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Volume 15 Issue 4 - Fall 2016


Are Dewormers Still Effective on Your Farm?

The American Sheep Industry Association’s Let’s Grow Program provided funding to facilitate testing for anthelmintic (dewormer) resistance on commercial sheep farms in three southeastern states:  Maryland, Virginia, and Georgia.

It is a well-known fact that worms have developed resistance to all the dewormers and dewormer classes currently available to sheep and goat producers. Resistance varies by geographic region. It is worse in the Southeast, because this is where internal parasites are most prevalent. It also varies by farm. It is worse on farms that deworm frequently. It tends to be worse on goat farms, as they usually have a greater need for deworming than sheep.

On average, worms have become most resistant to the benzimidazole or "white" class of dewormers which includes fenbendazole (SafeGuard®) and albendazole (Valbazen®). A high level of resistance is also prevalent among the avermectins, which includes ivermectin (Ivomec®), doramectin (Dectomax®), and eprinomectin (Eprinex®). Levamisole (Prohibit®, LevaMed®) is the dewormer most likely to still be effective on sheep and goats farms. Moxidectin (Cydectin®) is also still effective on many farms.

*Resistance varies by farm and geographic region and may be different on different from wheat is reported in this column.

There are two ways to test for anthelmintic (or dewormer) resistance:  the fecal egg count reduction test (FECRT) or the DrenchRite® test. To do a FECRT, you collect a fecal sample (2-4 g) from an animal, deworm it, then collect a second sample 7 to 14 days later. You compare the egg counts (eggs per gram of feces; EPG) between the two samples to determine how effective the drug was.  A fecal egg count reduction of 95% is desired. Below 95%, there are resistant worms in the population. Below 95%, a dewormer may still deliver an effective treatment, but increasingly it will be less effective at killing worms, reducing fecal egg counts, and saving clinically parasitized animals.

The first sample you collect should contain at least 200 EPG.  In fact, the data is more reliable with much higher initial fecal egg counts, as 200 EPG is a very low egg count for the barber pole worm. The barber pole worm is a very prolific egg layer. Ideally, you should collect and compare samples from 10 to 15 animals.  You need to do this for each drug you want to test. It’s also a good idea to collect samples from animals which do not receive treatment (a control group). Doing a fecal egg count reduction test on one animal or a few animals will give you a general idea if the drug works, especially if clinical signs are improved, but it will not tell you definitively if resistant worms are present.

There are several advantages to the fecal egg count reduction test. Producers can learn to do their own fecal egg counts. All that's really needed is an inexpensive student microcope (100x power) and a McMaster egg counting slide.  Most veterinarians and diagnostic labs can perform the test. Fecal egg counts can also be used to test the effectiveness of combination treatments, natural remedies, and other management practices aimed at reducing internal parasitism. Fecal egg counts can be used to identify resistant and susceptible animals.

For our Let’s Grow Project, we opted to go with the DrenchRite® test. The DrenchRite® test (or larval development assay/LDA) is a laboratory test that determines resistance to all four dewormer groups simultaneously from a single pooled sample of feces. The sample needs to contain at least 400 epg (preferably more) and should be collected from at least 8 animals.  When collecting fecal samples for either the DrenchRite® test or FECRT, animals with high FAMACHA© scores (3-5), low body condition scores (<3), and soiled rear ends should be favored. Lambs/kids tend to have higher egg counts than mature animals.

The DrenchRite® uses the same 95% cut-off value to determine the presence of resistant worms. It also identifies, via the larvae, the type of (strongyle) parasites which are infecting the animals. The University of Georgia is the only place that performs the test.  The DrenchRite® test is an expensive test ($450), but well worth the money if animals are dying from infective treatments or suffering from marginally effective treatments.










Calendar of Events


February 24
Frederick County Sheep Breeders Meeting

Lambing Discussion
Contact:  Peter Vorac



March 1-11

Regional NSIP Workshops

Workshops held in WV, PA and NY


March 14
Webinar: Is Sheep Milk Production in Your Future?
ASI Let's Program and Optimal Ag
Register at


April 14-15

Maryland Sheep breeders Sheep Shearing School

1942 Uniontown Road,, Westminster, Maryland



May 6-7

Maryland Sheep & Wool Festival




May 7

Junior Sheep & Wool Skillathon

Contact: Christopher Anderson

Phone: (301) 314-7187



Wild & Woolly is published quarterly by University of Maryland Extension. It is written and compiled by Susan Schoenian, Sheep & Goat Specialist, and Jeff Semler, Extension Agent with UME Washington County, and edited by Pam Thomas, Administrative Assistant. You may contact Susan or Pam at the Western Maryland Research & Education Center (WMREC), 18330 Keedysville Road, Keedysville, MD 21756; (301) 432-2767 x343 (Susan) or x315 (Pam); or  The cost to receive the newsletter via U.S. mail is $10 per year (payable to the University of Maryland). The newsletter can be accessed for free online at!newsletter/cno0. Internet users can subscribe to the newsletter listserv so that they’ll receive an e-mail message when a new newsletter has been posted to the web. To subscribe to the listserv, send an e-mail message to In the body of the message, write: subscribe sheepandgoatnews. To unsubscribe, send an e-mail message to the same address, but in the body of the message, write: unsubscribe sheepandgoatnews. Comments and suggestions pertaining to the newsletter are always welcome. References to commercial products or trade names are made with the understanding that no discrimination is intended or no endorsement by University of Maryland Extension is implied.

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