MRSA In Horses

• Staphylococcus aureus can also be found in the nose, intestinal tract or skin of a small percentage of normal, healthy horses, although the frequency with which it is found varies widely between studies.
• Studies in different areas of North America and Europe have identified MRSA in 0-10.9% of healthy horses, but on some farms over 50% of horses may carry MRSA.
• Most horses that are colonized with MRSA do not have an infection, and never develop an infection. However, they are at higher risk for developing an MRSA infection if they become sick, injured or stressed for another reason, such as when they are admitted to an equine hospital.
• Risk factors for MRSA colonization in horses in the general population have not been adequately studied. Horses that have been treated with antibiotics and that live on farms where MRSA is known to be present are more likely to be carrying MRSA when they are admitted to an equine hospital.
• As in human hospitals, MRSA can be spread between horses in equine hospitals. However, most MRSA infections in horses occur in animals that have never been in an equine hospital.

The vast majority of MRSA infections in horses in North America have been caused by one specific strain, called Canadian epidemic MRSA-5 (CMRSA-5) or USA500. This is a human MRSA strain, but it is uncommon in most people. It is suspected that this strain originated in humans, but it became adapted to horses. This strain is most commonly found in people that work with horses, which strongly suggesting that people and horses can transmit MRSA back and forth in both directions. More recently, a strain associated with pigs and cattle (ST398) has emerged as a common cause of MRSA infection in horses in some European countries. This is currently rare in North American horses, but has been found.

How Do People & Horses Get MRSA?

Transmission of MRSA to people or animals can lead to colonization alone, infection, or both.

In human hospitals, MRSA is most often transmitted on the hands of healthcare workers. This is probably the same in veterinary hospitals.

Transmission of MRSA in the community occurs through direct contact with high-risk, colonized or infected individuals. Outbreaks have occurred on sports teams, military bases and prisons where many people may have close contact with each other, hygiene may be less than ideal, and breaks in the skin may be common.

Horses can acquire MRSA from people or other horses. In colonized horses, MRSA is most often carried in the nose, so direct hand-to-nose contact between people and horses and nose-to-nose contact between horses are probably important routes of transmission. Horses may also acquire MRSA indirectly from other horses through contaminated surfaces such as feed bins, water bowls and fence rails.

MRSA can survive in the environment for a limited period of time, but the bacteria are susceptible to most commonly used disinfectants, if the surface/equipment is cleaned properly before the disinfectant is applied. Proper disinfection can be difficult on a farm because of the different types of surfaces that are present. Consult your veterinarian about the best way to disinfect various surfaces on your farm.

How is MRSA Diagnosed?

Because MRSA can cause so many different kinds of infections, and it’s impossible to tell for sure what kind of bacteria are involved based on how the infection looks, MRSA in horses must be diagnosed based on bacterial culture, which can take 1-3 days to complete. Molecular tests are now being used in humans, which can detect MRSA more rapidly (hours versus days), but these tests are not currently used in animals.

The body site most likely to be colonized with MRSA in humans is the nose, so a nasal swab is often cultured to check for MRSA colonization. Typically, the nose is also the site that is tested when horses are being screened for MRSA colonization, although this is rarely necessary except in equine hospitals or during outbreak investigations.

Molecular typing and classification of MRSA, which determines how closely related different strains are, is not routinely performed. This type of testing is typically only used for outbreak investigations or research and is not needed for treatment of individual horses.

How is MRSA Treated?

Infection: All MRSA strains are resistant to the entire family of beta-lactam antibiotics, including all penicillins and cephalosporins (e.g. ceftiofur (Excenel™ or Naxcel™)). Different MRSA strains may be resistant to other antibiotics as well; therefore, the bacteriuma must be tested in order to choose an antibiotic that will be effective. Despite the fact that MRSA is a multidrug resistant “superbug”, there are almost always reasonable treatment options. Local treatment of skin and soft tissue MRSA infections (e.g. lancing and flushing an abscess) is often very effective as well, and should not be overlooked, even if the person or animal is also treated with antibiotics.

Colonization: Decolonization therapy, including nasal ointment and/or oral antibiotics, is not needed or recommended for most people. It may be considered for people who are at higher risk for infection or in contact with individuals at higher risk for infection.

Most if not all horses eliminate MRSA colonization on their own within a few weeks as long as they are not re-exposed to the bacterium. We also do not have any proven methods to eliminate MRSA colonization in horses. Therefore, decolonization therapy with antibiotics in horses is not needed or recommended, but farm infection control practices (see below) are very important. Unnecessary use of antibiotics can lead to further development of antibiotic resistance, and can cause other problems such as antibiotic-associated diarrhea.

Infection Control For MRSA In Horses 

In general, MRSA colonization is uncommon in healthy horses, but you can never tell if a horse is carrying MRSA just by looking at it. Similarly, MRSA colonization is more common than average in people that work with horses. It’s prudent to assume that you and/or your horse are MRSA carriers, and take appropriate measures to reduce the risk of transmission.

Wash Your Hands! Hand hygiene is the simplest and most practical way to prevent transmission of MRSA between humans and animals. Regular handwashing with soap and water or use of an alcohol-based hand sanitizer is perhaps the most important infection control measure for MRSA.

What Should I Do If My Horse Is Infected With MRSA?

