Susceptibility Monitoring

Bayer carefully watches the development of resistance.

Our specialists not only test the susceptibility of various pathogens as part of a continuing resistance monitoring programme, but also watch publications on resistance to antibiotics worldwide.

Because published data on bacterial resistance are mostly obtained from diagnostic laboratories (sick animals and necropsy material), the findings will not always be congruent with the level of resistance of bacteria in the field. Surveys on resistance must therefore also cover bacterial isolates obtained from untreated animals in the field or from samples taken from animals at slaughter to give a reliable view on the actual resistance situation. A short overview of the resistance situation for the most important pathogens in ruminants can be seen below.

Pasteurella multocida and Mannheimia (Pasteurella) haemolytica
In Germany, a resistance monitoring programme brings together the findings from 29 different laboratories in nine provinces of the country. 39,289 antibiograms were included in a study (Trolldenier, 1995). In this study, 97% of the P.multocida  and 93% of the M. haemolytica isolates were susceptible to Baytril (Enrofloxacin).

In 1999 another such study was made on the basis of isolates from 10 laboratories in seven Germany provinces (Trolldenier, Kempf, 2001). 61 P.multocida isolates and 32 M. haemolytica isolates were tested. Susceptibility to Baytril in these cases was 100%.

In France, randomised clinical isolates of bovine origin were tested in 1988 and 1989 (Ganière, 1990). All isolates of M. haemolytica (n=20) and P.multocida (n=20) were 100% susceptible to Baytril (Enrofloxacin).

In the Netherlands, clinical isolates obtained from local AH services were tested in 1996 and 1997 (Mevius, Hartmann, 2000). Of the P.multocida isolates (n=96), 95% were susceptible to Baytril (Enrofloxacin).

Bayer's own data on European isolates (Pirro, 1999) showed susceptibility rates of 98.6% for P.multocida and 98.2% for M. haemolytica. Clinical isolates of M.haemolytica showed 97.1% susceptibility.

Salmonella spp.
The in-vitro activity of Enrofloxacin was tested in various pathogens of animal origin in the US (Watts et al., 1997). Among the tested bacteria, Salmonella spp. (n=82) were highly susceptible to Enrofloxacin and showed MIC_50/90 values of 0.03/0.06 µg/ml.

Bayer's internal monitoring (Pirro, 2000b) on 167 isolates of S. typhimurium of bovine origin from 1997 and 1998 showed 100% susceptibility to Baytril.

In France, a study that was done because of an increase in severe clinical cases of enteritis caused by S. typhimurium in adult cattle showed a susceptibility of 98% among the 161 isolates tested (Martel et al., 2000).

In Belgium the susceptibility of S. typhimurium of bovine origin from 1991 to 1993 was between 99% and 96% (Imberechts, 2000).

E.coli
E.coli isolates of bovine origin from 1988 and 1989 from France and Belgium were tested for susceptibility (Ganière, 1990). These clinical isolates had MIC₅₀ and MIC₉₀ values of 0.06 and 0.5 µg/ml.

Field isolates were collected by Martel (1990). Over 90% were susceptible to Baytril (Enrofloxacin).

E.coli isolates from cases of mastitis were tested by Pirro et al. (1997). Whilst isolates from Germany (n=73) were 95.9% susceptible to Baytril (Enrofloxacin), isolates from Italy (n=23) and Sweden (n=51) were 100% susceptible. MIC_50/90 was 0.03/0.06 µg/ml.

The in-vitro activity of Enrofloxacin was tested in various pathogens of animal origin in the US (Watts et al., 1997). Among the tested bacteria, E.coli (n=45) were highly susceptible to Enrofloxacin and showed MIC_50/90 values of 0.015/0.03 µg/ml.

As a part of Bayer's continuing monitoring programme, E.coli isolates from clinical cases in Germany were tested (Pirro, Schroeter, 2000). 85 isolates came from adult cattle and 73 from calves. Samples were obtained from diagnostic laboratories. Over 66% were multi-resistant to three or more antibiotics, but 88% were susceptible to Baytril (Enrofloxacin).

Mycoplasma spp.
Hannan et al. (1997) tested the susceptibility of 20 field isolates from the UK, Germany, USA and Japan. MIC₅₀/MIC₉₀ values were 0.1/0.25 µg/ml For M. agalactiae, MIC₅₀/MIC₉₀ values were 0.25/0.25 µg/ml. All strains were 100% susceptible to Baytril (Enrofloxacin).

These findings correlated with other findings (Poumarat, Martel, 1989; Devriese, Haesebrouck, 1991; Laak et al., 1993 and Friis, Szancer 1994).

The continuing monitoring shows that after more than 10 years of use in veterinary medicine Baytril still shows very good efficacy against relevant bacteria that cause enteritis, mastitis and respiratory tract infections.

Responsible use will help to maintain Baytril's high efficacy in the future.