Embracing antimicrobial stewardship benefits horses and humans.

Imagine your horse was sick or injured, and your veterinarian said no to antibiotics for that cough, runny nose, traumatic wound, gnarly looking lump, or another ailment you felt should be treated with an antibiotic. Would you be upset your horse wasn’t receiving what you believe is high-quality care, or would you appreciate that your vet is a good antimicrobial steward?

What Is Antimicrobial Stewardship?

The American Veterinary Medical Association (AVMA) defines antimicrobial stewardship as “actions veterinarians take individually and as a profession to preserve the effectiveness and availability of antimicrobial drugs through conscientious oversight and responsible medical decision-making while safeguarding animal, public, and environmental health.”

“In practical terms, this means only using antibiotics when absolutely indicated, for the shortest amount of time, choosing the most appropriate drug, and making decisions such as dose and route of administration that optimize efficacy,” says Sharanne Raidal, BVSc, MVSt, PhD, GradDipEd, FANZCVSc, Dipl. ECEIM, a professor at Charles Sturt University, in New South Wales, Australia.

Some of the many ways veterinarians act as antimicrobial stewards include:

• Maintaining equine health and welfare by focusing on preventive care that will minimize the need for antimicrobial drugs. This includes husbandry and hygiene, vaccination, and biosecurity and infection control strategies.
• Using evidence-based decisions and diagnostics to determine if an antimicrobial is necessary.
• Prescribing antimicrobials sparingly and judiciously with close and continual evaluation of the therapy outcomes.
• Selecting alternatives to antimicrobials when possible.
• Educating clients about the importance of antimicrobial stewardship and what conditions do not need antimicrobial drugs. 

“There are also tricks that we can use to enhance antibiotic efficacy,” says Raidal. “For example, some antibiotics work best when we are able to provide very high drug concentrations in the region of concern using intravenous regional perfusion.”

Developing Antibiotic Resistance

What does antimicrobial stewardship and antimicrobial resistance have to do with you and your horse? After all, it’s just one horse we’re treating. Surely resistance can’t develop from a single patient, can it?

Indeed, it can. In fact, scientists detected antimicrobial resistance almost simultaneously with the discovery of antimicrobials.

“Sir Alexander Fleming observed and reported antimicrobial resistance to penicillin in his original work back in the 1940s,” Raidal says. “In Fleming’s 1945 Nobel lecture he wrote, ‘The time may come when penicillin can be bought by anyone in the shops. Then there is the danger that the ignorant man may easily under dose himself and, by exposing his microbes to nonlethal quantities of the drug, make them resistant.’ ”

When it comes to resistance, Raidal emphasizes the importance of understanding the two main types:

1. Innate resistance

“This means that the bacteria are fundamentally unable to be killed by the antibiotic,” she says. “A good example would be that aminoglycosides, such as amikacin, have no effect against anaerobic bacteria. Because aminoglycoside uptake into bacteria is an oxygen-dependent process, anaerobes are innately resistant to them, and we therefore need to make a different choice for anaerobic infections.”

2. Acquired resistance

“This is the type of resistance we are concerned about,” stresses Raidal. “As Fleming warned, we select for this type of resistance by exposing bacteria to low, sublethal concentrations of antibiotic. This is the single most effective method of selecting for antibiotic resistance.”

Further, a small percentage of bacteria, by genetic chance, have mechanisms that confer resistance to antibiotics.

“By killing susceptible organisms, we allow these more resistant types to preferentially multiply,” she explains.

Misusing Antimicrobials: Not Always Neglect

Despite Fleming’s warnings, people have misused antibiotics in both human and veterinary medicine by underdosing, extending treatment durations unnecessarily, and addressing nonbacterial conditions with antimicrobials. As a result, multidrug-resistant bacteria and superinfections have emerged, defying even our strongest antibiotics.

