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

A three-year-old Standardbred gelding was presented to the hospital for evaluation and treatment of diarrhea (colitis). The previous week the horse had developed a mild hind-limb lameness which seemed to come and go, and it was decided to treat the horse with antibiotics “just in case” it had something to do with an infection. The horse was treated with ceftiofur (an antibiotic, often sold under the brand name Excenel or Naxcel) for five days. On the fifth day, the gelding developed moderate to severe diarrhea. The next morning the horse also had a fever. He was treated with anti-inflammatories and quickly referred to the hospital for intensive care.

On presentation, the gelding was very quiet. He had a very high heart rate, reddish gums and he was significantly dehydrated. Intestinal sounds could not be heard over the abdomen, indicating that the horse’s intestines were not moving normally, and there was a “ping” on the right side of the abdomen, indicating that there was gas accumulating in the cecum (part of the large intestine). Treatment with intravenous (IV) fluids was started right away to try to correct the dehydration and keep up with the amount of fluid the horse was losing in its diarrhea.

By the next morning the horse’s attitude was improved, but his gums were still an abnormal colour (“toxic mucous membranes”, see picture), indicating that there were inflammatory cytokines (substances released by cells when they’re in distress) and likely bacterial toxins in horse’s bloodstream. Also, despite the IV fluids, the gelding was still dehydrated, likely because he was pooling fluid from his body tissues in his intestine, as well as the more obvious loss of fluid in his ongoing diarrhea. This went on for another two days, despite intensive treatment in the hospital. On the fourth day, the gelding developed severe signs of colic. His large colon became progressively more distended with gas, and the contents of his small intestine started to back-up into his stomach. His heart rate became extremely high, and his pain could not be controlled with sedatives or anti-inflammatories. A belly-tap yielded a red-tinged fluid (normally belly fluid is light yellow), and the concern at that point was that the intestines had become twisted (which can happen in horses with diarrhea as a result of their abnormal intestinal motility). Despite the risks, it was decided to take the horse to surgery - but there was no twist in the bowel. The cause of the colic was that the large colon was severely distended with gas and fluid, and it was barely moving at all. The appearance of the large colon was consistent with extreme inflammation, and the tip of the cecum looked so bad that the surgeons decided to remove it because it was likely dead or dying.

The horse recovered from anesthesia, and IV fluid therapy was continued. Later that day, when the horse was offered some pellets, some intestinal sounds were detectable. The horse soon started to pass diarrhea again, but overall his attitude was much brighter, and his hydration status and (remarkably) blood protein levels remained stable.

Unfortunately the day after surgery the gelding became reluctant to move around the stall. Increased digital pulses were detected on the front feet, and the horse was sensitive to hoof testers – the gelding was developing laminitis. Despite additional treatment, the signs of laminitis became worse and worse. In the end the horse was euthanized, less than a week after being admitted to the hospital.

On necropsy, the entire large colon was severely thickened, filled with green-yellow fluid, and the mucosa (inside surface of the intestine) was ulcerated. Signs of severe acute laminitis were present in all four feet. A specific causative agent of the colitis could not be identified – tests for Salmonella and clostridial toxins were all negative. This is not too surprising as no agent is identified in over half of all adult horse colitis/diarrhea cases. But there is no doubt what set this terrible chain of events in motion – treatment with antibiotics, for a condition that may or may not have ever required antibiotic treatment in the first place.

We talk a lot about antibiotic-induced colitis/diarrhea in horses, but until you’ve seen it for yourself, it can be hard to believe that drugs used every day in both people and animals can have such a devastating effect on a horse. Antibiotics certainly do save lives, but unfortunately there are no “miracle cures” that are entirely without drawbacks. This case clearly demonstrates one of the most important reasons why we so strongly advocate prudent use of antibiotics in horses – their use should never be employed lightly. Although this is a “worst case scenario” that overall occurs uncommonly, the potential is there and should always be taken into consideration.

Photo credit: M. Anderson 2007

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European Antibiotic Awareness Day is an initiative of the European Centre for Disease Prevention and Control.  This year it falls on November 18.  The aim of the Day is to provide an annual opportunity for raising awareness about the threat to public health of antibiotic resistance and how to use antibiotics responsibly.

Responsible use of antibiotics can help stop resistant bacteria from developing and help keep antibiotics effective for the use of future generations.  Successful national public awareness campaigns are already resulting in more rational use of antibiotics and a reduction in levels of antibiotic resistance in Europe.

