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

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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Toxicology tests have confirmed suspicions that selenium overdose was the cause of the sudden death of 21 polo ponies in Florida. The pre-event supplement the horses received was suspected early in the investigation, and the compounding pharmacy that provided it confirmed that the selenium level turned out to be too high.  They would not say whether this was due to a pharmacy error, or if the wrong amount of selenium was specified on the original veterinary order.

While this is hopefully an extremely rare event, it raises questions about what we give horses and why. The amount of money that is spent on supplements for horses is astounding. It's questionable whether most (or any) are needed, even for the highest level performance horse - yet a large percentage of horses, including "backyard" pleasure horses, are regularly treated with a variety of unproven supplements. I'm not saying that all supplements don't work, but for most of them there's just no evidence that they do work. There are also issues with quality control, as has been clearly shown with probiotics and glucosamine supplements.

Using supplements is very clearly a "buyer beware" situation. Most supplements have not been shown to be effective. Most are likely safe, but toxic reactions to some compounds (especially some herbs), contaminants and mixing errors can occur. Supplements are also a concern in terms of positive drug tests if their contents are not very clear, if they contain small amounts of contaminants, or if the contents have similar properties to tested drugs.

Am I saying stop using supplements? No. What I'm saying is think before you buy. Don't fall for flashy sales pitches and testimonials. Don't use a supplement because "everyone else is using it." Remember the general rule: above all, do no harm.

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I just got back from the North American Veterinary Conference in Orlando, FL. One of the talks I gave at the conference was about probiotics, during which I spent a fair bit of time talking about various concerns with commercial probiotics. Part of this relates to a study we did in 2002 looking at probiotic product labels. We found that the label descriptions on most products were unacceptable, including many that misspelled the contents and others that claimed to contain organisms that do not exist.

It was quite funny when, immediately after that talk, I went to the tradeshow and saw a booth for a new probiotic product that is apparently quite popular in Italy. It claimed to contain four probiotic organisms, including Lactobacillus sporogenes. Unfortunately, Lactobacillus sporogenes does not exist!  There are no spore forming Lactobacillus species, and this particular bug was renamed Bacillus coagulans in 1939!  Some people market it as a Lactobacillus, presumably because lactobacilli are more widely thought of as potentially beneficial organisms. The person at this product's booth didn't particularly like the fact that I said the organism doesn't exist, but couldn't come up with a rebutal. The product also had no efficacy data or in vitro data suggesting that any of the organisms it contained had the potential to be beneficial, as is the case with most of these products.

I continue to be disappointed by the lack of good research behind most commercial veterinary probiotics. Hopefully, someday, companies will start spending some money on research instead of just marketing. There is more promise with some newer small animal probiotics, as it appears more research is being done, but that same degree of effort doesn't appear to be there for equine probiotics.

More information on probiotics can be found on the equIDblog Resources page.

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I commonly get asked about giving probiotics to horses, and what I recommend. While probiotics are often simply described as "good bacteria", a better definition is "live microorganisms that, when ingested in certain amounts, cause a positive health effect beyond that of their nutritional value." This definition makes it clear that there must be live microorganisms, they must be given at a certain dose and they must cause a beneficial effect. Unfortunately, while there are many, many probiotics available for use in horses, and lots of money spent on the marketing of such products, there has been basically no real research done on any commercial equine probiotic. That makes it difficult to make good recommendations. Studies have also shown that commercial probiotics often don't even contain what they say they do. So, what do I say when I'm asked? I say it probably won't hurt to give an adult horse a probiotic (one study in young foals showed that a probiotic actually caused diarrhea), but there is no guarantee that it will help either. One thing that I do recommend is always to read the label closely.

The label of a probiotic product should clearly state:

• Which organisms the product contains (and they should spell all the organisms' names correctly (don't laugh, it happens!)).
• How much of each organism is present (this is often measured in CFUs or colony-forming units).
• An expiry date.
• Whether the numbers of organisms are guaranteed to be present at expiry - sometimes the stated number of organisms is what was put into the product, with no understanding (or guarentee) of whether they will still be viable at the time of use.
• No excessive claims of effectiveness. (My general rule is that something that promises to cure everything, it probably cures nothing!)

While a good label is certainly no guarantee that the probiotic will work, it seems logical that companies that can't even get the label correct probably aren't making a good product either. We did a study on this a few years ago, for which we simply bought a variety of commercial probiotics and read the labels. I wasn't sure whether to be amused or disgusted by the results, but I certainly wasn't amazed:

• 35% of veterinary and 43% of human products improperly identified their contents. Many used vague (and useless) descriptions like "probiotic cultures." It wasn't even clear in some of the products if live bacteria were expected to be present.
• In 18% of veterinary products that listed bacterial species, the names of the organisms were misspelled.
• Bacterial species were misidentified in 35% of veterinary products, including claiming to contain bacteria that do not exist.

So, probiotic are very much "buyer beware." Until consumers start demanding proof from companies that their products are effective, we may not get much new information.

More information on probiotic use in horses can be found on the equIDblog Resources page.

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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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