Abstract
Diagnostic laboratory antimicrobial susceptibility data for bacteria isolated from clinical samples of cattle, sheep, and goats from 1994 to 2013 were evaluated retrospectively. Among bacteria from bovine mastitis, Staphylococcus aureus and Streptococcus uberis were the most commonly isolated organisms. Pasteurella multocida, Mannheimia haemolytica, and Histophilus somni were commonly isolated from the respiratory tract, while Escherichia coli isolates were frequently recovered from the intestinal tract. Isolates from mastitis were generally highly susceptible to the antimicrobials tested, except neomycin and oxytetracycline. Respiratory tract isolates were highly susceptible to trimethoprim-sulfamethoxazole, penicillin, florfenicol, and ceftiofur, while enteric bacteria were frequently susceptible to ceftiofur. Antimicrobial resistance trends over the study period were generally stable for small ruminant and cattle isolates. Multi-drug resistance was more common among respiratory isolates from small ruminants compared to those from cattle but more common in enteric bacteria from cattle compared to those from small ruminants. This information may guide clinicians when they are choosing empirical therapies for the treatment of ruminant animals in Atlantic Canada.
Résumé
Résistance antimicrobienne des bactéries respiratoires et entériques et de la mammite isolées des ruminants dans les provinces de l’Atlantique de 1994 à 2013. Les données de susceptibilité antimicrobienne des laboratoires diagnostiques, de 1994 à 2013, pour les bactéries isolées d’échantillons cliniques de bovins, de moutons et de chèvres, ont été évaluées rétrospectivement. Parmi les bactéries trouvées en lien avec la mammite bovine, Staphylococcus aureus et Streptococcus uberis ont été les organismes les plus communément isolés. Pasteurella multocida, Mannheimia haemolytica et Histophilus somni ont été communément isolés de l’appareil respiratoire, tandis que des isolats d’Escherichia coli étaient fréquemment récupérés du tractus intestinal. Les isolats de la mammite étaient généralement hautement susceptibles aux antimicrobiens testés, sauf pour la néomycine et l’oxytétracycline. Les isolats de l’appareil respiratoire étaient hautement susceptibles au triméthoprime-sulfaméthoxazole, à la pénicilline, au florfénicol et au ceftiofur, tandis que les bactéries entériques étaient fréquemment susceptibles au ceftiofur. Les tendances d’antibiorésistance pendant la période de l’étude étaient généralement stables pour les isolats des petits ruminants et des bovins. La multirésistance aux médicaments était plus commune pour les isolats respiratoires provenant des petits ruminants comparativement à ceux des bovins et plus commune dans les bactéries entériques des bovins par rapport à celles des petits ruminants. Ces données pourront guider les cliniciens qui choisissent des thérapies empiriques pour le traitement des ruminants dans les provinces de l’Atlantique.
(Traduit par Isabelle Vallières)
Introduction
Antimicrobials are used in food animals for treatment and prevention of bacterial diseases (1). However, there is considerable evidence that antimicrobial use in food animal production selects for resistance in commensal and pathogenic bacteria (2). This is a growing concern in veterinary and human health. Development of antimicrobial resistance (AMR), including multi-drug resistance, in bacteria in food-producing species is a particular concern as these organisms may be transferred in food to humans and also may serve as a reservoir for AMR genes. These resistant bacteria may lead to treatment failures in humans and animals and to increased costs of medical care and animal production (3).
For veterinarians, clinical microbiologists, public health officials, and government agencies, knowledge of the common bacterial causes of infection in food animal species and their antimicrobial resistance patterns is important in selecting optimal empirical and pathogen-specific therapy, guiding treatment protocols, and developing policy (4,5). Antimicrobial susceptibility patterns and bacterial distribution vary from herd to herd and from one region to another (4,5). It is therefore important to acquire regional or local data (6). Additionally, surveillance is important in identifying changes in AMR patterns as well as recognizing new or emerging bacterial diseases among food animals.