Responsible use of antibiotics includes use in people and in animals.  Here are some of the things you can do to help with regard to antibiotic use in your horses:

• Only give your horse antibiotics if directed to do so by your veterinarian.
• Make sure your horse gets the full dose of medication at the correct time(s) of day.  If you are having problems injecting medication or getting your horse to swallow pills, contact your veterinarian as soon as possible.  Your veterinarian may be able to give you advice on some "tricks" for getting your horse to take the medication, or sometimes the medication can be provided in a different form (e.g. a liquid instead of a pill, oral versus injectable).
• Always ensure your horse finishes the entire prescription.  There should be no leftover pills or medication.  Do not stop giving your horse the antibiotics just because it looks/acts like its feeling better.  This is a common mistake that can have disasterous consequences!  You should NEVER "save a few pills for the next time."
• Never give your horse antibiotics that were prescribed for you, another person or any other animal, whether they are expired or not.

This equIDblog entry was originally posted on the Worms & Germs blog on 18-Nov-09.

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At the recent symposium of the American College of Veterinary Surgeons, there was an interesting abstract presented about liposome technology.  Liposomes are basically teeny-tiny "bubbles" made up of the same basic components as cell membranes.  They are sometimes call nanoparticles because they are so small, but they are still much bigger than even large molecules, which gives them some interesting biological abilities.  When liposomes are injected into the bloodstream, they go all over the body, but they tend to accumulate where there is tissue inflammation, because blood vessels become "leaky" in inflamed tissue, allowing these nanoparticles to escape the bloodstream.  What's great about liposomes is you can put different things inside them (such as drugs), and they help deliver their contents to inflamed sites in greater concentration and over a longer period of time, while reducing the exposure of the other body tissues to whatever they're carrying.  This can ultimately help improve the efficacy and safety of the drugs they carry.  Liposomes also tend to accumulate in tumors, so they are sometimes used to deliver anti-neoplastic chemotherapy drugs to these sites.

Liposomes have actually been around for a long time.  They were first described by Dr. Alec Bangham in 1961, and since then have become valuable tools in biology, biochemistry and medicine.  The technology still has its share of problems that need to be worked out.  Some people may have acute adverse reactions when liposomes are injected intravenously.  Sometimes the body's immune system will "attack" the liposomes, taking them out of circulation prematurely.  Much work is still being done to find better ways to help liposomes target particular tissues.

In terms of treating infectious diseases, liposomes can be used to deliver antibiotics to infected tissues.  Because of the targeted delivery system, the toxic/side effects of drugs on the rest of the body can be reduced while still achieving the same (or higher) concentrations of drug at the site of infection.  An example of this is the drug Abelcet (Enzon Pharmaceuticals), which is a liposomal preparation of the very toxic antifungal drug amphotericin B.  Abelcet has been licensed for use in the US and Europe since 1995.

The abstract presented at the ACVS (C. Underwood et al.) described the use of liposomes containing a radioactive marker (99-m Technetium) in horses.  They injected 10 healthy horses with a dose of liposomes, monitored them for adverse effects (of which there were none detected), and then used the radioactive marker to determine where the liposomes accumulate in a horse under normal conditions.  Their ultimate objective is to determine if liposomes can be used to better deliver antimicrobials to areas of infected bone (osteomyelitis), which are typically very difficult to treat.  Liposome technology still has a long way to go before it's being used widely in horses, but this was an important first look at how liposomes can possibly be applied to equine medicine in the future.

Image credit: Kosi Gramatikoff (1999)

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If you have a horse, sooner or later you have a horse with a gash somewhere on its body.  Horses seem to have an uncanny ability to find the one sharp branch or protruding splinter of wood or nail in any paddock, pen or stall that no one else can ever find, no matter how hard you look.  But in the end, horses, just like people, sometimes get cuts.  Some cuts probably never even get noticed.  Thankfully, a lot of them heal by themselves with no interference from us.  In other cases, though, the cuts can get infected, and that's when you and your veterinarian need to step in and help that horse out.

Unfortunately, the "knee-jerk" reaction in many of these cases seems to be to put the horse on antibiotics.  Under some circumstances I don't disagree that this may be necessary, especially if the wound may involve tendons or a joint or deeper tissues.  In a lot of cases, however, this type of treatment may not be the best thing for the animal.  Use of local therapy for wounds, instead of systemic (e.g. oral or injected) antibiotics, should always be carefully considered.

Local therapy for wounds can include a variety of different types of treatments, such as drainage of discharge, removal of dead tissue (debridement), flushing of the wound (lavage), topical antibiotics, antiseptics, and sometimes newer (or older) compounds like honey.

