Antimicrobial resistance in horses | BioStar US

Antibiotic resistance in horses

Antibiotic drugs are important and often lifesaving in human and veterinary medicine.  The use of antibiotic drugs can create evolutionary pressures; in order to survive, bacteria spawn resistant genes so that they are not killed and continue to grow.  Some bacteria even pass their resistant genes to other bacteria.

Antibiotic resistance does not mean the horse’s body is resistant to antibiotics.  It means that the bacteria have become resistant to the antibiotics created to kill them.

The challenges with antibiotic resistance in horses

When bacteria develop resistance to antibiotics, veterinarians and owners are left with few choices as development of new antibiotics for animals is a long, slow process.  Veterinarians may resort to options dating back to the pre-penicillin ages.

The CDC outlines that 50% of the time antibiotics:

• are not used correctly,
• taken for the incorrect duration,
• dosed improperly,
• over-used, and
• over-prescribed.

Don’t treat a virus like a bacteria

Antibiotics don’t work on viruses.  Horses that have a fever, cough, and a nasal discharge may have a viral infection not a bacterial infection.  Pursue diagnostic testing with your vet to determine whether it is a virus or bacteria that is causing an inflammatory condition.

Tips for reducing antibiotic resistance in horses

1. Follow your vet’s prescribed dosage including duration the antibiotic is given.
2. Don’t use one horse’s antibiotic on another horse.  The dosage may be inappropriate or the condition you are treating may need a different antibiotic or no antibiotic at all.
3. Antibiotics lose effectiveness over time.  Don’t keep outdated antibiotics around for later use.
4. Keep accurate weights for your horses to help ensure accurate doses.
5. Antibiotics are not preventative medicines.

Microbes that are drug-resistant threats

In both humans and horses c.difficile is ranked as the top drug-resistant threat in the United States.  It has been classified by the CDC as urgent.  On the serious list are salmonella and Methicillin-resistant Staphylococcus aureus (MRSA).  Under the CDC’s concerning category is the Erythromycin-resistant group: Streptococcus, Esherichia coli and Enterobacteriaceae.

Petri Dish | BioStar US

Antibiotic resistance does not mean the horse’s body is resistant to antibiotics.  It means that the bacteria have become resistant to the antibiotics created to kill them.

Veterinarians at Merck Animal Health have identified another bacteria that has developed antibiotic resistance: Rhodococcus equi. This strain of bacteria can cause respiratory issues including pneumonia in young foals.

What’s in a name: Antimicrobials and Antibiotics

All antibiotics are antimicrobial, but not all antimicrobials are antibiotics.

The word “antimicrobial” is derived from the Greek words “against-little-life.”  The word “antibiotic” comes from the Greek words “against life.”

An antimicrobial is any substance of a natural, synthetic or semisynthetic origin that kills or inhibits the growth of microorganisms and causes little or no damage to the host. They are agents that act against various types of microorganisms including bacteria, viruses, fungi and protozoa.

Antibiotics are a low molecular substance produced by a natural microorganism that at a low concentration inhibits or kills other microorganisms.

In horses, it is the antimicrobial-resistant bacteria (not the viruses, fungi or parasites) that pose the biggest threat.

Antimicrobials classifications and actions

(Antibiotics target bacteria, antivirals target viruses, antifungals target fungi, antiparasitics fight parasites)

Penicillin, Chloramphenicol, Aminoglycosides: antimicrobial, antibiotic, antibacterial

Sulphonamides, metronidazole, Quinolones: antimicrobial, antibacterial, antiprotozoal (not technically classified as antibiotic because the sources are synthetic).

Ivermectin: antimicrobial, anthelmintic, antibiotic (its original source is a microorganism) but not antibacterial.

Types of Antibiotics

Broad spectrum antibiotics like tetracyclines, phenicols, fluoroquinlones, and cephalosporins act against a wide range of disease-causing bacteria, for drug resistant bacteria, and prior to identifying the actual disease-causing bacteria.

Narrow spectrum antibiotics have limited activity and are useful against particular species of microorganisms.  Among these are aminoglycosides and sulfonamides.

