Bad Horse Bugs

Antimicrobial resistance and the looming problems with MRSA infections.

MRSA. Salmonella. Enterococcus. If these bacteria ring a bell, here's why: They are just a few of the countless bad bugs that can infect horses and humans or are becoming resistant to the antibiotics we use to treat them. Elizabeth Santschi, DVM, Dipl. ACVS, a clinical associate professor at the Ohio State University, says horse owners must use antimicrobials responsibly to help reduce development of bacterial resistance against these drugs.

"This responsibility involves not just the choice of drugs, but also duration and concentration of a drug," she explains. "Antibiotics should be used at sufficient concentration for the appropriate amount of time to do the job. Where we get into trouble is when we use improper dose (generally too low) or use the drug for too short (of a) duration." If the antibiotic therapy has not eliminated the bacterial infection completely by the time you stop using the drug, the remaining bacteria are the most hardy--and this can lead to development of antibiotic resistance among these bugs.

"We're also handicapped in equine medicine and surgery because we have a limited number of antibiotics in our selection that are safe to use in horses and also economical," Santschi continues. "Some (effective) antibiotics are prohibitively expensive for a 1,000-pound horse; they would cost hundreds or thousands of dollars a day."

Amy Johnson, DVM, Dipl. ACVIM, a lecturer in clinical studies at the University of Pennsylvania, says that some drugs also require intravenous (IV) administration that might necessitate a hospital or layup facility visit, as opposed to availability as a pill the owner can give to the animal at home. This increases total treatment cost. And the longer the horse is hospitalized, the greater his chance of developing complications due to pathogen exposure, because the most resistant forms of microbes (such as Enterobacter, Enterococcus, and MRSA--methicillin-resistant Staphylococcus aureus), are often present in clinics. For instance, "MRSA infections are generally due to catheter site infections or surgical site infections," says Johnson.

Thus, many veterinarians focus on prevention instead of having to treat disease. "Today we don't treat every incision infection with systemic antibiotics," says Johnson. "Often we just use local treatment--opening up any pockets of infection and allowing these to drain. With time, the horse can heal itself. This is sometimes preferable to using an antibiotic. Every time you treat a patient (with an antibiotic), you end up with a more resistant population of bacteria.

"When we prescribe a course of antibiotic we insist it be given on the proper dosage schedule and the entire course completed," Johnson continues. "The owner should make sure the animal actually consumes the entire amount (if it's a pill), even if they have to use a dose syringe rather than put it in feed." Subtherapeutic dosing or missing a dose is not helpful, she says, because it allows the stronger bacteria to survive.

Johnson also discourages horse owners from treating their own animals without a veterinarian's supervision. "Rather than buying a bottle of penicillin or Naxcel at the feed store, they first need a proper diagnosis," she explains. "Many viral infections resolve spontaneously with supportive care, without antibiotics."

Josie Traub-Dargatz, DVM, MS, Dipl. ACVIM, a professor of equine medicine at Colorado State University, says there are not many up-and-coming drugs in the pipeline if bacteria become resistant to the options currently available.

Scott Weese, DVM, DVSc, Dipl. ACVIM, associate professor in pathobiology at the University of Guelph's Ontario Veterinary College, says that all too often the person treating a horse chooses the most powerful antibiotic, when a more common drug would do the job. This practice is not only expensive but also dangerous, because pathogens in the body could become resistant to that antibiotic, leading to future infections that are more difficult to treat.

"It's important to reduce antibiotic use, using these drugs only when we really need them," Weese explains. "What often happens, however, is the horse is put on an antibiotic and the infection doesn't get better, so it's put on another antibiotic and it still doesn't get better, so we culture it. Then we find out--a couple weeks down the road, when the infection is a lot worse--that we've been treating with something that isn't going to work. Getting cultures earlier is important in controlling antibiotic use."

Santschi also recommends isolating the organism (for proper diagnosis and determining antimicrobial sensitivity) before antimicrobial therapy begins. "If we just put the horse on a drug, the likelihood that we could grow (culture) the bug later if we need to would be much lower," she says.

"Staph" Infections

One organism that concerns vets and horse owners is Staphylococcus, in particular S. aureus--a bacterium found naturally in the nasal passages of about 30% of healthy humans and 10% of horses. These bacteria are opportunists, waiting for a chance to invade tissue and multiply. They usually cause no problems in a healthy body because the immune system keeps them in check. But if the person or horse becomes injured or sick, this pathogen can take advantage of weakened defenses and cause infection. Skin and soft tissue infections are most common, but occasionally other body systems are involved.