Don’t panic! The majority of MRSA infections can be treated effectively if they are diagnosed and appropriate treatment is started in a timely manner. A recent study reported that over 80% of hospitalized horses with MRSA infections survived to be discharged.

• Follow the treatment recommendations of your veterinarian very carefully. It is especially important to completely finish any antibiotic prescriptions, as directed, even if your horse seems to be better earlier.
• Avoid contact with the infected area of your horse. If possible, the area should be kept covered or bandaged. This also helps to prevent contamination of the environment and transmission to other animals or people. Wear gloves if you need to change the bandage and place all used bandage materials directly in the garbage.
• Wash your hands well after handling your horse, and especially after changing any bandages.
• Infected horses are often colonized as well, so also follow the recommendations for colonized horses below. Your horse may remain colonized in its nose or intestinal tract for a short period of time after the infection has resolved – even once the infection is gone, it does NOT automatically mean the horse is no longer infectious.

What Should I Do If My Horse Is Colonized With MRSA?

Horses that are colonized with MRSA should be considered possible sources of infection for other horses and people, and probably for other animals (e.g. dogs, cats) as well. High risk individuals, including young children, the elderly and individuals with a weakened immune system (e.g. HIV/AIDS, cancer or transplant patients), should not handle or come in contact with a colonized horse. It is also prudent to take extra precautions to prevent contact with horses that are sick or injured, that recently had surgery, mares in late pregnancy and very young foals.

• Colonized horses should be isolated whenever possible to prevent transmission of MRSA by direct contact (i.e. horse-to-horse, horse-to-human) and indirect contact (via contaminated items such as water buckets, feed troughs, blankets and tack). Transmission through the air is not likely a problem, but a horse that coughs or sneezes could potentially spread MRSA over short distances.
• Isolation strategies vary greatly between farms. Ideally, a colonized horse should be kept in a true isolation barn or stall, or in a barn without other horses. If this is not possible, keep the horse in a stall in an area with the lowest possible traffic (horse and human), and where there are no horses in the adjacent stalls.
• Do not allow colonized horses to stick their heads out doors or over dividers, in order to decrease the chance of nose contact with other horses or people.
• Horses can be isolated in their own pasture or paddock, as long as they don’t have over-the-fence contact with other horses. MRSA can probably only live outside for a few days, but it is prudent to rest a paddock or pasture used by a colonized horse for a longer time (e.g. one week) before putting any other horses in it.
• Colonized horses should have their own feed buckets, water buckets, hay nets and other items, which should not come in contact with other horses. When no longer needed, or when the horse is no longer colonized, these items should be thoroughly disinfected or discarded.
• People should handle colonized horses using “barrier precautions.” This is meant to reduce the risk of direct horse-to-person transmission of MRSA, and to reduce the risk of contamination of personal items (i.e. clothing).
• A disposable gown, designated coveralls or other form of protective outerwear should be worn whenever a person has contact with the horse or goes into its stall (even if the person doesn’t plan to touch the horse).
• Some type of overboot (e.g. designated large rubber overboots, disposable plastic overboots) should be worn in the stall. Otherwise, footwear should be disinfected immediately when leaving the stall.
• Disposable gloves should be worn for any contact with the horse, surfaces in its stall or objects in its environment. Gloves should be removed immediately upon leaving the horse’s stall. Always wash your hands or use an alcohol-based hand sanitizer immediately after removing your gloves.
• Items such as blankets and wraps that might be contaminated should be washed in hot water and dried using hot air in a dryer. Hot water washing will not necessarily kill MRSA – hot air drying is probably the most important step.
• Mark the horse’s stall clearly so people know that the horse is being isolated. This is particular important in public stables where many different people visit the barn on a daily basis.
• You can still train and ride your horse if it is colonized with MRSA, but prevent contact between your horse and other animals and people. The horse’s tack should be considered contaminated after use and disinfected accordingly.

Testing or treating normal, healthy horses for MRSA is not necessary, There is no evidence that it is useful in any situation, even if a horse is colonized with MRSA. However, MRSA should be considered in horses if they develop infections, particularly of the skin and soft tissues and particularly if they are colonized with MRSA.

If I Have MRSA, What Should I Do About My Horse?

• Wash your hands thoroughly before and after handling your horse, to help prevent transfer of MRSA to your horse, and transfer of MRSA from your horse if it becomes colonized.
• Do not kiss your horse, and avoid touching your horse’s nose.
• Testing or treating normal horses for MRSA is not necessary, even if a person in contact with the horse is infected or colonized with MRSA.

Is My Horse The Source Of My MRSA Infection?

Molecular classification of MRSA isolates is not performed routinely for sporadic cases of infection in humans or animals, which makes it very difficult to tell if a person may have acquired MRSA from their own horse or another horse, or if a horse acquired MRSA from a person or another horse. It’s important to be aware that even healthy horses can carry MRSA, and that MRSA is relatively common in people who work with horses. Simple precautions like washing your hands can help prevent MRSA transmission from unknown carriers of the bacteria. It is important that your physician know what types of animal contact you have.

It is impossible to completely prevent horses (or people) from being exposed to MRSA, because so many people and animals carry MRSA without any signs. However, proper use of antibiotics according to the prescription, and only using antibiotics when they are really needed are likely very important measures.