There is a lack of published data on the dynamics of antimicrobial resistance in bacteria from food animals, particularly over a long-term period. The objectives of this study were to identify the most commonly isolated bacterial species and their antimicrobial susceptibility from clinical samples from cattle, sheep, and goats submitted to a veterinary diagnostic laboratory in Atlantic Canada and to determine trends in their antimicrobial susceptibilities over a 20-year period.
Materials and methods
Antimicrobial susceptibility data for bacteria isolated from clinical samples from cattle, sheep, and goats from 1994 to 2013 were retrieved from the database of the Atlantic Veterinary College Diagnostic Services Bacteriology Laboratory (AVCDBL) in Charlottetown, Prince Edward Island. For each bacterial isolate, the host animal species was the only information available. Other data, such as antimicrobial treatment, age, gender, and breed were not available. From each clinical sample, the bacteria were isolated using standard clinical microbiological isolation techniques, and antimicrobial susceptibility was determined by the Kirby-Bauer disk diffusion method. Zones of inhibition were interpreted following Clinical and Laboratory Standards Institute standards (7). When CLSI zones of inhibition for bacteria-antimicrobial combinations in a particular host species were not available, the zones of inhibition for other animal species, humans, or different bacterial species were used. For bacteria isolated from mastitic milk samples from dairy cattle, antimicrobial susceptibility was determined to the following antimicrobials: ceftiofur, cephalexin, cloxacillin, oxytetracycline, penicillin-novobiocin, pirlimycin, neomycin, and trimethoprim-sulfamethoxazole. For bacteria isolated from the respiratory and intestinal tracts, susceptibility testing included the following antimicrobials: ceftiofur, erythromycin, oxytetracycline, penicillin, streptomycin, florfenicol, tilmicosin, and trimethoprim-sulfamethoxazole.
Data management and statistical analyses
Bacterial isolates and their antimicrobial susceptibility profiles were selected from the AVCDBL database. Only bacteria that commonly cause clinical disease in ruminant animals were selected. Data were tabulated using a computerized spreadsheet (Microsoft Excel, 2010). Antimicrobial susceptibility was presented as a proportion and only acquired resistance was reported (8,9). A bacterial isolate that was resistant to at least 1 antimicrobial in at least 3 antimicrobial classes was categorized as multi-drug resistant (MDR) (10).
Logistic regression was used to detect the increased or decreased antimicrobial resistance trend over time. Year was modeled as a continuous variable, and antimicrobial resistance (yes/no) for each bacterial-antimicrobial combination was the binary outcome. Linear relationship assumption between the year and the log odds of resistance was examined for each bacterium-antimicrobial combination by fitting quadratic polynomial for the year. The antimicrobial resistance trends were presented as odds ratios (OR). An OR > 1 indicated an increased AMR trend over the study period, while an OR < 1 represented a decreased AMR trend over the study period (11). The Wald test was used to determine the statistical significance of each bacterial species-antimicrobial trend. The level of statistical significance was P ≤ 0.05. All statistical analyses were performed using Stata 14 (StataCorp, College Station, Texas, USA).
Results
Antimicrobial resistance in bacteria from mastitis in dairy cattle
Staphylococcus aureus (n = 1532), Streptococcus uberis (n = 1171), and Escherichia coli (n = 716) were the most commonly isolated bacteria from mastitic milk samples. Overall, most of the mastitis isolates were susceptible (90% to 100%) to all the antimicrobials tested (Table 1). Escherichia coli showed reduced susceptibility to cephalexin (43.3%) and neomycin (62.5%); 70% to 90% of all the isolates were susceptible to oxytetracycline, while 96% of S. aureus isolates were susceptible to oxytetracycline. Among Gram-positive bacterial isolates 90% to 98% were susceptible to pirlimycin, except for S. uberis, for which only 82% of isolates were susceptible. Multi-drug resistance was not common among the mastitis bacterial pathogens; it was most observed in E. coli (12.7%) and least observed in both S. dysgalactiae and S. aureus (0.3%).
