Antimicrobial resistance (AMR), also known as antibiotic resistance, is a growing global concern that extends beyond equine medicine into all aspects of human and animal health.

The overuse and misuse of these critically important drugs can lead to the development of resistant pathogens, putting all animals and people at risk of potentially untreatable, life-threatening infections.

In recent years, the rise of resistant bacteria in horses has prompted a re-evaluation of antimicrobial stewardship practices within veterinary medicine. Factors contributing to antibiotic resistance in horses include unnecessary administration, inadequate dosages, and not completing a full course of antibiotics as prescribed.

These factors highlight how horse owners play a key role in responsible antibiotic management. To mitigate the risk of resistance, owners and veterinarians must work together to ensure appropriate treatment protocols and management for horses and other livestock.

By raising awareness and fostering proactive measures, everyone in the equestrian community can safeguard the health of horses and ensure the efficacy of antibiotics for future generations. Understanding and addressing AMR is not just an equine issue; it is a critical aspect of global health for humans and animals alike.

Antimicrobial Resistance

Antimicrobials, commonly known as antibiotics, are medications designed to target and destroy bacteria. Since their discovery in 1928, antibiotics have become a critical component of medical practice for humans and animals alike. Antibiotics are also used in livestock production to prevent disease and improve growth rates. [1]

Bacteria evolve and adapt to their environment easily due to their high mutation rate and their ability to transfer genes between species. Bacterial adaptation typically occurs in the face of challenging environmental scenarios, including exposure to antibiotics. [1]

When bacteria adapt to the presence of an antibiotic, they can continue to grow and proliferate despite antibiotic use. This phenomenon is referred to as antimicrobial resistance (AMR). [1]

The World Health Organization considers AMR one of the leading causes of human death worldwide and a major threat to global health. Estimates suggest that up to 1.27 million people died from AMR-related disease in 2019 alone. [1]

How Does Antimicrobial Resistance Develop?

The most common causes of antimicrobial resistance are exposing bacteria to low doses of antibiotic medications or not treating a bacterial infection for an appropriate length of time. [1]

In both scenarios, bacteria resistant to the antibiotic survive and proliferate. This means the infection continues and the growing population of bacteria carry the resistance mutations, passing them to their progeny over generations. [1]

Eventually, the proportion of resistant bacteria outnumber the susceptible bacteria and the antibiotic is no longer effective, requiring a different treatment protocol.

Bacteria can also transmit genes between different bacterial strains or species. [2] One bacterial species interacting with another may pick up resistance genes, resulting in a new strain of antimicrobial resistant bacteria.

Evidence also suggests exposure to antibiotics may increase the rate of gene transfer between bacteria. [2] This means antibiotics may actually promote the spread of resistance mutations in addition to increasing the proportion of resistance within the bacterial population.

Impact of Antimicrobial Resistance

The impact of antimicrobial resistance on horses, other animals, and humans cannot be overstated. Resistance affects both treatment of individual infections and the risk of infections on a global scale. [3]

In an infected individual, antimicrobial resistance means a different type of antibiotic is necessary to completely clear an infection. This antibiotic may have worse side effects or require longer treatment times, impacting individual patient outcomes. [2]

The risk of individual resistance is particularly relevant in hospitals, as a single case of resistance can rapidly become a widespread concern. If a resistant infection spreads to other patients in the hospital (nosocomial infection), the number of individuals carrying resistant strains increases. [1] This can rapidly amplify the number of patients who need more aggressive antimicrobial therapies.

In addition, since bacteria can transfer resistance genes between each other, strains can become resistant to antibiotics they have never been exposed to. [3] In a hospital setting, it is possible for many strains or species of bacteria to interact and transfer antimicrobial resistance, further complicating treatment.

The most serious consequence of antimicrobial resistance is the development of multidrug resistant bacteria. These bacteria have resistance to at least two classes of antibiotics. [1][3]

Treating bacterial infections that have multidrug resistance requires more intensive antibiotic protocols. They also have a high rate of treatment failure, which further perpetuates antimicrobial resistance as the surviving bacteria continue to proliferate. [3]

Bacterial species with multidrug resistant strains include: [1][3]

These species of bacteria affect both animals and people, and there are reports of zoonotic (animal to human) transfer of these infections. [1]

Multidrug resistant bacteria are a significant problem, as the development of new types of antimicrobials is slow. [2] Only two new classes of antibiotics have been approved since 2017, despite ongoing research and innovation in this field. [4]

Due to the slow rate of antimicrobial development, testing, and approval, maintaining the efficacy of our existing antimicrobial medications is critically important for the health of humans and horses alike.

