Recently published on the Veterinary Record Open was a study undertaken by Yoon-Hee Oh,Dong-Chan Moon, Young Ju Lee, Bang-Hun Hyun and Suk-Kyung Lim which evaluated the resistance of the P multocida bacterium against widely used antimicrobials to aid the most appropriate selection of drugs to use in affected animals. Resistance of bacteria is where therapeutic doses of antimicrobial medicine is administered, and bacteria continue to divide (APUA, 2014). There was an urgency to conduct this study due to the bacteria being one of the most significant causes of respiratory infection outbreaks in the Korean pig industry. Consequently, treatment failure due to resistant bacteria causes an increase in morbidity and mortality in pigs (Nedbalcová K. and Kučerová Z. 2013). Additionally, there is a concern of how the respiratory disease could pose a threat to the rest of the food chain.
In the present study, researchers used 454 isolates of the bacteria from all provinces in Korea over the period of 2010 to 2016 to evaluate the changes occurring in the resistance of P multocida to 18 antimicrobials used routinely to treat pigs. One of the reasons this study can be seen to be significant is the period of time that it took place over, along study over 6 years enables researchers to distinguish trends in the resistance and collate a large set of data that could aid other sectors such as veterinarians and surveillance stakeholders.
Strains of the P multocida were obtained from nasal swabs and lungs from diseased pigs from farms throughout Korea and used 5 isolates from each farm. These were then isolated on Columbia agar with 5% sheep blood. The researchers then tested the bacteria for the Minimum Inhibitory Concentration (MIC) by looking at their breakpoints, the concentration of the bacteria when it becomes susceptible to successful treatment (Mitka M, 2012). These were provided by the Clinical and Laboratory Standard Institute. The MIC90 refers to the MIC that inhibited 90 per cent of isolates.
When beginning to look the results of the research, it shows that P multocida isolates were defined to have multi-drug resistance (MDR) as they were found to be resistant to three or more different antimicrobials. From table 1, it can be seen that some isolates showed more resistance to some antimicrobials than others, with Sulphadimethoxine being the most frequently observed and Ceftiofur being the least. These figures are shown below:
Table 1 Resistance shown by the P multocida bacteria to different antimicrobials-§ indicates that a figure couldn’t be calculated because no Clinical and Laboratory Standards Institute interpretive criteria are available.
The second thing the results show in the report is that there was an increase in resistance to erolfloxacin over the period of the study increasing from 0 per cent of isolates in 2010-2011 to a resistance of 10.3 per cent in 2016. An explanation for this is suggested to be the increased usage of the antimicrobial over the past years, (AQPA, 2016). Although enrolfloxacin has been approved for the treatment of respiratory diseases in pigs, its cross-resistance with danofloxacin may have influenced the increasing MIC90 of danofloxacin as many isolates showed resistance to both antimicrobials.
An increase in the MIC90 values for neomycin, danofloxacin and penicillin 8 to 64 µg/ml, ≤0.125 – 2 µg/ml and, ≤0.125 to 0.25 µg/ml respectively was also observed which indicates that there is an increased amount of resistance to these.
Although there was no increase or decrease were observed for most antimicrobials, there was a frequent pattern for resistance seen from 50 percent of the isolates, with resistance shown with the following antimicrobials: sulphadimethoxine, chlortetracyline and oxytetracycline at 22.5 percent, sulphadimethoxine and oxytetracycline at 18.1 per cent and sulphadimethoxine on its own at 15.4 per cent. With this, MDR was found in 73 of the 453 isolates (16.1 per cent). It can also be seen that these figures were like those of other countries such as in Spain, where a study carried out by Lizaroano et al found 18.1 per cent of the isolates to have multi-drug resistance (Lizaroano et al, 2016).
With these results, it may then require a new antimicrobial to be looked at as therapeutic treatments to avoid promotion of multi-drug resistance increasing within the proportion of isolates. The study was the first of its kind on such a large scale, especially as clinical samples were collected from all nine provinces across Korea which allows for a more reliable and realistic study with results that can be used by the pig farmers and related professionals across Korea. It showed that the resistance rates in the present study for antimicrobials were higher than rates shown in previous studies in the EU (Garch El. et al, 2016) and North America (Sweeny MT et al, 2017).
The study concludes in highlighting the attention needed to the administration of antimicrobials in response the P multocida to prevent spreading of infection but also to prevent the increase of MDR and MIC. It may then lead to international collaborative studies which would help increase the knowledge on present resistance and formulate a better way of treating P multocida to reduce a future increase in resistance to antimicrobials. Further research, along with studies monitoring susceptibility of relevant pathogens would provide wider evidence and guidance when using antimicrobials to treat P multocida.
Animal and Plant Quarantine Agency (APQA). Antimicrobial use and antimicrobial resistance monitoring in animals and animal products. 2010. Gimcheon, South Korea, 2016.
El Garch F, de Jong A, Simjee S, et al. Monitoring of antimicrobial susceptibility of respiratory tract pathogens isolated from diseased cattle and pigs across Europe, 2009-2012: VetPath results. Vet Microbiol 2016; 194:11–22
Lizarazo YA, Ferri EF, de la Fuente AJ, et al. Evaluation of changes in antimicrobial susceptibility patterns of Pasteurella multocida subsp multocida isolates from pigs in Spain in 1987-1988 and 2003-2004. Am J Vet Res 2006; 67:663–8.
Nedbalcová K, Kučerová Z. Antimicrobial susceptibility of Pasteurella multocida and Haemophilus parasuis isolates associated with porcine pneumonia. Acta Veterinaria Brno 2013; 82:3–7.
Mitka M. Antibiotic Breakpoints. JAMA. 2012;307(10):1015. doi:10.1001/jama.2012.255