Equid Blog : Horse Health & Diseases : University of Guelph : Ontario Veterinary College : Equine Infectious Diseases

Methicillin-resistant Staphylococcus aureus (MRSA) is an antibiotic-resistant bacterium and a major concern in both human and veterinary medicine. This bacterium can be carried by healthy horses and is an opportunistic pathogen, meaning it can cause infection in certain situations, particularly when the body's normal defensive barriers (like the skin) are breached (e.g. by surgery, wounds, compromised immune system).

We've done several surveillance studies across North America and we consistently find MRSA in a small but not insignificant percentage of horses. In most studies, we see it carried in the nasal passages of 1-5% of horses. A couple of recent studies provide more evidence that this bug has spread widely in horses internationally.

In a retrospective study of bacterial infections in horses at the University Equine Clinic of Bern, Switzerland (Panchaud et al, Schweiz Arch Tierheilkd 2010), MRSA was on the most important bacterium isolated from affected horses. Additionally, screening of horses at admission to the hospital revealed that 2.2% were carriers.

In a retrospective study of samples submitted for bacterial culture from horses in Ireland (Abbott et al, Vet Rec 2010), MRSA accounted for 5.2% (20/383) of isolated bacteria, which was higher than the percentage of MRSA in samples from dogs (1.1%) and cats (0.7%). In the prospective part of the study, MRSA was isolated from only 4.6% (3/65) of infections where clinicians suspected that MRSA was the cause. Its interesting that it was isolated less commonly in cases where people thought MRSA might be the problem than overall.  In contrast, while MRSA was only found in 1.1% of culture samples from dogs, it was found in 8.1% of infections where clinicians suspected an MRSA infection. It's not clear why there was such a difference between horses and dogs. It doesn't mean the equine clinicians are clueless, it just supports the notion that there are few clear indicators of that an infection is being caused by MRSA. MRSA infections occur in horses without any obvious risk factors, which complicates diagnosis and management of the infection.

MRSA must be considered in any situation when a horses has an infection. There's evidence that it's more common in horses that have been recently treated with antibiotics or in a veterinary hospital, but the link is not particularly strong and MRSA infections do occur in horses with no identifiable risk factors. That's why culturing infected sites - rather than just trying different antibiotics and seeing what happens - is critical. Money and effort spent culturing infected sites is a good investment.

(click image for source)

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Methicillin-resistant Staphylococcus aureus (MRSA) is a huge problem in people and is an emerging pathogen in horses. Most earlier reports of MRSA in horses involved one strain, called CMRSA-5 in Canada, USA500 in the US, and sequence type 8 (ST8) as a more general term. This human-origin strain seems to be adapted for survival in horses, and in North America, this strain has accounted for most MRSA infections in horses and MRSA infections in people linked to horse contact.

Another MRSA strain, ST398, has recently emerged as a big problem associated with livestock (particularly pigs). This strain is very common in pigs internationally, and is a major cause of infections in people in some European countries. There are also a few reports of ST398 in horses. Most are from Europe, although we have found this strain in one horse in North America. At last week's ASM Conference on Antimicrobial Resistance in Zoonotic and Foodborne Pathogens in Toronto, Dr. Engeline van Duijkeren from the Netherlands presented a case of human ST398 infection linked to a horse.

In the reported case, a 16-year-old girl had a lesion on her foot that was initial diagnosed as a spider bite infection (a common misdiagnosis of early MRSA skin infections). It didn't respond to initial treatment and MRSA was isolated on culture. The girl didn't have any history of contact with pigs or cattle, but had close contact with a foal. That foal had previously been in an equine hospital because of a wound infection, but the wound was not cultured. The same MRSA strain that caused the infection in the girl, however, was found in the foal's nose (the prime site for MRSA carriage by healthy horses). Fortunately, the girl's infection responded to treatment once treatment was adjusted for MRSA.

