Antimicrobial resistance in swine production

Animal Health Research Reviews 9(2); 135–148 Antimicrobial resistance in swine production Frank M. Aarestrup1*, C. Oliver Duran2 and David G. S. Burch31National Food Institute, Technical University of Denmark, Bulowsvej 27, DK-1790 Copenhagen V,Denmark2Moss Veterinary Partners IDA Estate, Monread Road, Naas, Co. Kildare, Ireland3Octagon Services Ltd, Old Windsor, Berkshire, UK Received 15 September 2008; Accepted 22 September 2008; First published online 5 November 2008 AbstractLarge amounts of antimicrobial agents are still being used in modern swine production in manycountries around the world. This facilitates the emergence and development of antimicrobialresistance. Bacteria causing infections in swine have in several cases acquired resistance to anumber of the agents most commonly used for treatment, making it difficult to predict theefficacy of different antimicrobial agents without prior susceptibility testing. This review givesan overview of recent susceptibility data from different parts of the world and discusses theimportance of the development of resistance not only in the treatment of infections in swinebut also taking into account the human health implications of antimicrobial resistance.
Keywords: antimicrobial resistance, swine, treatment, Escherichia coli, Brachyspira, Actino-bacillus pleuropneumoniae, Streptococcus suis, Staphylococcus hyicus, Mycoplasma, Lawsonia,Clostridium, Pasteurella Emphasis is being placed on the need to target the use ofantimicrobials towards the specific pathogen and only Pork is one of the most commonly consumed food commodities globally. The production of pork ranges Antimicrobial resistance has emerged among bacteria from the highly intensive, volume and efficiency driven causing infections in swine in several countries. In some systems, to ‘backyard’ production with one or a few pigs cases this makes empiric therapy difficult, whereas it is per family. In both cases, diseases can greatly affect the still possible to predict the susceptibility of other patho- cost of production. Thus, especially in intensive and gens. This review gives an overview of the occurrence of large-scale production, the routine use of antimicrobials resistance among the most common swine pathogens, the has become an integrated part of the production system.
trends we currently observe and a discussion of the trends Antimicrobial agents have not only been used for treatment of clinically ill pigs, but also as part of theroutine management for prophylaxis and even growthpromotion. Sale of antimicrobials for use in swineproduction are reported to be worth an estimated 1.7 Most common pathogens causing infection in swine billion dollars, equal to 34% of the global animal healthantimicrobial market followed by poultry (33%) and cattle Precise estimates of the prevalence of porcine infections and the consumption of antimicrobial agents used to treat Because of the human (and animal) health aspects or control those infections are difficult to obtain, even associated with the development of antibiotic resistance, though most veterinarians have a fairly good idea of the as a consequence of this widespread use, medication incidence in farms under their care. However, only a very with antimicrobials has come under increased scrutiny.
limited number of countries report data on antimicrobialusage and/or prevalence of infections. In Denmark, amonitoring programme for antimicrobial use has been *Corresponding author. E-mail: [email protected] in place since 2000 ( Jensen et al., 2004), which monitors Mill. animal daily dosages
Fig. 1. ADDs used for treatment of weaners, sows and slaughter pigs in Denmark in 2007.
drug use at the farm and diagnostic level and also data are, however, available from diagnostic laboratories.
calculates the number of animal daily dosages (ADDs) Figure 2 shows the percentages of diagnosis of diseases of used for different age groups and by diagnosis. The usage the digestive system at Veterinary Laboratories Agency, of antimicrobial agents for the different age groups (http://www.defra.gov.uk/vla/reports/docs/rep_ and the different antimicrobial agents are calculated into vida_pigs99_06.pdf). The calculation is based on a total ADDs to get a better comparison because the activity of of 3188 diagnoses in the period 1999–2006. In 1999, the various antimicrobial agents differ substantially and E. coli accounted for almost 50% of all diagnoses of the amount necessary to treat e.g. a sow of 150 kg is diseases in the digestive system. This has, however, higher than the amount needed to treat a pig of 30 kg.