Let's compare some of the potential pros and cons of local versus systemic therapy:

• Treating what counts: Local therapy is targeted at the site of infection - the wound itself.  Some antibiotics that are too toxic to be given systemically, or can't be given safely at a high enough dose to be effective, can be applied directly to the wound at a higher concentration, delivering a more effective blow to the infecting organism.
• Not treating what doesn't count: Avoiding the use of systemic antibiotics decreases exposure of the bacteria that are part of the horse's normal bacterial flora, particularly those in the intestinal tract.  This helps to decrease the risk of disrupting the flora, which can otherwise result in antibiotic-associated diarrhea.  It also helps decrease the risk of other bacteria in the horse's body developing resistance to the antibiotic being used.
• "Taking out the trash": Drainage, debridement and lavage help remove all the "junk" in a wound by getting rid of dirt, pus and dead tissue.  Often times, these are the most important components of treatment, and likely have more of an effect on the outcome than any of the drugs that may be used.  Whatever can be physically removed from the wound decreases the amount of junk and bacteria with which the horse's body needs to deal, and also eliminates material in which bacteria can hide from the immune system.
• Cost savings: Often times local therapy involves more time for cleaning and bandaging, but less drugs, because you're only treating the infected site, not the entire horse.

• It's not easy: As difficult as it sometimes can be to give a horse needles or make it swallow medication, these methods are often chosen because they are perceived as the "easy way out." Local therapy for a wound can be a lot of work - it takes time, it sometimes means getting yourself a little dirty, and some owners don't like the "ick" factor of having to deal directly with the wound itself.  However, systemic antibiotics should not be used as a substitute for proper wound care.
• Every wound (and every horse) is different: Not every infected wound is amenable to local treatment.  Deep wounds can be especially difficult to treat this way, because the deepest parts of the wound simply aren't accessible.  Also, depending on the temperament of the horse and/or the location of the wound, the animal may not tolerate local therapy of the site without sedation, in which case it is best left to your veterinarian.  It is always very important to ensure that you can safely treat your horse.

Even for wounds that do require systemic antibiotic treatment for one reason or another, local therapy should not be neglected, and can be critical to achieving a successful outcome.  It's important to try to resolve infections (of any kind) as quickly and efficiently as possible in order to avoid complications associated with chronic infection, and minimize the risk of antibiotic resistance developing (if antibiotics are used).  That means using all the available tools at our disposal, including simple wound care and local therapy, to treat them. But remember:

• Always wear disposable gloves if you need to clean, treat, bandage or otherwise touch a wound, and wash your hands well with soap and water afterward.  This will help prevent bacteria from the wound from being transmitted to you, and bacteria from your hands from infecting the wound.
• Do not give your horse antibiotics of any kind before consulting your veterinarian.

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Horses sometimes need to be treated with antibiotics. That's an unavoidable fact. Some horses that are treated with antibiotics develop diarrhea, which can be fatal. That's another unavoidable fact. While those two situations can't be avoided, the risks can be decreased.

Addressing the first point, "sometimes horses need to be treated with antibiotics," involves various factors such as having a good preventive medicine program, good infection control and only using antibiotics when they are truly needed. We'll never absolutely eliminate the need for antibiotics, but we can reduce their use.

The second point is a little harder to address. Any horse being treated with antibiotics is at some risk of developing antibiotic-associated diarrhea. In addition to hopefully killing or inhibiting the bacteria at the site of the infection, antibiotics also reach the intestinal tract, where there can have effects on the complex resident bacterial population. Disruption of this normal bacterial population can allow "bad" bacteria to overgrow, resulting in diarrhea. How do we reduce the risk?

• Use antibiotics only when necessary and with the advice of your veterinarian.
• Avoid high-risk antibiotics (e.g. tetracyclines, erythromycin in adult horses) unless absolutely required.
• Use local (i.e. topical) therapy whenever possible instead or oral, intravenous or intramuscular administration. This limits the amount of antibiotic that makes it to the intestinal tract.
• Try to minimize other potential risk factors for diarrhea such as diet changes, high grain diets, transportation and other stresses.
• Monitor your horse closely and contact your vet if there are any signs of colic or diarrhea.
• It might be reasonable to avoid anti-ulcer drugs, since it's possible there could be increased risk of diarrhea while being treated with these drugs (possible, but not proven).

Probiotics are often used, but there is currently no evidence they are effective in horses. Mixed results have been obtained in people.  It's possible that certain probiotic organisms at certain doses may help reduce the risk of certain types of diarrhea in certain horses. We just don't know what "certain" means at this point. Probiotics probably won't hurt, but we can't have any confidence in them yet that they are really beneficial.  No other supplements have been shown to be effective, and there is little reason to suspect that any would be effective.

