Introduction
The development of resistance to antimicrobials used in veterinary medicine and the threat of resistance transference to man, especially of the fluoroquinolones (FQs), is a very important issue in the EU and of great concern to veterinarians, farmers and companies. The transference of resistance to man either via bacteria or genetic material is thought to be primarily by meat contamination, but other major routes do exist. The need for a baseline is essential, from which monitoring of resistance can be implemented, if new guidelines under consideration are introduced. Consistency of laboratory methods and interpretation are also vital so trends can be monitored and measured responses can take place.
It must be remembered that infections associated with Escherichia coli are frequently systemic in nature and the resulting septicaemia can be lethal. The infections frequently follow other infections such as mycoplasmosis or viral infections such as infectious bronchitis in chickens and turkey rhinotracheitis virus in turkeys. Treatment is necessarily by systemically acting antimicrobials and that gut only acting products of limited value.
Materials
Antimicrobial sensitivity data on E. coli isolated from chickens during 1999 by two centres of the Veterinary Laboratories Agency (VLA) were analysed. Lasswade provided data on 286 isolates mainly from Scotland Jan-Dec and Shrewsbury supplied data on 198 isolates from England and Wales from all VLA labs Jan-July and Shrewsbury alone Jan-Oct 1999. The isolates were from clinical cases, which may have been exposed to antimicrobial therapy prior to isolation and testing.
The sensitivity disc method was used with a 13mm zone of inhibition considered the break point.
Results
The results are summarised in Table 1.
Table 1. Antimicrobial sensitivity pattern (%) of UK chicken E. coli isolates
*Lincomycin/spectinomycin – 239 isolates
Antimicrobial
(Disc Conc. mg)
Lasswade
(n = 286)
Shrewsbury
(n =198)
Overall
(n = 484)
Apramycin (15)
98
98
98
Neomycin (10)
79
89
83
Spectinomycin (25)
-
88
88
Linco/spectin (150)*
91
-
91
Amoxycillin (2)
15
-
15
Ampicillin (10)
-
62
62
Doxycycline (30)
56
-
56
Tetracycline (10)
46
61
52
Trimeth/Sulpha (25)
72
82
76
Enrofloxacin (5)
99
98
99
Difloxacin (10)**
95
-
95
** Difloxacin – 71 isolates<
Enrofloxacin was consistently the most active (99%) of the systemically active antimicrobials, with difloxacin, another FQ, giving similar results (95%). These were followed by trimethoprim/sulpha (76%) and ampicillin (62%) but amoxycillin was reported as low (15%). Tetracycline (52%) and doxycycline (56%) were quite similar. Of the gut active compounds, apramycin (98%) was most active followed by lincomycin/spectinomycin (91%), spectinomycin (88%) and neomycin (83%).
If the disc strength is compared with levels achieved in serum either at the highest concentration after a bolus dose (Cmax) or at a steady state after administration via the drinking water for example it can be seen that the gut active products such as apramycin and spectinomycin almost have no serum levels. Amoxycillin has very high serum levels which demonstrates that the disc strength used was completely inadequate and is normally 10mg like ampicillin, hence the low sensitivity recorded.
Table 2. Disc strength in comparison with serum levels (mg/ml) in broilers
Antimicrobial
Disc Strength (mg)
Dose (mg/kg)
C. max. (mg/ml)
Steady State or 12hours (mg/ml)
Apramycin
15
43
-
0.04
Spectinomycin
25
33
0.05
<0.05
Amoxycillin
2
10
160
>15
Doxycycline
30
20
55
7
Chlortetracycline
10
25
0.35
0.14
Chlortetracycline
10
50
0.7
0.1
Trimethoprim /
25
9
0.24
0.11
Sulfadiazine
25
45
14.7
6.8
Enrofloxacin
5
10
1.9
0.52
Difloxacin
10
10
0.7
-
Danofloxacin
-
5
0.5
0.12
If the sensitivity is compared with other studies in the UK, it can be seen that there has been little change in the enrofloxacin sensitivity over the last decade but there has been a small fall in ampicillin and trimethoprim sensitivity. Isolates from slaughtered birds in Europe have similar results, whereas there seemed to be more resistance to tetracyclines, neomycin and spectinomycin in Canada. Turkeys from the US showed similar results except for spectinomycin and neomycin where the resistance was mush higher.