Treating wounds

Not all wounds may need antibiotics.  Recently Dean Hendrickson, DVM, Dipl.ACVS, professor in the Clinical Sciences Department at Colorado State University, gave a talk on wounds and antibiotic resistance at the 2019 British Equine Veterinary Association Congress (Beckstett, 2019).

He recommended debridement of the wound to get rid of bacteria.  “Get rid of the necrotic tissue, and don’t use antiseptics in your wounds, because they just create more necrosis, thereby encouraging the number of bacteria forming.”

He advised the audience, “we have a tendency in veterinary medicine to want to kill the bacteria in situ, but I am going to encourage you to remove them and throw them away and don’t let the patient have to deal with them at all.”

He recommended debridement dressings to help remove bacteria and dead tissues.  He spoke about the use of honey as both hypertonic and antimicrobial. Researchers have shown the efficacy of some honeys such as Manuka.  According to Hendrickson, “there’s some recent evidence out there that shows that local raw honey is more effective at killing local bacteria than medical grade honey.”

Dr. Hendrickson supports the use of silver dressings or triple antibiotic, which has not had much bacterial resistance over the years.  He also mentioned promising studies on stem cells in wound treatment. He noted that stem cells helped reduce the bacterial numbers in wounds.

Manuka Honey | BioStar US

Research conducted twenty years ago showed that manuka honey was able to inhibit around 60 species of bacteria, included E. coli and Salmonella strains.

Can Probiotics Help Wounds?

In an exciting study conducted by MIT researchers, a combination of antibiotic drugs and probiotics could eradicate two strains of drug-resistant bacteria in wounds (Li, Behrens, Ginat, Tzeng, Lu, Sivan, Langer, and Jaklenek, 2018).

Using an alginate coating and a polysaccharide called chitosan helped protect the probiotics from being broken down in the stomach.  The study used three strains of Lactobacillus (L.acidophilus, L.casei, and L. rhamnosus)  because these strains are known to kill methicillin-resistant Staphylococcus aureus (MRSA).  According to the study, When MRSA and Pseudomonas aeruginosa, another strain commonly found in wound infections, were exposed to the combination of probiotic strains and tobramycin, the pathogenic bacteria were wiped out.

According to Ana Jaklenec, a research scientist at MIT’s Kock Institute for Integrative Cancer Research and one of the senior authors of the study, “it was quite a drastic effect, it completely eradicated the bacteria.”

When researchers tried the same experiment with non-encapsulated probiotics, the probiotics were killed by the antibiotics, allowing the MRSA bacteria to survive.

The study authors noted that they plan to test this approach in animals and eventually humans.

One of the big take-aways of this study is that probiotics need to be protected from stomach acids to be effective.  Enteric coating, micro-encapsulation, and encapsulation ensures the active probiotic strains will survive the stomach.

If you are using a probiotic for therapeutic reasons, make sure the strains are active and are either enteric coated, micro-encapsulated or encapsulated to protect the strains from gastric acid.


We as horse owners must use antibiotics judiciously, not frivolously.  In some horses, antibiotic resistance has required multiple antibiotic therapies, increasing the recovery time and adding to the owner’s expenses.

horse hug | BioStar US

Dr. Michele Frazer of Hagyard Equine Medical Institute in an interview with US Equestrian said, “It’s important to remember that antibiotics aren’t benign.  Even aside from the drug resistance, they’re not benign.”

It is our responsibility not to treat these medications lightly or take them for granted.


Tigger Montague | BioStar USWith over 30 years experience in the equine and human supplement industry, Tigger Montague knows nutrition from the synthetic side as well as the whole food side. She started BioStar US in 2006 with formulas she created in her kitchen.  Before she started the company, she was an avid rider and competitor with eventing and show jumping, until she got hooked on dressage in the late 1980’s.  She has competed on horses she’s owned and trained all the way from training level to Grand Prix.


Beckstett, A. Managing Horse Wounds Without Antibiotics. Nov 2019.

Li, Z., Behrens, A., Ginat, N., Tzeng, S., Lu, X., Sivan, S., Langer, R., Jakelnec, A. Biofilm‐Inspired Encapsulation of Probiotics for the Treatment of Complex Infections. Advanced Materials. Oct 2018.

Oakford, G. Antibiotic Resistance is Real. Here’s How You Can Help. US Equestrian. Aug 2018.

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