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I often get asked "what's the best antibiotic?" The short answer (but the one people really don't want to hear) is "the one that works." It's obvious more complex than that, but, in reality, the best antibiotic to use for any given horse and any given disease IS the one that works, and the one that does so with minimal side effects, including both patient side effects (e.g. diarrhea) and microbial side effects (e.g. emergence of drug resistance).

Sometimes, certain drugs are referred to as "powerful" antibiotics, which really isn't a good description. What people are usually referring to is the ability of these drugs to work on bacteria that are resistant to many other antibiotics. These "powerful" antibiotics are not necessarily any better at treating an infection caused by a more susceptible bacterium, they are just able to work on bacteria resistant to other drugs.

When choosing an antibiotic, it's important to consider whether the bacteria involved in the infection are Gram-positive or Gram-negative. Gram staining is a simple technique used to divide bacteria into these two groups. Under a microscope, after staining Gram-positive bacteria appear purple and Gram-negative bacteria appear red (or pink). The differences in staining are a result of differences in the bacterial cell walls, which also play an important role in determining a bacterium's susceptibility to different antibiotics.  Part of describing the function of an antibiotic is stating whether it is effective against Gram-positive bacteria, Gram-negative bacteria or both. (There are other aspects such as whether there is effect on aerobic versus anaerobic bacteria (those that can versus cannot survive in the presence of oxygen), but I'll address that some other time).

Some antibiotics have broad spectrum activity, being able to kill Gram-positive and Gram-negative bacteria. At first that might seem like a good thing, and indeed it is when you are treating an infection with an unknown bacterium. However, broad spectrum activity is not desirable when you know exactly what bacterium you are trying to eliminate. If possible, it's much better to target treatment more specifically against the offending bacterium for various reasons, such as to reduce the risk of resistance emerging in other bacterial groups.

Some antibiotics are narrow spectrum. Some are most effective against Gram-positive or Gram-negative bacteria, with little activity against the other.  In general, we want to use an antibiotic of the most narrow spectrum possible. We can do this is a few ways:

Educated guess: A six-month-old foal with a lung abscesses most likely has Rhodococcus equi pneumonia. Even without a culture to confirm, we would usually treat the foal with drugs specifically targeting this Gram-positive bacterium, with little concern about Gram-negatives.

Gram stain: If the infection is in a location that can be sampled, then a Gram stain can be performed. This is very easy, quick and cheap, and can tell you whether you are dealing with a Gram-positive, Gram-negative or mixed infection.

Culture: This is the best method, but culture is not always possible and results may take several days. Culture can tell you the which bacterium is involved and exactly which antibiotics it's  susceptible to in the lab.

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Erythromycin is an interesting drug in horses, having saved the lives of countless numbers of foals but able to readily kill an adult horse. Erythromycin is an antibiotic of the macrolide class. It is effective against many Gram-positive bacteria (e.g. Staphylococcus, Streptococcus, Rhodococcus), but not Gram-negative bacteria (e.g. E. coli, Salmonella).

There are several very good aspects of erthyromycin, including its ability to kill bacteria that have invaded cells (intracellular bacteria) and the ability to treat abscesses (many antibiotics can't do that very well at all). Erythromycin is also available in an oral form, which makes it much easier to give. It is most commonly used for the treatment of Rhodococcus equi pneumonia in foals, almost always in combination with the antibiotic rifampin, and it's a highly effective treatment.

However, the news isn't all good.  Erythromycin is far from a benign drug, and should only be used in certain situations. Some of the problems associated with its use are:

Diarrhea: Any antibiotic can cause intestinal upset, ranging from mild diarrhea to fatal colitis, but erythromycin is a particularly high-risk drug. Diarrhea can occur in foals being treated with erythromycin, but this is not very common and when it occurs diarrhea is not usually very severe. However, giving oral erythromycin to an adult horse is a very high-risk proposition that can easily result in fatal colitis. In Sweden, there have been reports of fatal colitis in mares whose foals were treated with erythromycin - the hypothetical cause of this phenomenon is that mares were exposed to extremely low amounts of erythromycin from the foals' mouths or feces. Erythromycin should be avoided in adult horses at all costs.

Hyperthermia: This is a strange but rare side effect that is most often seen in foals being treated in sunny areas without access to shade. Some treated foals lose the ability to properly control their body temperature, and can develop extremely high fevers. The classic situation is an otherwise healthy foal that is responding nicely to treatment suddenly found down in the field one day with a life-threateningly high body temperature. This is uncommon, but it is a concern and treated foals should always have access to shade and be closely monitored.

Erythromycin can be a very useful drug is some situations. It's use is declining somewhat as related drugs like azithromycin are sometimes being used in its place, but it can still be a good treatment option in some cases. Because of the risks of adverse effects (and because of basic principles of antibiotic use), you should only use erythromycin under the direction of a veterinarian. Never use it in an adult horse unless you have no other choice, and realize there is a reasonable chance the horse will develop potentially fatal diarrhea.