changed considerably and diseases related to Lawsonia The number of ADDs used for treatment of different and Brachyspira seem now to be more important.
infections in sows/piglets, weaners and slaughter pigs in Whether this is due to a real change in the importance Denmark in 2007 is given in Fig. 1. It is very clear that the of the disease or changes in the diagnostic abilities is majority of treatments are for gastrointestinal infections however, unknown. The distribution of the most impor- in weaners (>170 million ADDs in 2007). It is noteworthy tant species among respiratory infections is given in Fig. 3.
that this implies that the approximately 25 million pigs In England, the most common bacterium is seemingly produced in Denmark each year on average are treated Pasteurella multocida and is followed by Actinobacillus for approximately seven days during their weaning period.
pleuropneumoniae. Conversely, in the USA based on General infections are almost exclusively in weaners, submissions to the Iowa State University Veterinary whereas respiratory infections are more commonly in Diagnostic Laboratory during 2006 (over 28,000 cases) slaughter pigs and not surprisingly treatment of repro- the frequency of A. pleuropneumoniae and Mycoplasma ductive and urogenital organs and the udder are in the hyopneumoniae diagnosis has declined in the last 5 years (Madson, 2008) (Fig. 4). Other respiratory bacterial The treatment incidence does, however, not give any pathogens, like S. suis, Haemophilus parasuis and information on the causative agent. The common bacterial Actinobacillus suis have been isolated more frequently infections are summarized in Table 1. They are divided by this diagnostic laboratory. These differences probably into primarily enteric, respiratory and other infections.
reflect the different nature of the production systems, There is some overlap, since e.g. Escherichia coli can eradication programs for major swine diseases and the be both septicemic and enteric, especially in neonatal influence of immunosuppressive viruses.
piglets, and Streptococcus suis can be isolated from the The number of bacterial and viral diagnoses during the respiratory tract as well as the central nervous system.
first 6 months of 2007 at the National Veterinary Institute Precise estimates of the prevalence of the various in Denmark is shown in Fig. 5 (http://www.dfvf.dk/ bacterial diseases are difficult to obtain. Practising Default.aspx?ID=21768). The most common swine patho- veterinarians often do not collect disease incidence data gen diagnosed is A. pleuropneumoniae, followed by in a systematic way and data obtained from diagnostic E. coli and S. suis. Thus some clear differences do exist laboratories may be biased by the fact that veterinarians between Denmark and England, but the general pattern mainly submit samples from difficult clinical cases. Some seems to be that the enteric pathogens Brachyspira, Antimicrobial resistance in swine production Table 1. Common bacterial infections and diseases in the pig Choleraesuis – septicemia diarrhea, death Porcine proliferative enteropathy (ileitis) Growers and finishers, 6–26 weeksAll ages in primary breakdown Gla¨sser’s disease (arthritis, pericarditis, time systemic-acting antimicrobials can be used effec- A. pleuropneumoniae and P. multocida, and the more tively. Piglet scours are usually less severe but almost all systemic pathogen S. suis are the most common and pigs suffer some form of post-weaning check. Diarrhea starts 4–5 days after weaning and can lead to dehydration Despite the lack of monitoring data from different and mortality in severe cases. The severity can be countries we have attempted to depict the basic patterns mitigated by a good stable temperature and clean of infections in Fig. 6, divided into enteric infections, environment, weaning at 4 weeks of age or older, not respiratory infections and general infections, especially mixing litters, carefully formulated diets and by the focusing on the first 24 weeks of the pig’s life, since this is addition of therapeutic levels of zinc oxide in the diet.