Table 1.
Antimicrobial susceptibilities in bacteria from mastitis in dairy cattle over a 20-year period.
| Susceptible % (95% CI) | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
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| Mastitis pathogens | N | CEFa | CEX | CXN | NEO | OXY | PIRa | PNNa | TMS | MDR (%) |
| Escherichia coli | 716 | 97.4 (95.9 to 98.3) | 43.3 (39.7 to 46.9) | IR | 62.5 (58.8 to 65.9) | 75.5 (72.2 to 78.5) | IR | IR | 93.7 (91.7 to 95.3) | 12.7 (10.5 to 15.4) |
| Klebsiella spp. | 200 | 96.5 (92.8 to 98.3) | 90.5 (85.6 to 93.9) | IR | 79.1 (72.9 to 84.2) | 71.1 (64.4 to 77.0) | IR | IR | 92.0 (87.4 to 95.1) | 4.0 (2.0 to 7.8) |
| Coagulase negative staphylococci | 413 | 99.0 (97.4 to 99.6) | 98.7 (97.1 to 99.5) | 99.3 (97.8 to 99.7) | 96.1 (93.8 to 97.6) | 82.6 (78.6 to 85.9) | 89.8 (86.5 to 92.4) | 99.5 (98.1 to 99.9) | 95.6 (93.2 to 97.2) | 2.4 (1.3 to 4.4) |
| Staphylococcus aureus | 1532 | 99.9 (99.5 to 99.9) | 99.6 (99.1 to 99.8) | 99.8 (99.3 to 99.9) | 81.9 (79.9 to 83.8) | 95.8 (94.7 to 96.7) | 98.1 (97.3 to 98.7) | 99.7 (99.3 to 99.9) | 99.5 (99.0 to 99.7) | 0.3 (0.1 to 0.7) |
| Streptococcus agalactiae | 56 | 98.2 (87.7 to 99.8) | 96.4 (86.3 to 99.1) | 98.2 (87.7 to 99.8) | IR | 85.7 (73.5 to 92.9) | 96.4 (86.3 to 99.1) | 100.0 | 100.0 | 1.8 (0.2 to 11.8) |
| Streptococcus dysgalactiae | 317 | 100.0 | 99.6 (97.8 to 99.9) | 99.7 (97.7 to 99.9) | IR | 75.1 (70.0 to 79.6) | 92.1 (88.6 to 94.6) | 99.7 (97.7 to 99.9) | 99.7 (97.7 to 99.9) | 0.3 (0.1 to 2.2) |
| Streptococcus uberis | 1171 | 96.8 (95.7 to 97.7) | 99.2 (98.5 to 99.6) | 97.6 (96.6 to 98.3) | IR | 81.3 (78.9 to 83.4) | 82.1 (79.8 to 84.1) | 99.7 (99.2 to 99.9) | 95.1 (93.7 to 96.2) | 1.7 (1.1 to 2.6) |
Available CLSI zone diameters.
CEF — Ceftiofur; CEX — Cephalexin; CXN — Cloxacillin; NEO — Neomycin; OXY — Oxytetracycline; PIR — Pirlimycin; PNN — Penicillin-novobiocin; TMS — Trimethoprim-sulfamethoxazole; IR —i ntrinsic resistance; CI — confidence interval.
The trends of antimicrobial resistance over the study period were stable for most bacteria and drug combinations. Significantly increased AMR trends (P < 0.05) were observed for trimethoprim-sulfamethoxazole and E. coli [OR = 1.06, 95% confidence interval (CI): 1.01 to 1.12], oxytetracycline and S. aureus (OR = 1.06, 95% CI: 1.01 to 1.11) and oxytetracycline and S. uberis (OR = 1.10, 95% CI: 1.07 to 1.13). A high proportion of E. coli isolates were resistant to cephalexin and neomycin, but the resistance to cephalexin (OR = 0.97, 95% CI: 0.94 to 0.99) and neomycin (OR = 0.73, 95% CI: 0.70 to 0.77) significantly decreased (P < 0.05) during the 20-year period. Significant decreased resistance trends (P < 0.05) were observed in coagulase-negative staphylococci (OR = 0.76, 95% CI: 0.65 to 0.89) and S. aureus (OR = 0.79, 95% CI: 0.76 to 0.82) to neomycin, and in S. uberis to pirlimycin (OR = 1.04, 95% CI: 1.01 to 1.07) and trimethoprim-sulfamethoxazole (OR = 0.91, 95% CI: 0.86 to 0.96).
