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

There are a few ways to administer dewormers to horses, depending on what product is being used:

• Giving a drug by nasogastric tube (stomach tube) used to be common, but is rarely done now because, while you can be sure the drug goes where you want it to go, it has to be done by a veterinarian and is not particularly pleasant for the horse.
• There are injectable versions of ivermectin, but they tend to cause significant pain and reaction at the site of injection, and are not generally recommended for horses.
• Most commonly, dewormers are given as oral pastes. These are easy to get and easy to administer - to some horses. Failure to get the whole dose into the horse is a problem (and some horses are remarkably good at avoiding what they don't want to swallow!).  This can result in underdosing, which can reduce the effectiveness of deworming and likely contributes to the emergence of drug-resistant parasites.

Another approach used by some is topical or "pour-on" administration of dewormers. Pour-on formulations are only available for cattle, so use of them in horses is "off-label". The ease of administration is appealing, but an important question is how well do pour-ons work in horses?

A study published in a recent edition of Veterinary Parasitology (Gokbulut et al 2010) compared oral, intravenous and pour-on ivermectin in horses. (Ivermectin is a common horse dewormer, sold under a variety of brand names.)  The researchers treated three groups of horses witheither ivermectin paste, bovine pour-on or bovine injectable formulations. They then looked at blood and hair/skin levels of ivermectin and fecal egg count reduction. Here are the highlights:

• Ivermectin concentrations were high on the skin and hair at the pour-on site, not surprisingly. Detectable concentrations were found on the skin at distant sites.
• Blood levels of ivermectin were lower in horses treated with the pour-on compared to the paste, but blood levels persisted longer with the pour-on, presumably because of longer, slower absorption. That's not necessarily a good thing, however. Prolonged exposure to low levels of an anthelmintic is a good recipe for resistance. What we want are high (therapeutic) levels for relatively short time to kill susceptible parasites and decrease the emergence of resistance.
• Oral ivermectin produced better reductions in fecal egg counts, meaning it did a better job eliminating parasites in the intestinal tract. Pour-on ivermectin did not work as well, but produced a longer effect.

What does this tell us?

Basically, it  means that pour-on ivermectin is absorbed and can reduce intestinal parasite levels. It can also provide a more long-lasting effect than oral ivermectin. However, it also has the potential for inadequate levels in the intestinal tract and theoretically a greater likelihood of resistance.

Since we have pastes with demonstrated efficacy against intestinal parasites and which are licensed for use in horses ("on-label"), it's hard to justify the use of pour-ons in this species. Perhaps in situations where a particular horse is extremely resistant to oral deworming and there is no way to get the dewormer paste into the horse by any method (after trying many different tricks), topical treatment of that specific horse could be considered. Such treatment of a rare single horse here or there is probably of limited consequence in the broader context, but if people start treating all horses on the farm like this just because one is hard to dose orally, or because they think it's easier, resistance concerns increase greatly.

Photo credit: Lesley Ward (click for source)

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We've taken the plunge into (the time- and money-sucking abyss of) pony ownership for our 3 daughters. Foxie Casie, a 4-yr-old Welsh cross, arrived yesterday, much to the delight of the girls and mixed delight and trepidation to their parents. I figured I might as well discuss some of the infectious disease-related issues dealing with acquisition of a new pony.

Vaccinations

• She came vaccinated against rabies and tetanus, with the last round having been done this spring. I'll re-do rabies and tetanus next spring. She'll get a rabies vaccination every year, but I will probably stretch out the tetanus vaccine since yearly vaccination is likely not necessary (especially considering tetanus vaccination is labeled for every 5 years in Australia).
• I always debate about the necessity of West Nile virus (WNV) vaccination, since it's very rare now in Ontario and I'm not aware of any mammalian cases in the general vicinity. I'll probably err on the side of caution and vaccinate her, but will do so in late summer, in advance of the true high risk period here. Typically, any WNV cases that occur here develop late August through October, so vaccinating the pony in late July, to get peak immunity during the main risk period, makes sense.
• I will probably not vaccinate her against eastern equine encephalitis (EEE) since it's so rare and hasn't been found around here.
• She lives with no other horses and won't be traveling off the farm in the near future, so there's no need to vaccinate against things like influenza, equine herpesvirus or strangles.

Deworming

• She was dewormed about one month ago. I'll check a fecal egg count soon and go from there. Since she's the only horse on the farm (the sheep are low risk for passing parasites to her), if we get started right and keep her from contaminating the pasture, a pretty conservative parasite control program with periodic fecal egg counts and deworming as needed should suffice.