Once over this period, there are usually few problemswith E. coli, except for cases of bowel edema, associatedwith verocytotoxic strains and sometimes after moving Antimicrobial resistance among the major pathogens into a new, colder house. The susceptibility pattern ofE. coli in different countries is shown in Table 2. A very high frequency of resistance is found in some countriesand the antimicrobial susceptibility of E. coli is difficult to predict, which means that the final choice of antibiotic E. coli primarily affect the younger pig. Neonatal scours has to be based on knowledge of the local situation can be severe and the piglets can die of septicemia. At this and preferably susceptibility testing. E. coli are also Percent of diagnosis
Fig. 2. Trends in percentage of selected bacterial pathogens from digestive diseases in pigs from the Veterinary LaboratoriesAgency in the period 1999–2006.
PRRSV and PCV2 infections. The antimicrobial suscept- ibility among Salmonella spp. is intensively surveyed because of the zoonotic importance of this bacterium. In general antimicrobial treatment is not recommended in animals because this might lead to resistance develop-ment and thus, human health problems. Specific data onthe occurrence of resistance in S. Choleraesuis are limited.
However, a high frequency of resistance has been reported from the US, Taiwan and Japan (Chang et al.,2002a, b; Esaki et al., 2004; Zhao et al., 2007) makingempiric treatment difficult. Recent data from the US in- dicate widespread S. Choleraesuis resistance to ampicillin, tetracyclines and sulfonamides, but susceptibility toaminoglycosides, trimethoprim/sulfamethoxazole, fluoro- Fig. 3. Percentages of respiratory diagnoses from the quinolones and cephalosporins (Madson, 2008). Thus, as Veterinary Laboratories Agency in the period 1999–2006 for E. coli treatment has to be based on local experience attributed to various bacterial species. The total number of Clostridium spp.
Clostridium perfringens type C is mainly associated with considered generally susceptible to polymyxins. These per-acute hemorrhagic and necrotic enteritis in young substances are not always included in routine suscept- piglets, which can be fatal. The disease is not very ibility testing, but could be a reliable drug for treatment of common, and is controlled mainly by sow vaccination.
Infections in older piglets and growing pigs is less severeand also in growing pigs and usually associated with type A strains. Clostridium difficile have recently emerged Many strains of Salmonella have a low pathogenicity in as a cause of infections in pigs (Songer, 2004). The pigs and are more of a concern for zoonotic transmission.
infections are associated with mild diarrhea and ill thrift in However, S. Choleraesuis is highly pathogenic in pigs and piglets and like in human medicine appears related to is associated with acute outbreaks of diarrhea, septicemia the use of antimicrobial agents, mainly cephalosporins.
and death especially in finishing pigs. This serovar is There are only very few reports on antimicrobial sus- rarely isolated in Europe but is commonly reported in the ceptibility from C. perfringens or C. difficile from pigs.
US and Asia. S. Typhimurium can cause diarrhea, wasting, In the 1970s in Wisconsin, USA, Rood et al. (1978) septicaemia and death. The incidence increases with examined 258 C. perfringens isolates from six pig farms Antimicrobial resistance in swine production Percent of diagnoses
1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Fig. 4. Percentages of 50,316 positive respiratory disease diagnoses from diagnostic samples from swine during 1994–2005that were attributed to various pathogens. The data are from the Department of Veterinary Diagnostic and production AnimalMedicine at Iowa State University, Ames, Iowa (Madson, 2008).
No. diagnosis
Fig. 5. Number of positive diagnoses from diagnostic samples from swine at the National Veterinary Institute in Denmarkduring the first 6 months of 2007.
routinely using antibiotics in feed and 240 isolates from without the availability of nitroimidazoles like metroni- five farms that did not. They found 78% tetracycline resistance and 23% macrolide resistance among isolatesfrom the antibiotic using farms in comparison to 25% tetracycline resistance and 0.8% macrolide resistance L. intracellularis is a relatively ubiquitous organism among the farms not routinely using in feed antibiotics.