Antimicrobial resistance in respiratory bacteria from ruminants
In cattle, 87%, 84%, and 77% of the total Pasteurella multocida (n = 238), Mannheimia haemolytica (n = 187), and Histophilus somni (n = 87) isolates, respectively, were recovered from the respiratory tract over the study period. In small ruminants, 71.4% and 73.8% of M. haemolytica (n = 133), and P. multocida (n = 42), respectively, were isolated from the respiratory tract.
Antimicrobial susceptibilities among these bacteria are presented in Table 2. In both cattle and small ruminants, M. haemolytica and P. multocida were frequently resistant to erythromycin and streptomycin, but were frequently susceptible (susceptibility ranged from 94% to 100%) to ceftiofur, oxytetracycline, penicillin, trimethoprim-sulfamethoxazole, and florfenicol. However, susceptibility was relatively less frequent to tilmicosin (susceptibility ranged from 62% to 85%) in the respiratory isolates from cattle and small ruminants as well as to trimethoprim-sulfamethoxazole in H. somni (86%) from cattle.
Table 2.
Antimicrobial susceptibilities in respiratory bacteria isolated from ruminants over a 20-year period.
| Susceptible % (95% CI) | |||||
|---|---|---|---|---|---|
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| Cattle | Small ruminants | ||||
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| Antimicrobials | Mannheimia haemolytica (n = 187) | Pasteurella multocida (n = 238) | Histophilus somni (n = 87) | Mannheimia haemolytica (n = 133) | Pasteurella multocida (n = 42) |
| Ceftiofura | 98.9 (95.8 to 99.7) | 98.7 (96.1 to 99.6) | 95.4 (88.3 to 98.3) | 100.0 | 97.6 (84.5 to 99.7) |
| Erythromycin | 47.1 (39.7 to 54.5) | 42.0 (34.5 to 50.0) | 89.1 (77.2 to 95.1) | 18.4 (12.5 to 26.3) | 26.8 (15.1 to 43.0) |
| Oxytetracycline | 90.9 (85.7 to 94.3) | 94.5 (90.7 to 96.8) | 85.1 (75.7 to 91.2) | 91.6 (85.7 to 95.3) | 95.2 (81.9 to 98.9) |
| Penicillin | 94.1 (89.5 to 96.7) | 97.9 (95.0 to 99.1) | 96.6 (89.6 to 98.9) | 97.7 (93.0 to 99.2) | 97.6 (83.8 to 99.7) |
| Streptomycin | 12.0 (7.9 to 17.6) | 23.2 (18.2 to 29.1) | 48.3 (37.8 to 58.9) | 16.2 (10.7 to 23.6) | 33.3 (20.4 to 49.4) |
| Trimethoprim-sulfamethoxazole | 97.3 (93.7 to 98.9) | 99.2 (96.7 to 99.8) | 86.2 (77.0 to 92.1) | 100.0 | 100.0 |
| Florfenicola | 100.0 | 100.0 | 100.0 | 99.2 (94.4 to 99.9) | 100.0 |
| Tilmicosina | 75.3 (68.3 to 81.1) | 85.1 (79.7 to 89.2) | 74.4 (63.6 to 82.8) | 76.1 (67.1 to 83.3) | 61.5 (44.8 to 75.9) |
| Multi-drug resistant | 17.1 (12.4 to 23.2) | 11.8 (8.2 to 16.5) | 14.9 (8.8 to 24.1) | 22.6 (16.2 to 30.5) | 38.1 (24.6 to 53.7) |
Available CLSI zone diameters.
