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

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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In equine hospitals, intravenous (IV) catheters are often placed in the large jugular vein in the neck of horses to make it easier for everyone (the horse included) to give the animal fluids and/or medication. However. any time the body's normal barriers (e.g. the skin) are broken (e.g. by placing a catheter through the skin into a vein), there is an increased risk of infection. Catheter site complications can range from mild inflammation of the skin around the catheter, to serious infection causing thrombosis (blockage) of the vein and abscesses.

Several routine practices are used to help reduce to risk of complications from catheters. Typically, a small (~2 cm X 2 cm) area of skin over the vein is shaved or clipped. Next, the area is usually cleaned with antibacterial soap, and then wiped several times with alcohol and a disinfectant like chlorhexidine or povidone iodine. This is done to help reduce the number of bacteria on the skin at the site where the catheter is placed so the site is less likely to become infected.

At some facilities, clipping  the neck before the catheter is places is often not performed. Also, sometimes owners request that the site not be clipped because they don't want it to be apparent that their horse needed a catheter because it was sick or had surgery. I've always been very reluctant not to clip, although there hasn't been a lot of objective evidence proving that clipping is important.

A recent study published in the Veterinary Record (Geraghty et al. 2009) has provided some information about the effects of catheter site preparation. In this study, researchers looked at different skin preparation methods. They cultured swabs from the skin before and after different procedures. 

The most important findings were:

• Clipping and shaving had a significant effect on bacterial numbers, even before disinfection. Disinfection resulted in essentially eliminating all detectable bacteria, whether the haircoat was clipped, shaved or left intact.
• Clipping, shaving or leaving the haircoat intact were equally effective if a disinfectant was used afterwards. However, clipping or shaving were still preferred over leaving the haircoat intact. (Clipping or shaving both reduced initial bacterial numbers, and this might be important in situations where the haircoat is highly contaminated or dirty).
• Clipping and shaving may be useful for reasons other than disinfecting the catheter site. They can make the vein easier to see, which may make it easier to place the catheter. Difficulty placing the catheter and repeated attempts to place the catheter can cause  additional tissue damage and most likely increase the risk of complications. Leaving the haircoat intact also may increase the risk of introducing foreign material (e.g. hair, dirt) into the body when the catheter is placed.
• Since it was possible to effectively eliminate detectable numbers of bacteria from an unclipped haircoat with proper disinfection, placing a catheter without clipping is acceptable "when rapid intravenous catheterisation is necessary as a clinical priority."  To me, this means that it's okay to place a catheter without clipping if the time needed for clipping might make a major difference to the chances of the patient surviving. This is not the case in situations where people just want to avoid evidence that their horse has had a catheter. While the true risk of not clipping is unknown, it makes no sense to not clip when there is no real downside (other than aesthetics) and there is a logical possibility that it could decrease the risk of complications.
• Chlorhexidine and povidone iodine were equally effective disinfectants. However, chlorhexidine has been shown to be more effective in people at preventing infections. It has a broader spectrum of activity against microorganisms, and is less inhibited by dirt and debris. The small size of the study might have limited the ability to detect a difference between the two products. Since there is no real reason to use povidone iodine over chlorhexidine, and chlorhexiinde has been shown to be better in people, it makes sense to use it in horses.

This study provides some very useful information. Because it looked at bacterial numbers on the skin, not complications/infections per se, we have to be careful not to over-interpret the results. This study (as usual for studies like this) only assessed the transient bacterial population on the skin, not the resident population. The resident population is deeper in the skin and harder to kill. So, even though no bacteria were found after certain procedures, we can't assume the area was truly sterile. Nonetheless, the transient bacteria tend to cause most of the problems, so this type of study is still useful.

Intravenous catheterization is a very important tool in equine medicine. While there are always risks, the benefits of having a catheter in place almost always greatly outweigh any risks. Careful preparation of the catheter site, good catheter placement technique and close monitoring of the catheter site help reduce the risk of complications.

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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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Diarrhea is a potentially life-threatening condition in horses. It's also frustrating from a diagnostic standpoint because, even with the most complete/comprehensive testing, a cause is only identified in a minority of cases. This is true for other species too, including people, and is a reflection of the complex nature of the intestinal tract and the numerous possible causes of intestinal disease. Diagnostic testing obviously costs money, so if it gives us an answer less than half the time anyway, it begs the question - why bother? Well, here are some points to consider:

• In some situations, you may find a cause that requires a different treatment, so diagnosis has a direct impact on patient care and probably the outcome.
• Many causes of diarrhea are infectious and it's important to know if other horses may have been exposed to a transmissible pathogen.
• Some causes of diarrhea, especially Salmonella, can also infect people, and it's important to know to what people have been exposed.
• It's also important to know whether a horse might still be shedding a transmissible pathogen after its diarrhea resolves. For example, horses with salmonellosis can shed Salmonella for a while (often a few weeks, sometimes longer) after they have recovered from their diarrhea. This could be a source of infection for other horses and people.
• Outbreaks of diarrhea can occur. It's much better to know early on what you are dealing with, rather than waiting until mulitple horses have been infected.

I consider diagnostic testing money well spent in cases of diarrhea. Some people don't want to test because they don't want to know, particularly about Salmonella. However, it's been proven again and again that the "head-in-the-sand" approach will backfire when it comes to infectious diseases.

