolated from dairy cows. The study by Bywater et al. (2004) also reported lower levels of antimicrobial resistance in Campylobacter from cattle compared with isolates from broilers and swine carcasses. Resistance to tetracycline is an exception, and a high prevalence of tetracycline resistance is often found in Campylobacter from food animals (e.g. Saenz et al. 2000; Van Looveren et al. 2001; Bywater et al. 2004). Some of the reported differences in antimicrobial resistance in Campylobacter from various food animal sources may be a consequence of susceptibility test methods used, different drug use practices, or other unmeasured differences in management or exposures. Even so, monitoring antimicrobial resistance in bacterial pathogens from food animals remains an essential component in the development of effective antimicrobial use practices in food animal production. Acknowledgements The authors thank Sandra House, Leena Jain, and Jodie Plumblee of the Bacterial Epidemiology and Antimicrobial Resistance Research Unit for excellent technical assistance. References Aarestrup, F.M., Nielsen, E.M., Madsen, M. and Engberg, J. (1997) Antimicrobial susceptibility patterns of thermophilic Campylobacter spp. from humans, pigs, cattle, and broilers in Denmark. Antimicrob Agents Chemother 41, Altekruse, S.F., Stern, N.J., Fields, P.I. and Swerdlow, D.L. 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