Multi-drug resistance was more common among the respiratory isolates from small ruminants compared with those from cattle. The frequency of MDR bacteria was higher in M. haemolytica (17.1%) than in H. somni (14.9%) and P. multocida (11.8%) in cattle. In small ruminants, MDR bacteria were more common in P. multocida (38.1%) than in M. haemolytica (22.6%). The antimicrobial resistance trends in respiratory isolates from ruminants over the study period were stable, with the exception of a significant increased resistance trend (P < 0.05) in P. multocida to erythromycin (OR = 1.15, 95% CI: 1.06 to 1.23).
Antimicrobial resistance in Escherichia coli and Salmonella spp
In cattle, 85.4% of the total E. coli (n = 489) isolates and 71.4% of the total Salmonella spp. isolates (n = 21) were recovered from the gastrointestinal tract with the remaining isolates recovered from blood, umbilicus, abscess, and respiratory tract. In small ruminants, 83.8% of the E. coli isolates (n = 74) and 5 Salmonella isolates were recovered from the gastrointestinal tract. A low proportion of ruminant isolates was susceptible to streptomycin and oxytetracycline. A high proportion of E. coli and Salmonella spp. isolates was susceptible to ceftiofur (Table 3). Multi-drug resistance was more common in enteric bacteria from cattle compared to enteric bacteria in small ruminants. The antimicrobial resistance trends in the enteric bacteria were stable over the study period, except for significant increased resistance trends (P < 0.05) in E. coli from cattle to ceftiofur (OR = 1.08, 95% CI: 1.04 to 1.13), trimethoprim-sulfamethoxazole (OR = 1.05, 95% CI: 1.01 to 1.08) and florfenicol (OR = 1.16, 95% CI: 1.10 to 1.23). Significant decreased resistance (P < 0.05) to streptomycin in E. coli was observed in both cattle (OR = 0.94, 95% CI: 0.90 to 0.99) and small ruminants (OR = 0.78, 95% CI: 0.70 to 0.87) over the study period.
Table 3.
Antimicrobial susceptibilities in enteric bacteria from ruminants over a 20-year period.
| Susceptible % (95% CI) | ||||
|---|---|---|---|---|
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|
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| Cattle | Small ruminants | |||
|
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| Antimicrobials | Escherichia coli (n = 489) | Salmonella spp. (n = 21) | Escherichia coli (n = 74) | Salmonella spp. (n = 5) |
| Ceftiofur | 86.4 (82.9 to 89.2) | 95.0 (67.7 to 99.4) | 95.8 (87.6 to 98.6) | 100.0 |
| Erythromycin | IR | IR | IR | IR |
| Oxytetracycline | 36.8 (17.3 to 61.9) | 36.8 (17.3 to 61.9) | 67.1 (55.3 to 77.1) | 100.0 |
| Penicillin | IR | IR | IR | IR |
| Streptomycin | 10.4 (7.9 to 13.6) | 11.1 (2.4 to 38.9) | 31.0 (21.1 to 42.9) | 0.0 |
| Trimethoprim-sulfamethoxazole | 51.6 (47.2 to 56.1) | 95.2 (69.1 to 99.4) | 95.9 (87.9 to 98.7) | 100.0 |
| Florfenicol | 72.0 (66.5 to 76.8) | 53.3 (26.6 to 78.3) | 88.1 (76.7 to 94.4) | 100.0 |
| Tilmicosin | IR | IR | IR | IR |
| Multi-drug resistant | 48.7 (44.3 to 53.1) | 33.3 (16.5 to 55.9) | 5.4 (2.0 to 13.7) | 0.0 |
IR — intrinsic resistance.
Discussion
Retrospective antimicrobial susceptibility studies provide helpful information to veterinarians in the selection of empirical antimicrobial therapy (6). This study examined the antimicrobial susceptibility patterns and trends in selected bacteria from clinical samples from cattle and small ruminants using data from a regional veterinary diagnostic laboratory over a 20-year period. Compared to previous studies, this study evaluated AMR data and trends from ruminant bacterial pathogens over a relatively long period (6,11,12).