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Antimicrobial Resistance in Horses

Horses have been recognized as a source of antimicrobial resistant bacteria since the 1970s. [5] Due to the widespread use of antibiotics in horses, they can be an important source of antimicrobial resistant bacteria in their environment. [3]

Horses can also acquire antimicrobial resistant bacteria through contact with: [1]

  • Other horses
  • Contaminated materials
  • Other animals such as rodents

Horses carrying antimicrobial resistant bacteria can spread the bacteria to other animals, including humans. Horses may shed bacteria in their feces, respiratory secretions, or infectious material, such as pus. [1]

Once in the environment, these bacteria may contaminate water, soil, feed, bedding, or other surfaces. In addition, insects and rodents can carry antimicrobial resistant bacteria, resulting in further contamination of new environments. [1]

The most common resistant bacteria associated with horses are: [3][5]

  • Methicillin-resistant Staphylococcus aureus (MRSA)
  • Extended spectrum beta-lactamase (EBSL) producing Escherichia coli
  • Multidrug resistant Salmonella

Methicillin-resistant Staphylococcus aureus (MRSA)

Staphylococcus aureus is a common bacteria found on the skin and nasal passages of most mammals, including horses. [5] MRSA refers to strains of S. aureus that carry antimicrobial resistance genes against the antibiotic methicillin. [2] These bacteria also have an inherent resistance to penicillin.

Studies first identified MRSA in horses in 1996, and since that initial report there have been several cases reported in equine hospitals worldwide. Most MRSA infections target wounds, surgical incisions, and intravenous catheter placement sites. [6]

In hospitals with MRSA-infected horses, there are reports of medical personnel carrying MRSA in their nasal passages, likely due to exposure to the infected horses. [2] There is also a report of human MRSA-related disease developing after association with an infected horse. [3]

One study out of the United Kingdom showed that 0.6% of healthy horses carry MRSA in their nasal passages. Although the prevalence is low, it highlights the potential risk of infection when immunocompromised, injured, or sick individuals interact with carrier horses. [6]

The most common antibiotic for treating MRSA in humans is vancomycin, however there are reports of vancomycin-resistant strains. [2] Vancomycin is considered a critically important drug in human medicine, so accessibility for animal use is limited. [1][2]

ESBL-producing Escherichia coli

Escherichia coli is a member of the normal gastrointestinal flora in most mammals. However, these bacteria can cause disease in immunocompromised animals or when introduced into locations other than the gastrointestinal tract. [5]

Certain strains of E. coli produce extended-spectrum beta-lactamases (ESBL), enzymes that break down beta-lactam antibiotics. [5] Beta-lactam antibiotics include: [1]

  • Penicillin
  • Ampicillin
  • Amoxicillin
  • Ceftiofur
  • Cefovecin
  • Cefquinome

Most infections of ESBL-producing E. coli in horses occur in wounds or soft tissues. [5] There have also been reports of these bacteria causing uterine, ocular, and joint infections. [5][6]

The prevalence of carrier horses may be as high as 27.3% in equine hospitals. [5] Studies also suggest that even hospitalized horses who are not treated with antibiotics have a higher risk of acquiring ESBL-producing E. coli compared to non-hospitalized horses. [5]

Interacting with recently hospitalized horses may also increase the risk of becoming an ESBL-producing E. coli carrier. [5][7]

The prevalence of ESBL-producing E. coli in healthy horses is low at around 6.3%. [5] However, up to 69.5% of E. coli strains identified in equine feces may have antimicrobial resistance to at least one class of drug. [6] This further highlights the need for prudent antimicrobial use, to prevent further development of antimicrobial resistance.

Multidrug Resistant Salmonella

Salmonella is a common cause of gastrointestinal disease in horses. [5] Many strains of Salmonella identified in horse fecal material show single or multi- drug resistance. [5] One study of fecal samples from diseased and healthy horses in Japan showed that 90% of the samples were resistant to one or more antibiotic types. [2]

Antibiotics with reported resistance in certain Salmonella strains include: [2][5]

  • Trimethoprim and sulfamethoxazole
  • Beta-lactam containing antibiotics
  • Chloramphenicol
  • Aminoglycosides like gentamicin, streptomycin, neomycin, and amikacin
  • Tetracycline

The possibility of multidrug resistant Salmonella can complicate treatment of gastrointestinal infections in horses. Selection of an effective antibiotic can be difficult, and there may be increased costs associated with prolonged treatment or use of more expensive antibiotics. [3]

Additionally, there is a risk of spread to other hospitalized horses due to the highly infectious nature of Salmonella. [2][5]

