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

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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The other day, I wrote about a few different types of surveillance that can be used in equine hospitals - active, passive and syndromic. Infectious disease surveillance, however, is not just for hospitals. Every horse farm should have some form of infection control program. In most situations, it doesn't need to be complicated, fancy or time consuming, but it means organizing some basic infection control policies and procedures. Syndromic surveillance is great for use on horse farms.

Syndromic surveillance involves looking for specific syndromes (e.g. particular clinical signs (not specific diseases per se) such as fever, cough, diarrhea, or off feed) that might indicate an infectious disease. These basic, easy-to-identify syndromes can be the early warning that something might be amiss, then closer examination can be used to determine if there is actually is a problem.

An important key to disease surveillance is it is only effective if something is done with the information collected. There has to be a plan based on the results of the syndromic surveillance, with regard to what needs to be done for the affected horse and on the entire farm.   For the horse, it usually means a visit from a veterinarian to determine what's wrong, and if it might be due to an infectious pathogen.  For the farm, it means making sure that horse doesn't have a chance to infect other horses (in case it is an infectious disease), and ensuring that the information is recorded so bigger problems (e.g. outbreaks) can be identified as early as possible. The plan for what to do when one of these syndromes is detected needs to be written down so it's very clear for everyone, and easy to find if someone's not sure what to do. One of the most important steps is making sure someone (i.e. someone "in charge") is notified.

Here's a scenario: A horse develops a mild fever overnight and doesn't eat all its grain.

Response 1 (what should happen): The fever and decreased appetite are identified and the person in the barn in charge of the surveillance program is notified. They record that a horse has a fever and talk about what to do with the horse so everyone knows. The horse is restricted to its stall until the cause of the fever is identified. An hour later, the same person in charge of surveillance gets reports from two other people about horses with fever. This obviously raises concern and they try to determine what is happening. They know that a new horse arrived a while ago and that the horses with fever were turned out with it. They immediately call the veterinarian, who suspects strangles may be the cause. All horses with a fever are isolated and tested. Other horses in the barn are closely monitored for signs of illness. Because horses with strangles spike a fever a couple days before they are able to transmit the disease, these horses do not spread the disease any further.

Response 2 (what usually does happen): The first fever and decreased appetite may or may not be identified. Since the horse doesn't really look sick, they decide to wait and see how it looks over the next day or two. A couple days later, someone notices the horse has enlarged lymph nodes. Then they call the vet, who diagnoses strangles. As the vet looks around the barn and talks to people, he/she finds out that a few other horses have big lymph nodes, and realizes that an outbreak of strangles is already well underway. Most horses may have been exposed at this point and a large outbreak is probably inevitable.

Which scenario would you want? Which response would be most likely in your barn?

Surveillance scares off many people just because of the name and the thought that it is expensive, complicated or requires special training. It doesn't. All it needs is people in a barn who communicate and some basic written policies on how to handle horses and potential diseases.

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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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