on pig farms. Various surveys have showed that 80–95% Post and Songer (2004) examined the susceptibility of of farms are infected. It is commonly associated with 80 C. difficile isolated from piglets with diarrhea. They diarrhea in growing pigs and primarily affects the ileum did not report full range MICs or percent resistance. The although the organism can be found in caecal and colonic data suggest that all C. difficile are resistant to bacitracin epithelial cells. Susceptibility testing is difficult because and ceftiofur and that some isolates have acquired the organism can only be grown in cell cultures. Based on resistance to macrolides, tetracycline, tiamulin and clinical experience tetracyclines, tiamulin, valnemulin, virginiamycin. Penicillins may be used for treatment of tylvalosin and tylosin seem to be effective in controlling C. perfringens infections, whereas treatment of infections the disease. However, there are currently limited data with C. difficile, as in human medicine, might be difficult, on the development of resistance or the establishment of Clinical incidence/risk (%)
Clinical incidence/risk (%)
Age (weeks)
Clinical incidence/risk (%)
Age (weeks)
Fig. 6. Disease patterns among pigs. (A) enteric diseases; (B) respiratory diseases and (C) systemic diseases.
breakpoints for L. intracellularis but Wattanaphansek Canada. Recent reports from the US suggest that et al. (2007) demonstrated high intracellular MICs to B. hyodysenteriae may be a re-emerging pathogen chlortetracycline, lincomycin and tylosin, particularly in (Duhamel, 2008). It causes severe diarrhea, commonly some US isolates suggesting that resistance can occur. In with mucus and blood and leads to rapid wasting and also comparison, carbadox, tiamulin and valnemulin all had dehydration and death. Brachyspira pilosicoli is wide- spread as a low-grade cause of mucoid diarrhea, eitheralone or in mixed infections and can be associated with PCV2 infections. There is only a limited number of Brachyspira hyodysenteriae, the cause of swine dysen- antimicrobial agents available for treatment of infectious tery, is a recurring severe problem in many countries in caused by Brachyspira spp. in pigs. The slow develop- Europe, but seemingly less important in the US and ment of immunity in infected pigs, the persistence of Table 2. Occurrence of antimicrobial resistance among E. coli isolated from infections in swine in different countries Percentage of E. coli that were resistant Hendriksen Boerlin Hendriksen Hendriksen Hendriksen Hendriksen Schro¨er Harada Hendriksen Hendriksen Hendriksen Hendriksen Hendriksen Hendriksen Hendriksen Table 3. Antimicrobial resistance among B. hyodysenteriae and B. pilosicoli in various countries Percentage of the isolates that were resistant B. hyodysenteriae in the environment and the under- have been almost pan-susceptible. However, as can be dosing of in feed medication in pigs with a reduced seen from Table 4, this is no longer the case, since appetite have undermined the effectiveness of available resistance to the beta-lactam antibiotics has emerged.
antimicrobial agents. Resistance to macrolides (tylosin) Resistance to tetracycline and other antimicrobials has and lincosamides seems to be very high in many countries also emerged, but most isolates seem still susceptible to and the most active agent are the pleuromutilins tiamulin fluoroquinolones, ceftiofur and florfenicol.
and valnemulin, where most isolates seem to besusceptible (Table 3). However, recently isolates with reduced susceptibility to pleuromutilins have emerged, Atrophic rhinitis is caused by a mixed infection of both among B. hyodysenteriae (Lobova et al., 2004; Bordetella bronchiseptica and P. multocida and usually Rohde et al., 2004) and B. pilosicoli (Pringle et al., 2006).
starts in young pigs from 7 to 10 days of age. Clinically It is essential that detailed susceptibility testing of there is sneezing and the bacteria colonize the nasal B. hyodysenteriae be carried out in all Swine Dysentery mucosa and the toxins, usually from Type D P. multocida, cases. Short antibiotic courses at effective doses and cause the destruction of the turbinate bones. The main enhanced pen and farm sanitation must be used at all nasal bones may grow unevenly causing twisting and times. Eradication of B. hyodysenteriae from farms can be foreshortening as the pig grows. The disease can be readily achieved and would be recommended to reduce effectively controlled by vaccination of the sows and the risk of antimicrobial resistance and inability to control antimicrobial therapy is rarely needed. Until now a low clinical outbreaks of swine dysentery.
frequency of acquired resistance to ampicillin, chloram-phenicol, tetracycline and TMP-sulfonamides has beenreported among B. bronchiseptica (Kadlec et al., 2004).