Recently van den Brom-Spierenburg et al. (2024) conducted a survey of equine veterinarians’ prescribing habits in cases of equine asthma. In that survey 249 veterinarians from 25 countries received descriptions of horses with mild, moderate, and severe asthma. Overall, 53% of the respondents reported they would use antimicrobials “at least sometimes” in at least one of the three equine asthma cases. The most common reason they chose to use antimicrobials at least sometimes was because they suspected a secondary bacterial infection. The most prescribed antimicrobials included trimethoprim sulfonamide combinations (80%), penicillins (40%), and tetracyclines (29%). However, a smaller number of respondents selected the critically important antimicrobials: cephalosporins (15 practitioners, or 11%), fluoroquinolones (9, or 7%), and macrolides (1, or less than 1%).

At first glance these results might seem to suggest that equine practitioners engage in “antibiotic misuse.” However, Clare Ryan, DVM, PhD, Dipl. ACVIM, an assistant professor at the University of Georgia’s College of Veterinary Medicine, in Athens, says, “Veterinarians almost never
intentionally misuse antibiotics, but practical factors may prevent them from closely following antimicrobial stewardship guidelines. For example, if owners do not approve the diagnostic tests needed to fully rule out a bacterial infection, the veterinarian may feel it is risky to withhold treatment. Veterinarians might also face significant pressure to treat with antibiotics from horse owners or farm managers, who may not be able to differentiate between a horse with asthma and a horse with bacterial pneumonia.”

The Real Threat of Antimicrobial Resistance

Nonetheless, antimicrobial resistance threatens all animal species, and we must take steps to improve our judicious use of antimicrobials. Tallon et al. (2023) reported that in 2019, bacterial antimicrobial resistance contributed to an estimated 4.95 million human deaths, with 1.27 million of these deaths directly attributable to antimicrobial resistance. By 2050, experts anticipate 10 million people will die each year because of antimicrobial resistance.

“There are already glaring examples of the impact antibiotic resistance has on the equine community,” says Ryan. “The bacterium Rhodococcus equi, which causes pneumonia in foals has, in some parts of the U.S., acquired resistance to more than one class of antibiotics. This means veterinarians can be left with few, if any, effective antibiotics for treating infected foals.”

Encourage Laboratory Testing Before Antibiotic Use in Horses

“One of the most powerful tools veterinarians have available when choosing antimicrobials is a bacterial culture with antimicrobial susceptibility testing, often referred to as culture and sensitivity,” Ryan explains. “It is currently the most accessible and practical way for veterinarians to identify which bacteria are causing an infection and help guide antibiotic selection.”

Researchers have shown, however, that veterinarians either do not or cannot perform culture and sensitivity before antibiotic use in some situations. Ryan says major roadblocks to performing this test include:

• The urgent need to institute therapy
• Financial constraints
• Sampling difficulties
• Length of time until results become available (three to five days)
• Concerns about relevance of in vitro (in the lab) results

“When testing is not performed, veterinarians must rely on information gathered from previously described cases and take their best guess about what bacteria are causing the infection,” says Ryan. “These educated guesses can easily be incorrect. Veterinarians are also then assuming that the offending bacteria are not resistant to typically used antibiotics.”

Raidal agrees, adding, “Sometimes we need to start treatment straight away because the horse is so sick that we can’t wait or because the clinical condition is so strongly supportive of infection that it is unwise to delay treatment.”

When Is Antimicrobial Testing Important?

She considers laboratory testing (cytology or culture and sensitivity) important for four reasons:

1. If testing does not support infection, veterinarians and owners can stop antibiotic treatment straight away.
2. When available, culture and sensitivity results reveal whether the first guess (i.e., empirical choice) was correct.
3. If the clinical condition is not responding or the veterinarian wants to de-escalate treatment (e.g., narrow the choice of antibiotic activity or select an oral antibiotic to save the horse from injections), the results will direct the second choice of treatment for that horse.
4. Every set of results helps build a record of which bacteria cause specific problems in treated horses and which drugs are likely effective in the equine population. In other words, each horse treated benefits from the information gathered from previous cases.

Ryan adds, “There are, of course, some situations when culture truly is not possible to perform. However, owners can contribute to good antimicrobial stewardship by discussing the value of performing a culture with their veterinarian whenever bacterial infection is suspected.

“Veterinarians can engage in conversations with clients to clarify how culture results could contribute to faster resolution of the infection, with less chance of treatment failure,” she says. “Even if a client initially declines culture tests, veterinarians can make it a habit to obtain consent to collect and store samples for later submission in case treatment failure occurs.”