In some ways, this case is not too surprising, since we know ST398 MRSA can cause disease in people, and since it is found in horses, transmission from horses to people was likely inevitable. However, it's the first report of human infection with this strain associated with horse contact. MRSA exposure is a potential risk for anyone working with horses, since we know that this organism can be found in a small percentage of healthy horses. We don't have great information about how to prevent horse-human transmission, but simple things like only using antibiotics when needed and attention to hygiene (especially hand washing) when working with horses are presumably important factors.

More information about MRSA in horses can be found on the equIDblog Resources page.

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At the ongoing ASM-ESCMID conference on methicillin resistant staphylococci in animals, Dr. Engeline van Duijkeren of Utrecht University (The Netherlands) presented a study on an outbreak of methicillin-resistant Staphylococcus aureus (MRSA) in their equine hospital.

From 2006-2008, several horses that underwent surgery at their hospital developed MRSA infections.  MRSA was also isolated from some healthy horses and personnel at the clinic. Early in the process, the hospital was closed for a thorough disinfection and the outbreak stopped, however another outbreak occurred later.  Further study again found people in the clinic that were MRSA carriers. Close to 15% of people in the hospital who handled equine patients were MRSA carriers, which is really astounding when you consider that less than 0.1% of the general population in the Netherlands carries MRSA. When they started testing horses coming into the clinic, they found that 9.3% of horses were carriers when they arrived. Weekly sampling of all hospitalized horses over a five-week period determined that 43% of all horses in the hospital carried MRSA at one point or another during their stay. Additionally, 53% of environmental surface samples were positive for MRSA, which is really not surprising if that many people and horses are carriers.

If horses keep coming into a facility carrying MRSA and people keep getting colonized, MRSA is hard to control. These experiences led the equine hospital at Utrecht to implement more stringent infection control practices to try to contain the problem, but the high MRSA rate in their referral population is going to pose a continual risk.

MRSA outbreaks in horses aren’t new. They’ve been reported by a few hospitals (including ours) and occur in many, many, (many!) more without ever being published. Since MRSA is present in the horse population, equine hospitals are at continual risk of MRSA outbreaks. If a large percentage of horses in the general population are carriers, the risk of outbreaks is higher.

MRSA is clearly a problem in horses in many areas. It’s important to realize that it’s a problem in the general population, not just horses in hospitals. Equine hospitals can amplify the spread of MRSA, but ultimately a lot (if not most) MRSA-positive horses originate from farms, not clinics or hospitals. Equine hospitals need solid infection control programs to reduce  the risk of outbreaks, but the risk will never be completely eliminated. Farms need good infection control programs to reduce the risk of spread of MRSA between horses and between farms, as well as from horses to people (and back). Antibiotics need to be used prudently since antibiotic use is a risk factor for MRSA carriage and infection.

More information about MRSA in horses can be found on the equIDblog Resources page.

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The April 2009 edition of the Equine Veterinary Journal includes a retrospective study of methicillin-resistant Staphylococcus aureus (MRSA) infections in horses (Anderson et al. 2009). This study looked at 115 horses with clinical MRSA infections (i.e. they weren't just colonized) from six different North American veterinary teaching hospitals. Here are some of the highlights (my comments are in italics):

• The age range of affected horses was from "born in hospital" to 31 years old. Many different breeds were affected: Any horse of any age is susceptible to MRSA.
• Community-associated (infections that likely originated on a farm) and hospital-associated (infections that likely originated in a veterinary hospital) were equally common. MRSA is not just associated with veterinary hospitals, and must be considered even when there is no history of hospitalization. I suspect that overall, many more MRSA infections occur on farms than in equine hospitals.
• Surgical site infections were most common in hospitalized horses, accounting for 58% of infections. No big surprise here. It makes sense because of the trauma associated with breaking the body's normal protective barriers (e.g. making a surgical incision in the skin) and probably other issues like antibiotic treatment, stress and contact with lots of hospital  personnel.
• Skin and soft tissue infections accounted for 29% of infections that originated on farms. This also isn't surprising - these types of infections are most common in community-onset infections in people as well.
• Joint infections accounted for 18% of community-associated infections. This surprised us and is a concern, because joint infections (of any kind) can be very severe. Detailed information about these infections wasn't available, but some of these could have been associated with accidental contamination of joints during joint infections.
• 84% of horses with MRSA infections survived. This is a very important finding. It shows that MRSA is a survivable infection in most cases. Prompt and proper treatment are the key.