P. multocida from pigs is generally susceptible topenicillins, ceftiofur, gentamicin, macrolides, fluoroqui- Many porcine bacteria can be found in the respiratory nolones, tetracyclines, trimethoprim-sulfonamides and tract, but also can be found systemically and cause florfenicol, even though some resistance seems to have meningitis, arthritis, pleurisy, pericarditis and peritonitis, emerged, whereas more resistance is observed towards streptomycin (Yoshimura et al., 2001; Lizarazo et al., 2006;Wallmann, 2006).
A. pleuropneumoniaeA. pleuropneumoniae can cause primary acute necrotiz- ing pneumonia on its own or in combination with M. hyopneumoniae, the cause of enzootic pneumonia, is M. hyopneumoniae. Some serotypes given in artificial endemic throughout the world, with most herds being infection studies can cause death within 24 h, due to the infected. On its own, it causes a relatively mild disease, toxic shock produced by its exotoxins. Treatment has the damage it does to the cilia lining the respiratory tract traditionally been using penicillins where the isolates and the immuno-suppressive effect it has in the lung Antimicrobial resistance in swine production permits a number of bacteria, especially P. multocida, to colonize the lung and cause broncho-pneumonia. Myco-plasma hyosynoviae is the cause of mycoplasmal arthritis.
Worldwide there have been very few reports on the antimicrobial susceptibility of Mycoplasma from pigs.
Some isolates seems to have acquired resistance to tetracyclines, fluoroquinolones and macrolides, whereasresistance to tiamulin has not been reported (Aarestrup and Kempf, 2006). Thus, based on the available suscept- ibility data, tiamulin seem to be the best choice forempiric treatment. More data on clinical efficacy does however, seem to be needed. However, for M. hyopneu-moniae, vaccines have become the main method of control with some countries vaccinating over 50% of thenational growing herd.
S. suisS. suis is widespread in many herds but do not always cause clinical problems. A large number of different serovars can be found, but the most important seem to be serotypes 2 and 7, with S. suis type 2 as the most common associated with meningitis in weaner and grower pigs.
Susceptibility data are presented in Table 5. Some variations in the susceptibility pattern can be observed between different surveys. A frequent occurrence of resistance to macrolides and tetracycline is found in most studies. In most reports a low frequency of resistance topenicillins is reported. However, resistance to this group of antibiotics seems to be emerging in some countries, potentially making treatment of S. suis difficult, since penicillins typically have been the drug of choice against H. parasuis causes infections in weaners and growers,especially polyserositis (Gla¨sser’s disease). In somecountries H. parasuis is almost pan-susceptible to all tested antimicrobial agents, whereas high frequencies of resistance seemingly have emerged in other countries (Table 6). Thus, compared to just a few years ago, when penicillins could almost always be expected to be effective against H. parasuis, treatment now has to be based on local knowledge and continuous monitoring.
Erysipelothrix rhusiopathiaeE. rhusiopathiae can cause arthritis and valvular endo- carditis following a septicemic episode, but is more commonly associated with the classic diamond-shaped skin lesions. In recent years, following partial depopula- tions and clean ups in herds to reduce respiratory disease,peracute outbreaks of erysipelas have been morecommonly seen. There are a number of effective vaccines available and treatment is rarely needed. E. rhusiopathiae is susceptible to penicillins, which is effective in treat- ment. There is some development of resistance to tetracycline, streptomycin and macrolides (Yamamoto Staphylococcus hyicusGreasy pig disease, caused by S. hyicus, is a sporadic disease affecting young pigs from 7 to 35 days of age. It isthought that the infection gets into the skin following trauma from fighting, rough concrete sharp protrusions, etc., which enables the organism to penetrate. It causes a generalized dermatitis and an excessive secretion of sebum and exudates, which causes a greasy dark cover-ing to the skin. A number of countries have reported data on antimicrobial susceptibility among S. hyicus (Table 7).