Image: 3D model of erythromycin molecular structure (source: www.3dchem.com)

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"Long-acting" penicillin, a combination of benzathine penicillin and procaine penicillin, is available for use in horses. The idea is that the drug releases penicillin slowly into the body so that a single injection lasts a few days. Good in theory - but not in reality.

The problem with long-acting penicillin is the penicillin levels achieved in the body (i.e. the concentration of penicillin in the blood and other tissues) aren't actually high enough to do anything (except perhaps to help encourage the development of penicillin resistance among bacteria). There are some exquisitely susceptible bacteria that might be killed by long-acting penicillin, but the vast majority of horses that get better after treatment with long acting penicillin probably get better despite treatment (i.e. they would get better on their own anyway).

If you need to use an antibiotic, use an effective one that has been prescribed by your veterinarian. Don't use long-acting penicillin.

As a paper looking at the pharmacokinetics of penicillin in the horse (Love et al, 1983) stated, "Fortified benzathine penicillin appears to have little value for antimicrobial therapy in the horse." That says it all.

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The latest issue of the Journal of Veterinary Diagnostic Investigation contains a case report by Dr. Glenn Songer's research group about a 14-year-old Quarterhorse that had been treated with ceftiofur (an antibiotic) because of suspected salmonellosis, and subsequently died of severe colitis (sometimes called colitis X, but I don't like that name). This pattern is all too familiar when it comes to horses: antibiotic treatment for an undiagnosed infectious disease results in death due to colitis (diarrhea). It's even more frustrating when you consider there is very little indication to treat adult horses with salmonellosis with antibiotics.

The colitis in this case was caused by Clostridium difficile.  The strain of C. difficile was ribotype 027 (also called NAP1, BI and toxinotype III, depending on the method used to type the strain), which is typically considered the most serious strain in people. It is often associated with outbreaks of disease, and has been blamed for the increased frequency and severity of C. difficile infections in people internationally over the past few years. This strain has also been found in dogs, cattle and pigs, as well as retail meat samples (e.g. from the grocery store).

While the information in this paper isn't particularly surprising, it should act as a reminder that C. difficile is an important problem in horses and that important strains of C. difficile can infect many different animal species. When you consider how big of a problem C. difficile is in human medicine, it should be a reminder that we need to take this problem seriously in horses too, and also continue to investigate whether people can be infected by horses. A small percentage of healthy horses shed C. difficile in their manure, and we don't really know if that poses a risk to people. When you consider how much C. difficile can be present in diarrhea, the huge volume of diarrhea that a horse can pass and the potential for human exposure because of the big mess that is made, we certainly should consider diarrheic horses as a possible source of infection for people, just as we do for Salmonella.

Antibiotic use is a well known trigger for C. difficile infection in people.  While disease in horses can occur even when antibiotics are not given, it is widely suspected that antibiotic use is a major risk factor for severe colitis due to C. difficile. Anecdotally, ceftiofur seems worse for this than many other drugsin some regions.  This is consistent with studies in humans that have shown that antibiotic drug class used can have a significant impact on the risk of C. difficile infection.

What should we take home from this report?

• Only use antibiotics when they are really needed. Antibiotic administration can lead to fatal complications, although these are rare.
• Consider C. difficile in all cases of diarrhea in horses.
• Consider all horses with diarrhea potentially infectious to other horses and people, and handle/house them appropriately.

Photo source: http://www.microvet.arizona.edu/Faculty/songer/diag.htm

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In people, as well as many other species like dogs and cats, oral antibiotics (e.g. liquids or pills that are swallowed) are very commonly used, because it's usually easier (and less uncomfortable) than giving antibiotics by injection with a needle.  A wide range of oral antibiotics are available for use in humans, and while antibiotic-associated complications such as diarrhea certainly can occur, oral antiboitics are relatively safe for most people.

So, why don't we use many oral antibiotics in horses?

Horses have a very different intestinal tract than people (and dogs and cats). A horse's intestinal tract is much likely to develop problems from antibiotic use, particularly antibiotic-associated diarrhea or colitis, which can be fatal. The root of the problem in these cases is  disruption of the normal bacterial populations that live in the intestine (the microbial "flora"), which can allow harmful bacteria to multiply and spread. This can occur with antibiotics given by any route (even by injection), but using oral antibiotics can result in higher drug levels in the intestinal tract, which creates a greater chance of causing problems. The likelihood of a horse developing complications from any antibiotic is probably a combination of what bacteria the antibiotic kills and how much makes it to the intestinal tract.

