Disease- and pathogen specific guidelines
A system-based approach to antimicrobial therapy is outlined in the following paragraphs. The guidelines provided in this section strike a balance between prudent antibiotic use and recommendations from the scientific literature. However, the equine literature is not always based on objective scientific studies. Clinical reports, antimicrobial susceptibility data and clinical experience are often used to formulate published recommendations because of the relative paucity of scientific data. As such, the scientific literature tends to be dominated by recommendations that promote the use of broad-spectrum drugs and antimicrobial combinations, especially penicillin with gentamicin. There is a need for more research to rationalize the
use of antimicrobials in equine medicine. The following tables provide recommendations for specific diseases/syndromes and pathogens. The tables have been drafted considering a combination of factors, including expected pathogens, expected susceptibility patterns and typical patient factors. The recommended doses for antimicrobial agents used in equine medicine are listed in Table 10.2. The scientific quality of the literature on which these tables are based is highly variable, as there is a general paucity of well-controlled studies on antimicrobial efficacy in horses. Many antimicrobial recommendations, particularly multiple drug combinations, have been passed down through the literature but are not based on any objective data. A common example of this is the combination of penicillin, gentamicin and metronidazole, which is sometimes used for the treatment of life-threatening conditions such as pleuropneumonia
and peritonitis. This triple antimicrobial combination is considered the most broad-spectrum
coverage possible for equine pathogens, with the exception of resistant organisms and Mycoplasma spp. However, this triple combination tends to be employed based on fears of missing a pathogen involved, economic value of the horse or lack of knowledge about the disease. The combination of a β-lactam with an aminogylcoside is a very broad-spectrum combination for sensitive organisms. However, some anaerobes, notably some strains of Clostridium and Bacteroides are not affected by β-lactams. Metronidazole treatment improves anaerobic coverage with better pharmacodynamic and pharmacokinetic characteristics for long-acting penetration into difficult to reach body sites. Nevertheless, most infections in horses are caused by aerobic Gram-positive and Gram-negative
bacteria, and thus this triple antimicrobial combination does not represent improved coverage. In fact, it is possible that the more antimicrobial treatments that disrupt the intestinal anaerobic population, the more likely the horse could develop antimicrobialassociated antimicrobialassociated colitis. Thus, it would be more prudent to put thought and effort into finding the cause(s) and choose antimicrobials with better pharmacodynamic and pharmacokinetic characteristics against Grampositive and Gram-negative infections. Equine anaerobic infections in most cases are more likely associated with mixed chronic infections (e.g. >5 days) in body
sites that can develop into low oxygen tension sites (e.g. pleura, peritoneum, deep wounds).
Our recommendations should be considered general guidelines that do not supersede information
obtained through culture and susceptibility testing from the individual patient. Most of them are disease-specific recommendations that can be used in situations where the specific agent or its susceptibility pattern is unknown, when samples are not submitted to laboratory diagnosis or while culture results are pending. Only few pathogen-specific recommendations are provided to guide antimicrobial selection when the causative agent has been identified and in vitro susceptibility data are available, as this situation is rather infrequent in clinical practice. In some situations, multiple options are presented in each category (first, second and last choice). This is because recommended drugs within the same category are presumed to be similarly appropriate and other factors such as cost, route of administration and patient factors (e.g. age, concurrent disease) should be considered for selecting the best antimicrobial option.
Furthermore, not all of the suggested antimicrobials are available in all jurisdictions and the use of certain compounds (i.e. chloramphenicol) is banned in some countries.
Disease- and pathogen specific guidelinesA system-based approach to antimicrobial therapy is outlined in the following paragraphs. The guidelines provided in this section strike a balance between prudent antibiotic use and recommendations from the scientific literature. However, the equine literature is not always based on objective scientific studies. Clinical reports, antimicrobial susceptibility data and clinical experience are often used to formulate published recommendations because of the relative paucity of scientific data. As such, the scientific literature tends to be dominated by recommendations that promote the use of broad-spectrum drugs and antimicrobial combinations, especially penicillin with gentamicin. There is a need for more research to rationalize theuse of antimicrobials in equine medicine. The following tables provide recommendations for specific diseases/syndromes and pathogens. The tables have been drafted considering a combination of factors, including expected pathogens, expected susceptibility patterns and typical patient factors. The recommended doses for antimicrobial agents used in equine medicine are listed in Table 10.2. The scientific quality of the literature on which these tables are based is highly variable, as there is a general paucity of well-controlled studies on antimicrobial efficacy in horses. Many antimicrobial recommendations, particularly multiple drug combinations, have been passed down through the literature but are not based on any objective data. A common example of this is the combination of penicillin, gentamicin and metronidazole, which is sometimes used for the treatment of life-threatening conditions such as pleuropneumoniaand peritonitis. This triple antimicrobial combination is considered the most broad-spectrumcoverage possible for equine pathogens, with the exception of resistant organisms and Mycoplasma spp. However, this triple combination tends to be employed based on fears of missing a pathogen involved, economic value of the horse or lack of knowledge about the disease. The combination of a β-lactam with an aminogylcoside is a very broad-spectrum combination for sensitive organisms. However, some anaerobes, notably some strains of Clostridium and Bacteroides are not affected by β-lactams. Metronidazole treatment improves anaerobic coverage with better pharmacodynamic and pharmacokinetic characteristics for long-acting penetration into difficult to reach body sites. Nevertheless, most infections in horses are caused by aerobic Gram-positive and Gram-negativebacteria, and thus this triple antimicrobial combination does not represent improved coverage. In fact, it is possible that the more antimicrobial treatments that disrupt the intestinal anaerobic population, the more likely the horse could develop antimicrobialassociated antimicrobialassociated colitis. Thus, it would be more prudent to put thought and effort into finding the cause(s) and choose antimicrobials with better pharmacodynamic and pharmacokinetic characteristics against Grampositive and Gram-negative infections. Equine anaerobic infections in most cases are more likely associated with mixed chronic infections (e.g. >5 days) in bodysites that can develop into low oxygen tension sites (e.g. pleura, peritoneum, deep wounds).Our recommendations should be considered general guidelines that do not supersede informationobtained through culture and susceptibility testing from the individual patient. Most of them are disease-specific recommendations that can be used in situations where the specific agent or its susceptibility pattern is unknown, when samples are not submitted to laboratory diagnosis or while culture results are pending. Only few pathogen-specific recommendations are provided to guide antimicrobial selection when the causative agent has been identified and in vitro susceptibility data are available, as this situation is rather infrequent in clinical practice. In some situations, multiple options are presented in each category (first, second and last choice). This is because recommended drugs within the same category are presumed to be similarly appropriate and other factors such as cost, route of administration and patient factors (e.g. age, concurrent disease) should be considered for selecting the best antimicrobial option.Furthermore, not all of the suggested antimicrobials are available in all jurisdictions and the use of certain compounds (i.e. chloramphenicol) is banned in some countries.
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