Global Perspective on Antimicrobial Use in Swine

Improved management and husbandry systems in swine farming have helped to prevent infectious diseases, including improvement of biosecurity, sourcing of high‐health breeding animals, utilization of “all‐in and all‐out” and multisite production. Further, increasing knowledge of feeding, development of feed additives, and efficient vaccines have added to the prevention and management of infectious diseases, potentially reducing the need for and consumption of antimicrobials in swine production. In spite of this, not all infectious diseases are controlled sufficiently. Problems in recruiting qualified staff and economic incentives to reduce production costs and increase productivity may lead to use of lower quality feed, increased stocking density, and lower management quality. In addition, preventive medication and antimicrobial growth promoters are used in some parts of the world. Therefore, there is and will be an ongoing need for treatment of pigs with antimicrobials.

Antimicrobial use in swine has always been substantial and in some cases, the swine industry has been considered overreliant on their use. As farms have evolved from small backyard operations to today’s substantially larger units, it is not surprising that antimicrobials have been used to help farmers maintain production under these major management, housing, and disease pattern changes.

Globally, antimicrobial use in swine is significant and accounts for a large part of the total antimicrobial use in animals. The amount of antimicrobials consumed by food animals worldwide is almost double that used in humans (Aarestrup, 2012) and the global antimicrobial consumption by food animals will increase substantially from 2010 to 2030 based on increasing demand for animal protein in low‐ and middle‐income countries. The production of pigs will be one of the main drivers of this increase (Van Boeckel et al., 2015).

Significant differences exist between countries, but consumption data suggest that managing antimicrobials in modern and intensive pig production is important in reducing the use of antimicrobials globally. This is in agreement with an OIE published report that highlights important animal diseases for which the development of a vaccine would potentially reduce antimicrobial use in swine. In total, eight porcine bacterial pathogens and three viral agents were identified: Streptococcus suis, Pasteurella multocida, Mycoplasma hyopneumoniae, Actinobacillus pleuropneumoniae, Escherichia coli, Clostridium perfringens, Lawsonia intracellularis, Brachyspira spp., porcine reproductive and respiratory syndrome virus, swine influenza virus, and rotaviruses (OIE, 2015). Overall, this makes antimicrobial use in swine highly relevant for the swine industry, the individual farmer, and global One Health. It must be remembered that antimicrobials have a cost and that farmers are unlikely to pay for them unless they can see a benefit from their use.

Responsible use calls for veterinarians and farmers to use antimicrobials “as little as possible but as much as needed.” There are a number of guidelines on how to use antimicrobials properly. At the same time, efforts are being made to reduce unnecessary use and forbid the use of critical human antimicrobials where suitable alternatives are available.

It is the aim of this chapter to provide insights into the use of antimicrobials in swine and to help with decision making regarding when to use antimicrobials, selection of active substance, route of administration, and dose.

Patterns of Antimicrobial Use in Swine

Globally, national information about administration, age groups, and indications for antimicrobial consumption is sparse, but more and more countries are setting up surveillance systems for antimicrobial consumption. Several studies have reported on the consumption patterns in swine, though primarily from Europe. Variation in antimicrobial use in relation to amount, age groups, and indications exists between herds, countries, and studies (Sjölund et al., 2016).

The postweaning production stage is exposed to the highest antimicrobial use across several European countries, though in countries like Germany and Sweden, the highest usage was in suckling pigs. Similar differences exist among countries in relation to the indications for which the antimicrobials are used. In Denmark and France, the majority of antimicrobials were prescribed for gastrointestinal diseases. In Germany, respiratory infections were the main indication for antimicrobial use in piglets, weaners, and fattening pigs, followed by intestinal diseases. In Austria, nonspecified metaphylactic or prophylactic treatment constituted the major indication, followed by respiratory and intestinal diseases. In Belgium, diarrhea was the main indication from weaning up to 70 days of age, while respiratory disease was the main indication in grower and finishing pigs.

The route of administration used also varies between age groups and countries. Oral administration at group level dominates, and numbers in the region of 86–98% of total consumption have been reported. In Denmark, antimicrobial use in pigs was administered orally for 24% of sows and piglets, 98% of weaners, and 74% of finishers. In contrast, Swedish herds received only 13% of antimicrobials orally.

Antimicrobial use can be therapeutic, prophylactic, metaphylactic or for growth promotion. Therapy is the treatment of infections in clinically sick pigs. Prophylactic use of antimicrobials has been defined as the treatment of healthy pigs to prevent disease from occurring, whereas metaphylactic use has been defined as concurrent treatment of both clinically healthy and diseased pigs belonging to the same group of animals. Finally, growth promotion is defined as the continual inclusion of antimicrobial agents in animal feed to improve growth (Aarestrup, 2005). This use has been banned in Europe, Canada, and the United States, but antimicrobial growth promoters are still used in many countries. Most prophylactic and metaphylactic treatments are administered orally in pigs. The relative occurrence of these two types of medications varies among countries.

