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October 2015
More on-farm dewormer resistance testing needed:  do an LDA!


Dr. Dahlia O'Brien
Virginia State University

As an Extension Specialist, I can’t tell you how many small ruminant producers I come across who have no idea what dewormers are still working on their farms.  After indicating that they have lost animals to parasites, and I follow up by asking which dewormer they are using, the answer is sometimes even more alarming.  Why is this alarming you ask?

Over five years ago, I collaborated with the University of Georgia and other specialists in the region to conduct a study in the Mid-Atlantic U.S. to evaluate the status of dewormer resistance on 33 farms in the region.  All samples were tested using the DrenchRite® Larval Development Assay (LDA). 


At the time, the results from this study indicated resistance to benzimidazoles (eg. Safeguard®, Valbazen®, Panacur® and Synanthic®) on all farms tested, ivermectin (eg. Ivomec® and Primectin®) on 82% of farms tested, moxidectin (Cydectin®) on 42% of farms tested, and levamisole (eg. Prohibit®, Rumatel®, Positive Goat Pellet, Strongid®) on 24% of farms tested.


So, when many producers are still using Safequard®, Valbazen® or even Ivomec®, I get a bit concerned.  I’m not saying these drugs aren’t still effective on some farms.  It's just alarming that so many farms still depend on these when the data above clearly indicates that most should have worms resistant to these drugs. 


The status of resistance found in the study mentioned above was very similar to that reported in an earlier survey of resistance performed in the Southern U.S. (in 2008), indicating that dewormer resistance in worms is a serious problem on many small ruminant farms throughout the Eastern U.S.


However, these results are over five years old and can no longer be considered current.  Even with extensive training on integrated parasite management and the use of targeted selective deworming throughout the U.S., I can only imagine that the current status has not improved and could in fact be very surprising.


In many FAMACHA© trainings, we emphasize the need to use effective dewormers.  We describe that the two most common tests available include the fecal egg count reduction test (FECRT) and the LDA (DrenchRite®). With the FECRT, the effectiveness of an anthelmintic is determined by comparing the fecal egg counts (FEC) of animals both before and after treatment or by comparing the FEC of treated and untreated groups of animals (control).


On the other hand, the LDA (DrenchRite®) offers tests for resistance to all dewormer groups in a single test without the requirement for a large herd size (10 animals minimum needed). My main purpose in writing this article is to encourage small ruminant producers to find out the status of resistance on their farm, so that when an animal goes down because of parasites, you will be able to treat it with a dewormer you are confident works. I’m also pleading with my extension colleagues, researchers and veterinarians to encourage more on-farm testing in their areas of the U.S. as well.   


Why do we need more testing? 


We need more testing because there are too many sheep and goat producers that are losing animals every year because they are using drugs that have low efficacy on the worms on their farms and they don’t even know it.  We need more testing because in an industry where we are encouraging our producers to selectively deworm their animals and only deworm based on clinical symptoms, an effective dewormer could mean life or death, especially in young animals. We need more testing because determining what dewormers still work is a piece of the puzzle in developing effective strategies for parasite control on sheep and goat farms.

There are pros and cons to both tests mentioned above.  Most producer farms I visit do not have enough animals present to test multiple dewormers in a single test or even to test a single drug using the FECRT.  Even on farms large enough, I have gone through 300-500 animals and not been able to test more than a single dewormer.


An accurate FECRT test requires >10 animals tested per dewormer in addition to a control group with a similar number of animals. Ideally, these animals should have FEC above 250 eggs per gram. Therefore, you have to identify enough animals with high FAMACHA© scores or with the presence of other signs associated with clinical parasitism to increase the chance that FEC will be above this threshold.


There is also the cost of pre- and post-treatment FEC which can be highly variable depending on the lab you use. The turnaround time for results also varies depending on the class of dewormer tested and how quickly FEC get done and the data analyzed. Based on all the reasons above as well as my personal experience conducting FECRT in the field, I recommend that producers do an LDA test to determine what drugs are still working on their farm.


An LDA only requires one pooled sample from at least 10 animals with FAMACHA scores > 3.  Even though this test cost approximately $450 for a producer, it gives you the results for all classes of dewormers in a single test and results are available in approximately 3–4 weeks.  Some might say that this is costly, but a FECRT can be just as, or more costly, depending on the number of drugs tested and the labor and time it takes to get it all done.


Finally, there is no doubt that clinical and subclinical parasitism affects production and profitability on many sheep and goat farms in the U.S.  We recommend that producers selectively treat animals (based on FAMACHA© scores or Five Point Check©), maintain good pasture and grazing management practices in conjunction with a variety of other tools to minimize/control the impact of parasites.


One of these tools is (not should be) knowing what dewormers are still working on your farm. There are only few dewormers FDA-approved for use in sheep and goats, and armed with this knowledge, producers can work closely with their veterinarian to establish a parasite control strategy that works best for them.  Every farm situation is different, so go ahead, do an LDA!


For information on getting an LDA done on your farm, contact Dr. Adriano Vatta at


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