Table 3. Comparative sensitivity (%) with other avian E. coli studies
Country
UK
UK
EU (slaughter)
Canada
USA (Turkeys)
Year Isolated
1999
1986-91
1995
1993
1995
No. Isolates
484
274
1154
294
1204
Antimicrobial
Apramycin
98
-
-
97
-
Neomycin
83
84
94
50
13
Spectinomycin
88
-
-
38
54
Ampicillin
62
84
66
58
67
Tetracycline
52
48
55
11
-
Trimeth/Sulpha
76
85
64
78
87
Enrofloxacin
99
-
97
99
99
Sources (Wray et al, 1993; Scheer et al, 1997; Laperle et al, 1996; Salmon and Watts, 2000)
Comparative data with salmonella isolated from the UK showed there to be a trend for greater sensitivity overall for salmonella species. S. typhimurium from turkeys had a much higher resistance to nalidixic acid than in chickens but unfortunately the enrofloxacin data is not routinely reported, although Davies (personal communication) confirmed there is little actual resistance.
Table 4. Comparative sensitivity (%) of E. coli and salmonella in the UK
*Source (VLA, 1999)
Antimicrobial
E. coli, 1999
Salmonella, 1998 (n = 1240)*
Apramycin
98
99
Neomycin
83
99
Ampicillin
62
81
Tetracycline
52
78
Trimeth/Sulpha
76
89
Enrofloxacin
99
-
Nalidixic Acid
-
92 (S. typhimurium, chick 95; turkey 29.
Conclusions
The high level of sensitivity to the FQs is very positive, as enrofloxacin has been used in poultry in the UK for over 7 years. The use of nalidixic acid should be replaced or additional to enrofloxacin as it confuses the issue of resistance. The low level of sensitivity to amoxycillin (15%) would be of concern but is probably associated with the low strength disc (2mcg) used, as ampicillin’s result was much higher 62% based on a 10mcg disc. This highlights the need for standardization of testing between laboratories. Apramycin is consistently active but it must be remembered it is only effective in the gut and would not be a suitable alternative to the FQs.
Various guidelines have been produced to advise on the responsible or prudent use of antimicrobials in veterinary medicine, and this has raised the awareness of the issue. The impact of the processors/supermarkets on requesting the restriction of the use and monitoring use of antimicrobials on the farm has also had a major impact. Salmonella have been monitored for some time in the UK and now other organisms such as E. coli and campylobacters will also be included from 1999 on a regular basis and once a baseline has been established major variations can be monitored and suitable actions determined.
References
Laperle, A., Nadeau, M. and Cantin, M. (1996) Profil de sensibilite de bacteries d’origine bovine, porcine et aviaire envers certains agents antibacterienne. Le Medecin Veterinaire du Quebec, 26, 1, 26-29
Salmon, S.A. and Watts, J.L. (2000) Minimum inhibitory concentration determinations for various antimicrobial agents against 1570 bacterial isolates from turkey poults. Avian Diseases, 44, 85-98
Scheer, M., Froyman, R., De Jong, A. and Altreuther, P. (1997) Antibacterial sensitivity monitoring of avian Escherichia coli isolates over 5 years. Journal of veterinary Pharmacology and Therapeutics 20 (Supplement 1), 181-182
Wray, C., McLaren, I.M. and Carroll, P.J. (1993) Escherichia coli from farm animals in England and Wales between 1986 and 1991. Veterinary Record, 133, 439-442
Veterinary Laboratories Agency (1999) Veterinary investigation surveillance report 1998 and 1992-1998.
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