People shouldn't panic when they hear "MRSA", but the importance of prompt diagnosis through culture of possibly infected sites can't be emphasized enough.  More information about MRSA in horses can be found on the equIDblog Resources page.

Photo: Skin and soft tissue infection (SSTI) of a horse's leg caused by Staphylococcus aureus.
 

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Methicillin-resistant Staphylococcus aureus (MRSA) is an emerging problem in horses. Owners often ask if they should have their horses tested to see if they are MRSA carriers. There's no simple answer that applies to all farms, but basic points that need to be considered include:

• MRSA is endemic in the horse population. A small percentage (<5%) of healthy horses carry MRSA at any given time, and it likely circulates regularly between groups of horses.
• MRSA infection only occurs in a small percentage of the horses that are exposed to the bacterium - most horses never develop signs of illness.
• MRSA can be transmitted to between horses and people.

So you can see why people might want to test their horses and there are few situations where screening really makes sense. Remember, though, that screening is only useful if you plan to do something about the results. If your horse is MRSA positive, what will you do? If the answer is nothing, or you'll just make sure you wash your hands well after handling your horse, I'd tell you that you should be doing that anyway and not bother with the test. However, there are a few scenarios when screening horses for MRSA is a good idea:

• Screening is a key component to any plan to eliminate MRSA from the farm. Eradication of MRSA can usually be accomplished quite effectively using screening and good infection control practices (and, if there are no clinically infected horses, not one dose of antibiotics!).
• Some farms routinely screen incoming horses for strangles before they are allowed to have contact with resident horses. This can be done for MRSA as well, to reduce the risk of MRSA getting onto the farm and silently spreading from horse to horse. It's not as straightforward for MRSA as it is for strangles, because we know less about the best  screening methods and, probably more importantly, people (not just horses) can bring MRSA onto the farm.
• When an MRSA outbreak is underway, or when efforts are being made to determine why a particular horse developed an MRSA infection, screening can be useful - but again, only if there's a plan to use the results.
• Our equine hospital screens horses for MRSA on admission, so that we can isolate carriers and reduce the risk of transmission to our highly susceptible hospitalized patients. It also helps us identify farms with MRSA problems that might benefit from implementing an eradication program. We also screen horses at the time of discharge to make sure MRSA has not been transmitted to them while they were in hospital.

Screening your average horse on your average farm is harder to justify. A single negative result might give you a false sense of security, because a horse that is negative today might be positive tomorrow, or it might positive already but the current testing methods couldn't detect it.  The use of good infection control practices is much more important than testing for day-to-day MRSA control.  Screening for MRSA is something that is usually only considered in specific circumstances.

More information about MRSA can be found on the equIDblog Resources page.

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Infectious disease surveillance is an important part of the infection control program. Equine hospitals with good infection control programs have put thought and effort into designing a surveillance program that is right for them. Like many (or most) aspects of infectious diseases, there is no "standard" program, because the risks and benefits vary greatly between facilities.

There are various types of surveillance that can be used. Here are some examples:

Active surveillance
This involves going out and "actively" collecting information that would not otherwise be collected for other reasons. This includes things like taking extra samples from horses for testing strictly for infection control reasons. This is discussed further below.

Passive surveillance
This involves using data that are already available. It's a cheaper and somewhat easier form of surveillance, but it doesn't provide as much detailed information as active surveillance. An example of this is compiling all the culture results from fecal samples that were already submitted for routine testing (as required by the case).  This helps us know what bugs tend to cause certain infections, and to what antibiotics they are usually susceptible.

Syndromic surveillance
Syndromic surveillance can be active or passive. It involves looking at the occurrence of specific clinical signs (syndromes), not specific diseases/diagnoses. For example, routine monitoring for fever in hospitalized animals is a useful tool. It doesn't tell us what the specific problem is, but it can alert us that something might be going on.