There are several strains of S. aureus. Some are susceptible to certain antibiotics and, as mentioned earlier, some are resistant. These particular species are categorized as being either methicillin-susceptible (MSSA) or -resistant. The MRSA strains are resistant to all the ß lactams--antibiotics in the family that includes some of the common antimicrobials, such as penicillins and cephalosporins. Some MRSA strains are also resistant to other families of antimicrobials, which makes these infections more difficult to treat.

According to Weese, MRSA is now found in about 0.2 to 3.5% of the human population, depending on geographic location. People who work with horses or livestock might have an increased risk of acquiring MRSA. Researchers conducting studies in various parts of North America and Europe have found MRSA in the nose, intestinal tracts, or skin of up to 10.9% of healthy horses, but on some farms more than 50% might carry MRSA.

"Most horses that carry it don't have any signs, so this complicates the picture and makes this disease more difficult to eradicate than something that always makes the horse sick," explains Weese.

This increases importance of general hygiene procedures such as washing your hands and taking proper care of wounds, along with general biosecurity and infection-control practices on farms.

Risk factors for MRSA colonization in horses have not been adequately studied, note the sources on this article, but horses that have been treated with antibiotics and/or live on farms where MRSA is known to be present are more likely to be carrying the bacteria. "Most MRSA infections in horses in North America are caused by one strain (called CMRSA-5 or USA500)," explains Weese. "This is actually a human MRSA strain but it is uncommon in people. Scientists suspect this strain originated in humans but became adapted to horses. It's most commonly found in people who work with horses, which suggests that humans and horses can transmit MRSA back and forth."

MRSA used to be considered primarily a hospital-associated organism in humans, because it developed where there were many sick people and extensive antimicrobial use. Now, however, it's found nearly everywhere. MRSA might cause more problems in equine hospitals than in the general equine community, but depending on the region it can infect horses just as easily outside a hospital's facilities as in them.

The risk of antibiotic-associated diarrhea further complicates treatment of bacterial infections, according to Santschi. A horse's gastrointestinal (GI) tract requires "good" bacteria to help ferment and break down food. The GI tract's bacterial population is very sensitive to antibiotics; if you kill too many of the normal, helpful bacteria, the horse might develop diarrhea, which can range from mild to fatal (due to dehydration and other complications). The diarrhea risk also can vary by geographic region, says Santschi.

This is because these bugs--both good and bad--in different regions don't have the same levels of resistance. "It's related to what flora is in your horse's GI tract, and there also seems to be an individual susceptibility, different from horse to horse. Most loose bowel movements resolve after the antibiotic is halted," Santschi adds. "Once in awhile we run into an overgrowth of a particularly virulent pathogen--such as Clostridium difficile or Salmonella--which can be a very serious situation (for a horse)."


Pathogenic bacteria can spread between horses through direct contact, shared feed or water sources, exposure to contaminated equipment, or contact with people who might transfer bacteria from horse to horse on their hands, clothing, or footwear. According to Traub-Dargatz, the most likely means of transmission and entry into the body (oral or nasal passage or through a break in the skin) can vary depending on the type of bacteria.

MRSA, for instance, is most often carried in the nasal passage; hand-to-nose contact between people and horses and nose-to-nose contact between horses are the most common routes of transmission.

"Like any other Staphylococcus sp. infection, it gets going most often when there's a wound, skin lesion, skin disease, or surgical incision," says Weese. "This bacterium rarely causes primary infection in a healthy horse."

While this pathogen is resistant to many antimicrobial drugs, the MRSA veterinarians see in horses still tends to be susceptible to at least a couple of drugs. "We usually have a few options for treating it," says Weese. "It's just a matter of knowing the MRSA is there, so we can choose those options. It's not an untreatable infection; we just need to get the right antibiotic."


According to Weese, the key steps in preventing MRSA infections are good hygiene, prompt/correct diagnosis when the horse has an infection, not using antibiotics unless they are required, and good infection-control programs for farms.

Johnson says that sometimes a horse will be admitted to a clinic with an existing MRSA infection, but more often infections are probably acquired while a horse is housed in the hospital (called nosocomial infections). "To prevent that, we use various precautions (such as barrier clothing and good hygiene), especially with young foals," she says. "We make sure that the people who are putting in the IV catheters wear sterile gloves and masks. And if we identify a patient that is positive for MRSA, everyone who goes into that stall wears masks, gloves, gowns, and shoe covers."

Traub-Dargatz says it's also wise to wash your hands after you work with a horse and before you touch your nose or eat--not only as a precaution for preventing MRSA transmission but also to avoid exposure to Salmonella and other disease agents. Other prevention methods include wearing disposable gloves when changing bandages on a horse, treating wounds, or dealing with sick horses.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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