Staphylococcus aureus was the most frequently isolated agent of bovine mastitis in this study and a high proportion of the isolates were susceptible to all the antimicrobials tested, including penicillin, which is comparable to other published studies (12–15). Streptococcal isolates were highly susceptible to all antimicrobials except oxytetracycline. β-lactams are the recommended antimicrobials for the treatment of intra-mammary infections caused by Streptococcus agalactiae and environmental streptococci (16,17). The proportion of mastitis pathogens that were MDR was low. Most isolates with multi-drug resistance were Gram-negative organisms, especially E. coli, which is similar to previous findings from a Canada-wide study (18). Apart from the therapeutic implications of MDR E. coli, the bacteria could also play a potential role in the transmission of antimicrobial resistance genes to other udder pathogens (16).
Resistance trends in most antimicrobial and mastitis pathogen combinations were stable. In some instances, resistance decreased over the 20-year period, although most of the antimicrobials have been used to treat mastitis for decades (17). This stable trend is consistent with previous studies in North America (18–21). Increased resistance trends for trimethoprim-sulfamethoxazole and E. coli as well as oxytetracycline and S. aureus and S. uberis were found in this study. This increased resistance trend to both antimicrobials for these bacteria could be because these antimicrobial drugs are commonly used for systemic treatments of dairy cattle on Canadian dairy farms (17). The findings from our study support the evidence presented by the National Mastitis Council’s Expert Group that antimicrobial resistance in mastitis pathogens is uncommon with little evidence of increasing resistant trends (13).
Respiratory isolates from cattle, sheep, and goats and their antimicrobial susceptibility patterns were similar over the study period. However, AMR, as well as MDR bacteria trends over the study period, changed more in cattle compared to small ruminants. This may be because of differences in antimicrobial usage patterns in cattle compared to small ruminants. Dairy and beef cattle are more likely to have bacterial disease challenges, possibly due to management choices, such as increased animal stocking density, and are more frequently exposed to antimicrobial drugs compared to sheep and goats (22).
Respiratory diseases are a major cause of morbidity and mortality in dairy, beef, and small ruminant production systems (23–25). Mannheimia haemolytica, P. multocida, and H. somni were the most common respiratory pathogens isolated in this study. While P. multocida was frequently isolated from cattle, M. haemolytica was more common in small ruminants. Histophilus somni was the least common respiratory pathogen in cattle and was not isolated in small ruminants. These 3 organisms are commonly isolated from cases of bovine respiratory disease complex (26). A high proportion of these bacteria was susceptible to ceftiofur, florfenicol, oxytetracycline, trimethoprim-sulfamethoxazole and penicillin in both cattle and small ruminants. Similar antimicrobial susceptibility patterns have been reported in other studies in North America (5,26–28) and in Europe (29). However, ceftiofur should be reserved for situations in ruminant respiratory diseases in which other antimicrobial options are not available. Third generation cephalosporins are critically important in human medicine (30), but ceftiofur is a third generation cephalosporin that is approved for both systemic and intramammary treatment in cattle in Canada (17).
While there was variation in proportions of isolates that were susceptible to the antimicrobials tested among both cattle and small ruminant bacteria, most antimicrobial resistance trends over the course of the study were stable. However, an increased antimicrobial resistance trend was observed in P. multocida to erythromycin and is in agreement with a similar older study over a 4-year period (1988–1992) in Michigan (26). Conversely, a study from Oklahoma (27) reported a decreased resistance trend to erythromycin in both M. haemolytica and P. multocida over a 9-year period (1994–2002).