New horse infectious disease issues

• She didn't have much of a trailer ride and appeared healthy when I saw her the day before, so shipping associated diseases like pleuropneumonia ("shipping fever") are pretty unlikely.
• Diarrhea's always a concern. Risk factors for diarrhea include shipping, stress, diet change and antibiotic use. Well, she's been shipped, presumably stressed, had her diet changed and been introduced to pasture, so she has some risk factors. She's not exposed to other horses, which helps, and she has no need for antibiotics (at least at the moment). So, basically, I'll give her a good diet (good hay and not-very-lush pasture) and keep an eye on her. My diarrhea research background probably makes me look at her feces more than the average person, but so far, so good.

I'm sure there'll be more to come... hopefully not much from the actual infectious disease standpoint. 

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TheHorse.com has a good article about deworming, which includes six good parasite-fighting tips from Dr. Craig Reinemeyer. Check out the full article for all the details. Here are the key points and some comments:

1. Stop focusing on the wrong things

It's easy for people to get hung up on a few concepts or topics and not see the big picture or the real problems. We need to think in terms of the horse(s), not the parasites. We're trying to optimize horse health. That doesn't necessarily mean killing all parasites, nor does it mean relying soley on drugs to control them. Conceptually, we need to think about parasite control, not just deworming.

2. Plan parasite control around parasite biology, not a calendar

There is no "standard" deworming program. Every program needs to be tailored to the geographic region, farm and horse. What happens here is Ontario is very different from what happens in Florida in terms of parasites.  For example, the risk of transmission in horses turned out on pasture in Florida in February is certainly different from those turned out in snowy fields in Ontario at the same time of year. Even in the same region, what happens on one farm may be very different than what happens on another. When designing a parasite control program, you need to consider what is going on with worms in the horses and in the environment to determine the optimal approach.

3. Don't encourage resistant parasites

Most people know antibiotic-resistant bacteria are a bad thing, but often concerns about anti-parasitic resistance are ignored. We only have a limited number of antiparasitic drugs, and there aren't many more coming in the near future. Heavy use or misuse creates an environment where resistance may emerge. It's being seen with certain bugs and certain drugs, and needs to be taken seriously - now.

4. Don't treat all horses the same

As I mentioned above, you need to tailor parasite control to something that is relevant for the region, farm and horse. Different horses are at different risks of exposure. Some horses are naturally more resistant to parasites, so these animals don't need to be (and indeed shouldn't be) treated the same as more susceptible horses. Deworming should be tailored to the needs of the individual horse. Yes, it takes more effort and some thought, but it's worth it.

5. Practice evidence-based parasite control

That means use all available information to make decisions. It includes knowing what parasites are a concern on the farm (which requires testing), what antiparasitic drugs have been effective on the farm (which requires testing to determine treatment success), an understanding of farm management practices, and information from research about deworming strategies and drugs. Performing fecal egg counts is a critical component of this, and something that is not done nearly enough.

6. Be prepared to change

Something that works today may not work in the future. You can't get stuck in your ways and assume that since something worked in the past, it will always work.

Image source: www.ponytalesblog.com

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A report in the September 1 edition (Swor et al, 2009) of the Journal of the American Veterinary Medical Association describes three horses with ivermectin toxicity. All three animals were adult Quarter horses from the same farm, and were among five horses that received a single tube of ivermectin paste (Bimectin) each. The tubes were the last in a box of tubes, the rest having been given to other horses on the farm with no problems.

Approximately 18 hours after all five horses were treated, three started to show signs of depression, drooling and ataxia (e.g. wobbly, unstable gait, poor balance). One horse was severely affected, and was sent to Texas A&M's equine hospital. The neurological abnormalities progressed in that horse, it became recumbent (i.e. unable to stand) and blind, and was euthanized. Nothing remarkable was detected on necropsy. Testing of the brain revealed an ivermectin concentration similar to the levels found in dogs with ivermectin toxicity. (Normally, ivermectin should not be detectable in the brain after deworming, because it should not be able to cross the blood-brain barrier.) The other two horses were less severity affected. They also ended up being hospitalized but recovered.

This is the first report of complications developing in adult horses treated with appropriate doses of ivermectin. Earlier reports of such complications have involved young horses receiving overdoses.

So what happened? A couple of possibilities were considered and dismissed:

Too much ivermectin in the product: Some residual paste was tested and it did not have excessive ivermectin levels.  Also, other horses treated with tubes from the same box were fine. Therefore, it's very unlikely that too much ivermectin was given.

A genetic sensitivity to ivermectin: All of the affected horses were unrelated.