The prevalence of Salmonella shedding in the feces is low, with only 0.8% of otherwise healthy horses shedding the bacteria. Horses that have been recently hospitalized, have health problems, or have recently received antibiotics have a higher risk of shedding Salmonella into the environment. [8]

Preventing Antimicrobial Resistance

The first step in preventing antimicrobial resistance is only using antibiotics when necessary. [1] The most common use of antibiotics in equine medicine is treating wounds or injuries. Respiratory infections and diarrhea are also commonly treated using these medications. [3]

However, most superficial wounds, viral respiratory infections, hoof abscesses, and other minor ailments do not require antibiotics. [3] A veterinarian must evaluate the horse to determine whether antibiotics are necessary for treatment and establish an appropriate treatment plan. Owners should only administer antibiotics under the direct advice of a veterinarian.

If antibiotics are necessary, careful selection and proper use is critical for preventing antimicrobial resistance. Veterinarians must consider: [1]

  • Bacterial properties: Certain bacteria have an inherent resistance to some classes of antibiotics. Knowing the type of bacteria causing the infection is important when choosing an antibiotic.
  • Antibiotic properties: Some antibiotics are dose-dependent, meaning they require a specific concentration of antibiotic at the site of infection to be effective. Others are time-dependent, meaning that they require an adequate concentration over a prolonged period for efficacy.
  • Sensitivity results: Since antimicrobial resistance is increasing in bacterial populations, veterinarians often send out bacterial samples for sensitivity testing. These tests subject the bacterial population in question to a variety of antibiotics to determine which antibiotics are most effective at targeting that population. Veterinarians use these results to design their antibiotic protocol.
  • Route of administration: Veterinarians can give antibiotics systemically (whole body) or target their administration to a specific location. For example, veterinarians can flush an infected uterus with an antibiotic preparation to deliver the medication to the site of infection directly, rather than treating the whole horse.
  • Side effects: Many antibiotics have side effects in horses. Veterinarians must consider the risk of adverse reactions when designing their treatment protocol.

If your veterinarian prescribes antibiotics for your horse, ensure that you follow the recommended dose and length of treatment. Even if your horse’s symptoms resolve or there are no longer signs of infection, continue the antibiotic treatment for the recommended number of days and dosage. [3]

Summary

Antimicrobial resistance (AMR) is a growing concern within human and animal health, posing a threat to horses as well.

  • Overuse and misuse of antibiotics in horses can lead to the development of resistant pathogens
  • Horses can carry several antibiotic-resistant pathogens without being affected themselves
  • Resistant pathogens have the potential to affect treatment outcomes, costs, and time
  • Key preventative measures include using antibiotics only when necessary, and using them for the correct length of time and at the correct dose
  • Owners should always consult their veterinarian before giving antibiotics to protect their horse’s health

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References

  1. Kabir. A. et al., Antimicrobial Resistance in Equines: A Growing Threat to Horse Health and Beyond—A Comprehensive Review. Antibiotics. 2024.
  2. Traub-Dargatz. J. L. et al., Antimicrobial Resistance: What’s the Big Deal? Importance of Antimicrobial Resistance to the Equine Practitioner. Proceedings of the Annual Convention of the AAEP. 2002.
  3. Traub-Dargatz. J. L. and Dargatz. D. A., Antibacterial Drug Resistance and Equine Practice. Equine Veterinary Education. 2009.
  4. WHO Releases Report on State of Development of Antibacterials. World Health Organization. 2024.
  5. Maddox. T. W. et al., Antimicrobial Resistance in Bacteria from Horses: Epidemiology of Antimicrobial Resistance. Equine Veterinary Journal. 2015. View Summary
  6. Maddox. T. W. et al., Cross-Sectional Study of Antimicrobial-Resistant Bacteria in Horses. Part 1: Prevalence of Antimicrobial-Resistant Escherichia Coli and Methicillin-Resistant Staphylococcus Aureus. Equine Veterinary Journal. 2012. View Summary
  7. Maddox. T. W. et al., Cross-Sectional Study of Antimicrobial-Resistant Bacteria in Horses. Part 2: Risk Factors for Faecal Carriage of Antimicrobial-Resistant Escherichia Coli in Horses. Equine Veterinary Journal. 2012. View Summary
  8. Traub-Dargatz. J. L. et al., Fecal Shedding of Salmonella Spp by Horses in the United States during 1998 and 1999 and Detection of Salmonella Spp in Grain and Concentrate Sources on Equine Operations. Journal of the American Veterinary Medical Association. American Veterinary Medical Association. 2000. View Summary