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This post originally appeared (in modified form) on www.wormsandgermsblog.com on January 1, 2009.

An important step in diagnosing infectious diseases and determining the optimum approach to treatment and management is rapid and accurate diagnostic testing. Many different testing methods are used, particularly bacterial culture (at least for bacterial diseases). Molecular testing has revolutionized the field of microbiology, and is making inroads into the field of diagnostic testing. Polymerase chain reaction (PCR) testing is a very powerful tool that can be used to detect DNA or RNA from specific microorganisms.  This technique can be very useful, but it can also be easily misused or misinterpreted.

The potential PROS of molecular diagnostic testing include:

• Rapid turnaround time: Testing can take as little as a few hours versus a few days for other tests like bacterial culture.
• Sensitivity: Organisms that are difficult or impossible to grow in a lab can be detected, and they can often be detected at lower levels than with other diagnostic methods.

The potential CONS of molecular diagnostic testing include:

• Sample contamination: This is a common concern with highly sensitive molecular tests - even a minute amount of contamination in the sample can cause a false positive result.
• Test inhibition: Samples from complex biological sites (e.g. stool) can contain substances that interfere with the many complex molecular reactions upon which the tests rely. Without good (and proven) methods to prepare the sample, this can result in a false negative result.
• Biologically irrelevant results: Some bacteria that cause disease are also commonly found as part of the normal microflora in healthy animals - simply finding it does not tell you that it is necessarily relevant to the problem. For example, Clostridium difficile can be found in the intestine of normal, healthy horses, but the diagnosis of C. difficile diarrhea requires detection of the bacterial toxins in stool samples, not just the bacterium itself.  A molecular test that simply identifies the presence of C. difficile, even if it identifies strains that possess the genes to produce toxins, tells you nothing about whether the bacterium was actually producing toxins in the animal.
• Lack of validation: This is a common problem with many (if not most) molecular tests. Some companies, especially those that just run molecular tests, offer a huge array of completely unvalidated and sometimes illogical tests.  It is also important to remember that tests must be validated for each species in which they are used - a test that works well in people will not necessarily work on a sample from a horse or a dog.

Molecular testing can be useful in some situations. If you are unsure, here are some things to ask the lab:

• Do they have a validated test that provides relevant results?  If they don't have good data (ideally published data) that their test is useful, accurate and reproducible, I'd avoid it.
• Do they have a quality control program, which includes running positive and negative control samples with each test batch?

Finally, as with any test that we use in veterinary (or human) medicine, it's important to evaluate all  results in the context of what is happening with the animal - treat the patient, not the test result.

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Welcome to equIDblog, a new public resource all about equine Infectious Diseases! This site, sponsored by the Ontario Veterinary College Teaching Hospital (OVCTH), is designed to provide high quality information about infectious disease topics related to horses.

The main equIDblog page is regularly updated with posts about equine infectious disease, in a blog format. It provides information and commentaries about recent and important issues pertaining to this field. New posts will be going up every day or so, so check back regularly. Periodically there will also be "case posts", which describe a horse with a particular infectious disease, with an emphasis on explaining what the problems and issues are with similarly affected horses.

Don’t miss the equIDblog Resources page, which contains detailed information sheets on various infectious disease topics which can be downloaded and/or printed.  More information is available on specific diseases, as well as issues such as vaccination and antibiotic use.

Also on the Resources page is an infection control section, focused on equine hospitals. There you’ll find infection control resources, including hospital protocols and information sheets developed by the OVCTH for clients. 

You can also register on the main page to receive automatic e-mail notification when new posts go up, so you'll never miss the latest updates!  We hope that you'll find this site educational and informative, and we welcome any comments and suggestions for future blog topics.

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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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When people talk about prevention of infectious diseases on horse farms, they often use the term "biosecurity." I don’t.  I prefer to use the term "infection control." Here’s why:

• Biosecurity is a term often used in association with rearing of food animals, particularly chickens and pigs. In these types of facilities, new animals are rarely or never introduced to an established group, control of personnel access is very strict, and significant efforts are made to prevent exposure of animals to new infectious agents.
• In my opinion, we do not (and cannot) practice true biosecurity on horse farms because of the way horses are managed - the movement of horses on and off farms, movement of people that work with horses, and diseases that are always circulating in the horse population. The simplest daily, weekly or monthly activities - going to a show, buying a new horse, going off-site on a ride, bringing in visitors/farriers and other people that have had contact with horses - in combination with the fact that even healthy horses can carry  potentially harmful bacteria and viruses, means that there is always an risk of exposure to infectious agents on horse farms.
• We are not able to completely exclude infectious agents like bacteria and viruses from entering horse farms (i.e. making them "biologically secure"). However, what we need to strive for is to limit the problems that develop when these agents do enter the farm. For example, we know that the majority of horses carry equine herpesvirus in a dormant state in their bodies. We can’t eliminate it, but we try to reduce the risk of problems like herpesvirus abortion in mares by vaccinating mares while they're pregnant.

While it may be somewhat an issue of semantics, I think it’s important that horse owners have a realistic mindset. There is always, and will always be, infectious disease risks when dealing with horses in any situation. Some risks are avoidable while others are not.  The key is to implement measures to reduce the inevitable impact of infectious disease exposure on our horses.

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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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