Antimicrobial Resistance in Swine Production

An overview of recent susceptibility data of common pig pathogens to antimicrobial agents used in pig production has been published by F.M. Aarestrup and colleagues. They went on to discuss the development of resistance and its implications for human health. For ThePigSite, editor Jackie Linden, summarises some of the important points raised in their review.

20 February 2009

6 minute read

Frank Aarestrup of Denmark's National Food Institute and veterinarians Oliver Duran from Ireland and David Burch from the UK published a paper entitled 'Antibiotic resistance in swine production' in Animal Health Research Reviews last year.

The authors set out to give an overview of the occurrence of resistance among the most common pathogens in pigs, followed by the trends they have observed and a discussion on possible future trends.

"Large amounts of antimicrobial agents are still being used in modern swine production in many countries around the world," the report says. A survey carried out in year 2000 indicated this amounted to $1.7 billion. It is because of the implications of this widespread use on human and animal health that has caused greater scrutiny of the use of medications for farm animals generally.

Common Pathogens Causing Infections in Pigs

The authors point out that there are few estimates of the prevalence of porcine infections and the use of antimicrobials to treat them.

Perhaps the best picture is from Denmark, which has had a monitoring programme for antimicrobial use since 2000. The data show that the majority of antimicrobial treatments are for gastrointestinal infections in weaners, amounting to more than 170 million doses in 2007.

That programme gives no indication of the causative agents of the infections, however. Although usually presented with the more hard-to-diagnose infections, reports from diagnostic labs offers some clues about the most common infections. Comparisons with historical data can highlight some interesting changes in the prevalence of the various pathogens. For example, in 1999, Escherichia coli accounted for almost half of the digestive diseases in pigs in the UK but by 2006, it had been overshadowed by Lawsonia and Brachyspira.

Antimicrobial Resistance among Major Pathogens

Enteric infections

For E. coli, the report says, " A very high frequency of resistance is found in some countries and the antimicrobial susceptibility of E. coli is difficult to predict." Knowledge of the local situation and susceptibility testing are required but the authors indicate that polymyxins can be reliable treatment for E. coli.

Salmonella rarely causes disease in pigs - with the exception of S. cholerasuis, which is quite commonly reported in the US and Asia. The issue with Salmonella is otherwise linked to foodborne disease in humans. "In general, antimicrobial treatment is not recommended in animals because this might lead to resistance development and thus, human health problems," according to the paper.

Clostridium perfringens is uncommon in pigs and is usually controlled by vaccination. Cl. difficile has recently been found to infect pigs. There is very limited data on the susceptibility of these clostridia to antimicrobials.

Widespread on pig farms, the susceptibility of Lawsonia intracellularis to antimicrobials is difficult to establish but it appears that resistance is developing in US strains.

Brachyspira hyodysenteriae, the cause of swine dysentery, and related B. pilosicoli, are showing resistance to a number of commonly used antimicrobials. As a result, the authors strongly recommend susceptibility testing, followed by a short course of an appropriate antibiotic combined with enhanced pen and farm sanitation.

Respiratory and systemic infections

Actinobacillus pleuropneumoniae used to be effectively treated with penicillins but resistance has developed. The authors say that most isolates seem still susceptible to fluoroquinones, ceftiofur and florfenicol.

For atrophic rhinitis caused by Bordetella bronchiseptica and Pasteurella multocida, antimicrobial therapy has largely been replaced by vaccination of sows. Both organisms have developed some level of antimicrobial resistance.

There is little data on the susceptibility of Mycoplasma hyopneumoniae and M. hyosynoviae but tiamulin is usually effective. Control of M. hyopneumoniae is now widely achieved by vaccination, with up to half of growers in some countries receiving the vaccine.

Penicillins are the traditional drug of choice against Streptococcus suis but reports of resistance are emerging in some countries.

Haemophilus parasuis causes Glässers disease. Until recently, it could be treated reliably with penicillins but it is now showing a high frequency of resistance.

Because of the availability of effective vaccines against Erysipelothrix rhusiopathiae, antimicrobial therapy is rarely required these days but penicillin remains an effective treatment.

A high frequency of resistance has developed in Staphylococcus hyicus, the cause of greasy pig disease. Isolates are still generally susceptible to florfenicol, fluoroquinolones and gentamicin.

A methicillin-resistant strain isolate of Staphylococcus aureus (MRSA) has emerged among pigs. The authors say, "It is difficult to predict the susceptibility and the potentially continued emergence of MRSA, which might not only have implications for human health but also make it more difficult to treat infections is swine."

General Principles for Disease Control

"Disease control is not only about using medicines," the authors clearly state. It is important to get the environment right for both the pig and for disease prevention.

Disease control comes down to two approaches: avoiding infectious agents and avoiding clinical disease. Infectious agents can be avoided by starting up a new, healthy herd or by depopulating, thorough cleaning and re-stocking with clean stock. Avoiding clinical disease requires a range of aspects covering herd management, pig housing and environment and establishing immunity.

Conclusions

The authors highlight that the data demonstrate how antimicrobial resistance varies widely between and within farms. They suggest that the final choice of treatment has to be based on local knowledge, which requires regular susceptibility testing of the pathogens.

For some pathogens - for example, Mycoplasma, A. pleuropneumoniae, Pasteurella multocida and S. suis - it is easier to predict future susceptibility despite some evidence of emerging resistance. For others, including E. coli, Streptococcus and Brachyspira, resistance is developing both rapidly and unpredictably.

Some of the newer antimicrobials are still effective against most isolates but these should be given priority for human health "and their use in food animal production should be limited or avoided as far as possible," according to the report.

The authors conclude their paper that the way forward seems to be to ensure improved production systems with less dependence on antimicrobial agents and to implement more continuous monitoring at the national, regional and down on farm level to assist the veterinarian in the best choice of treatment.