In general a high frequency of resistance is found to macrolides, tetracycline, sulfonamides and streptomycin, whereas the isolates still seem to be susceptible to flor- fenicol, fluoroquinolones and gentamicin. It is difficult to predict the susceptibility of S. hyicus and treatment has to be performed according to knowledge of the specific farms and routine testing of the pathogen.
Staphylococcus aureusS. aureus is an important opportunistic pathogen for most animal species and causes a variety of different infec- tions including skin infections, septicemia, osteomyelitis, arthritis and pneumonia. Recently, a special methicillin- resistant S. aureus (MRSA) isolate (CC398) has emerged among production animals, primarily swine in many countries (Wulf and Voss, 2008). This type has gained intensive attention because it might colonize healthy swine and spread to humans through direct contact, such as farmers and veterinarians. However, S. aureus ispotentially an important pathogen for swine and also MRSA of CC398 has been observed as a cause of infections in pigs (van Duijkeren et al., 2007). There isonly limited information on the susceptibility of S. aureus from infections in pigs. Unpublished data from Denmark and data from The Netherlands (van der Wolf et al., 2008) suggest that resistance to macrolides, streptomycin and tetracycline is frequent, whereas the isolates are in general susceptible to TMP-sulfonamides and fluoroqui- nolones. It is, however, difficult to predict the suscept- ibility and the potentially continued emergence of MRSA, which might not only have implications for human health, but might also make it more difficult to treat infections in General principles for disease control in Disease control is not only about using medicines.
Frequently, what has gone wrong is the production system; hence the challenge is to correct the underlying management problems. Post-weaning diarrhea is the classic example. If the temperature of the weaning accommodation is kept high and constant and drafts are avoided, there is normally little trouble. The ‘correct’ environment is very important to the pig and disease Antimicrobial resistance in swine production Table 6. Antimicrobial susceptibility of H. parasuis Percentage of isolates that were resistant Table 7. Occurrence of antimicrobial resistance (%) in S. hyicus from different countries (from Aarestrup and Schwarz 2006) Percentage of isolates that were found resistant 1Both healthy and diseased animals.
–, Not tested.
prevention. In general, two approaches can be used, avoid the infectious agents and avoid the clinical disease.
More details can be read in Burch et al. (2008). Avoiding Small closed breeding finisher herds, which are family the infectious agent can be achieved by starting up a herd owned, often do better than farms where pigs are looked free of infectious diseases or by carrying out depopulation after by employees. In addition, avoid mixing pigs of and repopulation with clean stock. Once established, it is different ages and/or immunity status, such as those crucial to avoid buying animals from farms with diseases coming from different farms. Avoid stress by using as well as ensuring strict biosecurity when entering the production systems based on reduced moving and mixing farm. Avoiding the clinical disease might be more difficult.
of pigs. The benefit of raising pigs segregated by the The production systems in every pig-producing region of parity of the sows is also well established since this the world and almost each farm are different and have reduces the pathogen transmission between groups of their own problems. However, even though this is a pigs with similar immune status. Another important man- complex situation, some basic principles still apply. Thus, agement point is the age at weaning. One of the main there are three key areas for avoiding clinical disease, problems with enteric diseases in pigs comes from which need to be addressed: herd management, pig weaning the piglets before their immune system is housing and environment, and immunity.
consequences should also be taken into account.
Recently, the World Health Organization has developed Particularly for respiratory diseases, a reduction in pigs a list of critically important antibiotics for human health per airspace has resulted in less severe infections, (WHO, 2007) and it is recommended that the use of these although some of these benefits can also be reached agents in food animal production be limited as much as with correct ventilation and management. Increased pig density in pens or barn has also been linked withincreased stress and disease transmission resulting inhigher mortality and reduced growth.