What Antimicrobial Alternatives Do We Have?

We must fiercely protect our remaining antimicrobials, our sources say, because few, if any, alternatives are on the horizon.

“Most of the antimicrobial agents we use in horses were originally developed for use in people,” Ryan explains. “New antimicrobials are rarely discovered, and of those, even fewer prove safe and effective enough to successfully make it to the market. As resistance spreads, we continue to lose effective antimicrobial drugs to combat potentially deadly infections.”

“Economic drivers for drug development do not favor the production of new antibiotics, and certainly not at prices that will enable us to treat animals in our care at prices their owners can afford,” adds Raidal.

Thus, whenever possible, our sources say we should embrace strategies that avoid the use of antimicrobials, which is exactly what some researchers are doing. Here is a short summary of a few recently published research studies attempting to spearhead antimicrobial stewardship.

Using honey as an antimicrobial

In the case of skin wounds, Manuka honey has received increasing attention (see TheHorse.com/woundcare#honey). For example, Gandini et al. (2024) examined the effect of a pectin-honey hydrogel to prevent surgical site infections (SSIs) following abdominal surgery (laparotomy). These abdominal-incision infections occur relatively frequently, complicate healing, and represent an additional financial burden for horse owners.

 The researchers randomly divided 36 horses undergoing emergency abdominal surgery into two groups. They treated half the horses with the hydrogel on the body wall (linea alba) closing the subcutaneous tissues and skin, and they did not treat the other half with the hydrogel (controls). Gandini and colleagues saw an overall SSI occurrence of 19.4%, but only one out of 18 horses (5.5%) treated with the hydrogel developed an infection. In contrast, six out of 18 (33.3%) horses in the control group developed SSI. Horses in the control group were 8.5 times more likely to develop a surgical site infection.

Antimicrobial powers of orthobiologics

Veterinarians are also looking at stem cells and platelets for their antimicrobial properties. In a literature review Pezzanite et al. (2023) reported that both stem cells and platelets exert potential antimicrobial effects, particularly platelet lysate products. These are created by pooling platelets collected from multiple donor horses and lysing the platelets to release their antimicrobial peptides (short chains of amino acids). In preliminary studies researchers have shown these products effective against both gram-positive and -negative bacteria, as well as biofilms produced by Staphylococcus aureus. Scientists have also studied commercially available lysate products’ effects in joint infections.

Plant-based antimicrobials

On an experimental stage, researchers are looking for novel plant-based antimicrobials. In a recent study Emiru et al. (2024) used strangles as an example. Strangles, caused by the bacterium Streptococcus equi, is often treated with penicillin; however, resistance is now developing. Emiru et al. cited research showing 12.5% of S. equi isolated from upper respiratory tract samples had resistance to penicillin. Hoping to identify plant-based antimicrobials, the researchers evaluated the antibacterial activity of ethanol extracts from the stem bark, leaves, and roots of the velvet bushwillow plant, Combretum molle, against S. equi cultures isolated from clinical strangles cases. All tested extracts had significant antibacterial activity against clinical isolates of S. equi in vitro, with the root extract performing better than the stem bark and leaf extracts. Of note, bark and root extracts both had a greater inhibitory effect on S. equi growth than penicillin G.

The researchers concluded, “These findings support the traditional social claims that the plant can be used to treat equine respiratory conditions and suggesting its high potential to be developed as an effective antibacterial drug.”

Raidal acknowledges the importance of this type of research but adds, “There is no limit to what we might achieve; however, it remains unwise to expect that we can use available resources in a foolish manner and expect additional fixes to become available. We need to look after what we’ve got!”

Take-Home Message

The AVMA has an extensive list of ways equine veterinarians can embrace the judicious use of antibiotics (available at tinyurl.com/bdhkf2zw). In a nutshell, our sources say the best way to preserve antimicrobial efficacy involves choosing the right antimicrobial for the right patient, at the right time, with the right dose, and for the right route, causing the least harm to the patient and future patients. “Veterinarians and horse owners all have the same goal: to treat the sick or injured animal in front of us,” says Ryan. “As an equine community, we need to take it one step further and support practices that protect future generations of horses and their humans from antibiotic resistance.”