I'll outline the active surveillance component of the Ontario Veterinary College's active surveillance program here (I'll get to the other types some other time):

Salmonella

• All horses that are at high-risk for shedding Salmonella in their feces (i.e. horses with colic) and all horses that are at increased risk for becoming infected (i.e. horses undergoing anesthesia and surgery) are tested.  Horses with diarrhea - the highest risk group for shedding Salmonella - are also tested as part of the routine case work-up.
• Stalls that have housed diarrheic horses are quarantined until they have been tested for Salmonella. The stall environment is sampled and culture is performed. Stalls are not opened until negative culture results are received.

Methicillin-resistant Staphylococcus aureus (MRSA)

• A nasal swab is collected from all horses at the time of admission, weekly during hospitalization (if they stay that long) and at the time of discharge.

Other

• Additional active surveillance for Salmonella and MRSA can be conducted at the discretion of the Chief of Infection Control (i.e. me). If I have any concerns about a particular disease, I institute additional active surveillance.

Who pays for it?
Active surveillance costs (as opposed to routine diagnostic testing like Salmonella culture in a horse with diarrhea) are covered by the Hospital. It's a cost of doing business. Financially, it also makes sense, because it is cheaper to prevent infections than it is to deal with hospital-acquired infections and outbreaks.

Don't be afraid to ask about the infection control program at your equine hospital. A hospital should be more than willing to tell you what they're doing to help reduce the risk of your horse developing an infection.

More information on equine hospital infection control can be found in on the equIDblog Resources page.

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As part of the 2008 Conference of the American Association of Equine Practitioners, equIDblog's own Dr. Maureen Anderson gave a presentation about MRSA in horses and people. While there, she gave in an interview for the The Horse, which can be found here. This short video gives some good general information about MRSA in horses and people that work with horses.

More information about MRSA can also be found in our MRSA archives and on the equIDblog Resources page.

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As we encounter more infections caused by antibiotic-resistant bacteria (e.g. MRSA), we need to explore treatment options other than antibiotics. While we usually focus on "new" treatments, sometimes we can look back in time for ideas to treat infections. An old treatment method that is getting increasing attention these days is the use of honey. While not used much in equine medicine at this time, honey may be a safe, effective and affordable treatment option in many cases. Click on the picture to view a video by Dr. Karol Mathews, a critical care specialist at the Ontario Veterinary College. Everything that is discussed regarding dogs and cats can apply equally to horses.

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Methicillin-resistant Staphylococcus aureus (MRSA) is an emerging problem in horses.  This antibiotic-resistant bacterium can cause disease in both horses and the people in contact with them. In the early 2000s, virtually all reports of MRSA in horses in North America, and many from Europe, involved one particular "strain" of MRSA. This strain goes by different names in different places, but in Canada its called CMRSA-5, and in the US it's called USA500. This is actually a strain that originated in people but which seems to be well adapted to horses.

Recently, there was a report in the journal Veterinary Microbiology describing the isolation of a different MRSA strain from 10.9% of horses tested on admission to a veterinary teaching hospital in Belgium. This strain, called ST398, is getting a lot of attention lately, because it is being commonly found in pigs (and perhaps cattle).  It is also emerging as an important cause of disease in people in Europe. In the Belgian study, the horses carrying ST398 were from different areas (not just one farm or town), which suggests that the strain is widespread in the horse population in that region. The source of ST398 in these horses is unknown, but it is reasonable to suspect that it may have moved from pigs to horses. The impact this will ultimately have on horses and people that work with horses, in Belgium, Europe and other parts of the world, is unclear, but one has to assume that this strain could become a problem, just like CMRSA-5.

More information on this topic can be found in the previous equIDblog post entitled "Horses and MRSA".  More information about MRSA can also be found on the equIDblog Resources page.

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You may notice a recurring theme on equIDblog anytime we talk about infectious disease control, particularly when it comes to zoonotic diseases (those that can be transmitted between animals and people): an emphasis on handwashing. There is increasing emphasis on hand hygiene (i.e. hand washing and use of alcohol hand sanitizers) education in hospitals because the hands of healthcare workers are a major (if not the most important) means of disease transmission between patients. Despite hand hygiene being easy, cheap and effective, people rarely wash their hands as often as they should, and they often don't do it properly.