The multi-drug resistance in M. haemolytica and P. multocida in this study is consistent with the findings of a similar study from Kansas State University, USA (5). The development of multi-drug resistance in both bacteria is reported to be mediated by small plasmid-derived DNA and conjugative and nonconjugative transposons (31). Recently a chromosomal-based mobile genetic element, referred to as an integrative conjugative element, was associated with multi-drug resistance in North American isolates (32). The development of multi-drug resistance in either bacteria would be economically challenging to ruminant animal production systems within this region, as respiratory disease complex is a major disease of both cattle and small ruminants (28).
Escherichia coli was the most commonly isolated enteric organism from both cattle and small ruminants in this study, which is consistent with other studies (33,34). Escherichia coli can be a primary pathogen, a commensal, or a cause of coinfections with other bacteria and viruses. Furthermore, some types of E. coli are important food-borne pathogens (35). Enterotoxigenic strains of E. coli are a primary bacterial cause of calf scours, while other strains are major causes of extraintestinal infections in humans and animals (36). In this study, enteric bacteria were frequently resistant to oxytetracycline and streptomycin, which is similar to findings in Europe and North America (6,35,37), and may be due to the prescribing practices and administration of these antimicrobials in ruminant animal production in these regions.
A decreasing trend of resistance to streptomycin in E. coli from cattle and small ruminants was found in our study. This may be because streptomycin is unavailable for use in Canada. Since use of streptomycin in dairy production was prohibited in Canada 3 decades ago, the selective pressure exerted by streptomycin is declining (35). Increased AMR trends were seen to ceftiofur and florfenicol in E. coli. This is different from the stable or decreased AMR trends to ceftiofur and florfenicol reported from 2 studies from the United States (35,38). The increased AMR trend to ceftiofur found in this study in E. coli warrants continued monitoring.
The high proportion of MDR E. coli isolates from ruminants found in this study is similar to findings reported from western Canada (6), the USA (33,35), and Europe (39). This is not surprising because E. coli is ubiquitous, colonizes a wide range of hosts, and easily acquires AMR genetic elements (33).
The use of antimicrobial resistance data generated from the diagnostic laboratory has limitations. The Kirby-Bauer disk diffusion method was used for antimicrobial susceptibility testing over the study period, rather than micro-broth dilution methods used in other surveillance studies. This may limit the use of these data for comparison with other studies, and may have also limited the detection of smaller changes in antimicrobial resistance. Also, as there are regional variations in antimicrobial susceptibilities of common bacteria, comparison or extrapolation of our findings to other studies or regions should be done cautiously (4). Additional sources of bias in our study include the possibility of repeated clinical sample submission from the same farm or by the same veterinarian. Some of the isolates may have been from animals that were treated with antimicrobials prior to sampling, possibly selecting for more resistant pathogens, and increasing the proportion of resistant isolates and the frequency of MDR. It should be noted that some isolates in this study may be normal microflora and not pathogenic, especially for bacterial species such as E. coli.
This study provides valuable information on antimicrobial susceptibility patterns of commonly cultured mastitis, respiratory, and enteric bacteria from cattle, sheep, and goats from the Atlantic region of Canada. This information may guide clinicians in choosing empirical therapies for the management of these diseases of cattle and small ruminants in Atlantic Canada. We caution that selection of antimicrobials categorized as highly important to human health should be based on strong scrutiny and in most cases as a last resort. In conclusion, a high proportion of the bacteria isolated were susceptible to the antimicrobials tested over the study period, especially bacteria isolated from bovine mastitis and samples from small ruminants. Generally, AMR trends in isolated bacteria were stable over the study period in both cattle and small ruminants. Stable trends and higher susceptibility among specific pathogen and antimicrobial combinations may indicate predictable susceptibilities that should result in a high probability of therapeutic success when these antimicrobials are used to treat infections without prior antimicrobial susceptibility testing.
Acknowledgment
We are grateful to Robert Page of University of Prince Edward Island for the retrieval of the culture and susceptibility data. CVJ
Footnotes
Use of this article is limited to a single copy for personal study. Anyone interested in obtaining reprints should contact the CVMA office (hbroughton@cvma-acmv.org) for additional copies or permission to use this material elsewhere.
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