Diagnostic Aspects of Antimicrobial Use in Swine

Several guidelines relating to the responsible use of antimicrobials in veterinary practice have been published, including standards for the prudent use of antimicrobial agents in veterinary medicine in the OIE Terrestrial Animal Health Code 2017 (OIE, 2018) and guidelines issued by the European Commission (European Commission, 2015).

According to the OIE, the first key step is to perform a proper clinical examination of the animal and decide whether antimicrobial treatment is necessary or if the disease should be managed another way. The second step is to select the most appropriate antimicrobial drugs based on clinical experience and diagnostic laboratory information, including pathogenic agent isolation, identification, and antimicrobial resistance assessment where possible. The third step is to provide a detailed treatment protocol.

The initial steps in the timely and accurate diagnosis of infectious swine diseases are often challenging. Diagnostic samples must be transported from farms to laboratories and then tested. The time lag between sampling and obtaining the result has been reported as one of the important reasons why antimicrobial susceptibility testing is not commonly performed before antimicrobial treatment (Carmo et al., 2018). Therefore, veterinarians need to treat pigs empirically in order to prevent disease progression. Knowledge of the prevalence of a disease in a given country can help when making an empirical diagnosis and guide the initial selection of an antimicrobial substance. Rapid and affordable point‐of‐care tests (pig‐side or pen‐side tests) for selected diseases for which antimicrobials are most commonly used will improve antimicrobial usage. Development of diagnostics has been reported as one of the areas that must be prioritized in animal research in order to reduce antimicrobial use (WHO, 2017).

For diseases in pigs, clinical decision making in relation to antimicrobial treatment should be based on evidence, using diagnostic test performance measures, knowledge of disease prevalence, post‐test probabilities, and scientific knowledge of the efficacy of different antimicrobial treatments or alternative (i.e., nonantimicrobial) interventions. Clinical decision making is a complex process potentially influenced by many factors and clinical decisions on antimicrobial treatment and preventive strategies in relation to diseases in pigs are likely to be a combination of clinical experience, judgment, and scientific knowledge.

For a diagnostic method to be useful, it must be accurate, simple, and affordable and it should also be demonstrated that a diagnostic test has an impact, resulting in changes related to prevention or antimicrobial treatment strategies. Lack of impact is an important reason why antimicrobial susceptibility testing is not commonly performed before antimicrobial treatment, as the results are not considered to help in the clinical decision‐making process and/or selection of the antimicrobial to be used (Carmo et al., 2018).

Aspects including selection of pigs to test, diagnostic sample, and diagnostic technique must be considered. These considerations are in no way trivial and require professional effort in each case. Interpretation of laboratory examinations must be made in relation to the clinical signs, pathology, and history from the farm. Modern techniques like PCR testing of swabs, oral fluids, fecal materials or pathological material are in general sensitive. Also techniques like MALDI‐TOF identify many bacteria after an unspecific culture. Many infectious diseases in pigs are caused by opportunistic bacterial organisms so the simple detection of such bacteria in a sample does not necessarily demonstrate the cause of disease or a need for antimicrobial treatment. Such bacteria include Mycoplasma hyorhinis, Streptococcus spp., Staphylococcus spp., Escherichia coli, and Glaesserella parasuis.

Quantitative or semiquantitative techniques should be preferred when possible, including quantitative PCR or culture. Additional characterization by virulence determination in enterotoxigenic E. coli should be performed. Service limitations in local laboratories may make such diagnostic tests difficult to obtain and increase the need for empirical antimicrobial treatments.

Laboratory investigation should include antimicrobial resistance assessment of bacteria. This may not be possible for some important bacteria like Lawsonia intracellularis or difficult for some slow‐growing bacteria like Brachyspira spp. Further, some bacteria like Glaesserella parasuis or Mycoplasma spp. may be difficult to culture and can only be detected by molecular techniques, providing no possibility for antimicrobial resistance assessment. Molecular techniques may help with this in the future but currently, it is not possible to predict the clinical effect of an antimicrobial based on demonstration of resistance genes. Additional elements concerning susceptibility testing are outlined in the section on susceptibility testing below, and see also Chapter 2.

Mixed infections pose a special situation and are very common for both enteric and respiratory diseases in swine. This may result in use of more broad‐spectrum antimicrobials or even using two different substances in some situations. For enteric infections, it is also now well documented that the infections involved can change from one batch to the next within the same herd. This has implications for both selection of antimicrobials and diagnostic strategy, since pigs representing more than one batch ideally should be included in the diagnostic workup.

The concept of primary and secondary infections is also important and must be taken into consideration when selecting antimicrobials for treatment of mixed infections. Antimicrobial therapy should always be targeted against the primary infections while secondary infections like some Streptococcus spp. or Trueperella pyogenes do not need to be taken into consideration if efficient treatment of the primary infection is performed at the beginning of a disease episode in a pig. In other situations, the primary infection is a virus infection such as PRRS virus or PCV2 virus, in which case treatment may by targeted at the secondary bacteria. Secondary bacteria may also cause more severe disease, for example Mycoplasma hyorhinis or Pasteurella multocida, and in those situations the treatment must also be targeted at these secondary infections.

Administration of Antimicrobials in Swine

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