Ask Your Veterinarian Presented By Equistro: Where Are We With Dewormer Drug Resistance?

Q: Veterinarians have been getting the word out for a while about the importance of rotating deworming products and focusing on high shedders. Can you explain why, and do we know yet whether this strategy is reducing drug resistance?

A: For years, veterinarians and horse owners have correlated rotational deworming of horses with any number of paste dewormers on a predetermined schedule to be adequate control of internal parasites. With the absence of any new class of deworming products over the past 20 years and with the overuse of dewormers available, anthelmintic resistance has become a problem and needs to be addressed. Understanding that all dewormers vary in their clinical effectiveness on different species of parasites and at different stages of their lifecycle, we need to be more critical of which dewormers are used. Other factors that influence this decision include length of time each dewormer is expected to be effective and timing of when each treatment should occur.

A targeted selective approach to deworming is the new recommendation to aid in slowing anthelmintic resistance and managing intestinal parasites based on individual animals rather than a ‘blanket’ treatment of the entire herd. When designing a farm’s (or individual horse) program, we must consider the egg reappearance period of each dewormer, individual horse susceptibility, climatic factors, and current anthelmintic resistance. The egg reappearance period (ERP) is the time from deworming until eggs are detectable in fecal float, and varies between the three classes of dewormers.

The ERP helps determine the timing between treatments, and varies based on which product is used from 4 weeks up to 8 weeks. Horses possess varying levels of natural immunity to internal parasites, resulting in 25 percent of horses in a herd producing 80 percent of the eggs found on pastures. This level of individual susceptibility can be determined by using fecal egg counts per gram (McMasters), following a period of three to four months (winter or summer) without deworming, and then designate each horse as a low (<200epg), moderate (200-500epg), or high (>500epg) egg shedder. These groupings allow us to treat individuals based on need, instead of overusing dewormers.

Rood and Riddle Equine Hospital’s Dr. Dale Brown

Climatic factors provide a great deal of natural parasite control, and need to be considered when timing deworming. Small strongyle larvae cannot survive on pasture above 85 F, and the development of eggs to larvae is suspended under 45F. These factors result in a period of time during the summer (June, July, and August) and winter (December, January, and February) when deworming is not necessary in all horses, except for the high egg shedders. Anthelmintic resistance can be determined by performing a fecal egg count reduction test (FECRT) for each class of dewormer. A fecal egg count should be done prior to administering the dewormer and repeated 14 days post treatment. Resistance can be defined as a less than 90 percent reduction in fecal egg numbers following a specific dewormer. Once a farm’s parasite population has been identified as being resistant to a dewormer, that product should no longer be used on that farm for that parasite.

By being selective in choosing when to treat, what to treat with, and which horses to treat, the targeted selective approach can decrease deworming costs by up to 50 percent and at the same time provide superior control of parasites and decreased resistance. Anytime you reduce the exposure of parasites or bacteria to a drug, it slows the rate of resistance built up to that particular drug.  Only time will tell how much we’ve slowed it down.

Dr. Dale Brown was raised on a cow/ calf farm in Girard, Kansas. After working on a Quarter Horse breeding farm during high school, he decided to attend Kansas State to pursue an Animal Science/ Agribusiness undergraduate degree. He earned his degree in 2001 and continued his schooling at Kansas State obtaining his Veterinary Medicine degree in 2006. Dr. Brown completed his Ambulatory Internship with Rood & Riddle in 2007. He joined Rood & Riddle as an associate in 2007 and in 2013 he became a shareholder. Dr. Brown’s special areas of interest include reproduction, neonatal medicine, herd health, and public yearling sales. When not seeing patients, Dr. Brown enjoys spending time with his wife Kelly, his daughter Samantha and his twin boys, Wyatt and Clayton. He also is an avid college sports enthusiast.

The post Ask Your Veterinarian Presented By Equistro: Where Are We With Dewormer Drug Resistance? appeared first on Horse Racing News | Paulick Report.

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