• Some oral antibiotics can be used relatively safely in horses. Trimethoprim sulfa (TMS) is very commonly used, and quite safely for the most part, in many areas (including Ontario).
• Chloramphenicol and enrofloxacin (Baytril) are also given orally to horses, but these drugs should be reserved for problems for which they are specifically indicated.
• Some antibiotics can be used more safely in foals than adults, such as erythromycin. While diarrhea can occur in foals, erythromycin (and related drugs in the macrolide class) are commonly used without problems in young stock. Macrolides are higher risk drugs in adult horses. I've used them a few times in adults when I've had no other choice, but it's always a scary prospect.
• Never use an oral antibiotic that is not known to be relatively safe for use in horses. One of the first cases that I saw during my residency was an expensive racehorse owned by a physician. He had cultured a minor foot wound and grown a Staphylococcus that was susceptible to cloxacillin. He got some cloxacillin and gave it to the horse orally - something no veterinarian would do. The horse developed severe diarrhea and died despite intensive treatment.

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Lawsonia intracellularis is an important cause of disease in weanling foals, causing a disease called proliferative enteropathy. Antibiotics, mainly erythromycin,are usually used as part of the treatment for this condition. However, little is known about the true antibiotic susceptibility of this bacterium. The problem is that Lawsonia does not grow in culture plates in a lab like most other bacteria with which we deal - it can only grow in cells, which makes it very difficult to test for  antibiotic resistance.  Antibiotic therapy for this disease is therefore chosen based on basic knowledge about the bacterium and anecdotal information about how animals respond to treatment.  Obviously, this is not the ideal situation.

A little more information is now available on this topic. In a recent edition of Veterinary Microbiology, researchers from the University of Minnesota published a study of the activity of different antibiotics on L. intracellularis. The study, by Suphot Wattanaphansak and colleagues, only involved 10 isolates of the bacterium, all of which were from pigs. (Lawsonia infection is an even bigger problem in pigs than it is in horses).  They only tested with a few drugs that are used in horses, and they used an unvalidated (but reasonable) test, so it's very important to be careful (and conservative) when extrapolating the results to Lawsonia in horses. However, they did have some potentially equine-relevant results. One important finding was that there was variation in susceptibility to some drugs between isolates, meaning some strains were susceptible and some were resistant to certain drugs. Although this isn't really surprising, it emphasizes the fact that the same treatment may not be suitable for all cases - antibiotic resistance can be present, and if it is it's critical to identify and address the issue.  This particular study showed that tetracycline, a drug sometimes used in horses, had "intermediate activity" against Lawsonia... not great, but not horrible. Unfortunately, they didn't test erythromcyin or chloramphenicol, which are two drugs more commonly used to treat Lawsonia in foals.

This study doesn't tell us much more about how to treat horses with this infection, but it does emphasize the need to test equine strains against drugs used in horses to see if we are doing things right, whether resistance to important equine drugs is present, and to determine if there are better and faster ways to detect resistance.

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

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Abscesses are common problems in horses. They can range from easy-to-identify-and-treat foot abscesses to abscesses that are very difficult to diagnose and even harder to treat (e.g. brain abscesses, abdominal abscesses).

An abscess is an accumulation of pus.  Pus is basically dead bacteria, partly broken-down cellular debris, and white blood cells. Sometimes there can be a thick capsule of tough, fibrous tissue surrounding an abscess, particularly if it has been there for a long time (see picture left). Most abscesses occur as the result of a local infection that the body was able to contain (at least partly), but could not eliminate altogether, and that didn't "drain" naturally to an open space (e.g. outside the body). Over time, some abscesses may continue to grow and become quite large, which can cause many different problems depending on where the abscess is. "Sterile" abscesses can also occur, but are much less common - these lesions are not associated with a bacterial infection, but may occur following a severe trauma or in association with certain tumours.

Because abscesses are usually caused by an infection, it's logical for people to think about using antibiotics to treat them. However, antibiotics are not very effective at treating abscesses because:

• Abscesses have very little blood supply. A good blood supply is needed to deliver antibiotics to an infected site.
• Many antibiotics do not penetrate into abscesses well because of the tough capsule and the type of fluid within the abscess. Some do a better job than others, but none are great.
• Even if an antibiotic is able to penetrate an abscess, it may be only minimally effective due to the amount of pus and bacteria that are usually present.

 So, if we can't use antibiotics effectively, what do we do? The most effective treatment for any abscess is incision and drainage. This involves lancing the abscess to remove as much of the infected pus as possible. It also allows for local treatment such as flushing with antiseptics.  Antibiotics should only be used when there is no way to drain (lance) an abscess, such as with lung abscesses.