Most of the research about hand hygiene that has been published has focused on its use and impact in human hospitals, but this area is now also being studied more with regard to animals and veterinary medicine. A study published earlier this year in Veterinary Microbiology provided more evidence that hand hygiene is a critical infection control measure when dealing with animals. The study looked at MRSA carriage rate in veterinarians who work with horses. In addition to finding a high rate of MRSA carriage among these veterinarians (which was consistent with other reports indicating that equine vets are at higher than average risk for exposure to MRSA), the study looked at factors associated with MRSA carriage. Vets that reported routinely washing their hands between farms and those that reported washing their hands after contact with potentially infectious cases had a significantly less likely to be carrying MRSA. That should come as absolutely no surprise, but it's one more piece of evidence that we need to pay more attention to this routine infection control measure, in human hospitals, in veterinary environments, out in the barn, and even in households.

Remember, the 10 most important sources of infection are the fingers on your hands!

Click here for instructions on how to wash your hands properly.

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Many people in the horse world have heard the hype about methicillin-resistant Staphylococcus aureus (MRSA) in horses. MRSA can cause infection in horses, just like it can in people, dogs, cats and many other animals. It’s usually what we call an “opportunistic” pathogen, meaning it usually takes advantage of a person or an animal that is already sick or injured, like someone who’s in the hospital and has just had surgery. And because MRSA is resistant to many different antibiotics, the infection can be difficult to treat. The big concern with MRSA in recent years is that infections are now sometimes occurring in people who aren’t sick, and who don’t have wounds or incisions, which is where MRSA usually likes to move in. It’s very important to find out from the start if an infection is being caused by MRSA, so that it can be prevented from spreading to other people and animals, and so that it can (if necessary) be treated with the right kind of antibiotic.

Horses are a bit of a special case when it comes to animals and MRSA. When researchers look at the DNA of MRSA from a dog or a cat, it usually turns out to be one of the common human MRSA strains (usually called a “clone”) from the same area. This means that the dog or cat probably picked up the MRSA from a person somewhere.  When researchers look at the DNA of MRSA from horses, however, they often find a different clone, which seems to be more common in horses and people who work with horses than in people in general. A very similar situation has also been discovered in pigs. The worry is that this “horse MRSA clone” can survive in and be transmitted between horses better than the human MRSA clones. That means that in order to control MRSA, just controlling it in the people won't do the trick - we need to take steps to stop the spread of MRSA in horses specifically as well.

Here are some key points to help reduce the risk of your horse (and you!) getting MRSA:

• Always wash your hands with soap and water (or use an alcohol-based hand sanitizer) after handling a horse, and before handling another horse.
  • This is especially important if you have touched a horse’s nose, or any cuts or wounds that the horse may have.
  • Don’t go down the row of stalls in the barn and pet every horse on the nose! They love the attention, but this is a great way to spread MRSA if it’s there!
• New horses coming into the barn, or animals coming back from a hospital, should be kept separate from all the other animals and only dealt with after all the other horses, for 3-4 weeks.
  • This is an important measure for controlling many infectious diseases, not just MRSA.
• If your horse has a cut that looks infected, cover it with a bandage of some kind and contact your veterinarian. Your veterinarian can culture the wound to determine if it is an MRSA infection.

This equIDblog entry was originally posted on the Worms & Germs blog on 06-May-08.

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Click on any of the highlighted links below for more information about these horse-related infectious disease topics. Topics that are not highlighted are in development and coming soon. New information will be added as it becomes available, so be sure to check this page regularly for the latest updates.

All information sheets found on this page can be freely downloaded, printed and distributed. The authors only request that this website (www.equIDblog.com) is acknowledged as the source.  The downloadable files on this page can be opened with Adobe® Reader®.  To get the latest version of Adobe® Reader® for free, click here.

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The following are infectious disease control resources that have been developed at the Ontario Veterinary College (OVC), as well as information regarding the infectious disease control polices of the Ontario Veterinary College Teaching Hospital (OVCTH) in Guelph, Ontario.

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