Why is antimicrobial resistance such a big deal, and what can you do about it? 

None of us remembers living in a world without antimicrobial drugs. Yet, less than a century—a mere lifetime—ago, people and animals routinely died from conditions that today require no more than a tube of antibiotic ointment or a short course of antibiotics to cure.

According to the Centers for Disease Control and Prevention (CDC), simply using antibiotics creates resistance. How? Antibiotic resistance is a complicated process that scientists do not fully understand. But, simply put, in any population of microbes, some naturally resist the drugs used to control or kill them. Then, when an infection arises and antimicrobials are used to kill disease-causing microbes, the commensal (good) microbes that protect the body from infection also die. This leaves the drug-resistant microbes to proliferate unchecked. And, some bacteria pass their resistant genes to other bacteria, further spreading antibiotic resistance.

In humans the CDC estimates that 2,049,442 illnesses and 23,000 deaths occurred due to antibiotic resistance in 2017 alone and, of these, 250,000 illnesses (12%) and 14,000 deaths (61%) were due to the bacterium Clostridium difficile.

As far as microbes that affect both humans and horses, C. difficile ranks No. 1 among drug-resistant threats in the United States, with an “Urgent” CDC classification. On the CDC’s “Serious” list are Salmonella and methicillin-resistant Staphylococcus aureus (MRSA). Erythromycin-resistant group A Streptococcus falls in the “Concerning” category. Also of concern is Escherichia coli and related bacteria (Enterobacteriaceae), says J. Scott Weese, DVM, DVSc, Dipl. ACVIM (LAIM), veterinary specialist at the University of Guelph, in Ontario, Canada, and a partner at the National Collaborating Centre for Infectious Diseases.

Wendy Vaala, VMD, Dipl. ACVIM, senior equine technical service veterinarian at Merck Animal Health, who’s based in Alma, Wisconsin, says another bacterial species that has developed antimicrobial resistance (AMR) is Rhodococcus equi, which causes pneumonia complicated by abscess formation in young foals. “R. equi is frequently treated with one of two macrolide antibiotics—clarithromycin or azithromycin—and rifampin,” she adds. “In recent years these antibiotics have been used with increasing frequency in foals suspected of having R. equi pneumonia. In many cases treatment is started prior to the onset of any clinical signs. As a result there has been an increasing number of reports of R. equi isolates that are resistant to these antibiotics.”

Indeed, antimicrobial-resistant infections of all kinds continue to challenge veterinarians and compromise equine health. So what do we do that contributes to AMR? What is your veterinarian doing to curb it? And what can you do to help minimize antibiotic resistance? Read on to learn what these veterinarians recommend.

What’s the Difference Between Antibiotics and Antimicrobials?

When we talk about antibiotics and antimicrobials, what do we mean? Simply defined, microbes are organisms too small for the eye to see. They’re found everywhere on earth and include bacteria, viruses, fungi, and parasites. Most are harmless. Some are even beneficial. But many (known as pathogens, germs, bugs, etc.) can cause disease in humans, animals, and/or plants. All types of microbes can develop resistance to the drugs (antimicrobials) created to destroy them.

Among antimicrobials, antibiotics target bacteria. Antivirals, antifungals, and antiparasitics (aka anthelmintics) fight viruses, fungi, and parasites, respectively.

Antimicrobial-resistant bacteria—rather than viruses, fungi, or parasites—pose the major threat in horses. “We rarely use antivirals and antifungals for horses, and parasites are not typically considered microbes in this context,” Weese says.

While AMR refers to all types of microbes, for this article’s purpose it refers to antibiotics.

Why Is AMR So Concerning?

Once bacteria develop resistance, we’ve lost our ability to control their proliferation. This throws medical options for treating infectious diseases back to the early 1900s, before penicillin became widely used. There simply isn’t “just another antibiotic.”

“New classes of antibiotics are screened first for use in human medicine,” says Vaala. “Only if a drug is deemed unsuitable for humans is it likely to be considered for use in animals. This additional filter will further slow the development of new drugs for the veterinary field.”

The CDC says that while antibiotics are life-saving drugs, up to 50% of the time they aren’t “optimally prescribed.” This means they’re used incorrectly, dosed improperly, or taken for the wrong duration, all of which promote AMR.

The food chain can also promote AMR bacteria when antibiotics are used inappropriately in food animals (for growth rather than to treat infectious disease), subsequently passing antibiotic-resistant bacteria to humans.

Human medicine plays a pivotal role in AMR, as well, because many harmful organisms can spread not only from animal to animal but also from human to human and from animals to humans (in ways besides the food chain) and vice versa.

What Are Veterinarians Doing About AMR?

A multifaceted problem, AMR calls for a multifaceted response. It requires the cooperation of everyone involved in diagnosing, making treatment decisions for, and treating bacterial infections. This includes doctors and veterinarians, livestock producers, and horse and pet owners.

“Veterinarians are trying to implement the concept of antimicrobial stewardship, which involves improving antimicrobial use practices,” Weese says. “The American College of Veterinary Internal Medicine (ACVIM) Consensus Statement from 2015 highlights some of those.” You can search for this document at onlinelibrary.wiley.com.