If an abscess can be lanced, it should be, with very few exceptions. That may involve paring out a foot abscess, lancing a strangles-infected lymph node, or opening a large abscess from a wound. Lancing of abscesses should always be done by a veterinarian. While it may be tempting to "pop" an obvious abscess yourself, there may be important blood vessels or nerves in the area that need to be avoided. The abscess that you see may be only one part of the problem, and other diagnostic tests or treatments may be needed. A veterinarian can pick the best part of the abscess to lance, to ensure the best possible drainage, and a veterinarian can provide proper sedation and local anesthesia for the horse during the procedure. An ill-placed stab at an abscess can make things a lot worse.

Photo: A large abdominal abscess from a horse that was drained and opened to show the very thick outer fibrous capsule, and many thinner, internal loculi. (M. Anderson)

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Penicillin is an old, but still very useful, antibiotic. It is effective for treating many kinds of infections and is relatively cheap and safe. However, allergic reactions to penicillin, though rare, are reported in both people and animals, including horses.  Signs of a reaction may include hives, sweating, weakness and collapse shortly after treatment.

What many people may not realize is that a horse that has a reaction to a penicillin injection is not necessarily allergic to the penicillin.  Most of these animals actually react to procaine, which is a component of the white penicillin that is given by injection into the muscle. If procaine reaches the blood supply too quickly (e.g. if the injection accidently goes directly into a vein instead of the muscle), it can cause a very sudden, dramatic reaction including signs of hyperexcitability, manic behaviour and rearing over. Horses that have procaine reactions typically recover quickly, but they can injure themselves when falling.

It is important to differentiate penicillin allergy from procaine reaction. If a horse has a penicillin allergy, it should never be treated with penicillin (of any kind) ever again. Diagnosing a horse with a penicillin allergy is therefore not something that should be done lightly or unnecessarily, because penicillin is a very useful drug. A small percentage of individuals with penicillin allergy are also allergic to the related antibiotic class, cephalosporins (including the common drug ceftiofur (Excenel, Naxcel)). Only a small percentage of horses are allergic to both classes, so we don't automatically ban the use of both drug classes in a horse with a penicillin allergy.  However, it does warrant caution when a cephalosporin is used in the animal for the first time, and a veterinarian should be present in case there is a reaction. If a horse has had a severe allergic reaction to penicillin, cephalosporins should probably be avoided anyway, to be on the safe side.

If you are going to use penicillin, you should ensure that you know how to inject it properly. Improper injection technique increases the risk of a procaine reaction.

White penicillin must NEVER be injected intravenously.

While penicillin is cheap and easy to get, but as with any antibiotic, it should only be used on the recommendation of your veterinarian.

Photo: Penicillium notatum, the mold from which penicillin was originally produced
(photo source: Dept of Biology, University of North Carolina at Charlotte)

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I'm trying not to sound like a broken record when it comes to antibiotics, but it's a very important topic so you'll see many posts on the subject. When you consider that antibiotic-associated diarrhea is a potentially life-threatening problem in horses, and that antibiotic-resistant bacteria are becoming more common in general, it should be obvious that we need to limit the use antibiotics to when they are actually needed, and potentially effective.  Horses with respiratory disease are commonly treated with antibiotics.  However, as discussed in a previous post ("Snotty-Nosed Horses: What To Do?"), there are many causes of respiratory disease in horses, and most of them are not bacterial.

A common cause of nasal discharge and coughing in horses is inflammatory airway disease (IAD, which is somewhat similar to asthma in people). This condition is different from heaves, which typically occurs in older horses and causes more severe clinical signs.  However, horses with IAD  may cough and/or have clear nasal discharge. They may also have decreased athletic performance. They do no have a fever, decreased appetite or other signs associated with an infection. However, antibiotics are (unfortunately) commonly used to "treat" these horses, which makes very little sense.  A few years ago a study was done looking at antibiotic treatment history (i.e. prior to diagnosis) in 55 horses diagnosed with non-septic inflammatory airway disease at the Ontario Veterinary College. Here are the highlights:

• 69% of horses were given an antibiotic for treatment of their respiratory disease. 52%  received more than one antibiotic, and 19% received 3 different antibiotics.
• Horses were treated with antibiotics for extended periods of time, with an average of 9.9 days.
• No testing was performed to detect bacterial infection in any horse prior to treatment.
• 21% of the horses that received antibiotics did not have any abnormal clinical signs beyond poor performance - not even nasal discharge.
• Horses with a history of coughing were more likely to have received an antibiotic, but cough is a very non-specific clinical sign that does not necessarily indicate an infection, let alone a bacterial infection.

