In your war against parasites, you faithfully deworm your horse every six to eight weeks–just like you’ve been doing for the past 10 years. Beware! Recent reports of parasite resistance across the country indicate that the deworming medication you’ve given your horse may not be working. Now is the time to evaluate the efficacy of your deworming plan and choose effective deworming medications. With a battle plan, you can be confident that you are really winning your war against the worms.

But, the battlefield has changed, and you need to make sure the weapons you employ are still effective. A number of changes in the parasite population over the years has yielded a varied response to medications. Different species of worms, such as the small strongyle and tapeworm, have become a more serious concern, while others, such as the large strongyle, have become less important. Additionally, widespread use of deworming drugs has led to resistance within some of these parasite populations.

Weapons Check
When waging war on worms, the most reliable monitoring tool available is measurement of parasite eggs passed in your horse’s feces, called a fecal egg count. Although there are some factors that can cause inaccuracies in these measurements (such as parasites that can be present in larval forms without passing eggs), a high egg count is a solid indicator of a significant parasite load. In addition, a reduction in egg count following deworming means the deworming drug is working.

To evaluate the effectiveness of the dewormers you choose to include in your deworming program, ask your veterinarian to collect a fecal sample prior to deworming. If egg counts are high, he’ll repeat this test seven to 14 days following administration of the medication (depending on what class of drug you’re evaluating). An 80 to 100 percent decrease (again, depending on the class you drug you’re testing) in that number indicates that the deworming medication is still effective on your farm.

Based on this type of testing, along with an evaluation of the type of parasites most likely to attack your horse, your vet can help you determine what deworming medications should be incorporated into your deworming schedule. Twice yearly fecal egg reduction tests can then be used to determine whether your deworming program is still effective.

Strategic Planning
It’s obvious that the “cookbook” approach to deworming horses is no longer appropriate. Instead, you must consider your horse’s individual situation and determine if resistance is a problem on your farm. The following outlines goals and strategies for your deworming program. With this information in hand, you can refer to our dewormer guide (see p. TK) for more information about specific drugs.

Goal #1: Target populations of parasites most likely to impact your horse’s health. Large strongyles are no longer your primary target. Focus instead on small strongyles, bots and tapeworms. Take the following steps to determine what dewormers are effective for your situation:

• If you’re currently on an interval deworming program that uses benzimidazoles or pyrantel as part of your rotation, ask your veterinarian to perform fecal egg reduction tests on your farm following administration of these dewormers to determine whether they’re still effective. Based on these results, decide which deworming medications you’ll continue to use.
• Consult with your veterinarian regarding the prevalence of tapeworms in your area, as they are much more common in some areas of the country than others. Based on this information, decide how often you should include praziquantel in your program. It may be recommended as frequently as twice a year, or as infrequently as every other year.
• Include ivermectin or moxidectin at least twice each year, in the spring and fall, for bots. If you determine that you have resistance problems to benzimidazoles and pyrantel, you may need to use these dewormers more frequently, as they’ll be your only remaining effective options. The frequency of administration will depend on your horse’s level of risk for parasite exposure.
• If you have foals, ask your veterinarian for deworming advice. Foals are more likely to be affected by ascarids, a worm that’s unlikely to affect your older horses, and may not respond as well to ivermectin or moxidectin.

Goal #2: Reduce your horse’s exposure to parasite larvae in the pasture. As resistance problems become more widely recognized, it’s no longer possible to rely on medications administered every six to eight weeks as a guaranteed method for parasite control. These days, basic pasture management to control exposure simply can’t be overlooked. Take the following steps to keep your horse’s parasite exposure to a minimum:

• Keep stalls and paddocks clean, and remove manure from pastures on a regular basis.
• Don’t spread manure on horse’s grazing areas.
• Mow or harrow pastures regularly, according to growth. Grass heights should be maintained below five inches.
• Avoid overcrowding. One to two acres per horse is ideal.
• Maintain groups of horses on pasture segregated by age group. Youngsters (especially yearlings and 2-year-olds) are likely to shed larger numbers of parasite eggs and may require more frequent deworming than their older counterparts. Avoid exposure of your mature horses by turning them out separately.

Goal #3: Decrease the frequency of deworming to avoid development of resistance. More is not better. In fact, if you administer deworming agents frequently, you’ll increase the chances that parasite populations will be become resistant, and you’ll be left with fewer options for waging an effective war against the worms. Take steps to allow deworming fewer times per year:

• Assess your horse’s risk. If he lives in a stall that’s cleaned twice a day with access to a paddock or pasture that’s also cleaned regularly and only has exposure to a small number of other horses that are all dewormed on the same carefully analyzed schedule and maintained in the same conditions, his risks are low. You may be able to deworm as infrequently as twice each year. If he lives in a large pasture with a herd, all on different deworming schedules, his risks are high.
• If you determine that his risk is high, take steps to lower it. (You’ll either need to change his management situation or continue frequent deworming with medications you’ve determined are effective, based on fecal egg counts. High-density turnout in a large pasture is one situation in which a daily dewormer administered to all horses can help if pyrantel is still effective. It will help reduce parasite loads on the pasture and minimizes the need for ivermectin and moxidectin administration.

The following chart outlines basic facts about the commonly available deworming medications. Use this information to design your own horse’s deworming schedule.

Barb Crabbe, DVM, is a graduate of the University of California at Davis and is now in private practice at Pacific Sporthorse in Oregon City, Ore.. A lifelong horsewoman, she is a graduate of the U.S. Dressage Federation’s (USDF) “L” Education Program, a USDF silver medalist and a competitor through Intermediaire I. An award-winning equestrian journalist, Dr. Crabbe is a frequent contributor to Dressage Today.

This article first appeared in the March 2009 issue of Dressage Today magazine.