As can be seen from the examples provided in this reviewthe occurrence of antimicrobial resistance varies greatly between countries and even regions and individual herds.
Thus, the final choice of empiric treatment has to be Understanding the development of immunity in a herd or based on the local situation. This requires regular group of pigs will allow better control of diseases on farm.
susceptibility testing of the pathogens involved in the Excellent colostrum intake in the first 6 h of life will diseases to guide the veterinarians. Especially enteric ensure good protection against many piglet infections.
bacteria, such as E. coli, have in some cases developed Grouping of pigs to ensure a common immune status will resistance to all available antimicrobial agents and the reduce the susceptible population and reduce infections.
susceptibility of the infecting bacterium is therefore Vaccination can also be successfully used, especially totally unpredictable. This is, however, also the case against infections caused by C. perfringens, E. rhusio- for staphylococci, where multiple resistant isolates are pathiae, Mycoplasma, Lawsonia and virus infections.
recently more frequently observed. Thus, treatment hasto be based on knowledge at the individual farm.
For Brachyspira it is especially worrying that resistance Choice of antimicrobial agents for therapy is emerging to the currently most active compoundtiamulin.
The licensing of veterinary medical products was until the It is for some bacterial species to some extent still last couple of decades to a large extent using limited possible to predict the susceptibility. Thus, Mycoplasma documentation for clinical efficacy. This has now changed are still susceptible to tiamulin, and most A. pleuropneu- and clinical trials are today required for licensing.
moniae, P. multocida and S. suis isolates are susceptible However, there are very few independent studies that to penicillins. Resistance to this group of antimicrobial have compared the different available compounds for the agents has emerged making it important that at least same disease. Furthermore, information of clinical failure national monitoring is performed to follow the trends in due to the development of resistance is almost absent in Some of the more recently approved antimicrobial Antimicrobial susceptibility testing is practically useful classes such as the cephalosporins and fluoroquinolones in determining whether an antimicrobial should be used are still active against a high frequency of isolates and can to treat a condition, but should not be used as an absolute therefore easily be preferred in many cases. However, as result, only a guide. Susceptibility testing can be difficult previously mentioned these antimicrobial classes are also and requires the use of standard methods and use of considered critically important for human health and correct breakpoints for determining whether an isolate their use in food animal production should be limited or should be considered resistant or susceptible. Optimally, avoided as far as possible. This makes it even more difficult for the practising veterinarian to choose the most among the target pathogen and high clinical efficacy optimal treatment taking both the welfare of the animal should be chosen for empiric treatment. Based on the and the human health considerations into account. The clinical experience and routine examination of clinical most optimal way forward seems to be to ensure a more samples and susceptibility testing this treatment might be optimal production system with less dependence on antimicrobial agents and to implement more continuous However, choosing the right antimicrobial agent for monitoring at the national, regional and down to the farm treatment of infections in food animals is not only about level to assist the veterinarian in choosing the most the susceptibility of the animal pathogen. Using anti- microbial agents for treatment of infections in foodanimals might also select for resistance that might be transferred to humans and thereby cause human healthproblems (Aarestrup et al., 2008). Thus, whenever This work was supported in part by grant 274-05-0117 initiating treatment of food animals the human health Antimicrobial resistance in swine production resistance among bacterial pathogens and indicator bacteriain pigs in different European countries from year 2002– Aarestrup FM and Schwarz S (2006). Staphylococci and 2004: the ARBAO-II study. Acta Veterinaria Scandinavia streptococci. In: Aarestrup FM (ed.) Antimicrobial Resis- tance in Bacteria of Animal Origin. Washington, DC, USA: Jensen VF, Jacobsen E and Bager F (2004). Veterinary ASM Press, pp. 187–206. (ISBN 1-55581-306-2).
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