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

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.

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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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