“Also, the American Association of Equine Practitioners’ (AAEP) Prudent Drug Usage Guidelines are a pending source of information,” he adds. “Essentially, antimicrobial stewardship focuses on only using antimicrobials when they are needed, using the right drug for the right situation at the right dose, and taking practical measures to reduce the risk of disease.”

The present AAEP Guidelines also stress continuing education for veterinarians that focuses on using antimicrobial drugs appropriately. Veterinarians can then not only use what they’ve learned but also pass updated, pertinent information on to their clients.

Vaala says the detailed updated guidelines for judicious antimicrobial use in horses will be available on the AAEP website later this year.

What Can You Do?

Both Weese and Vaala say three commitments on your part will impact AMR in a positive way:

1. Do all you can to prevent disease and, therefore, antibiotic use.

“If horses don’t get sick, there’s no pressure to use antibiotics,” Weese says. “Disease prevention practices—good infection control and biosecurity, vaccination, and good management—are all important factors.”

“Good husbandry and biosecurity practices can help reduce the risk of contagious diseases from spreading between horses,” Vaala adds. “For instance, new arrivals should be isolated from resident horses for a minimum of three weeks to ensure they aren’t ill with transmissible diseases.”

2. Reserve antibiotics for bacterial ­infections.

Antibiotics don’t work on viruses. They also don’t work on other noninfectious inflammatory conditions, says Vaala.

“Viruses can be treated with antiviral agents, although those drugs are rarely used in the horse for a variety of reasons, including lack of specific dosing information and expense, as well as the fact that many viral infections are of short duration and resolve with time, rest, and good nursing care,” she says. “Without appropriate diagnostics, many equine viral infections are treated unnecessarily with antibiotics.”

She recommends trying to identify the bacteria involved and establish which antibiotics are most effective against them. 

“A horse with a fever, cough, and nasal discharge might have a viral infection or a bacterial infection,” she continues. “There are tests to help differentiate between viral and bacterial causes, and veterinarians are encouraged to pursue diagnostic testing whenever possible to determine the cause of an inflammatory condition.”

For example, she says, veterinarians can collect a nasal swab and submit it for qPCR testing (which looks for DNA) against a panel of viral (equine influenza virus, equine herpesvirus-1 and -4, equine rhinitis virus) and bacterial (Streptococcus equi, the causative agent of strangles) pathogens. If they suspect a lower respiratory infection they can submit a transtracheal aspirate for bacterial culture. Fecal cultures can help diagnose gastrointestinal bacteria such as Salmonella or C. difficile. As an additional diagnostic they might also measure serum amyloid A (a protein the liver produces in response to inflammation) levels to rule out the need for antibiotics.

Horses ill with influenza don’t need antibiotics, of course, only requiring them if they develop a secondary bacterial infection, says Vaala.

“A wound is another example of a condition that may not require systemic antibiotic therapy to heal. And not all cases of diarrhea should be treated with antibiotics,” she continues. “Also, when surgery is required, perioperative (around the time of surgery) antibiotics are used carefully, and the duration of treatment will be based on the procedure being performed and the likelihood of bacterial contamination. In general, veterinarians are trying to restrict the use of antibiotics to those cases that have a bacterial component that requires drug treatment for ­resolution.”

3. Get—and follow—your veterinarian’s advice regarding proper antibiotic protocol.

Always consult your veterinarian for the correct type, dose, and duration of prescribed treatment for any horse’s particular condition.

Veterinarians receive intensive training in the mechanism and efficacy of individual antibiotics, dosage regimens, drug interactions, and side effects. Therefore, they’re most qualified to prescribe a protocol that minimizes the risk of AMR by treating the infection effectively and efficiently.

Many of us have been guilty of abusing antibiotics at one time or another, either in treating our horses, ourselves, or family members. Vaala provides a short list of “never-dos” that can guide you in proper and effective antibiotic use:

1. Never use one horse’s antibiotic on another horse. The mechanism or dosage might be inappropriate or the horse’s condition inaccurately assumed.
2. Never stop giving the antibiotic before the prescribed end of treatment. It takes a certain amount of time for the targeted bacteria to succumb to the antibiotic so, if you stop treatment early, you’re letting those bacteria live—and multiply.
3. Never skip doses. Antibiotics need to maintain a constant level in the horse’s body, and skipping a dose can allow so-called superbugs to develop and proliferate.
4. Never stray from FDA-approved antibiotics. Don’t use illegally compounded formulations of antibiotics.
5. Never hesitate to have your horse re-examined if his condition isn’t improving with a prescribed antibiotic.
6. Never keep outdated antibiotics for later use. They lose effectiveness over time and, again, a horse’s condition could be inaccurately assumed.

Take-Home Message

Antimicrobial resistance develops when antibiotics are used improperly. Because a finite number of these drugs exist, we all must do our part to ensure they remain effective into the future, for both our horses’ sake and our own.