The most likely explanation is compromise of the blood-brain barrier.  As mentioned above, the blood-brain barrier should keep ivermectin from reaching the brain, but if this important barrier was somehow damaged, then ivermectin could have reached the brain and caused the signs of toxicity. Although there was nothing to indicate that another condition was present that would affect the blood-brain barrier, this is still the most likely cause. The authors suspected that a toxin from a plant may have been responsible for weakening the barrier, such as toxicity from silverleaf nightshade. Some infectious diseases can also affect the blood-brain barrier, but there was no evidence of these. In the end, a definitive cause could not be identified.

This report shouldn't scare people away from using ivermectin or other dewormers. This is a rare complication from use of a very common drug. Rather, it should remind us that there are potential risks with virtually all drugs (including supplements), and that the cost-benefit of using them must always be considered. It's also another good reason to use dewormers prudently. Ivermectin toxicity should be considered in any horse that develops neurological abnormalities after treatment, and any potential adverse reactions should be reported so the extent of the problem can be determined.

Image source: www.horsechannel.com

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Many peoples' idea of a parasite control program is to give their horse a dewormer and assume everything's taken care of. Many aspects of appropriate, logical and prudent anti-parasite programs are ignored with such an approach, which can lead to problems.

One often overlooked aspect of parasite control is regularly performing  a simple, cost-effective and important test called a fecal egg count.  This test involves identifying and counting different types of parasite eggs in a fecal sample by examining it microscopically. Results are reported in eggs per gram (EPG) of feces. In general, a low egg count (less than 200 EPG) indicates a low parasite burden, while a high egg count (500-1000 EPG, or greater) indicates a large number of parasites are present.  High fecal egg counts suggest ineffective deworming (e.g. too long between deworming, inappropriate deworming drugs, underdosing of dewormers, failure to get the horse to swallow the dewormer), a lot of parasites in the environment or parasite resistance. Evaluation of fecal egg counts can give a general idea of the parasite burden in a particular horse or on a particular farm, indicate the effectiveness of the current parasite control program, and allow deworming recommendations to be tailored for a particular horse or farm. People recommend regular testing with deworming when egg counts are greater than 200 EPG, however, I don't think there is a consensus as to the best approach. Certainly, high EPG counts indicate a need for treatment or a change in the deworming program.

The fecal egg count reduction test (FECRT) is a way to assess parasite resistance to dewormers. Basically, it means performing a fecal egg count before and after (usually 7-14 days after) deworming. Egg counts should go down to close to zero after effective deworming. Failure to see this decrease indicates that the horse was not properly treated (e.g. it didn't actually swallow the dewormer or it wasn't given enough dewormer) or that some of the parasites may be resistant to the drug used. Early detection of resistance is very important so that the deworming program can be changed before health problems from uncontrolled parasites develop.

The main limitation of fecal egg counts is that some parasites, particularly tapeworms, do not produce many eggs, or they only produce eggs intermittently.  Therefore, low EPG counts do not mean that there are no or few parasites of any type. That's why even deworming programs based on fecal egg count test results still use some "scheduled" deworming to eliminate these other parasites. Despite the limitations, fecal egg counts give excellent information for most of the parasites of concern, and can help a great deal with the design of effective, responsible deworming programs and evaluation of parasite resistance.

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The Horse Trust from the UK has issued a call to "stop over-worming".  They've launched a campaign to get horse owners to take a "more informed and strategic approach" to deworming because of the growing problem with resistance to dewormers. Now's a good time to do this because many people start more aggressive deworming programs in the spring.

There's no doubt that parasites are a potential problem in horses and that dewormers are an important part of parasite management. However, good parasite management and frequent use of dewormers are not synonymous.

Drug resistance in parasites doesn't get a lot of attention, particularly compared to drug resistance in bacteria. However, there is increasing evidence that resistance to dewormers is a problem in some regions, including right here in Ontario. It may actually be a very widely disseminated problem, but we don't know the full extent of it because testing parasites for resistance isn't as easy and routine as testing bacteria.

People often think that they are providing excellent management by deworming their horses very frequently. Just like people are starting to pay attention to the concept of prudent of use antibiotics, we need to think about prudent use of dewormers. These are important drugs and we need to make sure that we don't contribute to their demise.

• Deworming should be done according to a program developed by you and your veterinarian. There is no "standard" program because the needs of different facilities and horses vary.
• A good deworming program isn't necessarily a frequent one.
• Fecal egg counts are a test of a horse's manure to look for parasite eggs. This type of testing can (and should) be used to determine who needs to be dewormed, and testing after deworming can be used to detect resistance to dewormers.
• Decreasing the use of dewormers can help reduce the risk of resistance and save you money at the same time. How many other things that are good for you save you money as well??
• There are other very important management practices, such as pasture management, that play an important role in parasite control. Trying to rely on solely on drugs is bound to fail in the end.

Photo: An adult roundworm (Perascaris equorum) from a horse.

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