Veterinarian Addresses Questions Regarding Parasite Resistance

Horse owners across the country are asking questions about parasite resistance and seeking answers on the best way to ensure their horses are protected from parasite infestations. Dr. Robert Holland, DVM, PhD, Associate Director, Outcomes Research at Pfizer Animal Health offers advice to concerned horse owners.

Why are parasites such a huge problem in horse health, especially with the convenience and availability of dewormers for all types of parasites?

Parasites can cause many severe health risks to horses including life-threatening colic. In addition, they may cause other problems such as weight loss or a loss of body condition, a decline in performance or act as a predisposition to secondary causes. Horse owners have easy access to a variety of dewormers on the market. However, simply administering a dewormer may not ensure that the horse is getting the necessary protection. If the dewormer is not used correctly, meaning given in proper dosage and timing, this can cause them to not be as effective as they could be. An inadequate treatment dosage allows residual worm populations to survive and encourages resistance.

What can horse owners do to ensure their horse is being properly dewormed?

The best approach is for horse owners to consult with a veterinarian about a deworming program that is most appropriate for each horse or farm. Currently parasite resistance is not widespread in every area or chemical class, which means veterinarians must assess the risk of parasite infestation for that horse or herd and treat each case individually. Horse owners should also understand the importance of proper dosage. In many cases, assuming every horse in the barn is the standard 1100 pounds has lead to under dosing. While weight tapes are not 100 percent accurate, using one prior to each worming can help decrease the risk of under dosing. Veterinarian can also help educate horse owners on the three different chemical classes among the dewormers and the importance of rotating among these classes.

What do you think about "parasite resistance"? Is product rotation necessary or does it just build-up resistance to all parasites?

There are three chemical classes of dewormers – the Benzimidazole class, the Pyrantel class and the Macrocyclic Lactone class. Horse owners should keep in mind with parasite resistance that it does not mean dewormers are no longer effective. Rather, it means the chemicals are less effective than they use to be previously against a specific type of parasite. It is also important to know that resistance is not widespread among all areas of the country. Currently there are no new chemical classes for horses on the horizon. Therefore, rotating among the three chemical classes – not just between brand names of dewormers – is still a safe and effective way to ensure the horse is protected from parasites.

What can the industry do to better protect horses?

There is currently one standard to determine effectiveness or resistance to a dewormer – Fecal Egg Count (FEC). Monitoring the herd with FEC exams helps determine high and low egg shedders. A FEC is a relatively inexpensive test that can provide information on general health, possible parasite burden and the effectiveness of the deworming program. However, it’s important to know that while FEC exams are the best that veterinarians have to work with right now these tests also have limitations. A number of factors can influence the test results, making it less than a precise measurement of parasite burden in a horse or a herd. Incorporating an FEC exam is a good monitoring system, however it is only one part of a deworming program for a horse or farm.

What do you recommend to reduce environmental contamination, when early and adult stage larvae are in the barn, the pasture, and continue be ingested?

The best solution when it can be done is to rotate pastures. Pastures can naturally decontaminate in 8 weeks during summer, late spring and early autumn. Larvae live longer during cool, stable temperatures, so natural decontamination for them can take up to 6 months. Other key factors that are recommended are not to overstock pastures, meaning one horse per every two acres. Also, horse owners can maintain healthy pastures with frequent mowing, manure collection and herd rotation. Stalls and feeding area should be cleaned daily and corrals and paddocks at least twice weekly.

Has a process been developed to accurately detect parasites so treatment can be more precise?

The Fecal Egg Count exam (FEC) is really the gold standard when it comes to testing for parasitic infection in the horse. However, as mentioned previously, the fecal egg count exam also has significant limitations; knowing how to correctly interpret results is a key step in using them to design, or alter, a farm’s parasite protection program. Fortunately, total eradication of fecal egg shedding in horses on pastures is not a necessary goal for a parasite protection plan. A general rule of thumb is that the proper objective of parasite control is to maintain the parasite burden at a low level, rather than to eliminate parasites completely. This middle ground avoids over-treatment, limits the cost of parasite control and helps maintain partial immunity to overwhelming infection.

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