​Maryland Small Ruminant Page

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.