Antibiotics are important drugs, but we overuse them (in people and in animals). We need to be careful with when and how we use to ensure they have the maximum beneficial effect and to reduce the risks of side effects or increasing bacterial resistance.

More information on inflammatory airway disease is available in the consensus statement on IAD in horses from the American College of Veterinary Internal Medicine (ACVIM).

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I often get asked "My horse has been exposed to a horse with strangles. Can I treat him with antibiotics to prevent infection?" It's a reasonable question, but unfortunately there is no clear answer.

Strangles is an infection caused by the bacterium Streptococcus equi subsp equi, which is often simply called Strep equi or S. equi for short.   This pathogen is always circulating somewhere in the equine population, and it can cause sporadic cases of strangles in individual horses, or large  outbreaks in groups. It is transmitted mainly by nasal secretions and pus from infected horses when they come in contact with other horses.  The classical and most prominent feature of strangles is the formation of large abscesses in the lymph nodes between the jaw bones.

Treatment of strangles with antibiotics is typically frowned upon because antibiotics do not penetrate abscesses well. Draining the abscesses is the key to treatment in most situations. However, antibiotics can kill the S. equi if they are used before abscesses develop. Therefore, if a horse has been exposed to strangles and is in the very early stages of developing an infection, but it does not have any abscessed lymph nodes, then treatment with antibiotics could be useful.

The downside of this kind of treatment is, while it stops the immediate infection from developing, the horse does not develop immunity against S. equi like it would if its body had to fight the infection. This is not a problem if good infection control measures are in place to prevent further S. equi transmission. However, if transmission is not controlled, then the horse is at risk of being re-exposed, and could be infected again after the antibiotics are stopped.  This can lead to a vicious cycle of exposure-treatment-susceptiblity-exposure-treatment...  I've seen farms where this goes on for a long time with horses receiving multiple courses of antibiotics, and they often eventually getting the disease anyway.

Antibiotics can be useful in horses with early disease (i.e. fever, no lymph node enlargement) IF they are used as part of an overall infection control program that has a heavy emphasis on implementation of sound infection control measures. If potentially infected horses can be rapidly detected and isolated, thus minimizing the risk of exposure to other horses, then early antibiotic treatment of horses that are developing strangles can be useful. Antibiotic treatment alone, without concurrent use of good infection control measures, is bound to fail. While uncommon, antibiotics can be associated with adverse effects in horses, so we want to make sure that we are only using them when they are needed.

More information about strangles can be found 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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The discovery of antibiotics was one of the most important medical advances in history, and these drugs have had an immense impact on human and animal health. While antibiotics have saved countless lives, their use can also be associated with some very serious side effects and complications.  For example, in horses, antibiotic-associated diarrhea (colitis) is a major issue.

By the nature of their intestinal tract, horses at among the highest risk species for developing serious diarrhea associated with antibiotic use.  The intestinal tract of a horse is full of billions of bacteria of different types. These bacteria are important for normal digestion, and also help to prevent infection by "bad" bacteria such as Salmonella and Clostridium difficile.  Antibiotics can disrupt the balance of this complex bacterial population.  The imbalance itself can cause problems, and can also make it easier for bad bacteria to move in, grow and cause disease.

Here are some important points to remember about antibiotics, particularly in horses:

• Antibiotics can only kill bacteria. They should not be used when a bacterial infection is not present and not likely to occur. Antibiotics are too commonly misused for viral infections in horses, which puts the animals at unnecessary risk.
• Any antibiotic can cause diarrhea, but some drugs are considered higher risk. Erythromycin and tetracyclines are considered high-risk in most areas of North America. Certain drugs, such as lincomycin and oral penicillins, are such high risk that they should never be used in adult horses.
• Both oral and injectable antibiotics can also cause diarrhea. Drugs that are injected can still reach the intestinal tract and affect the bacteria there. Some drugs, like tetracycline, are actively pumped into the intestinal tract from the bloodstream, resulting in relatively high concentrations in the intestine, even if the drug was given by injection.
• While some antibiotics can be purchased over the counter in some areas, antibiotics should never be given without the direct recommendation of a veterinarian.
• If your horse is being treated with antibiotics and develops diarrhea, contact your veterinarian immediately.
• There is no known way to reduce the risk of antibiotic-associated diarrhea in horses, other than to avoid unecessary use of antibiotics. Some people treat horses with probiotics or yogurt, but currently there is no evidence that this is beneficial (but it probably doesn't hurt, at least in adult horses).

Another major concern with antibiotic use is the development of antibiotic resistance in bacteria, but that's a topic for another post (or two, or three or more!).

More information about Clostridium difficile and probiotics in horses can be found on the equIDblog Resources page.

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