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The DrenchRite™ Assay

by Sue Howell & Bob Storey  (retired)
University of Georgia College of Veterinary Medicine

Most producers with grazing animals are acutely aware of the effects of parasites on their animals, as well as the problems associated with resistance to the dewormers (anthelmintics) used for parasite control.


The most important small ruminant gastrointestinal nematode is Haemonchus contortus (Barber pole worm), which affects many species of animals. The barber pole worm is literally a blood-sucking worm that can cause severe anemia and death. 

The DrenchRite™ Assay is a test performed to detect drug resistance in Haemonchus contortus parasites of small ruminants, camelids, and some exotic animals. This specialized and extremely useful test allows us to determine the resistance status for all of the different drug classes commercially available for parasite control.  

The DrenchRite™ test utilizes eggs isolated from the feces collected from of a number of infected animals in a herd.  The isolated eggs are placed in a specially made 96 well assay plate. These plates have doubling concentrations of the different drugs in them, along with “no drug” wells to serve as controls.

The eggs are incubated in the plate for 7 days to allow hatching and subsequent development of the larvae to the 3rd stage, which is the stage the animal ingests on the pasture and serves as the source of infection.  At the end of the incubation period, the plate is examined using a microscope. The parasites present are identified by species, and the stage of development for each larva is determined and compared to the larvae that develop in the control wells. Using this information, we can make a determination of susceptibility, suspected resistance or resistance for each of the three classes of dewormers currently available.

For the producer, the DrenchRite™ Assay answers two critical questions regarding each individual herd/flock: 1) What parasites does my herd/flock have?; and 2) Which drugs are effective on my farm?  Identifying the species of parasites infecting the animals is a key part of the test. The DrenchRite™ assay provides a reliable method of detecting drug resistance to the parasites in your herd.  It is easy for the producer.  

All that is required is to properly collect and ship the feces to the laboratory.  We provide specific collecting and shipping


instructions, to ensure that a viable sample will be received in the laboratory for the test.  

The special handling required is due to the nature of the Haemonchus eggs. These eggs are very temperature sensitive, so storing the feces in the refrigerator or directly on ice packs for more than 24 to 48 hours will actually keep the eggs from hatching, thereby invalidating the assay.


Yet if the feces are just stored in baggies with air at summer temperatures, the eggs will hatch out before we can set up the assay, again invalidating the assay. The set-up for this assay is labor-intensive, so we ask clients to please call the laboratory to pre-schedule the assay before collection to ensure we can perform the assay once it arrives in the lab.

Please contact Dr. Adriano Vatta ( at Louisiana State University School of Veterinary Medicine, Department of Pathobiological Sciences for further information.  Dr. Vatta's laboratory offers veterinary laboratory services and advice in the field of internal parasite resistance in sheep, goats, cattle, exotic hoof stock, dogs, cats, and horses.

DrenchRite™ Plate

Originally published August 2012. Reviewed and updated June 2024 by Adriano Vatta.

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