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Update on Managing Serious Wound Infections in Horses

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Christine King
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This fourth and final article of the series continues the discussion of advances in the management of serious wound infections in horses, this time with the focus on wounds involving soft tissues. The preceding articles focused on wounds involving joints and other synovial structures 1,2 and wounds involving bone. 3 Wound infections can be a considerable challenge in this era of multidrug-resistant bacteria. Examples include these variants of common equine wound pathogens: 4 • methicillin-resistant Staphylococcus aureus (MRSA) and S. epidermidis (MRSE) • vancomycin-resistant Enterococcus spp. (VRE) • heteroresistant vancomycin-intermediate S. aureus (hVISA) • penicillin-resistant Actinobacillus spp. • aminoglycoside-resistant Escherichia coli • Pseudomonas aeruginosa strains that are resistant to most commonly used antibiotics Note that resistance is now documented in equine isolates for vancomycin5-8 and imipenem7,9,10—drugs we are advised in veterinary medicine to reserve for use only with documented multidrug-resistant pathogens. Multidrug-resistant strains of bacterial pathogens are still relatively uncommon in equine practice, but their increasing prevalence and diversity—particularly in hospitalized patients but also on-farm—spurs us to seek treatment approaches that extend beyond a reliance on antibiotics.4 The usual response to a serious wound infection is to change or add antibiotics, increase the antibiotic dosage(s), add local or regional delivery to systemic administration of antibiotics—in other words, to continue to rely on antibiotic drugs primarily. Certainly, if culture and sensitivity results indicate that a change of drug is needed or a polymicrobial infection is present which requires the addition of a different class of antibiotic drug, then these measures must be implemented immediately. In addition, local and regional modes of antibiotic delivery have undoubtedly improved our ability to manage difficult infections in horses, such as septic arthritis1,2 and osteomyelitis.3 However, because of the nature of serious wound infections, other factors beyond antibiotic sensitivity and effective delivery become crucial to a successful outcome. A healthy body has the circulatory and immunological resources to deal with most instances of bacterial invasion, so in most wounds bacterial infection remains localized and is dealt with quickly and effectively by the patient, either alone or with the aid of veterinary treatment. So with a serious wound infection, it is wise to begin with a question: Why has this infection persisted and progressed?
The first two articles in this 4-part series explored the question, Why do some infections persist and progress despite seemingly appropriate treatment?, as it pertains to wounds involving joints and other synovial structures. 1,2 Of the many possible reasons, most serious wound infections involve at least one, and usually a combination, of these factors: 3 1. extensive contamination, or bacterial burden that overwhelms the patient's resources 2. refugia which protect the bacteria from host defenses and antibiotic drugs 3. immunocompromise 4. poor perfusion 5. antibiotic insensitivity of the wound pathogen(s) These same factors, often in combination, also contribute to the persistence of wound infections that involve bone. In horses, wounds that involve bone range in severity, complexity, and long-term impact from those containing a thin sequestrum on the surface of the third metacarpus/tarsus that resolve with routine wound care after sequestrum removal, to septic osteomyelitis at the site of internal fixation that results in failure of the fracture repair and potentially in euthanasia. Yet in all cases, the principles of successful treatment are the same: • debride the devitalized or irreparably damaged bone and soft tissue • preserve and protect the vascular supply to bone and soft tissue • maintain or restore structural integrity at the site • control infection through appropriate local/regional and systemic antibiotic therapy • protect the wound from further contamination, desiccation, maceration, and trauma There are a number of review articles on the management of wounds involving bone in horses. 4-7 So, rather than plowing the same ground, this article examines some advances in wound care over the past 15 years as they relate to wound infections involving bone.
The first article of this series began an exploration of the reasons why some wound infections persist and progress despite seemingly appropriate treatment.1 Examined in that article were the impacts of extensive contamination, bacterial refugia, immunocompromise (intrinsically and extrinsically imposed), and poor perfusion. This article continues the theme with an examination of antibiotic insensitivity of the wound pathogen(s).